Posted: 12/13/2014 2:51:50 PM EDT
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Say you have a 2-pole breaker rated at 30 amps.
Is the total amps divided between poles or shared? Under what conditions will it trip? If.... - any pole exceeds 15 amps - any pole exceeds 30 amps or - cumulative amperage through both poles exceeds 30? |
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Short answer: Each pole should individually be 30 amps, and it should trip if either pole exceeds that.
Longer answer (a bit off topic): It's very easy to understand this if you think about it in terms of a 2 wire 230 VAC circuit where there is no neutral wire. To make electricity seem more simple you can think of it as water in pipes. The flow volume = amps, water pressure = volts, water inertia (time it takes to get that water moving when the valve is opened) = inductance. A neutral wire is just a "return line" in a way, and your line 1 & line 2 are both pressurized (they alternate from positive to negative pressure alternatively). So if you have a circuit with no neutral L1 is positive while L2 is negative and they are connected together. When L1 is positive the "water" is flowing from L1 into L2 through the panel. In this case both poles ALWAYS have the same current (amps) unless there is a short to ground. In the case of a 3 wire circuit where there are 2 hots and a neutral the "flow" from L1 can either go into L2 (at 230 volts "pressure difference") or into the neutral at 115 volts. In this case, a poorly designed device or improperly wired device could double the allowable current on the neutral wire without tripping the breaker. IE, both L1 & L2 are flowing at the max of 30 amps but they are flowing into the neutral, you can have 60 amps going through the neutral (bad). In this case the device shouldn't be wired into a 230 V circuit because each line is being used as an independent 115 VAC line and they should be on separate circuits... |
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In typical residential split phase service (120/240), L1 and L2 are 180 degrees out of phase. In a 3-wire system, the most you can have flowing in the neutral is the max current on one of the hots with zero current on the other hot. So if 30A on L1 and 0A on L2, the Neutral will carry 30A. If you've got 30A on L1 and 20A on L2, then you'd have 10A on the Neutral.
The only way you'd get L1 and L2 to combine and put all their current on the Neutral is if some asshat wired everything wrong at the service entrance... Of if someone tied a 120V inverter to both L1 and L2 in the service. All 240V appliances use a 2-pole breaker. So the main breaker is not the only 2-pole in the system. And yes, a 30A breaker is 30A per pole. |
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Quoted:
In typical residential split phase service (120/240), L1 and L2 are 180 degrees out of phase. In a 3-wire system, the most you can have flowing in the neutral is the max current on one of the hots with zero current on the other hot. So if 30A on L1 and 0A on L2, the Neutral will carry 30A. If you've got 30A on L1 and 20A on L2, then you'd have 10A on the Neutral. You're right, I totally spaced that one... 
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Quoted:
You're right, I totally spaced that one... Quoted:
Quoted:
In typical residential split phase service (120/240), L1 and L2 are 180 degrees out of phase. In a 3-wire system, the most you can have flowing in the neutral is the max current on one of the hots with zero current on the other hot. So if 30A on L1 and 0A on L2, the Neutral will carry 30A. If you've got 30A on L1 and 20A on L2, then you'd have 10A on the Neutral. You're right, I totally spaced that one... The common US system has a couple of names. Edison Circuit Split phase 120/240 V single phase service 110V, 115V, & 220V all went away many years ago. 230 V has a very specific use now. It show up in residential apartments instead of 240 V if the feed to the building is 3-phase. The whole 'trick' to the system (and the split phase) name comes from how the pole transformer secondary feeding loads is connected. A not atypical system ises a 7.2 kV (7,2000 volts) primary for the distribution wiring. The secondary of the trannsformer is a single winding that is 240 V from end to end. The widing has a 'center tap' in the middle of the winding. his point is grounded and declared neutral. That makes each half of the winding produce 120 V, while still having 240 V available across the two hot lines hooked to the ends of the winding. We have 120 V for safety on smaller circuits, while maintaining easy access to 240 V for larger loads. Ever notice all those fancy safety provisions on European 240 V equipment? Things like fuses in plugs. 240 V is far more dangerous when humans become part of the circuit. The additional precautions become worth the cost. Europe has no ready source of copper so thinner wires carrying less current (and thicker insulation for the higher voltage) became their standard for economic reasons. |