Piggy-back fuseholders - a right way and a wrong way?

[Pjones]

A correct graphic would really be nice at this point.

I had to pull out my daughters best wrighting sticks for this one...

The problem with sending your 12 VDC to the common side of the Piggy back Fuse holder (side A) is that it can overload the circuit that the original Fuse is trying to protect. In my example within the picture you can see that, if installed incorrectly then the Piggy back Fuse holder could allow 20 Amps to enter a 10 amp circuit. You may get it to work after you install it but if something shorts out or fails then you may cause a lot of damage that could have been avoided if you simply ran your Piggy backed circuit in series through the original Fuse. Let's keep in mind that a Fuse is not there to make the equipment operate, it is there to protect the circuit when it fails.

This is where my knowledge is limited, I'm not a car guy, I know voltage setups in buildings and equipment. I've only ever played around with automotive wiring for pleasure so my experience is limited in this area in the sense that I don't know if all manufactures follow the same standards and practices. There is reason that the automotive industry would do things a little differently but a lot of the principles will apply regardless.

Fuses are run in series all the time. This is common in high voltage and low voltage systems, both AC and DC as well. They are designed to pass through their rated capacity and fail once the amperage exceeds the rated amount by a specified amount and or time, dependng on what the Fuse specs include.

Fuses are sized to protect the circuit not the load. If your power window motor winding burns out and shorts to the chassis then this could cause the power wires to light up like a glow stick. The Fuse is sized to protect the wire and components from failure, once the Fuse blows the motor has already failed. If a 10 amp Fuse is installed in a circuit its most likely because that circuit can't handle any more than 10 Amps. The manufacturer doesn't want the motors in rush current to cause the Fuse to open when you use the window so the Fuse won't be sized exactly to the operating current of that device. Instead they will put the biggest possible Fuse into that circuit so that these nuisance trips don't occur and the circuit still remains protected as designed. Increasing the load beyond the fuses rated ampacity is risky, dangerous, and asking for an unexpected failure. Rule of thumb is that this failure won't happen the first time you use it. For instance, keeping with the power window circuit for example, your window motors will draw a modest amperage when you use them in the fall, but when you try to roll down all the windows at the same time to clear the ice off of them in the winter the motors will pull much more amperage as they struggle to break through the ice. The Fuse and circuit need to be sized for this even though it may only happen once a year (depending where you live), for the rest of the year the Fuse would appear to be over sized. If you have bypassed the circuits 10 amp Fuse in order to pull more amperage, then when this time comes for that circuit to require the protection it was supposed to have, you will have melted wires or caused a switch to fail. Instead of simply replacing the Fuse and moving your Piggy back to a different circuit you now have to rewire and trace down everything that got damaged.

Putting the Piggy back Fuse in series with the original Fuse will retain the circuits original intended protection and provide a means for you to create a second circuit to have an equal or lower rated ampacity. If you run wire from the Piggy back Fuse to your dash cam and the wire is only rated for 5 Amps protection then you should NOT be installing a second 10 amp Fuse into the Piggy Back slot but rather the proper 5 amp Fuse should be used. The original 10 amp Fuse should remain in the original circuit slot. This new 5 amp Fuse will have no effect on the original 10 amp Fuse that is still protecting the original circuit.

I suspect when people are saying "the original Fuse size should be used" that there was some lost in translation confusion or misunderstanding as the info was passed along. My suspicion is that people have been installing these on circuits that were already fully loaded and it was causing the Fuse to open (blow) , this could lead to people putting larger fuses in to stop this but that creates a hazard. So the manufactures probably specified something like "must use the original size Fuse" implying that the original size Fuse was to be used to serve the original circuit, but was misunderstood to mean that the original size Fuse must be used in both circuits. As I said above, if the wires or relays that are installed into your new circuit can't handle the full load of the original size Fuse then the correct (smaller) size Fuse should be used.

Its a long post but I think I covered most of the topics that have been discussed...

Sent from my SM-A520W using Tapatalk

 

[Nigel]

I had to pull out my daughters best wrighting sticks for this one...
....

