FAQ:12v Wire sizes and Fuses Made Easy

[eggsalad]

In these days of LED lighting, the chart can (and should) go to smaller gauge wire. My trailer uses all LED lighting. In modern technical parlance, LEDs don't draw squat.

.

Can you tell me what brand/model converter you use? and what size fuses for your LEDs? Most of what I read are 30 and 20 Amps blade fuses.

I'm not sure what you mean by "converter". The LEDs are 12v; the batteries provide 12v. No need to convert anything.

As for fuses, I don't use fuses to protect the LEDs. I'm not horribly concerned about them. I use a 10A fuse to protect the *battery* against excessive draw from a short circuit.

[KCStudly]

As for fuses, I don't use fuses to protect the LEDs. I'm not horribly concerned about them. I use a 10A fuse to protect the *battery* against excessive draw from a short circuit.

Fuses aren't there to protect the load (light, fan, whatever...) or even the battery (worst that can happen to that is that it goes dead, unless of course it freezes and you try to charge it up quickly... but that's a whole other problem). Fuses are there to protect the wires from burning you and your camper to the ground if the load gets too high due to a short.

Each wire run should have an appropriately sized fuse to protect the wire from burning out. That's what the wire size vs. circuit length chart is for; use it and fuse it!

An over size fuse is like having no fuse at all; the wires could still burn out and start a fire because they could burn thru before the fuse does.

[grantstew8]

Kc is on the money. Our electrician said "it's really easy to start a fire with 12v if you don't have a fuse. It's easier with DC" I'm no expert but I've taken his word for it.

I guess that's why my car has a couple of dozen fuses

[MtnDon]

If the wires are large enough and not fused you can weld with DC.
If the wires are small enough and not fused they act like resistance heating element wires.

[bdosborn]

I defer to the experts on the forum.

[KCStudly]

Fair enough, but he also said,

All of my lighting wiring, from the switch panel out, is done with 20/2 red/black zip cord.

Bruce, by your own recommendation this wire is not suitable for 10 amps at typical TD circuit lengths, and this is backed up by other available ampacity charts that suggest that 10 amps on this gauge wire is only good for a circuit of a few feet.

I'm not hung up or ornery about this, I'm just saying let's make sure we watch out for everyone here who looks to us for definitive advice, especially the novice.

My fuse panel will be in my tongue box. Some of my lighting circuits are more than 20 ft round trip by the time you add up all of the zigs and zags. I would not be comfortable with 20 ga wire and a 10 amp fuse on most of my circuits. None of the ampacity charts that I have seen would support that approach. Perhaps if your circuit panel was on the bulkhead and none of your circuits strayed very far from there, then it would check out, but I am assuming that this is not the case for most viewers that might be reading this advice.

[bdosborn]

I defer to the experts on the forum.

[eggsalad]

Is this a bad way to do things? I don't think it is, if his fuse is right at the battery and all his wiring is rated at 10 amps continuous

Bruce

Edit Note: Oh yeah, I forgot about the 20/2 wire, thanks for pointing that out. Using 20/2 is crazy.

One good point, and one false point. The ampacity of 20 ga. wire is 5A over the (maximum) 6' my wiring runs. The maximum load, with all 3 fixtures lit, is approximately 1 amp. I should change the fuse to a 3A.

I'd really like to understand why you think using 20/2 is crazy. In terms of ampacity, it's way overkill for the load. There was virtually no voltage loss over the 6' run. One respondent to this thread said "yeah, but what if some future owner changes to an incandescent bulb?" Well, the only fixture that would accept an incandescent bulb is the LittleLight. With the original 50W halogen lamp, the draw is still under 4A. Besides, nobody will own this trailer besides me.

I won't defend my position any further. My electrical system is based on sound science, combined with 25+ years professional experience building custom, low-voltage lighting. I'm sorry you felt it necessary to resort to questioning my sanity.

[eamarquardt]

If the wires are large enough and not fused you can weld with DC.
If the wires are small enough and not fused they act like resistance heating element wires.

Depending on what yer trying to accomplish the above can come in handy.

Cheers,

Gus

[bdosborn]

I defer to the experts on the forum.

[MtnDon]

If the wires are large enough and not fused you can weld with DC.
If the wires are small enough and not fused they act like resistance heating element wires.

Depending on what yer trying to accomplish the above can come in handy.

Cheers,

Gus

I've welded a broken spring hamger with 2 - 12 volt batteries in series and cooked hot dogs along the trail with jumper cables too.

[eggsalad]

I'd really like to understand why you think using 20/2 is crazy.

- I couldn't find any quality, listed/rated wire in 20#. I am not a lucky person, I need all the protection I can get in life.

I used this stuff. It's GPT wire:

http://www.allelectronics.com/make-a-store/item/WRB-20/20-AWG-RED/BLACK-AUTO-ZIP-CORD/1.html

- You're right, 20# will work with the appropriate fuse, but do you have any manufacturer ampacity tables that actually show the wire rating? I couldn't find any for 20# wire but I can post links to manufacturer ampacity tables for all the wire that I posted. I know that wire is rated for the amps I've shown, and that includes inside insulated walls.

You sure are right about finding "ampacity" tables for 20 gauge wire! I dug and dug, and finally came up with this interesting document:

http://www.gmupfitter.com/publicat/electrical.pdf#page=22

This is the General Motors Upfitter Guide. An Upfitter is an aftermarket company that makes additions to completed, or partially completed vehicles sold by an auto manufacturer. One example might be companies that make Ambulance bodies. Another, more relevant, example would be RV manufacturers.

According to the chart on page 23 of this .pdf, 20 gauge GPT wire carrying 3 amps at 12VDC can have an effective run of 36 feet! In the case of trailers, that would be 18' away from the battery! (18' from B+ to device and 18' from the device to B-). that's a whole heck of a lot of distance! Even if you bump up to a 10A load, the device can be located 5.5' from the battery.

To paraphrase the old saying... What's good for General Motors is good enough for ME!

[GerryS]

Too bad their cars aren't as good as this guide....

[Shadow Catcher]

I guess I have enough experience to understand that what I want today may not be what I want tomorrow, and plan for expansion and changes and all of the wiring is 14 ga or larger. One of the tasks with Compass Rose is to clean up the rats nest of wiring going into the Progressive Dynamics load center in the back, Most of it is not my fault, it came that way.

[working on it]

Too bad their cars aren't as good as this guide....

minus one on that...All GM, all the time!