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I have a 15a breaker and 12g wire from the breaker.

If I add a 15a outlet to this circuit with 14g wire it should be ok since the 15a breaker should protect that wire.

But if the 12g wire at the breaker won't heat up as quickly as the 14g, how does the breaker see the overheat of the 14g wire?

I assume the 14g wire is then protected by the 15a outlet?

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The breaker doesn't know about the heating of the wire.

The breaker has its own internal "specimen wire" that is in series with the wall wire. It is measuring how fast its "specimen wire" heats up, and will trip at the appropriate time.

Literally, the "specimen wire" is a bi-metal strip, which provides both the sense and the muscle to pull the hammer on the breaker trip.

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The breaker senses the current through the breaker. It does not care what size wire is connected to it, nor "sense" that in any way.

Thus, the 14Ga wire is protected by the 15A breaker - neither the 12Ga wire nor an outlet play any role in the protection.

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It's the current that trips the breaker, not the heat of the wire. You could have 1/0 copper going to a 15 amp breaker (it won't fit) and draw 20 Amps and it would trip the breaker.

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The breaker limits the current to the whole circuit. The breaker actually reacts to the heat in the breaker. Higher current in the breaker creates more heat in the breaker.

The rules about minimum wire size allowed is evaluation of the ability of the insulation to not melt under maximum load. Larger wire creates less heat when current flows through it.

Without taking a full hour and a whiteboard and a bunch of math teaching the relationship of voltage/resistance/current/watts/load/heat the important factor is that wire has resistance, resistance reduces the voltage available at the receptacle. Bigger wire has less resistance, and less voltage is lost on the wire feeding the load. Voltage lost on the wire is directly related to heat, 3.4 btu's per watt/hour.

So for a theoretical imaginary 15A load on a circuit with #14 wire that is +/-100' from a breaker the voltage available at the end of the wire might be reduced from 120v to 116v, the heat created on the wire would be around 60 watts, or around 200 btu's. If the circuit was installed with #12 wire the voltage might be 118v, only creating 100 btu's in the walls.

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