Nice post, and accurate

In my car, the engine compartment fusebox is of the bus bar style and also contains a number of relays which switch the main circuits on/off. The passenger compartment has individual supplies to each fuse, some from switches as in your diagram and some from relays. The switches, relays and wire supplying the power should all be protected by the original fuse. Two months ago I had to replace 3 failed relays that were soldered into the car's main computer board, they controlled the power windows, central locking and windscreen wipers all of which had stopped working, all supplying individual fuses in the fuse box. If I had needed to buy a new computer board because I couldn't solder, or the relays had burned rather than just failed to switch then by the time the alarm system was reprogrammed by a main dealer to match the engine ECU it would have cost around $3000. And of course if you really mess up you can set fire to the car and loose everything.

Do it properly and you don't need to worry.

(My relays died through corrosion, not overload.)

 

[Pjones]

Just to further clarify, because I've seen it mentioned a number of times in here now, just because the dash cam only draws 0.2 or 0.5 amps doesn't make it any more ok to install the piggy back incorrectly (with fuses ran in parallel). You need to plan for a worst case scenario, that is when a fuse is needed and what it's designed to protect against. When the 0.5 amp devices circuit shorts out it could be moving a lot more that 0.5 amps. I've seen dead shorts that will blow a fuse instantly and very minor shorts that are found because it adds to the current draw on the circuit. Things fail in different ways and you won't always see it coming. To get to my point though, if I haven't beat this horse enough, unless the fuse is pulling directly off a buss bar and it is fastened in a way that will allow the connector to handle additional current then the possibility of a short combined with the operating current of the original circuit could be enough to cause damage or a hazard. It's only 0.5 amps until something happens.


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[Ralph2]

A correct graphic would really be nice at this point.

Reading an automotive electrical schematic.. is probably beyond most folks capacity.

Ralph, I see your point now. So what should I probe with?

The only safe way to check for line voltage is to be using a multi meter. Looking for a specific wire tucked into the maze of bundles coming out of the fuse block is really really hard. If.. and that too is hard.. you can find an electrical schematic of your car / model and year it will identify the color of the wire.. Trust me.. you are better off finding a source for your attachment at the fuse block by adding one of the add a fuse things..

But.. to eliminate the cover problem and you are determined to splice into an existing wire. It is relatively easy to find an ACC source by finding the wire at the destination of say the cigarette lighter.. An always on source gets trickier.. find something that is powered all the time.. then find the motor or light and splice into the wire powering that.

 

[LMJ]

I had to pull out my daughters best wrighting sticks for this one...

The problem with sending your 12 VDC to the common side of the Piggy back Fuse holder (side A) is that it can overload the circuit that the original Fuse is trying to protect. In my example within the picture you can see that, if installed incorrectly then the Piggy back Fuse holder could allow 20 Amps to enter a 10 amp circuit. You may get it to work after you install it but if something shorts out or fails then you may cause a lot of damage that could have been avoided if you simply ran your Piggy backed circuit in series through the original Fuse. Let's keep in mind that a Fuse is not there to make the equipment operate, it is there to protect the circuit when it fails.

This is where my knowledge is limited, I'm not a car guy, I know voltage setups in buildings and equipment. I've only ever played around with automotive wiring for pleasure so my experience is limited in this area in the sense that I don't know if all manufactures follow the same standards and practices. There is reason that the automotive industry would do things a little differently but a lot of the principles will apply regardless.

Fuses are run in series all the time. This is common in high voltage and low voltage systems, both AC and DC as well. They are designed to pass through their rated capacity and fail once the amperage exceeds the rated amount by a specified amount and or time, dependng on what the Fuse specs include.

Fuses are sized to protect the circuit not the load. If your power window motor winding burns out and shorts to the chassis then this could cause the power wires to light up like a glow stick. The Fuse is sized to protect the wire and components from failure, once the Fuse blows the motor has already failed. If a 10 amp Fuse is installed in a circuit its most likely because that circuit can't handle any more than 10 Amps. The manufacturer doesn't want the motors in rush current to cause the Fuse to open when you use the window so the Fuse won't be sized exactly to the operating current of that device. Instead they will put the biggest possible Fuse into that circuit so that these nuisance trips don't occur and the circuit still remains protected as designed. Increasing the load beyond the fuses rated ampacity is risky, dangerous, and asking for an unexpected failure. Rule of thumb is that this failure won't happen the first time you use it. For instance, keeping with the power window circuit for example, your window motors will draw a modest amperage when you use them in the fall, but when you try to roll down all the windows at the same time to clear the ice off of them in the winter the motors will pull much more amperage as they struggle to break through the ice. The Fuse and circuit need to be sized for this even though it may only happen once a year (depending where you live), for the rest of the year the Fuse would appear to be over sized. If you have bypassed the circuits 10 amp Fuse in order to pull more amperage, then when this time comes for that circuit to require the protection it was supposed to have, you will have melted wires or caused a switch to fail. Instead of simply replacing the Fuse and moving your Piggy back to a different circuit you now have to rewire and trace down everything that got damaged.

