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

Yes the fuse is meant to protect the whole circuit including the input to the fuse box. If you wire it your way you're basically sharing the load of the existing fuse. If you wire it the other way, so power comes from the input side of the existing fuse, you're adding more potential load than what the wires are made to handle.

For a dash cam either way is probably fine but IMO better safe than sorry. Plus if there is some kind of short, you can easily draw much more power than the 1 or 2 amps of the dashcam, and that becomes dangerous

Thanks so in summary my concerns are valid.
The dead short was precisely my concern, i.e my additional circuit dead shorts and wired the wrong way would = the combined value of both fuses, i.e. potentially way more than Mercedes designed the suppy to the fusebox to handle.
So in Summary Nigel is absolutely correct.
Hmmm funny these piggyback manufacturers dont spell this out big style!
 
go wiring up a fridge like this and it would be a valid concern, a dashcam though has very minimal current draw
Agreed; except if your add on circuit shorted to ground, and you were using the original circuit to its maximum?
Or am I wrong?
 
Agreed; except if your add on circuit shorted to ground, and you were using the original circuit to its maximum?
Or am I wrong?

if it shorted to ground it would blow the fuse, the fuses aren't there to protect the device, they're there to protect the vehicle
 
Agreed; except if your add on circuit shorted to ground, and you were using the original circuit to its maximum?
Or am I wrong?
Car fires are normally caused by shorting to ground where the short starts off at high resistance through a poor contact to dirty/painted and corroded metal and the resistance slowly drops with the wires gradually heating up, eventually the insulation around the wire gets so hot it catches fire, drips burning drops of plastic onto the carpets and destroys the car, since cars are made out of flammable materials, unlike home furnishings which are self extinguishing.

So the possibility that you can put two loads on a supply cable by putting the fuses in parallel instead of in series is genuinely a cause for concern.
If you use a 1amp fuse for your camera and add it to a 30amp circuit then there is unlikely to be a problem, but use a 10 amp fuse tap with a 10 amp fuse on a 10 amp circuit and your short could cause double the designed load before blowing the fuse and potentially maintain double the load for long enough to cause a fire. More likely the fuse will blow before the fire, but car fires do happen.
 
if it shorted to ground it would blow the fuse, the fuses aren't there to protect the device, they're there to protect the vehicle

Exactly my point ... if wired the wrong way around and it shorted to ground it would blow the fuse, but before it did , it could put the combined load of both fuses on the wires that supply the fuse box for that circuit. Which would be more than the wire was rated at. No?
Also if wire chaffing was the issue, the fuse doesnt always blow instantly. Of course when the load is greater than the fuse it will blow.
 
Car fires are normally caused by shorting to ground where the short starts off at high resistance through a poor contact to dirty/painted and corroded metal and the resistance slowly drops with the wires gradually heating up, eventually the insulation around the wire gets so hot it catches fire, drips burning drops of plastic onto the carpets and destroys the car, since cars are made out of flammable materials, unlike home furnishings which are self extinguishing.

So the possibility that you can put two loads on a supply cable by putting the fuses in parallel instead of in series is genuinely a cause for concern.
If you use a 1amp fuse for your camera and add it to a 30amp circuit then there is unlikely to be a problem, but use a 10 amp fuse tap with a 10 amp fuse on a 10 amp circuit and your short could cause double the designed load before blowing the fuse and potentially maintain double the load for long enough to cause a fire. More likely the fuse will blow before the fire, but car fires do happen.

Totally agree with your logic.
And I also agree its highly unlikely to happen with a tiny dashcam. But I cant see why people argue about doing it a way that negates the fundamental point of having a fuse of a given value?
Not to mention the legal implications if you do it. Et Al grenfield towers, bet a few people are having sleepless nights.
If I was one of those contractors ... I would have liquidated every asset, and be bobbing along on my new Yacht in the Caribbean. :)
 
It's not the size of the dashcam that matters, if you get a short then the power wont be reaching the dashcam.
It is the size of the fuse and wire you use that matters.
 
