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I would like to mount some external flood-lights on a motorhome with a 12V DC system. I may need to install new positive wires to make sure they are sufficiently thick.

I wonder if I also need to install thicker negative wires, or if I can simply connect to the existing negative wires, without worrying that I overload them?

Someone told me that the negative wire in a 12V DC system can be very thin, but I haven't been able to find any information on this.

Is this complete nonsense and should the negative and positive wires be the same gauge? Or can the negative wire be thinner than the positive wire by some ratio?

If possible, please give a reference for your answer.

Thanks!

questiondude
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    Ground it via the chassis? – winny May 22 '22 at 09:52
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    Instead of down-voting it would be more constructive if people ask for clarifications in the comment. It comes across as a hostile attitude to simply down-vote like that. – questiondude May 22 '22 at 10:00
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    Of course I have searched for answers. But the person who told me about the negative wire being thinner was convinced he was right, although it did sound strange to me. I found another answer here on StackExchange that said there could be a single negative wire, but it should be able to carry the combined current from all the positive wires. That makes more sense to me. And I suppose that connecting directly to the chassis would also work, as @winny suggests above. I don't know very much about electrical circuits, that's why I'm asking to be sure I do it correctly. Thanks! – questiondude May 22 '22 at 10:03
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    Whoever said the negative can be thinner is either wrong or referring to some specific cases in three phase AC systems which may allow neutral to be thinner. No such thing for your 12 V DC system. The negative wire needs to be as thick as the positive. And as stated previously, the chassis is a very thick conductor which you already have paid for and readily installed in your car. – winny May 22 '22 at 10:07
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    @winny, if you write that as an answer, I will accept it! It is a legitimate question for those of us who know very little about electrical systems. The person was talking specifically about 12V DC systems. If someone follows his advice I suppose the current can burn the negative wire, and that could start a fire or something, right? – questiondude May 22 '22 at 10:13
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    If you might ever move the lights onto nearby poles or trees, then you would need a negative wire, and it's usually easier to run two wires at the same time. In case that fits the scenario. – Andrew Morton May 22 '22 at 10:49
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    Your lighting will actually be more efficient and be usable with thinner wires if you boost the voltage at the 12V power source up to 48V (or use an inverter to 110V AC). Then the line that you run to the lights would be 48V and you would use 48V lights, or alternatively 4 x 12V lights in series. The higher voltage makes using a longer run work just fine. With an all-12V system, you run out of cable length fast, and then you have to increase the gauge (of both conductors being same) to prevent light dimming or faster battery draining. I think the 48V sys. would be cheaper than the 12V system. – MicroservicesOnDDD May 23 '22 at 03:32
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    @MicroservicesOnDDD it would have to be a pretty big motorhome for the saving in cable to pay for the cost of fitting more expensive lights and a DC-DC converter. Then it would be a pain to maintain by being non-standard. Motorhomes are 12V and mains - not anything in between (except big ones with 24V vehicle electrics but even those use 12V in the living area. With LED lighting the current isn;t usually massive, and the acceptable voltage drop can be a little higher than with incandescent lights – Chris H May 23 '22 at 12:24
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    @winny I can see some logic in an argument that a short tail of negative wire to the chassis doesn't have to be full thickness on voltage drop grounds. But when I wired mine I just used thick enough wire on both -1.0mm² gave a worst case of just under 1V drop on both lines combined at the last light with all of them on, which is tolerable and there are more lights than I'd ever use at once so the lights are in the right place – Chris H May 23 '22 at 12:30
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    @ChrisH A valid edge case! – winny May 23 '22 at 12:36
  • Always remember that we call these things "circuits" because current flows in circles. All of the wires in the complete circle must be able to safely handle the current. – John Doty May 23 '22 at 15:29
  • Since this refers to a motorhome, one must consider whether there is an actual return wire at all, or if the vehicle chassis is the return path... – MikeB May 23 '22 at 15:53
  • @ChrisH -- I modify dollar store light bulbs for fun, and at 2 x 100W for a dollar and change, that's cheap. I also make my own boost converters, so that's cheap, too. Of course if we were told the length of the run and how many amps are intended to be drawn, then we could figure out if you were right about me being wrong. ;-) But you have your own installation, so I'll defer to you -- though I have to ask, why did you only find expensive lights? ;-) – MicroservicesOnDDD May 24 '22 at 19:12
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    @MicroservicesOnDDD if you can build the hardware that does make it easier. There's a lot of choice of 12V "vehicle" lighting, internal and external. That brings the price down, though less so for external where 48V is more common and more of the cost is in the weatherproofing which is the same for 12V or 48V - though most of the 48V stuff I'm seeing is high-power, and with motorhomes you often want low power external lighting, e.g. to find your way back to the door, or to sit with your back to the vehicle reading, not to light up everyone else's units. – Chris H May 24 '22 at 20:42
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    My LED light-hacking is mostly at the 3.7--5V range, though I have played with some at 12V. – Chris H May 24 '22 at 20:43

