If a 2 or 3 pull 20 amp breaker has 12 amps on each phase, how is the total amp per phase not added up?
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Poles.
Each phase may pull a different amount of current, but the amount you are “reading” on a meter is the RMS value (Root Mean Square) which is a fancy way of saying its the “average” amount of power.
Since the different poles are out of phase with each other (cycling at different intervals) the actual total amperage in the circuit is not going to be the total of all three phases.
In a single-pole USA breaker, the hot wire is varying from +120 to -120V relative to neutral. Current flows from the hot wire to the outlet, and returns via a neutral wire. Both the hot and neutral wires carry nearly the same current. The breaker cuts off the +/- 120V hot wire. The neutral wire does not need a breaker; it’s always tied to ground.
In a double-pole USA breaker, there are two hot wires varying from +120V to -120V, such that one pole is +120V when the other is -120V. Current flows from one hot wire to the outlet, and returns via the other hot wire. Both breaker poles carry nearly the same current. There’s a couple ways to arrange these breakers:
* If you had a breaker on only one hot wire, an overcurrent would trip the breaker and stop current flow, but the other wire would continue to be +/- 120V, which is hazardous. This setup is not allowed.
* If you had a beaker on both wires, but they aren’t mechanically tied together, slight variations in the breaker performance would potentially cause one breaker to trip earlier than the other, which again would leave one wire at +/- 120V. This setup is also not allowed.
* If you have a multi-pole breaker, where all poles are tied together, as soon as one breaker activates it trips the other pole(s), cutting power to all the hot wires on that circuit. This is the only acceptable setup for a multi-pole circuit.
Trace the path
Take a 2 pole 20A breaker. If you put a 15A load on it that’s just wired across the phases then when Phase A is positive relative to ground you’ll have current flowing from Phase A into Phase B, then the next half cycle the current flows from Phase B back into Phase A. The sine wave peaks at 21.2A (15A RMS) each time
Each phase has a current equivalent to 15A RMS in this scenario so it doesn’t trip the breaker. Current flows up one phase and down the other then down one and up the other, but matched currents
The total power is increased but its better to think of that as a result of the increased voltage. A single phase 120V 15A load is 1,800W. The split phase(technically not two phase) 240V 15A load on the 2 pole breaker is 3,600W
Its actually important that we increase the voltage to increase the delivered power because double the current requires a wire that’s over twice the area to stay a safe temperature
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