I would be concerned about the crappy Chinese fuse tap or crappy Chinese fuse failing, not the dash cam
 
A Ham radio friend bought a cheap fuse assortment which was made in China, and on testing them discovered that none were anywhere close to their rating :eek: I can't say this is the usual result but I now replace unknown fuses with a brand I know and trust, and even those get a visual inspection for apparent conductor size now :rolleyes:

While there is a correct and proper way to use a fuse tap, I'm with Jokin on it not being terribly relevant regards which side is tapped. No automotive circuit will use close to the fuses rated capacity- there is always extra margin allowed there to eliminate the chances of a fuse blowing in normal use. That is standard engineering practice, just the same as ensuring the wiring feeding the fuse can carry more current safely than the fuse us rated for. So for instance a known 12A load would have a 15A fuse fed by wiring which can carry 18A+ ;) Adding a 1.5A load to that circuit will likely give no trouble even with the load added to the wrong side of the factory fuse. With a larger load that concern would be valid.

What is more a concern (and something done more frequently) is tapping the wrong fuse to get the desired operation :mad: You should never tap a circuit related to car safety (such as exterior lighting, wipers, ABS, or airbags), nor should you ever tap a circuit related to car operation (such as fuel pump or engine computer). Always ask yourself what will happen should that car fuse blow while driving :cautious: Even with the added safety margin the car engineers put in the circuit, there is always a small chance something will go wrong and you do not want that to affect safe car operation in any way. This may leave you with only circuits which do not give the desired operation which means you'll either have to live with that or design an added circuit which does give the operation you desire even though that will be troublesome and costly.

Dashcams are for safety and it makes no sense to make your car less safe by wiring the cam unsafely. Tap only larger capacity circuits which will not detract from safety should the car fuse blow, and never use more than a 5A (lower is much preferred) fuse for the cam side of the tap. Follow these two rules and the odds are great you will not have problems even if better methods are possible :cool:

Phil
 
Yes the fuse is meant to protect the whole circuit including the input to the fuse box. If you wire it your way you're basically sharing the load of the existing fuse. If you wire it the other way, so power comes from the input side of the existing fuse, you're adding more potential load than what the wires are made to handle.

For a dash cam either way is probably fine but IMO better safe than sorry. Plus if there is some kind of short, you can easily draw much more power than the 1 or 2 amps of the dashcam, and that becomes dangerous
Hi again in my last post I failed to mention that I have worked as an electrical engineer in the past my secondary job was that of a fire control room officer in the merchant navy and as such I am very conversant in both subjects. First of all the method of fitting a piggyback connection as I described is the correct way and the only way if you want your fuses to work i.e they are the safety device for any electrical circuit.
Again people on this site seem to believe that it is OK to wire a high capacity item together with a low capacity item for example a dashcam and spotlights I can only suggest that they look at their house fusebox where you will see that lights, sockets, cooker, etc are on separate circuits for a very simple reason it is to prevent a house fire. When you apply that logic to a vehicle the same thing applies. The main vehicle circuits are also subdivided in the same way from very high to very low electrical current the addition of spotlights to a car would require an independent fuse and a relay to work without setting your car on fire. This site is primarily is to support the hardwiring of a dashcam people seem to be befuddled by the use of piggybacks they are like a spur from a circuit therefore using spotlights and dashcam on the same circuit fine if you want to risk setting fire to your vehicle better to use your dashboard lights even better use an unused fuse location to which you add let's say led daytime running lights. Remember that electric wiring faults are the foremost cause of fire that is before cigarettes became more popular. If you are unsure of how to wire up a dashcam then for your own safety I suggest you get someone qualified to either fit the dashcam or fit an additional fusebox.
 
To those that want to put it in backwards... because you are worried about the size of the supply "inside" the fuse box.. I suggest purchasing some welding cable and running that from the battery to the dash cam..
 
