Quote:
Originally Posted by twindman
So on a standard 50A plug and service, isn't there still only 1 neutral wire and would have the same problem?
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First some basics. On a standard 50 amp 240/120 vac system there are two hots using a common neutral. The voltage of the hot wire referenced to neutral constantly cycles between positive 120 volts rms and negative 120 volts rms (peak voltage is actually much higher than 120, but 120 vac rms is sort of an average). Since the voltage is constantly reversing, the current is also constantly reversing. That means on the first half of each cycle the current flows from hot to neutral. On the second half of each cycle the current reverses and flows from neutral to hot.
But hot 1 and hot 2 are 180 degrees out of phase. That means when hot 1 voltage is positive with respect to neutral, hot 2 voltage is negative with respect to neutral. Since there is only one neutral wire, that would mean that current would be trying to flow both directions thru the neutral wire at the same time. But that can’t happen. The result is that only the DIFFERENCE between the two currents actually flows thru the neutral wire. The excess current is diverted thru the hot drawing the higher current. Example; if hot 1 was pulling 20 amps and hot 2 was pulling 10 amps, the neutral would only be carrying 10 amps (the difference between the two). The neutral wire can never be carrying more than 50 amps. And only then if one hot was pulling 50 amps and the other hot pulling no current. A unique aspect of this is that if both hots were each pulling the exact same amount of current then the neutral wire would be zero current!
Now, if you had the two 30 amp outlets, and both of those hots were on the same phase, then both voltages have the same positive to negative timing, and the current of the two always flows the same direction. So instead of the neutral flow being decreased by the two offset of the hots out of phase, now the neutral flow is the combined flow of both.
As stated above on a standard 50 amp system the neutral flow can never exceed 50 amps and is typically something less. And all wiring is sized for 50 amps.
But on a system using two 30 amp outlets on the same phase, if both hots were pulling 30 amps the neutral would be carrying 60 amps. The likelihood of doing that is probably low. But you can see it is possible.
I know this is a pretty drawn out description. And possibly (likely?) confusing. Something much easier explained by drawing the circuits on a blackboard.