Putting the Piggy back Fuse in series with the original Fuse will retain the circuits original intended protection and provide a means for you to create a second circuit to have an equal or lower rated ampacity. If you run wire from the Piggy back Fuse to your dash cam and the wire is only rated for 5 Amps protection then you should NOT be installing a second 10 amp Fuse into the Piggy Back slot but rather the proper 5 amp Fuse should be used. The original 10 amp Fuse should remain in the original circuit slot. This new 5 amp Fuse will have no effect on the original 10 amp Fuse that is still protecting the original circuit.

I suspect when people are saying "the original Fuse size should be used" that there was some lost in translation confusion or misunderstanding as the info was passed along. My suspicion is that people have been installing these on circuits that were already fully loaded and it was causing the Fuse to open (blow) , this could lead to people putting larger fuses in to stop this but that creates a hazard. So the manufactures probably specified something like "must use the original size Fuse" implying that the original size Fuse was to be used to serve the original circuit, but was misunderstood to mean that the original size Fuse must be used in both circuits. As I said above, if the wires or relays that are installed into your new circuit can't handle the full load of the original size Fuse then the correct (smaller) size Fuse should be used.

Its a long post but I think I covered most of the topics that have been discussed...

Sent from my SM-A520W using Tapatalk

Excelllent. Thanks for the explanation in detail. IMO too many people just wnat to get their cam running without realizing they could mess up their car's electronics badly.

Just to confirm what you are Nigel are saying, the diagram marked 12v with the red arrows by user Nir https://dashcamtalk.com/forum/threads/piggy-back-fuseholders-a-right-way-and-a-wrong-way.7524/ at the start of this thread is incorrect. and the fuse tap should be insterted so that the 12V goes through the other side instead of the one where its currently marked 12V?

 

[LMJ]

Just to further clarify, because I've seen it mentioned a number of times in here now, just because the dash cam only draws 0.2 or 0.5 amps doesn't make it any more ok to install the piggy back incorrectly (with fuses ran in parallel). You need to plan for a worst case scenario, that is when a fuse is needed and what it's designed to protect against. When the 0.5 amp devices circuit shorts out it could be moving a lot more that 0.5 amps. I've seen dead shorts that will blow a fuse instantly and very minor shorts that are found because it adds to the current draw on the circuit. Things fail in different ways and you won't always see it coming. To get to my point though, if I haven't beat this horse enough, unless the fuse is pulling directly off a buss bar and it is fastened in a way that will allow the connector to handle additional current then the possibility of a short combined with the operating current of the original circuit could be enough to cause damage or a hazard. It's only 0.5 amps until something happens.


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Thanks again. Thinking statistically, I think too many people are not focusing on "tail risk" from running fuses in parallel. If you think about risk management, its the outlier event that is gonna cost you a lot of money.

 

[Lola]

I had to pull out my daughters best wrighting sticks for this one...

The problem with sending your 12 VDC to the common side of the Piggy back Fuse holder (side A) is that it can overload the circuit that the original Fuse is trying to protect. In my example within the picture you can see that, if installed incorrectly then the Piggy back Fuse holder could allow 20 Amps to enter a 10 amp circuit. You may get it to work after you install it but if something shorts out or fails then you may cause a lot of damage that could have been avoided if you simply ran your Piggy backed circuit in series through the original Fuse. Let's keep in mind that a Fuse is not there to make the equipment operate, it is there to protect the circuit when it fails.

This is where my knowledge is limited, I'm not a car guy, I know voltage setups in buildings and equipment. I've only ever played around with automotive wiring for pleasure so my experience is limited in this area in the sense that I don't know if all manufactures follow the same standards and practices. There is reason that the automotive industry would do things a little differently but a lot of the principles will apply regardless.

Fuses are run in series all the time. This is common in high voltage and low voltage systems, both AC and DC as well. They are designed to pass through their rated capacity and fail once the amperage exceeds the rated amount by a specified amount and or time, dependng on what the Fuse specs include.

