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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!

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    \$\begingroup\$ Ground it via the chassis? \$\endgroup\$
    – winny
    May 22 at 9:52
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    \$\begingroup\$ 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. \$\endgroup\$ May 22 at 10:00
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    \$\begingroup\$ 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! \$\endgroup\$ May 22 at 10:03
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    \$\begingroup\$ 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. \$\endgroup\$
    – winny
    May 22 at 10:07
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    \$\begingroup\$ @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? \$\endgroup\$ May 22 at 10:13

3 Answers 3

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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.

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  • \$\begingroup\$ Marine suppliers have tinned (coated) copper wire to reduce the corrosion problem. \$\endgroup\$ May 22 at 11:11
  • \$\begingroup\$ 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. \$\endgroup\$
    – Ben Voigt
    May 23 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.

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    \$\begingroup\$ 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! \$\endgroup\$ May 22 at 11:03
  • \$\begingroup\$ 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. \$\endgroup\$ May 22 at 11:10
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    \$\begingroup\$ @questiondude No worries mate! Glad I could help. \$\endgroup\$
    – winny
    May 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.

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  • \$\begingroup\$ Thanks I appreciate it! \$\endgroup\$ May 23 at 14:19

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