Wire Gauge For Interior use? - Fiberglass RV


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Old 07-25-2009, 01:17 PM   #1
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Hi all,
I need to wire a Compact II and would like to know what is the best gauge wire for 12v systems:

12v lights will be installed

Shurflo water pump

(Marine deep cycle battery)

12v outlet

12v line to fridge

The trailer is NOT charged by towing car. Car is not set up for that.

There were remnants of various system wires when I got the trailer, a 1974. Those cylindrical glass-type fuses were used on each line.
I'd like to upgrade, and read it's o.k. to put in a heavier wire than the load requires.
The heaviest wire in the trailer was the line to the fridge.

I have a spare jumper cable; could I use this to run from battery to the interior, say to a distribution box?

Could someone refer me to 12v installation instructions? I'd like to do this the right way.

Thank you all for your guidance!!
Fran
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Old 07-25-2009, 06:07 PM   #2
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I use nothing smaller than 12 Gauge. The heaver you go the better it is so 10Gauge would be even better. I use single wire for each item wired, not Romex, and I ran the same size Ground to the frame or central buss for each item that was wired.
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Old 07-25-2009, 11:36 PM   #3
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12 gauge stranded wire is a good general-purpose size for most 12 volt trailer wiring, and can carry up to 20 amps. Use black wire for the ground (negative) side of the battery and white for the positive side.

I like to use 10 gauge wire for connecting the battery to the load center where the fuses are. 10g wire is good for loads up to 30 amps.

I should add that my 50 and 55 watt solar panels are wired into my load center with 14g wiring.
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Old 07-26-2009, 12:45 AM   #4
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I don't know how everyone else feels about this, but I think stranded wire would be better than solid because of the flexing from the trailer moving. Comments?
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Old 07-26-2009, 10:38 AM   #5
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Peter, I do the opposite. Having just come off a long career in audio manufacturing, where 95% of all wiring is for dc current, I can tell you that the standard is the opposite. DC wiring has white as the + and black as the -. Your solar produces DC, as you know.

I have seen trailers done both ways, even with the same manufacturer. Both my Burros were opposite. ALWAYS CHECK!

The AC wiring in both was partially Romex and partially twisted pair.. (wierd) done to AC standard.. the DC wiring in the 13 was to DC standard, and in the 17, it is to AC standard.

I do all my DC to the DC standard. It really makes no difference to the electricity, it does not care what color jacket it wears, but it helps visually identify what you are dealing with, at least when working on your own installs.



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Old 07-26-2009, 02:48 PM   #6
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White +
Black -
12 gauge
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Old 07-26-2009, 04:32 PM   #7
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I submit my apologies for extreme oops!-ness. I knew "the dark side is always negative" rule and still wrote it down wrong. I've also added that it should be stranded wire, which is standard for automobile and RV 12 volt wiring.
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Old 07-27-2009, 07:03 AM   #8
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I don't know how everyone else feels about this, but I think stranded wire would be better than solid because of the flexing from the trailer moving. Comments?
Most definitely stranded wire is desired in any location where it might flex!
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Old 07-27-2009, 08:02 AM   #9
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I am curious as to why everyone is running #12 wire. Unless you have a 20A draw, it would not be needed and be overkill. #14, though not lots smaller, is much easier to work with.

I do agree that if it will flex at all, that stranded wire should be used.
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Old 07-27-2009, 10:11 AM   #10
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I was wondering when someone would mention the 12 gauge wire. If the wire is not for the purpose of charging, where a voltage drop would be of concern, then size the wire to the breaker.

Dean
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Old 07-27-2009, 11:17 AM   #11
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The reason we do not have DC wiring in our houses is that DC has a tremendous loss over a short distance. The larger the wire the less the loss so that is why we use 12 Gauge or larger when wiring DC in the campers.
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Old 07-27-2009, 11:45 AM   #12
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The attached chart shows the AWG sizes that are recommended for a 3% voltage drop. The distances include the entire circuit, so that a load 10 feet from the battery or breaker panel would be read as 20 feet, if you have a 2-wire connection. If the load is grounded to the frame, then I would guess it would be measured as the distance of the positive wire to the load, plus that of the negative wire, if any, from the load to the frame.

All wiring in an RV or boat should be stranded, including the 120V AC.

BTW, am I the only one who uses red for positive and black for ground...?


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(Source: Managing 12 Volts by Harold Barre)
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Old 07-27-2009, 07:43 PM   #13
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...BTW, am I the only one who uses red for positive and black for ground...?
No, not for 12V -- that's what I use too. Makes it easier to tell the 12V stuff from the 110V when all you see is a bunch of wires, doesn't it?

As another aid to tracing things out, I use red/black ZIP cord to each 12V fixture. Hams use it a lot for their rigs, and on a fiberglass trailer getting to the frame with a good connection is often harder than just running a black negative return wire.

The advantage of the zip cord as contrasted to separate red and black wires is that while tracing why one fixture isn't getting full voltage I know which black wire is associated with each red.
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Old 07-27-2009, 08:05 PM   #14
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The reason we do not have DC wiring in our houses is that DC has a tremendous loss over a short distance. The larger the wire the less the loss so that is why we use 12 Gauge or larger when wiring DC in the campers.
No, the loss is the same for AC and DC over the same distance.

Loss is primarily determined by the current flow and the gauge of the wire. More current or smaller wire leads to more loss. Thus for the same power, which is volts times amps, you encounter less loss if you use high voltage and thus less current.

That is the reason long-distance transmission lines are very high voltage. Even in my neighborhood the "local" high-tension voltage is 16,000V, which is much higher than the voltage in my house.

The real reason why we have AC at the house is that it has historically been much easier and cheaper to convert high voltage AC to "reasonable" house voltage than it is to convert a similar high voltage DC to that same DC voltage.

Different countries have come to different conclusions as to what is a reasonable voltage for houses, generally based on what allows a light accidental touch without lethal effects. Obviously you don't want to grab on to bare wires -- you will suffer severe burns and if the current goes through your torso you are probably dead -- but a light touch confined to a small area will probably not be lethal. YMMV

BTW, we can make the same argument for why we went from 6V systems in our cars in the early '60s to 12V -- the higher voltage allowed the manufacturers to use lighter gauge wire to run the lights, fans, and whatnot without excessive losses.

There is a move afoot to increase the voltage again to 28V, again for the same reason -- less loss in the wire and connectors because the current will be reduced.
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