Fuses are sized to protect the circuit not the load. If your power window motor winding burns out and shorts to the chassis then this could cause the power wires to light up like a glow stick. The Fuse is sized to protect the wire and components from failure, once the Fuse blows the motor has already failed. If a 10 amp Fuse is installed in a circuit its most likely because that circuit can't handle any more than 10 Amps. The manufacturer doesn't want the motors in rush current to cause the Fuse to open when you use the window so the Fuse won't be sized exactly to the operating current of that device. Instead they will put the biggest possible Fuse into that circuit so that these nuisance trips don't occur and the circuit still remains protected as designed. Increasing the load beyond the fuses rated ampacity is risky, dangerous, and asking for an unexpected failure. Rule of thumb is that this failure won't happen the first time you use it. For instance, keeping with the power window circuit for example, your window motors will draw a modest amperage when you use them in the fall, but when you try to roll down all the windows at the same time to clear the ice off of them in the winter the motors will pull much more amperage as they struggle to break through the ice. The Fuse and circuit need to be sized for this even though it may only happen once a year (depending where you live), for the rest of the year the Fuse would appear to be over sized. If you have bypassed the circuits 10 amp Fuse in order to pull more amperage, then when this time comes for that circuit to require the protection it was supposed to have, you will have melted wires or caused a switch to fail. Instead of simply replacing the Fuse and moving your Piggy back to a different circuit you now have to rewire and trace down everything that got damaged.

Putting the Piggy back Fuse in series with the original Fuse will retain the circuits original intended protection and provide a means for you to create a second circuit to have an equal or lower rated ampacity. If you run wire from the Piggy back Fuse to your dash cam and the wire is only rated for 5 Amps protection then you should NOT be installing a second 10 amp Fuse into the Piggy Back slot but rather the proper 5 amp Fuse should be used. The original 10 amp Fuse should remain in the original circuit slot. This new 5 amp Fuse will have no effect on the original 10 amp Fuse that is still protecting the original circuit.

I suspect when people are saying "the original Fuse size should be used" that there was some lost in translation confusion or misunderstanding as the info was passed along. My suspicion is that people have been installing these on circuits that were already fully loaded and it was causing the Fuse to open (blow) , this could lead to people putting larger fuses in to stop this but that creates a hazard. So the manufactures probably specified something like "must use the original size Fuse" implying that the original size Fuse was to be used to serve the original circuit, but was misunderstood to mean that the original size Fuse must be used in both circuits. As I said above, if the wires or relays that are installed into your new circuit can't handle the full load of the original size Fuse then the correct (smaller) size Fuse should be used.

Its a long post but I think I covered most of the topics that have been discussed...

Sent from my SM-A520W using Tapatalk

Hi PJones, If I'm reading your diagram correctly (think I am) the last two diagrams, wrong and right, the only difference is the direction you plug the piggy-back into original bus slot. The wrong way is in parallel the right way is in series ?

What is confusing to me is the right way picture/diagram showing the two fuses being offset when in reality they aren't, they are just two fuse slots in the piggy- back one above the other. Does this mean that the piggy-back is turned right or left to conform to the diagram to be correct ?

I had calculus 1-4 and it wasn't this confusing!

 

[LMJ]

http://www.bcae1.com/fuses.htm

"Fuse Taps:
Many times, you'll need to get power to a new accessory but won't want to run a wire to the battery. The following is one solution that works relatively well where there is sufficient space in the fuse panel. You pull a fuse, insert the tap and connect the accessory to the red wire. It sounds simple enough but there are a few things you need to know to ensure that there are no problems. You need to make sure that the fuse that you're using for the tap is rated well above the current required for the device/equipment you're installing. The wire in the vehicle is likely barely big enough for the fuse that's protecting it. You wouldn't want to use this in a slot that's supposed to have a 5 amp fuse for a 15 amp device. There are two ways to install this tap. If you install it with the terminal nearest the bottom-left of the image in the 'hot' side of the fuse slot (the side of the fuse holder that has 12v when no fuse is in the slot), each fuse will be pulling directly from the vehicle wiring. In this configuration, you must not use a fuse larger than the fuse that the slot originally had. If you install it with the terminal nearest the red wire in the hot side of the fuse slot, it's going to be a bit safer because you will pull current through both fuses and if the main fuse (the one nearest the terminals on the tap) is the original fuse size, you will never be able to pull too much current through the wiring in the vehicle. these are mainly for use to supply low current devices like radar detectors, LED light bars, and chargers for MP3 players. For devices that draw more than 5 amps, I'd suggest running a dedicated wire to the battery (with the appropriate fuse at the battery)."


