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

Interesting answer you got, but there is a clear assumption that the source has plenty of power for both devices, as would be the case in a domestic fuse box, and since you asked it in an "Electrical Engineering" forum rather than an electronic engineering forum, maybe that is not surprising.

See if you still get the same answer if you point out that in my car, the fuses being used in the fuse box are supplied not by a bus but individually by thin wires, some of which come from a relays soldered to the car's main computer PCB, and some from the ignition/stalk switches on the steering column.
 
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)."
 
Interesting answer you got, but there is a clear assumption that the source has plenty of power for both devices, as would be the case in a domestic fuse box, and since you asked it in an "Electrical Engineering" forum rather than an electronic engineering forum, maybe that is not surprising.

See if you still get the same answer if you point out that in my car, the fuses being used in the fuse box are supplied not by a bus but individually by thin wires, some of which come from a relays soldered to the car's main computer PCB, and some from the ignition/stalk switches on the steering column.
You have taken your fuse box apart then? Traced all the wires and where they go and come from..
 
You have taken your fuse box apart then? Traced all the wires and where they go and come from..
Much easier to look at the circuit diagram for the car!
 
Much easier to look at the circuit diagram for the car!
Ummm.. We do know that the weight of the line on a schematic is not indicative of the size of the wire. :)

I can not vouch for how your vehicle's fuse box is wired but here is a schematic for mine.. a 2011 Toyota Highlander. It shows a main supply powering segments.. turned on by relays..

In keeping with the main concern expressed by some about overloading the circuit.. the choke point would.. in my opinion.. be the relay and how much load it can handle. In my case the ACC circuit supplies a total of 40 amps.. (a value that would never in practice be reached). So.. taking a nominal amount of power directly from the ACC circuit to power a dashcam would not be a concern to me..
 

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Lol, here we are discussing about how to install it right... I don’t think this falls anywhere close to that category. Their solution may result in a completely unprotected circuit if they aren’t careful, not to mention a bunch of other possible hazards.

It’s been so long that I’ve looked at this thread I have forgotten where we left off. I know I said I would get back with examples but i never have had the time to do that. Forgive me if I repeat myself...

I think the most accurate answer is, it depends. It depends on your supply wire size and how the car was wired. I’ve found some circuits are powered from a main feeder while others are not. It depends on the make and model and year. For the best suited solution for your vehicle it is going to require some research, because I know some of you on here are into more than dash cams, this info will probably get applied to audio equipment, lights, Radios, and other accessories.

The safest solution if you don’t want to research the wiring setup in your vehicle and you don’t want to risk overloading the circuit that you are tapping into then the solution for you would be to run the second fuse in series with the circuit you are pulling power from. If the dash cam pulls so little current that you believe unloading the circuit is unlikely then running through the original fuse is probably also negligible. Never attach to a circuit that will cause a dangerous situation if the original fuse were to fail or cause the whole circuit to stop functioning.

research will need to be done before adding anything to the fuse in a parallel installation with the original fuse so that you are sure the wire feeding that fuse does not become overloaded. If it can handle the additional amperage then proceed with your install, if it can not then find a different circuit or run a new circuit. If installing in parallel then calculate the allowable load for your vehicles wire feeding the particular fuse that you are attaching to and make sure the total amperage of your fuses combined do not exceed the ampacity that your wire is rated for. Do not blindly install the supplied high amperage fuse without first confirming that it will maintain proper protection to your circuit in the event that it’s required, if it’s too large of an amperage then change it to one that is properly sized. Make sure that any relays on that wire will also be able to handle the total combined ampacity.

There is not a simple “this way is right, that way is wrong” answer because different vehicles are wired different from each other. What works in one vehicle make/model may not work at all in another. We can find examples all day for car wiring diagrams that support each method of install and they would all be correct for that method. We could also find examples of why each method is not the best or correct way to install, those would also be correct. It really depends on how the vehicle was constructed.