3 Answers3

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The proper wire gauge for a particular application is determined by a few factors:

  1. The current (amperes) that the wire will carry. This is the most important consideration. Keep in mind that the positive and the negative wires carry the same current (the electric charge is a conserved property in physics, see also Kirchhoff's laws)
  2. The acceptable voltage drop. This applies to both the positive and the negative wire, but if you want to get minimum voltage drop for minimum price, wires have to be equally thick.
  3. Adequate behavior of the fuses. Too thin a wire may not be able to melt the fuse (but will happily start a fire instead).
  4. Mechanical properties. Sometimes the wire has to bear some mechanical tension or stress. You may want to use thicker wire than the gauge determined from p.1 and p.2. Equal for the positive and the negative wire, too.
  5. Corrosion safety margin. Yes, the positive wires corrode faster. This may be a consideration for marine setups. In a motorhome (or in a non-motor home, for that matter), visible wire corrosion is a clear sign that you have to act quickly and fix the water ingress (and then the wires as well).
  6. The possibility of using existing metal parts as a second wire. Widely used in cars, where almost all metal parts are connected together and used as a negative wire. This is why the negative wire is sometimes not used at all. In a motorhome, however, metal parts are usually not available so one uses negative wires for everything.

In short, use positive and negative wires of equal gauge, determined by points 1,2,3 - there are plenty of online calculators for them.

If in any doubt, consult an electrician.

psmears
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fraxinus
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  • Marine suppliers have tinned (coated) copper wire to reduce the corrosion problem. –  May 22 '22 at 11:11
  • WRT #2, voltage drop depends on length. The negative wire may not have to be as long to reach "a" grounded terminal (your #6), and if it isn't as long, it doesn't need to be as thick to achieve tolerable voltage drop. – Ben Voigt May 23 '22 at 20:53
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Whoever said the negative conductor can be thinner is either wrong or referring to some very specific edge-case.

Thermally, and most wiring is thermally limited, losses are proportional to P = I^2R, regardless of voltage so given same length and cross sectional area, the negative lead will dissipate as much heat as the positive.

In your case of a car, the obvious solution is to use the chassis. Steel has much higher resistance than copper but the cross sectional area is huge so it’s very favorable to use as negative return.

winny
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    Thanks! At the exact same moment you wrote this, someone else wrote a very long and detailed answer, so I marked that as the "correct" one, because I can only mark one as the "correct" answer. But I greatly appreciate your answer as well and of course up-voted it! – questiondude May 22 '22 at 11:03
  • It may be that, using the chassis as -ve, there is a case for a negative wire as well to maintain some connectivity if e.g. chassis bolts come loose, poor maintenance, etc. In that case perhaps you can get away with a thin wire. But not if it's normally carrying the full load current. –  May 22 '22 at 11:10
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    @questiondude No worries mate! Glad I could help. – winny May 22 '22 at 12:49
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Current flows in loops. Exactly the same current is on the negative as the positive.

The negative wire does not have any special characteristic that would make it heat up any less than the positive.

So your safety minimum wire sizes are the same.

However, many vehicles are metal chassis, tied to the battery - (negative) terminal. While steel is not a great conductor, it has great cross-section, so most people model the "chassis conductor" as infinite conductance.

In that case, the positive has to run the whole length but negative can shortcut to the nearest place on the chassis. So if you have a long wire run, and you are optionally enlarging wire size to reduce voltage drop, you have less concern with the negative, and would do your size increase mainly on the positive.

Harper - Reinstate Monica
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