Hi since my last post I believe it would be better for me to post a step by step guide I note that piggybacks are mostly rated to 20 Amps, my personal preference would be to keep to 10 Amps with a maximum of 15 Amps, purely for safety reasons and unknown places of manufacturing. Right here we go wiring any item with a 5 Amp fuse, tools required a volt meter or cheap electric probe.
STEP 1
Find the nearest fusebox to where you wish to install the item.
STEP 2
Find a 5 amp fuse or empty fuse location ( note if using an empty location you will first have to determine that it is live )
STEP 3
We assume that we will use an existing 5 Amp circuit. Pull out the fuse and replace with the piggyback.
STEP 4
We need to determine that the piggyback is fitted correctly. First using the voltmeter set at 20 volts or the test probe. Connect the earth or ground lead to a suitable earth.
STEP 5
( Important to get this right )
Both fuses are to share the same live + or hot whatever you want to call it. As we have connected the earth or ground of the tester all you need to do is touch the red lead/cable to see if it is live.
If it is live it is placed in fuse box the wrong way round it is then a simple matter of moving the piggyback the opposite way round. If you do not get any indication either way it is due to not having the earth or ground lead of the tester is positioned correctly.
STEP 6
Now we have established the correct position of the piggyback. This seems to be the thing that is confusing people we have now established a usable spur from the live / hot side of the fuse only and this means that we now have 2 yes two separate usable circuits.
Replace the fuse of the circuit that you have linked to with the piggyback to check that you have got this right use the test meter or probe on the red lead if it has no power OK if it has you have the fuse in the wrong location slot.
STEP 7
Wire up the accessory according to the instructions the live / hot feed is the red wire on the piggyback. All you need to do is put the fuse in the piggyback the accessory you have installed should work perfectly if not check that all joints are clean dry and tight. Tip when all is working OK coat the joints with nail varnish and let it dry before covering them with silicon boots or tape.
Congratulations you have now have two separate circuits independently fused by only sharing the live / hot side of the piggyback.
 
Nice write up.. I would add to your #2 ( note if using an empty location you will first have to determine that it is live ) That it is live under the conditions you want.. as in, with or without the key in the ACC position.. and that in some vehicles power will be lost after a period of time.. or, if there is logic to shut off a circuit if there is an unexpected load.
 
Nice write up.. I would add to your #2 ( note if using an empty location you will first have to determine that it is live ) That it is live under the conditions you want.. as in, with or without the key in the ACC position.. and that in some vehicles power will be lost after a period of time.. or, if there is logic to shut off a circuit if there is an unexpected load.
Hi Ral
Nice write up.. I would add to your #2 ( note if using an empty location you will first have to determine that it is live ) That it is live under the conditions you want.. as in, with or without the key in the ACC position.. and that in some vehicles power will be lost after a period of time.. or, if there is logic to shut off a circuit if there is an unexpected load.
Hi Ralph yes you are pefectly correct and the use of any accessorie, for instance if you hardwire a satnav you need to be aware that when you start the engine you will create a power surge and destroy the satnav in a very short time, easy to place a switch in the circuit just before the satnav and switch on after the engine is running. Also my apologies in my step by step guide in step six I think I forgot to say that when first placing the piggyback into the fusebox it needs to have a fuse in it before it can be tester to determine the correct way either by turning the piggyback around or putting the fuse in the alternative location. The main thing is that the red live / hot + must not have any power at this stage. Ralph is also correct in trying to determine in what position the ignition key should be in. I think that we need to use a simple rule in that when you fit the accessorie think what do I have in the vehicle that meets the same or similar requirements and fuse size and choose that one to fit your piggyback.
Also when the piggyback is in position and fuse for the original circuit is still functioning correctly. I think that I have now gone as far as I can in fully explaining how this is designed as an easy option as opposed to the old wire splice or more modern junction box to add another power circuit. Before I sign off I think that I should warn people that doing a sit fix on a vehicle must be done correctly otherwise you will find out that if not you may well have invalidated your insurance don't believe me read the small print and you will soon discover that there are a hundred more reasons for refusing a payout than there are fulfilling one. For those that wish to do a haphazard job at any cost I recommend that you keep your locat fire department on speedier. Ancient Chinese proverb
You can educate those who listen in a reasonable time stupidity lasts a lifetime.
I will keep an eye on this site for a short while and will happily advise when possible this is due to other commitments.
 