If I am reading this correctly, P Jones and Nigel and others in agreement with them are correct. The guy installs high amp car audio equipment and if you read his stuff he clearly knows what he is doing. I didn't email him - one of you might ask him to participate here. However, the paragraph above is good to know if I understand it correctly.

 

[LMJ]

Hi PJones, If I'm reading your diagram correctly (think I am) the last two diagrams, wrong and right, the only difference is the direction you plug the piggy-back into original bus slot. The wrong way is in parallel the right way is in series ?

What is confusing to me is the right way picture/diagram showing the two fuses being offset when in reality they aren't, they are just two fuse slots in the piggy- back one above the other. Does this mean that the piggy-back is turned right or left to conform to the diagram to be correct ?

I had calculus 1-4 and it wasn't this confusing!

It means the hot is flowing in through the other side first. So the current passes the original fuse first then through the new fuse. The other way around, the current splits and each takes its own path.

I'd rather lose $300 on a dash cam than spend $3,000 at the dealer getting them to diagnose and replace the wiring in my vehicle.

 

[Ralph2]

There are far too many cooks in this kitchen.. One of the better schematics is this.. and if you imagine the power flowing up from either leg.. you can see how one way it ALL goes through the original fuse.. and the other way by-passes the original fuse.

 

[Pjones]

Hi PJones, If I'm reading your diagram correctly (think I am) the last two diagrams, wrong and right, the only difference is the direction you plug the piggy-back into original bus slot. The wrong way is in parallel the right way is in series ?

Correct. Installing the piggy back so that the new fuse is in series with the existing fuse is the safest way to install the device.

What is confusing to me is the right way picture/diagram showing the two fuses being offset when in reality they aren't, they are just two fuse slots in the piggy- back one above the other. Does this mean that the piggy-back is turned right or left to conform to the diagram to be correct ?

Yes, the correct method has simply rotated the piggy back 180 degrees.
I have drawn pictorial representation of how the piggy back works. Because we live in a 3D world but my drawing is 2D the drawing will not look physically the same as the component in your hand. In actuality the piggy back is simply swapped end for end so that power enter from the same side as the red wire when in the correct orientation. (I ran out of room to draw the diagram differently so that the two fuses line up.) the important thing to notice is that the new fuse takes power from the down stream side of the existing fuse when installed correctly so that the two fuses run in series.

I had calculus 1-4 and it wasn't this confusing!

Your questions about the wiring diagram are common for those who don't use them very often.



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[SawMaster]

Power flow should go like this: Existing power to car fuse@ fusebox > car fuse at original rating > fuse tap on downstream side of car fuse> new fuse for new device at it's correct rating > new device (called series wiring).

Done this way, it's impossible to overload the car wiring as the car fuse will blow before the car wire feeding it smokes. The worst which can happen is the car device uses nearly all the fuse capacity so when the new device is added on top, the car fuse blows during normal operation. That is solved by trying another different car fuse to tap into. Non-worst-case: the new dashcam fuse should blow first if there is a short in it without blowing the car fuse.

I still believe it very unlikely that tapping on the hot side of the car fuse by mistake will cause an issue as long as a 2A fuse is inline with the dashcam. Dashcams typically draw around 1/2A, perhaps slightly more during startup, while other devices may draw near the full 2A constantly. No car designer is going to leave that little of a safety margin (1/2A) without going up one wire size on the larger capacity circuits. The cost to repair one wiring failure under warranty could easily cover the cost of one larger gauge wire in many thoudsands of cars when built. It's their job on the line if they use too small of a wire and warranty repairs become commonplace. So I think any increased load at cam start-up (1A or so) would likely still be within design limits even if just barely. Should the cam PS short out, the temporary car wire overload will be momentary until the 2A dashcam fuse blows, thus possibly heating the wire but probably not melting the insulation. Note that this paragraph is just my opinion based on my own experiences along with those shared by trusted individuals. It is not recommended practice to tie in on the hot side of the car fuse; taps should be on the downstream side.

Myself, I don't use "add-a-fuse" devices; rather I use inline fuses after making my tap (either at a car fuse or elsewhere), but I'm always on the downstream (fused) side of whatever I'm tapping into. Or I run my own large fused 'bus' directly to the battrery as I've usually got a whole lot of things to be powering and I want the best possible source for everything. I'm very familiar with car electrics so I sometimes do things I wouldn't recommend for others.

