How much current draw


Old 12-10-06, 07:39 PM
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How much current draw

I understand I can run 20amps on a 12 guage wire but I have a cable that is 12/3, ie, red, white, black, green. So my question is, can I run 20amp on both the red and black?

I'm confused because I would think the return white wire would still only be rated for 20amps total but if the other two ran 20 then that would put a 40amp load on the white wire.

What am I misunderstanding?
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Old 12-10-06, 07:50 PM
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It is actually the opposite. The white will only carry the imbalance of the two circuits of a multi-wire circuit.
For instance, if the black is carrying 10 amps, and the red is carrying 7 amps, the white is only carrying 3 amps.


This is provided the multi-wire circuit is wired correctly. Meaning the two hots are on different "phases" in the panel.
Old 12-10-06, 08:00 PM
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As long as the circuit is arranged so that there is 240 volts between the red and the black and 120 volts between each of them and the white then the white will carry only the difference between the current on the red and black wires instead of the sum of the two currents. This is because the two loads will cancel each other out by behaving as a 240 volt load. How you arrange the circuit to do that is to study the circuit diagram in your panel cabinet and attach the black to the buss bar that is on the left side of the panel and the red to the buss bar that is on the right side of the panel. In many but not all panels every other breaker down each side is on the same buss bar so if the two breakers are full sized and adjacent to each other each will be on a different buss bar than the other. The best way to assure this is to use a full sized double pole breaker but that means both loads will be deenergized instead of only the one that is faulted or is overloading the circuit.

The common name for this circuit is an Edison circuit. Many people oppose the use of Edison circuits in homes for fear that someone will open the white conductor believing that the circuit is deenergized and get a shock. The use of a double pole breaker will prevent that but so does compliance with the code requirement that the continuity of the neutral (white) may not depend on a device in such a circuit. The neutral is spliced all the way through the circuit with the splices including short jumpers; commonly called pig tails; to connect the neutral to the receptacles, luminaires, and other loads of the circuit. My belief is that anyone who is unwilling to follow standard safety procedures and check for voltage and current prior to working on a circuit should keep their hands out of electrical work entirely.
Old 12-10-06, 08:08 PM
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Yes, if you make sure each are on opposite legs of the incoming supply. That way, the current on the neutral will be the difference in current carried on each of the hots.
Old 12-10-06, 08:11 PM
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Or you know why we charge... OOPS, never mind.
Old 12-11-06, 05:40 AM
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The common name for this arrangement is "multiwire circuit". This is the common trade term for the circuit and is also the name used in the National Electrical Code for rules on permitted use and application of the circuit.
Old 12-11-06, 09:46 PM
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I understand. Didn't think about the phasing. Thanks guys.

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