Again people on this site seem to believe that it is OK to wire a high capacity item together with a low capacity item for example a dashcam and spotlights I can only suggest that they look at their house fusebox where you will see that lights, sockets, cooker, etc are on separate circuits for a very simple reason it is to prevent a house fire. When you apply that logic to a vehicle the same thing applies. The main vehicle circuits are also subdivided in the same way...
The problem here is that some people, including you, seem to think that their car fusebox works the same way as their house fusebox. Unfortunately it doesn't, the house fusebox is actually a distribution panel, the car fusebox isn't.

In the house fusebox, each fuse is supplied by a busbar capable of supplying as much power as the main fuse protecting it, thus it is not possible to overload it. In a car fusebox the fuses are supplied by individual wires from various sources, the 5 amp fuses are likely to by supplied by 5 amp cable, quite likely from a 5 amp relay or 5 amp transistor, put a 10 amp fuse in (or a wrong way around fuse tap with 2x5 amp fuses) and you can draw double the current that can safely be supplied and either damage things or set fire to the car. Even worse some of these circuits are safety critical; causing a failure in the ABS, stability control, electric steering systems etc. could kill someone. Car manufacturers always fit the minimum rated cable required for each circuit in order to save weight and reduce costs. Saving weight improves performance and fuel consumption, it's important, reducing costs improves profits.
  • You should not bypass the circuit's main fuse by installing the fuse tap the wrong way around.
  • The fuse tap should tap into the circuit after the existing fuse so that the entire circuit (both before and after the fuse) is still correctly protected by the original fuse as it was designed to be.
 
The problem here is that some people, including you, seem to think that their car fusebox works the same way as their house fusebox. Unfortunately it doesn't, the house fusebox is actually a distribution panel, the car fusebox isn't.

In the house fusebox, each fuse is supplied by a busbar capable of supplying as much power as the main fuse protecting it, thus it is not possible to overload it. In a car fusebox the fuses are supplied by individual wires from various sources, the 5 amp fuses are likely to by supplied by 5 amp cable, quite likely from a 5 amp relay or 5 amp transistor, put a 10 amp fuse in (or a wrong way around fuse tap with 2x5 amp fuses) and you can draw double the current that can safely be supplied and either damage things or set fire to the car. Even worse some of these circuits are safety critical; causing a failure in the ABS, stability control, electric steering systems etc. could kill someone. Car manufacturers always fit the minimum rated cable required for each circuit in order to save weight and reduce costs. Saving weight improves performance and fuel consumption, it's important, reducing costs improves profits.
  • You should not bypass the circuit's main fuse by installing the fuse tap the wrong way around.
  • The fuse tap should tap into the circuit after the existing fuse so that the entire circuit (both before and after the fuse) is still correctly protected by the original fuse as it was designed to be.
Hi if you had read my postings in full you will note that I was an electrical officer in the merchant navy. I worked my way up to chief electrician. This means I have a full working knowledge of electrical systems from 12 volt batteries emergency power generation right through to massive turbine generators my first ship that I went to sea on could produce enough power to supply a town of 75000 people not just housing factories offices the lot.
During my 50 years at sea I have worked on every kind of system that it is possible to from a single high capacity 2 volt battery cell to multi megawatt generation. This included both AC and DC systems in varying voltage levels up to and including three phase electricity. What I am trying to impart is my knowledge for people to do a job correctly without endangering themselves I have tried to do this in a simple manner as not everyone is electrically minded. You implied I was getting confused between household Systems and vehicle systems WRONG I have worked on both many many times you're post is not giving anyone the confidence to carry out a very very simple task safely and is causing confusion to those with limited knowledge of the subject. You are correct in that you need to be wary of cable size and that some are related to a vehicle safety system it is for that very reason that I suggested that a spare fuse location be used and to check that it is live when the ignition is operated to the position you need depending on what use you are putting it to. As for the piggyback method see my earlier advice on the subject plus note this apart from the spotlights for reasons that I mentioned before most accessories that you fit in a car nowadays have a very low current draw. The main exception to the rule is an additional 12 volt socket now if you look at the red wire on the piggyback it is fairly thick and should handle the promised
20 Amps my concerns are with the ability of the piggyback itself to withstand 20 Amps the point being that the fuse has to be the weakest link. Generally the cable that most accessories have is fairly light therefore you now have a double safety feature if the fuse is a bit on the slow side to blow you have a small degree of protection from the piggyback cable being that little bit more heavy duty. I apologise for the misspelling in my last post I was using my wife's phone and it was not until later after posting my reply I realised that her spell checker has dyslexia.
 