Phil

 

[RyuZaKi]

For switched power in my car, i use fuse tap + 4 pin relay + fuse block.

relay diagram:
pin 85 = ground
pin 86 = fuse tap ( ACC slot)
pin 30 = Battery +
pin 87 = fuse block

this setup won't cause much stress/load on original ACC wire in the fuse box since the tapped power only act as switch.

the fuse block i use

 

[Lola]

There are far too many cooks in this kitchen.. One of the better schematics is this.. and if you imagine the power flowing up from either leg.. you can see how one way it ALL goes through the original fuse.. and the other way by-passes the original fuse.

Hi Ralph2, In your diagram of piggy- back you show a connection between the 2 fuse slots as being on the far side away from the downstream red wire. Is this the way all fuse taps are designed/made, one connection between the two fuse slots it being on the side away from downstream ?

Just trying to make sure I totally understand.

 

[Ralph2]

Hi Ralph2, In your diagram of piggy- back you show a connection between the 2 fuse slots as being on the far side away from the downstream red wire. Is this the way all fuse taps are designed/made, one connection between the two fuse slots it being on the side away from downstream ?

Just trying to make sure I totally understand.

Yes. That is the way they are all made. And.. it means that depending on which way you orient the fuse tap you get the "hot" leg either passing ALL the electricity through the first original fuse or.. have each fuse act independently. There is much controversy over which orientation is correct. I favor the orientation that supplies each fuse separately.

In practical terms it means very little. The fuse is there to open the circuit should there be a short in the wiring downstream. A fuse is not designed as a throttle to the flow of electricity. However by installing a fuse tap so that all the flow goes through the first fuse you can develop a situation where it does throttle. Supposing your draw on the normal circuit may go up to 6 amps and it is protected with a 10 amp fuse... No problem.. but you add a fuse tap and your second circuit has a 4 amp draw. To protect this circuit you protect it with a 10 amp fuse. Now.. you have a possible 10 amp load all going thru the original 10 amp fuse.. and now at capacity it may well fail.

As most of the discussion in the forum is about adding a power supply for a camera.. again in practical terms either way is fine as the camera draws such a small amount of power it is extremely unlikely that the original fuse can not handle the extra load.

There is also some controversy about the size of that second fuse.. and here is where wire size comes into play. The wires supplied to most of the camera gadgets are designed by the manufacturer for the load expected.. and are "small" thus one needs to pick a fuse that will fail.. before the wire.. acting as a fuse.. fails.. should there be a short in the system. Excessive flow through a small wire will heat it.. to the point of failure.. which could start a fire.

 

[Pjones]

...There is much controversy over which orientation is correct. I favor the orientation that supplies each fuse separately.

In practical terms it means very little. The fuse is there to open the circuit should there be a short in the wiring downstream. A fuse is not designed as a throttle to the flow of electricity. However by installing a fuse tap so that all the flow goes through the first fuse you can develop a situation where it does throttle. Supposing your draw on the normal circuit may go up to 6 amps and it is protected with a 10 amp fuse... No problem.. but you add a fuse tap and your second circuit has a 4 amp draw. To protect this circuit you protect it with a 10 amp fuse. Now.. you have a possible 10 amp load all going thru the original 10 amp fuse.. and now at capacity it may well fail.

But that is exactly the point of a fuse. If you overload the circuit then it fails before you cause damage to the circuit. When you wire the fuses in parallel then you have taken control away from the fuse and it can no longer do its job properly. Fuses are installed in series all the time without issue. The fuses in the car were specifically chosen by their design engineers to allow the circuits to operate safely, by installing the fuse in parallel you are changing their design and taking away the ability for the original fuse to protect the circuit.

Let's go back to this house panel example, although it was incorrectly applied before we are going to fix that... You have power that comes in from the pole outside, enters your main breaker. This main breaker protects the wires, that supplie the panel, from damage. From the main breaker power goes to a buss bar. Each circuit from that buss bar has its own breaker because each circuit has different wires that the breaker is protecting and those wires can't handle the current that the main breaker can supply.

