Neutral Imbalance in Multiwire Branch Circuits (Two Hots, One Neutral)

In the latest episode of Electrician U, a question came in from a viewer that needed some help understanding a topic. The question centered around sharing a neutral between 2 circuits. As this is a common practice in the construction industry, Dustin came prepared to drop some knowledge on the subject in an ongoing effort to better our viewers understanding!

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First, lets discuss Multiwire Branch Circuits. This is where we have multiple hot conductors paired with a single neutral. This installation can be in the form of Non-Metallic Cable (with a black, red, white, and grounding conductor) or even with MC Cable/standard pipe & wire method. Under normal circumstances, this is a perfectly acceptable practice, provided (among other things) that you have connected the hot conductors to TWO SEPARATE phases. One hot to A phase and the other hot to B phase for example. This way you don’t overload the neutral with return current from the same phase. Another thing that the NEC requires us to do is make sure that the breaker handles for the multiwire branch circuits have a common handle. This can be accomplished via a two (or three pole, if it is a three phase multiwire branch circuit) pole breaker OR other approved breaker handle tie. By doing this, if you shut off one breaker, you should not have any return current from any of the other circuits in the multiwire branch on the shared (or common) neutral conductor.
To make it a bit easier to understand, imagine, that while the black hot conductor if pushing the current thru the wire/load, the white conductor is pulling it thru. And if the red/white set were doing the same thing, then the current that is on the white conductor of a multiwire branch circuit would cancel it out and we wouldn’t have any issues. It would be the same, in essence, as a 240v 2 pole circuit with two hots. Now, this would only be true if the exact same load was on each circuit. Let’s say 10 amps. What would end up happening is the 12 ohms worth of resistance (120v divided by 10a = 12 ohms. Basic Ohm’s Law) would be doubled across double the voltage (12 Ohm’s + 12 Ohm’s on a 240v circuit now). The current is still able to make the complete loop from source, thru load(s) and return to source, therefore making a viable electrical circuit, and everything operates as it should.
Let’s get back to the statement of the exact same sized load being on each circuit and unpack that for a minute. We have discussed what would happen if the loads were balanced. But what if the loads were Imbalanced? Say 10a on one circuit and 15a on the other. Well, the neutral conductor (grounded conductor) would carry that imbalance of 5a back to source. This doesn’t necessarily pose any problem other than the fact that there are 5a of current flow on something other than the hot conductor. If we were to look at it from two single branch circuits (with the same amount of current on each), in lieu of a multiwire branch circuit, it would look much the same. 10a flowing out on the black wire, thru the load and returning on the white conductor, and the same with the red/white set. But the point where those neutral conductors came together (at the neutral bus bar) it would become 0a worth of current flow as they would cancel themselves out. So as an overall picture, the neutral is only carrying the unbalanced amount of current!
A big issue that can arise is when we do not use hots from different phases. Instead of each carrying 10a worth of current working in tandem (one pushing the other pulling for example) they are now both trying to push that 10a of current since they are both traveling the same way. So, while the breaker may be fine (10a worth of current on it) the neutral, at the point they came together, would have 20a worth of current on it since there was nothing to carry the other 10a away!

#electrician #electrical #electricity