Remember, the fuse is not to protect your equipment from over amping under normal operation, your equipment should have protection built into it to protect from that already, it is there to protect the wires from catching on fire or melting in the event of a problem or accident. The last thing you want to have happen is for your car to catch fire following an accident because the wires on that circuit became damaged and allowed too much current to flow through the feeder wire before the circuit protection was able to preform its job (or pick any other potential cause of failure). That is only a possibility with a parallel install, but if sized correctly then it should still protect against that. I just witnessed an accident where the front end of a vehicle was mangled and the driver was unconscious for about three minutes. That would have been three minutes of smoke filling the vehicle. Three minutes where the person or their infant would not be able to escape the flames. This is just one of many very possible scenarios since accidents occur every day. Whatever method you find works for you please make sure you install it with safety as your number one concern for you and the future owners of the vehicle.




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Lol, here we are discussing about how to install it right... I don’t think this falls anywhere close to that category. Their solution may result in a completely unprotected circuit if they aren’t careful, not to mention a bunch of other possible hazards.

It’s been so long that I’ve looked at this thread I have forgotten where we left off. I know I said I would get back with examples but i never have had the time to do that. Forgive me if I repeat myself...

I think the most accurate answer is, it depends. It depends on your supply wire size and how the car was wired. I’ve found some circuits are powered from a main feeder while others are not. It depends on the make and model and year. For the best suited solution for your vehicle it is going to require some research, because I know some of you on here are into more than dash cams, this info will probably get applied to audio equipment, lights, Radios, and other accessories.

The safest solution if you don’t want to research the wiring setup in your vehicle and you don’t want to risk overloading the circuit that you are tapping into then the solution for you would be to run the second fuse in series with the circuit you are pulling power from. If the dash cam pulls so little current that you believe unloading the circuit is unlikely then running through the original fuse is probably also negligible. Never attach to a circuit that will cause a dangerous situation if the original fuse were to fail or cause the whole circuit to stop functioning.

research will need to be done before adding anything to the fuse in a parallel installation with the original fuse so that you are sure the wire feeding that fuse does not become overloaded. If it can handle the additional amperage then proceed with your install, if it can not then find a different circuit or run a new circuit. If installing in parallel then calculate the allowable load for your vehicles wire feeding the particular fuse that you are attaching to and make sure the total amperage of your fuses combined do not exceed the ampacity that your wire is rated for. Do not blindly install the supplied high amperage fuse without first confirming that it will maintain proper protection to your circuit in the event that it’s required, if it’s too large of an amperage then change it to one that is properly sized. Make sure that any relays on that wire will also be able to handle the total combined ampacity.

There is not a simple “this way is right, that way is wrong” answer because different vehicles are wired different from each other. What works in one vehicle make/model may not work at all in another. We can find examples all day for car wiring diagrams that support each method of install and they would all be correct for that method. We could also find examples of why each method is not the best or correct way to install, those would also be correct. It really depends on how the vehicle was constructed.

Remember, the fuse is not to protect your equipment from over amping under normal operation, your equipment should have protection built into it to protect from that already, it is there to protect the wires from catching on fire or melting in the event of a problem or accident. The last thing you want to have happen is for your car to catch fire following an accident because the wires on that circuit became damaged and allowed too much current to flow through the feeder wire before the circuit protection was able to preform its job (or pick any other potential cause of failure). That is only a possibility with a parallel install, but if sized correctly then it should still protect against that. I just witnessed an accident where the front end of a vehicle was mangled and the driver was unconscious for about three minutes. That would have been three minutes of smoke filling the vehicle. Three minutes where the person or their infant would not be able to escape the flames. This is just one of many very possible scenarios since accidents occur every day. Whatever method you find works for you please make sure you install it with safety as your number one concern for you and the future owners of the vehicle.




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Thank you so much for your response. I will take my dashcam installation more serious than BlackVue (y)
 
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