So how does that apply an how could it be relevant? The main breaker is similar to the original fuse that you are trying to piggy back off of. This fuse needs to remain unaltered because the circuit it is protecting requires its protection at that specific rating in order to be safe. The piggy back fuse is similar to the breaker coming off of the buss bar. (If you want to see a representation of this then plug 10 of the piggy backs together all in the same direction and you will see the common side looks exactly like a buss bar while all the red wires that come off create multiple circuits.) The main circuit will always require the same protection because you haven't changed any components within that circuit to allow for additional load. All the circuits being added get installed after that main breaker (original fuse)

As most of the discussion in the forum is about adding a power supply for a camera.. again in practical terms either way is fine as the camera draws such a small amount of power it is extremely unlikely that the original fuse can not handle the extra load.

The circuit probably can support an additional small load but you are guessing about that. If you are wrong about your guess then you either melt a wire if you install the fuse incorrectly or you simply cause the original fuse to fail as designed if you install the piggyback in series (the correct method for most situations). In addition to this you are anticipating that nothing will fail causing your original small load to now become a large load.

 

[Ralph2]

So how does that apply an how could it be relevant? The main breaker is similar to the original fuse that you are trying to piggy back off of.

Sorry but your analogy is wrong. If.. there is a "main breaker" it is the relay (normally in a box under the hood) feeding a particular buss.. that becomes the hot side in your fuse box. To keep with your analogy.. From the main breaker.. the various circuit breakers in your home panel take power from the buss and pass it on to a junction box.. where it may turn on a light.. etc.. it then may continue on and supply many other outlets.. ALL protected by one circuit breaker, if the collective load reaches the rated amperage of the "breaker".. it will trip.

Same with your car.. your fuse (similar to the house breaker) takes power from the internal buss.. passes it through the fuse and down the cold side leg.. where it then goes to a light.. and continue on to other similar uses.

When you use an "add a fuse thing".. ALL you are doing is making a new connection to the hot side of the buss.. passing it through a new fuse and on to your application.
Should you prefer to flip the fuse 180 degrees.. then a similar analogy to your home panel would be.. From the circuit breaker out put.. add a wire.. going to the inlet side of a new breaker.. and then a wire from the outlet side of the breaker to your application. This of course can not be done in a normal home panel.. but a similar example.. and commonly done.. would be to add a wire to an existing breaker.. then take that output to a new distribution (panel) box.. where you now could add new breakers.. supplying applications in.. say your garage..

 

[Pjones]

Sorry but your analogy is wrong. If.. there is a "main breaker" it is the relay (normally in a box under the hood) feeding a particular buss.. that becomes the hot side in your fuse box. To keep with your analogy.. From the main breaker.. the various circuit breakers in your home panel take power from the buss and pass it on to a junction box.. where it may turn on a light.. etc.. it then may continue on and supply many other outlets.. ALL protected by one circuit breaker, if the collective load reaches the rated amperage of the "breaker".. it will trip.

I apologize for bringing the house analogy back, I thought I had a way to explain it so that it would make more sense but it's really just creating confusion. The analogy was intended to help visualize the scenario easier by applying it to something that people may be more familiar with. It becomes apparent to me that my good intention has had unintended results when a relay is being considered as a protection device within the analogy. Having to swap functions of components in order to visualize the circuit better just adds confusion and blurriness. My intentions with posting the original diagram was to help people who are unfimiliar with electrical circuits to preform these installations safely. If it starts causing confusion then its time to stop and rethink how to explain and discuss this in a way to minimize this confusion. It might be best to just do away with the analogies all together to avoid further confusion but I would like to first try and clarify some of the points that you brought up in your post.

For those who are not interested in the banter you can scroll to the bottom and see the spoiler in bold.

I was unfortunately having to multitask as I was typing my last post while trying to play with my kids so my attention was split and I didn't proof read anything to make sure it was clearly described (I still haven't and at this point but I don't intend to make any changes to it, and I'm trying to wright this before another child wakes up). When I was using the house analogy I was trying to keep it very simple so that people with limited knowledge of this setup would hopefully still be able to follow along with as little struggle as possible. With that intent in mind, I described the original fuse as the main breaker and the piggy back fuse as the breaker connecting off a hypothetical buss bar. This was to assume that the main fuse was served by a wire from the battery or a switch. I wanted to use a simple circuit because additional components upstream of the fuse will have little effect on what we are discussing so it was easier to cut them out of the discussion to avoid confusion. In the vehicle if the original fuse connects directly to a true buss bar with a hard connection, and that buss bar is protected by its own fuse, then installing the piggyback fuse in a parallel setup would most likely not cause any harm. in that case it would be similar to adding a new circuit directly off of the buss bar itself providing that the connection point between the original fuse and the buss bar is capable of supporting that new additional load. It sounds like this buss bar setup is not the situation in this case so I have been basing my information as though the fuse box has individual wires feeding the hot side of each fuse.