The problem here is that some people, including you, seem to think that their car fusebox works the same way as their house fusebox. Unfortunately it doesn't, the house fusebox is actually a distribution panel, the car fusebox isn't.

In the house fusebox, each fuse is supplied by a busbar capable of supplying as much power as the main fuse protecting it, thus it is not possible to overload it. In a car fusebox the fuses are supplied by individual wires from various sources, the 5 amp fuses are likely to by supplied by 5 amp cable, quite likely from a 5 amp relay or 5 amp transistor, put a 10 amp fuse in (or a wrong way around fuse tap with 2x5 amp fuses) and you can draw double the current that can safely be supplied and either damage things or set fire to the car. Even worse some of these circuits are safety critical; causing a failure in the ABS, stability control, electric steering systems etc. could kill someone. Car manufacturers always fit the minimum rated cable required for each circuit in order to save weight and reduce costs. Saving weight improves performance and fuel consumption, it's important, reducing costs improves profits.
  • You should not bypass the circuit's main fuse by installing the fuse tap the wrong way around.
  • The fuse tap should tap into the circuit after the existing fuse so that the entire circuit (both before and after the fuse) is still correctly protected by the original fuse as it was designed to be.
I think.. we are starting to split hairs here. I do not believe that individual fuses are fed by individual wires. Instead a whole bank is fed by relays that turn on whole segments of the fuse box.. for example all the ACC supplied fuses come on when the key is in the ACC or running position.

From the perspective of the manufacturer of "piggy back" fuses the placement described by Broadman is correct. If the manufacturer thought Nigel was correct they.. would have designed the piggyback fuse thing differently.
 
Hi Ral

Hi Ralph yes you are pefectly correct and the use of any accessorie, for instance if you hardwire a satnav you need to be aware that when you start the engine you will create a power surge and destroy the satnav in a very short time, easy to place a switch in the circuit just before the satnav and switch on after the engine is running.

Now I am very worried! But is the concern justified ??
Note: I am only converting my std. moulded cigar lighter cable into a simple USB2.0 Type A to Micro B cable (i.e. power source to Dashcam) purely so I can swap this lead, (that powers the Dashcam); between my proposed USB ACC. Ign. switched power outlet and my External Battery Bank, namely when I need “Parking Surveillance” mode.

Thus it is effectively hardwired.
My corncen is the above mentioned power surge!

My wiring conversion, effectively removes any possible Clever circuitry that the Dashcam manufacturer may have put in the Power Cigar Plug part, e.g. some kind of suppressor/smoothing capacitor to “dampen” these start up power surges?
Q. But would they have put any Smoothing circuits in?
Q. You say it wrecks SatNavs, so I assume such surges could also harm Dashcams too, is that correct or not?
Q. Would the Dashcam have this inside it, or do they expect that to be inside their overpriced, and in my case unrequired, hardwiring kits?


Ref. Hardwiring, yes I guess I am, but I am using a USB Power Source/ Charging Port.
Like this
GEREE Universal Rocker Style Car USB Charger - with Blue LED Light Dual USB Power Socket for Rocker Switch Panel https://www.amazon.co.uk/gp/B01J9QA1MCQ. Would they have put any form of surge suppression in this USB charger port?
And is surge protection a std. thing to put in a USB power charging port?

I guess being chinese built, there is little chance of finding this out; especially if it is not a std. thing to do.

Yes I appreciate one could put a switch in and turn it off before starting; but what a mare, opening the glove compartment
Every start ... never going to happen! And you know the time you forget, is going to be the time you would needed it! :).

Q. When you start up, is it a voltage or current surge that happens, if both which or both does the damage?

Apologies for being over enquiring but as they say “the devil is in the detail” :):)

Q. Would Mercedes have put a delay or surge suppression for the cigar lighter circuit, or would that be in the socket or the cigar lighter plug?
 
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