Ohm's Law and Trailer Brakes

[Steve Braverman]

I feel kind of silly asking this after posting the link to Ohm's Law as a response to a post regarding a six to twelve volt conversion. I am installing a trailer brake controller in my truck, and the instructions say to use 10g wire, however, the wires coming out of the controller are 12-14g. I always think of the weakest link analogy. What's up?

[Chuck Richardson]

Steve--I'm not sure what your question is, but 10g wire has a larger current capacity than the 12-14, and thus would not be the weak link. If the controller was properly engineered, the 12-14g wire would be adequate. I don't know how long the new wiring is, but a smaller gauge would mean that you could wire for a longer distance with less resistance.

[Steve Braverman]

I'm aware that 10g is larger than 12, but if the wires coming out of the controller are 12, what good is using 10g?

[Mark Shaw]

The 12-14 gauge control wires are most probably set up for a vehicle with brake control wiring already in the dash harness. I would go ahead and use the 10 gauge for any wire runs to the rear of the vehicle.

[Chuck Richardson]

As Mark says, the smaller (10) gauge allows you to do long runs. If you used 12-14g on a long run, the resistance losses might not meet the design parameters of the controller and the brakes.

[Jim Bollman]

The short piece of 12-14g limits the current you could put down the wire. What you are trying to do is reduce resistance, so a short piece of 12-14g connected to a long piece of 10g is less resistance than a long run of 12-14g.

If you need 10g or not I don't know.

[Steve Braverman]

Thanks Chuck and Jim, that makes sense. I finished the install today, and I used all 10g wire. Better to er on the side of caution.

[Barry Wolk]

Good choice. However, since the magnets were originally designed to operate on a lower voltage system they will be more powerful at the higher voltage. They will work fine at 12-volts. That's where the controller comes in. It determines the amount of current sent to the brake coils. For your first trip I'd advise setting the dial down and work your way up to where the brakes feel good.

[Guest simplyconnected]

Not a silly question at all, Steve. Not all wire is created equal, either. Ampacity capacity depends on these very important things:

*The size and type of metal used for the wire. (yours is stranded copper, not aluminum for example)

*The type of insulation covering the wire,

*How many wires are in the same harness (raceway or conduit), or if it is in "FREE AIR."

The concern is with the wire's ability to shed heat (so the insulation doesn't melt). The same wire size may carry more amps when it is in 'free air' and percent-duty cycle allows time for cooling. That's the discrepency you ran into. Certainly, #10AWG THHN safely carries 30amps CONTINUOUSLY. Some wire sizes are drastically reduced, like battery cables. They carry 500-amps or more, but for a short time, and they're in free air.

Your brake company knows you won't energize the brakes all the time, at 'full blast'.