When you use an "add a fuse thing".. ALL you are doing is making a new connection to the hot side of the buss.. passing it through a new fuse and on to your application.

It's not connecting to a buss bar though. Unless I miss read somewhere, people are saying that their fuse boxes are fed by multiple wires connecting to each fuse. By adding the piggy back to the hot side then you are creating a situation where that wire that feeds the original fuse can get overloaded.

Should you prefer to flip the fuse 180 degrees.. then a similar analogy to your home panel would be.. From the circuit breaker out put.. add a wire.. going to the inlet side of a new breaker.. and then a wire from the outlet side of the breaker to your application. This of course can not be done in a normal home panel..

There are times where we do things similar to this but not inside the main panel. Having to explain the situation and then try to apply it to the original topic is asking for confusion and will most certainly drift this thread far away from the intended direction.

but a similar example.. and commonly done.. would be to add a wire to an existing breaker.. then take that output to a new distribution (panel) box.. where you now could add new breakers.. supplying applications in.. say your garage..

I had just about finished typing out a long post about how this theory worked because the fuses are then in series but at the end I was finding it still really didn't.... The problem is that the circuit breaker that feeds the sub panel is in the wrong location to be accurate in that scenario. It really needs to be a main panel, not a sub. The other problem is that it's an analogy, and that isn't really working out too well because i'm having to simplify a household system to apply to an automotive scenario and the two aren't really the same (close but not close enough that you can nit pick at the little details because... it's a house...)

With continuing to simplify minor things to keep this less confusing... A main panel has a main breaker that is sized to stop the feed entering your house (from the pole outside) from overloading and also to protect the panel from damage. Because we have wires entering the fuse box we can't use a scenario where the circuit breakers feed is supplied by a buss bar. The best way I can think to use the analogy is to consider the original fuse as the main breaker and pretend that the panel is the original load (which it has no load by itself, so again the analogy thing is failing as we are having to again swap functions of components to make this work and thus becoming more confusing... we'll have to pretend that it does have load by itself in this scenario, i'm sorry its just easier that way, I think). The piggy back fuse would be similar to adding a circuit breaker onto that panel. Because the wires feeding the main panel can only handle the same ampacity that the main breaker shows, that main breaker needs to see all the current that travel those wires in order to protect them. If you bypass that main breaker in order to add additional load then that main breaker can no longer do the job that it was designed to do. This is the same for the original fuse that is protecting its circuit as it was designed. Without first confirming that the circuit can in fact handle additional load safely then the additional load needs to be wired so that the original fuse sees all the new amperage from that load.

The general theory in the analogy works but if anybody wants to nit pick the details then we shouldn't keep using analogies. My apologies again for trying to reintroduce them.

 

[Ralph2]

It's not connecting to a buss bar though. Unless I miss read somewhere, people are saying that their fuse boxes are fed by multiple wires connecting to each fuse. By adding the piggy back to the hot side then you are creating a situation where that wire that feeds the original fuse can get overloaded.

Yes it is. One of the legs of the fuse goes directly into the buss bar.. this is the hot side.. the other leg of the fuse attaches to a wire coming out the back of your fuse box where you can not see it and goes to your application. With an Add a Fuse thing that leg is wired to both fuses.. the original and the new one. You place the original fuse back into one slot.. your original circuit is protected as it was.. And your new fuse now provides power to the pig-tail coming out of the add a fuse thing.

There is no magic here.. it is the only way it can work. Study a schematic of an add a fuse or take one apart.

And for what it is worth.. a typical fuse box is fed from a distribution center that in turn is fed from your main battery. Depending on the position of the key relays send power from the distribution center (typically close to the battery under the hood) to power always on circuits ACC circuits etc. that in turn through individual fuses power the various applications.

 

[Lola]

I thought I understood what was going on, but both of you guys seem set in the way you do this and it has become confusing after the last two posts. I recognize that you both agree on it not mattering because of the small amount of amps used by the camera, however it does matter a great deal since I will use the method under discussion to do other electrical work where it maters a great deal especially if higher amperage is in the scheme of of things.
I hope you keep on discussing this untill an agreement comes to the front in language mere mortals unused to doing this sort of thing is well versed in the correct procedure to follow.