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Sizing wire for AC circuits

Posted by Administrator on 7/8/2015 to ABYC and USCG standards
The ABYC standards for wire sizes in AC circuits are outlined in E11 Table VI. Allowable current depends on the insulation rating of the wire, whether the wire is in an engine room or not, and how many wires are in the bundle.  The temperature rating of the insulation is the primary deciding factor and for the purposes of this post we are only looking at wire with a 105 deg C rating, which is the rating of all the boat cable we sell on this site.  The wire is derated when it is in an engine room because it assumes the space will be 20 deg C hotter than the outside.  The wire is also derated as more wires are added to the bundle as the cumulative heat of a bundle of wires affects how the insulation performs.

From experience I have found that certain wire sizes work for most situations.  Take for example 10 awg wire.  I normally use this for 30 Amp mains.  With 105 deg C wire the rating for a single wire outside an engine room is 60 Amps, but in an engine room it is reduced to 51 Amps, and if four to six conductors are in the bundle it is reduced to 30.6 Amps in an engine room. 

I usually use 12 awg wire for 15 Amp circuits.  This wire by itself outside an engine room is good for 45 Amps, in an engine room it gets reduced to 38.3 Amps, but if four to six conductors are in a bundle then the rating is reduced to 26.8 Amps in an engine room.  As the bundle gets bigger the rating gets reduced even more, with 7 or more conductors the rating in an engine room is 19 Amps.  So when I use 12 awg wire for 15 Amp circuits I am being cautious but it also means that I don't have to keep watching how many wires are in the bundle and I can wire the boat without constantly worrying.

In many cases I could get away with 14 awg wire for 15 amp circuits but it does fall just below the 15 amp rating with 7 or more conductors in a bundle in an engine room.

When considering how many wires are in a bundle we are counting current carrying conductors, the ground wire doesn't count.  Don't forget though that as you get near the circuit breaker panel all the wires come together in one big bundle, so even on a small boat you can end up with quite a few.

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