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

[Nigel]

For example, if the device uses 3 amps, then the fuse might be 5 amps and the feed wire would be 7 amps. I'm trying to see if anyone knows this is true or not, or if car manufacturers set all feed wires to say 40 amps.

As an example, in my car, a nice fuse to tap into for powering a dashcam is the electric windows fuse. It has the advantage that if I turn the engine off while waiting at a red light, the electric windows keep working, they only turn off when I open the door to get out. The power for this fuse comes from a relay which is controlled by the car's main computer, which is also watching the doors for turning on interior lights and triggering the alarm, and of course it doesn't use a huge 40 amp relay when a 20 amp one will do, and since the relay is smaller there is no point wasting copper on huge wiring which would make the car more expensive and hurt performance by adding weight, the wiring loom is the third heaviest component in the car after the engine! If I took an extra 9 amps from that fuse for charging a battery, it would probably work fine, until one day I tried to open the window while the battery was charging and the window was stuck due to ice, then depending on which way around the fuse tap was installed, it would either blow the fuse leaving me unable to open my windows, or it would melt the relay and maybe start a fire.

In some cars, the power may go through the fuse before going to the relay, but in mine it goes through the relay first, mainly because they could then fit separate fuses for left and right windows while only needing one relay, and relays are more expensive than fuses.

Unless you know how your car is wired, it is not safe to take a large amount of power from a fuse tap, and 9 amps is a lot of power, 110 watts.

 

[Ralph2]

Finding an empty fuse slot does little good in most cases as one can not.. find the wire that is part of the harness going to an unused appliance. To provide power to any additional gadget will require.. a) using a fuse tap and tying into that new circuit.. or b) tapping into a known sources of power at the destination.. as in finding the power supply to the cigarette lighter and attaching your lead to that wire or, tapping into something like the brake lights.. wires that are "relatively" easy to trace and function is known.

Using choice b) though will mean you are sharing the amperage available through the original fuse.. option a) means that you can supply your new appliance through its own fuse.

It is important to remember the purpose of the fuse is to prevent your wire(s) from overheating / melting and starting a fire should there be a short or excessive demand in the circuit... Thus.. to do any good it should be installed as close to the source as one can get. A thin wire.. will heat quickly and must be protected with a low number fuse.. a thick wire conversely needs a higher number fuse.

 

[Captain]

all this thread gave me was a headache

 

[TexMex]

all this thread gave me was a headache

Ditto

 

[Deleted member 37895]

What I've done is wire the fuses to both be in parallel, IE with blade A on the input and B on the load side. But I use lower power fuses. For example when I tapped my cigarette lighter, I replaced the 20 amp fuse with a 5 amp and a 15 amp.

 

[LMJ]

I esp like your point #4. Too many people discount that the wires are of difference guage and the amp rating of the original fuse bear some correlation to the guage. It's not cost effective in mass production terms for a car manufacturer to use one guage size across the whole vehicle.

 

[LMJ]

https://www.autogenius.info/lexus-rx-330-2004-2006-fuse-box-diagram/



So based on the discussion about supply wires to the fuse being of various gauges and tending to correlate to the original fuse in AMPs, which fuse should I tap? I’m limited to the passenger cabin and some candidates are fuel filler door (#37) or read windows (#35 and #36). Those three are always powered. Any comments? Should I select a different fuse? I plan to add use a 2AMP fuse for a Viofo A129 Duo cam. For ACC (power then ignition is on only), I plan to tap the cig lighter (#63) or extra power outlet (#64).

 

[Ralph2]

Tapping into a fuse has no bearing on wire size. Your supply is coming from an internal buss that is built into the fuse box (the hot side your fuses touch). Wire size needs to be considered only from the fuse tap and on.. the pigtail attached to the fuse tap is a good indicator of the minimum size.. going larger is overkill.. when going with a smaller wire diameter then the expected current draw (and length ) needs to be considered. A good guide.. is to use the same size wire as attached to the gadget you are wiring in.

