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I'm wiring up a trailer brake controller unit on my 79 Jeep Wagoneer and I want to run 10Ga. supply wire to the unit. I bought 20 feet of 10Ga. wire while in town today and ran said 10Ga. wire to the trailer hitch for my trailer brakes. Unfortunately, my Wagoneer is a little longer than I thought and I don't have enough 10Ga. to run to the battery for the control unit power supply.

My plan is to run 10Ga. from the battery to the supply side of a 30A DC breaker I'm installing on the firewall. Then, Off the load post I plan on running two 14Ga. wires and then joining them at the 10Ga. butt connector where they join the harness for the controller.

My big question is, will two slightly lighter wires carry the same load as one larger wire? And if so, what's the math on figuring what gauge of two wires is needed to compensate for the lack of a single heavier wire?

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While this may seem OK in theory, it is not a good idea in practice and here is why it could be unsafe:

You are probably assuming that the current will split equally between the two wires, which under ideal conditions it might (or at least be very close). In the real world however corrosion is often a problem and where the wire enters the butt connector one wire may have more corrosion than the other, In such a case one wire will carry more current than the other, and a 14 gauge wire can not safely carry 30 A.

Much better to simply get some more 10 gauge wire than risk having your vehicle catch fire.

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  • \$\begingroup\$ Ah, I see now. So that explains why I see those strips with posts to connect dissimilar wire sizes. \$\endgroup\$
    – Ofeargall
    Jan 23, 2012 at 5:02
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    \$\begingroup\$ It should be noted that in the usual case this level of corrosion would result in little to no current flowing through the wire, and the tailer lights not working. Accidentally cutting or breaking one of the wires would produce the same result. \$\endgroup\$ Jan 23, 2012 at 6:05
  • \$\begingroup\$ @KevinVermeer, Thank you. Excellent point indeed. Yet another reason to regard one wire better than two. Since the intended application is for an electric brake controller, the last thing I want is an unbalanced load/failure. One wire it is. \$\endgroup\$
    – Ofeargall
    Jan 23, 2012 at 15:59

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