FAQ:12v Wire sizes and Fuses Made Easy

[bdosborn]

I defer to the experts on the forum.

[Joe G]

I made a few wire gauge charts to use at work, and thought I'd post them for the benefit of everyone here as well.

Joe G

30 Amps through an 18 AWG wire? Your chart looks badly undersized, what kind of wire are you using?

Bruce

Keep in mind that would be for a run of no longer than 5 feet - the first chart (10% voltage drop) is based on the recommendations of my wire supplier (Waytek). Their 18 ga. has a resistance of 6.9 ohms per 1000 ft, so that's 0.0345 ohms in 5 feet - 0.0345 ohms at 30 amps would drop the voltage 1.035 volts or 8.6% in a 12 volt system. According to them, that's within industry accepted standards. In fact, according to that, you could go as high as 34 amps for 5 feet.

I agree with you that 18 ga seems too small for the job, and a 10% loss is a lot, so that's why I use the second chart (5%) which calls for 14 gauge for a 5 foot run at 30 amps.

The charts show what a dramatic effect the length of the wire has on it's ability to carry current.

I use high temp polyethylene coated (GXL) wire, but these charts apply to vinyl coated wire as well.

I know you know your stuff, Bruce. You do fine work and I respect you for that. I'm not in any way trying to be combative here. I just wanted to let you know where I'm coming from on this.


Joe G


P.S.
I encourage you to check my math on this, because I have been wrong before. If my figures are incorrect, I will gladly revise the charts immediately.

Here is the data I was working with:

Wire resistance per 1000 ft:
18 ga = 6.9 ohms
16 ga = 4.7
14 ga = 2.8
12 ga = 1.8
10 ga = 1.1
8 ga = 0.7
6 ga = 0.4
4 ga = 0.3
2 ga = 0.2

amps x ohms = voltage drop

[bdosborn]

I defer to the experts on the forum.

[brian_bp]

I understand Bruce's points, which result mostly from Joe's method of considering only voltage drop, and not insulation temperature, as the selection factor. Rather than a simple 14 ga minimum (which still wouldn't keep people from running some very hot cables at the highest current points), maybe the combinations of current and gauge which would be expected to run too hot for common insulations could be flagged in the chart with something like a change in text style.

I'm thinking of someone picking cable, and wanting to both have safe wire temperatures and acceptable power loss, without overkilling the amount of copper.

In an auto accessory store I saw an interesting demo board with a few samples of cable (in different gauges), with digital displays showing both the current and length (selectable by the customer with sliders) and the voltage at the load end (calculated, of course... the board didn't actually run a test), to illustrate why sufficient conductor area is important and excessive size doesn't do much good. It was set up to sell Monster Cable, which is amusing because that stuff is rarely chosen for any rational reason.

[brian_bp]

...Here is the data I was working with:

Wire resistance per 1000 ft:
18 ga = 6.9 ohms
16 ga = 4.7
14 ga = 2.8
12 ga = 1.8
10 ga = 1.1
8 ga = 0.7
6 ga = 0.4
4 ga = 0.3
2 ga = 0.2

amps x ohms = voltage drop

The resistances look generally reasonable; for a comparison with sources provided, perhaps look at the Wikipedia page for American Wire Gauge. That also lists a version of current capacity, with temperature dependence.

[Joe G]

Thanks for the great input, Guys.
I have revised my original post to include a much simpler chart that reflects your recommendations.

On the topic of speaker wire: At one of my wiring suppliers, high temp GXL wire costs less than an equivalent length of speaker wire does. 14 gauge GXL is about $12 for a 100 ft spool, and 14/2 speaker wire is $29 per 100 ft.

You could get two 100' spools of 14 GXL (red and black) for $ 23.86
and two ten foot rolls of 10 gauge PVC for $ 8.64
plus about $5 for shipping, and you would have all the wire you need for up to two trailers for about $37 from DelCity.net.

They also have way better prices on terminals, switches, circuit breakers, fuse blocks, split loom, wire ties, and connectors than any auto store, hardware store, or building center.

Speaker wire is for speakers - use PVC or GXL wire - it's cheaper, safer, and easier to work with.



Joe G

[daveleb55]

Folks,
Be conservative! No one wants an electrical fire starting in the wall, igniting the Styrofoam insulation, and turning your teardrop into a Viking funeral pyre!
As a general rule of thumb, I figure what the wire size should be, and then go to the next size up to have a margin of safety. If you are close to the maximum for a given length and size, go to the next size up.

For example, if you have an appliance that draws 15 amps, the wire should at least be rated for 20, and the fuse should be 20amp as well. Automotive fuses are easy to find and plentiful, and are cheap insurance against electrical fires.

My two centavos,

Dave & Kathleen.

[bdosborn]

I defer to the experts on the forum.

[ArtMini]

OK I think I understand all this and will re-read it when I'm ready to wire. I know the wire is easy to find but where do we get a fuse box, and how do we charge the marine battery on the trailer? I used to have a custom van years ago and the marine battery was wired right into the alternator.

Art

[Esteban]

A tip for finding things on the DelCity website is to click on the site map. Their printed catalog is very well organized and illustrated. They'll send you one at no charge. When mine arrived it had an enclosed discount coupon code good for a short time.

DelCity fuse panels

Or you could do a Google search for "Blue Sea 5026" or "Blue Sea 5028" for a marine fuse block with a built in negative bus. They're much pricier than the DelCity fuse panels.

I'm using the 12 circuit Blue Sea 5026 for my 12 volt panel (because I bought it before the DelCity catalog arrived with a range of less costly fuse panels).

HTH

how do we charge the marine battery on the trailer?

I second this question. I'd like to see a discussion, with graphic illustrations, of how to charge the teardrops battery from both the tow vehicle and shore power.

[artfd]

The stud size is 3/8". The hex nut is 9/16" This is the size of fuse you will need. I found these for sale at an Autozone or Advanced Auto Parts store in Albuquerque at $5 each. Branches of the same in Ohio do not stock these.

[dh]

I ran into an issue at work today, so while its fresh in my mind, I'll share it here.

Make sure your switch is rated for at least as many amps as the breaker that feeds it. A circuit is only as a strong as its weakest link. So if you use 110v light switches on your AC side, do not put a 15A switch on a 20A circuit. Same for the DC side.

Had a guy replace a 277V 20A light switch with a 120V 15A light switch. It welded the contactors and started smoking, but never tripped the breaker.

[Dant]

This is very helpful, starting with Bruce's initial post.

I think this answers my unposted question about how to build a simple 12 VDC only system for my CT conversion.

So... I'm planning on a battery bank of 2 Trojan 6 volt golf cart batteries wired in series for 12 V. From the battery I use 10 gauge wire to a fuse block, then simply wire each circuit from the block using appropriate size wire considering the factors outlined.

Right?

I'm planning on a very simple, boondocking only system, for lights, a fan, and charging a small tablet computer, electric tooth brush and other small items I can use a small inverter for if necessary.

[bjeppson]

What do you do about grounding the trailer battery? Is this grounded to the trailer frame?

Bob

[droid_ca]

Great topic Thank you for sharing all this valuable information