 

[LMJ]

Ralph, Im not as well versed as you but help to understand a few things. If the wire supplying the circuit has no bearing, then why when I look behind the fuse box I see wires of various thickness going into the fuse box? Don't these wires continue on the the vechicle device they power (lets say for example the fuel filler door)? That wire is much thinnner than one for power windows. If I am adding a fuse via tap, a thin wire is less able to handle the extra amps needed than a thick wire? Where am I wrong in my understanding?

 

[Nigel]

Ralph, Im not as well versed as you but help to understand a few things. If the wire supplying the circuit has no bearing, then why when I look behind the fuse box I see wires of various thickness going into the fuse box? Don't these wires continue on the the vechicle device they power (lets say for example the fuel filler door)? That wire is much thinnner than one for power windows. If I am adding a fuse via tap, a thin wire is less able to handle the extra amps needed than a thick wire? Where am I wrong in my understanding?

Ralph thinks that those thin wires are carrying power out of the fuse box and that there is just one big wire carrying power in, which may be correct for his car but certainly isn't correct for mine.

Since the A129 will never take more than 0.3 amps of 12 volt power you don't really need to worry about which you use.

 

[LMJ]

Nigel, thanks. So based on https://www.autogenius.info/lexus-rx-330-2004-2006-fuse-box-diagram/ and focusing on in cabin fusebox only, do you think its a good idea to tap the fuel filler door (#37) as a permanent hot and the extra power outlet as as the ACC (#64). Total amps leading into the fuse box would then be 7.5A plus as you said 0.3A or maybe max on 0.4 (for the dash cam). Just a bit leery as I dont know what guage the wire leading into the fuse box is. OR you think its better to go for #35 or #36 (power windows)?

Also what would you do in a situation where the fuse box sits near the parking brake but the cover wont go back on if I run a fuse tape (in this case two taps)? Even the cover is dusty when I removed it to take a look. Concered about no protection for the fuses and the cover wont fit back on with the taps.

 

[Nigel]

Nigel, thanks. So based on https://www.autogenius.info/lexus-rx-330-2004-2006-fuse-box-diagram/ and focusing on in cabin fusebox only, do you think its a good idea to tap the fuel filler door (#37) as a permanent hot and the extra power outlet as as the ACC (#64). Total amps leading into the fuse box would then be 7.5A plus as you said 0.3A or maybe max on 0.4 (for the dash cam). Just a bit leery as I dont know what guage the wire leading into the fuse box is. OR you think its better to go for #35 or #36 (power windows)?

Also what would you do in a situation where the fuse box sits near the parking brake but the cover wont go back on if I run a fuse tape (in this case two taps)? Even the cover is dusty when I removed it to take a look. Concered about no protection for the fuses and the cover wont fit back on with the taps.

Seems reasonable, I might try #68 for the permanent.

I do like to put the fuse tap the correct way around so that the total load goes through the original fuse and thus can't overload the wiring without blowing the fuse, then you know everything is safe but with the possibility of blowing the fuse, very unlikely to happen when only taking 0.3A.

As for the cover, I would probably cut a hole in the cover so that it will fit back on and then add an extra cover over the hole. If you only need an extra few mm clearance then the extra cover might just be a flat sheet, otherwise maybe a flat sheet of aluminium with a bulge beaten into it. Fuses don't mind a bit of dust, but it's good to keep wet mud etc. out and you don't want someone accidently knocking the taps out with their feet.

 

[Mtz]

Since the A129 will never take more than 0.3 amps of 12 volt power you don't really need to worry about which you use.

My measurements show about 0.48A when GPS (and Wi-Fi on).

enjoy,
Mtz

 

[Nigel]

My measurements show about 0.48A when GPS (and Wi-Fi on).

enjoy,
Mtz

0.48 amps * 5 volts = 2.4 watts.
2.4 watts / 12 volts = 0.2 amps.

