I have been wiring new circuits and breakers for years and feel comfortable doing it. I got some advice from a Home Depot guy that has me thinking:
I need to wire two 20 amp circuits in my kitchen a long way from the box. He suggested I us 12-3 wire and use the black for one circuit and the red for the other then use the single white for the neutral for both circuits similarly with the copper ground wire.
Won't that result in possibly 40 amps on the white neutral leg of the circuit (at full load) - more than the 12 gauge wire is rated for?
If done right , one hot wire on each side of the circuit box then with one side loaded, the more current you draw from the other side then the less current you will have on the neutral wire. As far as the code goes I don't know if this is legal or not. If both hot wires are comming from the same side of the line then you would have too much current in the neutral.
The missing part of the story is that you should connect these to a two-pole breaker: red on one pole, black on the other.
Not if you wire it to the two-pole breaker. If wired to a two-pole breaker, the white only carries the DIFFERENCE in current between the red and the black.
1) I'm fairly certain it isn't in code, for the application you have stated, and anyone who comes along to do repairs or modifications might not know what you have done.
2) The red and the black wires carry current that is 180 degrees out of phase. If only the red wire is carring current, then the white wire will be carring the red wire current back to the box.
If the black wire is carrying current, then that current can be SUBTRACTED from the current in the red wire.,... for instance... assume the red wire is supplying 20 amperes and the black wire is supplying 20 amperes.... The resulting current in the white wire would be zero amperes. ..... and if the appliance on the red wire were suddenly switched off, the white wire would be carrying 20 amperes (from the black wire back to to box).
You see, you won't burn up stuff.
If I am wrong, and it does meet code,.... I still think it would be more reasonable to run two 12-2 w gnd for your 115V 20A outlets..... Probly not cost any more and you could keep track of it better.....
Even wire from your breaker box to utility transformer is wired that way - a common neutral. Current on your 20 amp neutral wire will not exceed 20 amps - if the load is mostly resistive. A high reactance load (motors, fluorescent lamps, computers, etc) are why some circuits wire this way using a larger neutral.
One thing you do want to do AND to understand why. You want the circuit breaker to be a dual 20 amp type. IOW if one circuit trips, then both circuits trip. I don't remember if this is required by code, but you want to do it anyway.
Dual 20 amp breaker accomplishes two things. First, it trips both hot wires so that anyone working on the wall receptacle (even the neutral wire) is not surprised. Second, it verifies that both circuits will be on different phases. If both circuits were accidentally wired to same phase, then a neutral wire fire is possible. Number two is especially important. As long as both hots are on different phases, then the neutral wire will not be overloaded. The only way to absolutely verify this will be wired properly is a dual 20 amp breaker where each side of the 20 amp breaker will be on the opposite phase.
Another who has the code article number is now encouraged to post that number and quote that code.
You should use a tied (aka 220V) breaker, so that all the power gets shut off to the outlet when it is switched off. Think what would happen if you only shut off one breaker before you started working on it, the other half would still be live.
If the circuit supplies a single device that uses both hot legs (e.g. a duplex receptacle with one hot leg feeding each half of the duplex), the Code requires that there be a single disconnect for both legs. A double-pole breaker is not mandated: two single-pole breakers, side-by-side, connected by an approved handle tie, will satisfy the requirement of a single disconnect.
Note, though, that even if the Code doesn't require a single disconnect, it's still a good idea.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
One more reason why I consider the dual 20 amp breaker essential. Another could move one wire to another breaker - for whatever reason (including another circuit's wire was cut too short). By doing so, he has put both 20 amp circuits on the same phase, without realizing it.
What do humans do to prove something is working properly? We put a light bulb on each wall receptacle, see the light illuminate, and then assume everything is OK. And everything is OK until we connect multiple appliance to each circuit. Now that neutral wire is carrying 30+ amps; cooking inside the walls without anyone's knowledge that it could even happen.
This a third reason for connecting via a dual breaker. It sends a message to any future electrician - these wires are connected here to different phases for good reason. The alternative could be a house fire generated by an overloaded neutral wire because someone did not know of or understand why those two wires must be on separate phases.
The > If the circuit supplies a single device that uses both hot legs (e.g.
Note, as I mentioned earlier, _some_ panels do not guarantee opposite legs for dual breakers. You need to examine the panel backplane and/or measure voltage unless you know for sure your panel doesn't do that.
Or better yet, just run two 12-2 cables and be done with it. Simple, straightforward, no confusion for anyone doing repairs, etc. In other words, besides being creative and maybe saving a couple of bucks, why use a shared neutral?
Doug Chill out. The Home Depot chap was correct in that it is a perfectly acceptable practice to run multi wire branch circuits. Every north American service or feeder is an Edison circuit. They are used for almost every "home run" in commercial work.
You do have to know what you are doing to work with multi wire branch circuits safely. If you don't know what you are doing then you should not even take the cover off of an electrical box, let alone remove the door assembly off of a panel cabinet. I have to say that the idea that electrical construction techniques should be dumbed down to avoid injury to ignorant harry home owner types irritates me. Anyone that does not know how to test circuits for voltage before working on them should not be doing electrical work.
Unlike some other posters on here I am not opposed to sweat equity electrical work as long as the person doing it has taken the time to learn enough theory and technique to do the work safely. Anyone who is not willing to invest the requisite time to learn to work safely, and yet still insist on working with electricity, should be wearing one of Jeff Foxworthy's signs so that the rest of us can stay clear of the after affects of their utter stupidity.
I just ran across a problem in a friend's panel and thought it was on topic with this.
He had(or did) a multiwire branch circuit to feed the receptacles on a floor. He wired both ungrounded conductors(the hots) to a single tandom breaker.
I took a few drawings to explain how he could be cooking the neutral since it carries the unbalanced current back. It wasn't easy, since he was first embarrished that I had to point out the ground for the circuit cannot hang freely, unterminated in the panel board.
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