0.48A at USB voltage is 0.2 amps from the 12 volts car voltage, the voltage converter in the hardwire kit is only about 80% efficient so it takes about 0.24 amps from the car battery.

 

[LMJ]

And the reason you prefer #68 over #37 again has to do with a thicker wire going into the fuse box? As that is much higher amp and likely to be fed by a much heavier guage wire?

You gave me a great idea. The cover is about $6 or $7 from the dealer. Assuming there will be enough clearence, after I cut the original I can tape the new one on top of it. That way the plastic grooves on the original will ensure it latches on to the fuse box properly. And I retain the diagram embossed on the cover of all the circuits and their purposes as well as fuses in amps.

 

[Nigel]

And the reason you prefer #68 over #37 again has to do with a thicker wire going into the fuse box? As that is much higher amp and likely to be fed by a much heavier guage wire?

You gave me a great idea. The cover is about $6 or $7 from the dealer. Assuming there will be enough clearence, after I cut the original I can tape the new one on top of it. That way the plastic grooves on the original will ensure it latches on to the fuse box properly. And I retain the diagram embossed on the cover of all the circuits and their purposes as well as fuses in amps.

I wouldn't really expect #37 to have power once the car has been locked with the radio key code - don't want people to be able to steal the fuel just by using a wire coat hanger through the window seals to press the fuel cover release button!

Different brands of car can be quite different so it is hard to answer these questions without knowledge of the model of car, for example I never thought about retaining your fuse diagram because the fuse diagram for my car comes printed on a separate sheet of plastic so that if they fit a different selection of fuses/features they can put the appropriate diagram in, presumably with your car they only mould one version of the embossed cover.

 

[Ralph2]

Ralph, Im not as well versed as you but help to understand a few things. If the wire supplying the circuit has no bearing, then why when I look behind the fuse box I see wires of various thickness going into the fuse box?

Those wires you are looking at are the wires coming OUT of the fuse box.. supplying your various applications in the car.. And they.. as you noticed.. are different sizes because of the different power demands.

There is a large wire from your battery going to a fuse distribution center under the hood (typically). From there relays (and some fused circuits) send power to various components, one of which is the fuse box under the dash. It.. gets several types of power.. power to a buss rail that is on all the time.. power to a buss that is on under ACC conditions only.. power to a circuit that is timed on some vehicles.. There is a lot of technology going into the supply side of the fuse boxes that you do not see.

The fuses you see make the connection from the power (hot) side to those wires that you have noted to complete the circuit. If.. you can positively identify the right wire coming out of the fuse box there is no reason why you can not splice into that wire to add your circuit. That is what a lot of folk essentially do by finding the wire going to the cigarette lighter.. because at its destination it is easy to identify.. and splicing into it. But.. finding the right wire is tricky at its source in the bundle coming out of the fuse box.

AND probing with a 12 volt tester is dangerous.. some circuits on sensing a current draw may activate things like airbags.

 

[LMJ]

Nigel, regarding #37 great point. I assumed it was always on as I sat in the car with the ignition off an pressed the button. I assume a better way to test is to sit in the car and then turn the alarm on then try to open it. Nevertheless, you still think the higher amp circuit is a better idea for wiring reasons? OR you agree with Ralph, it doesnt matter? I'm so confused.
Ralph, I see your point now. So what should I probe with?

 

[Lola]

A correct graphic would really be nice at this point.

 

[LMJ]

A correct graphic would really be nice at this point.

Theoretically, Nigel's way would be like putting an in line fuse down the line which is tapped. But it seems everyone is setting up the tap as if its the second branch of a two branch tree limb. Or you can think of the latter a thin plastic water pipe that splits into two. Is the additional pipe to supply the dash cam gonna break the pipe which splits into two? The straw which broke the camels back for lack of a better way to explain it? I tried hard to look for an automotive electricans write up on the Internet but could not find any.

https://www.evilution.co.uk/info/fuse_taps.htm