Calling all electricians- Sharing neutral on split wired circuit ?

[Andrew Seemann]

Yep, a split/multi wire circuit sort of functions like a 240 volt circuit. Basically where the neutrals tie together between the two loads, the current goes to the other circuit, and the neutral only carries the difference in load back to the breaker box.

This also happens in the main breaker box in your house and in sub panels. If the loads are exactly balanced, there theoretically would be no current traveling back to the breaker box in the neutral, so again in theory, you have less resistance from wire length. Whether this has any practical impact in a regular house or shop circuit is kind of debatable though. Myself, I don't like them because they are kind of a pain, and I have too many memories of getting zapped from current backfed from the neutral of unexpected split wire circuits in old plug fuse fuseboxes.

Note that the opposite is true if you are on the same leg of the breaker box, then the neutral would be carrying both loads, which is bad.

[sean meltvedt]

David, another advantage of a multi wire circuit-120 & 240 availability. My shop is wired using double pole gfci breakers to provide both 20 amp 120 and 20amp 240 outlets. (Of course the circuit can only support 20 amps, so one 240 load or 1or2 120 loads)
cheers
Sean

[Rollie Meyers]

I know it is not intuitive but Jim and Rollie are both quite mistaken and Wade is correct. If the load on both legs is the same then zero current will flow in the neutral wire, even at full load. The worst case is that full load current from one leg will flow in the neutral wire and that is only if the other leg is completely unloaded.

If you have a common neutral w/ both lines on the same leg, YOU WILL fry the neutral if the load is high enough, you misread what I said. Multiwire circuits are fine, it's just that GFCI requirements make it harder to use multiwire circuits, if AFCI's are expanded to garages & shops then that just adds to the issues. I don't refer to the legs as phases unless it is a 3 phase panel.

[Art Mann]

I was under the impression that we were all talking about using 240VAC plus neutral and ground to energize two 120VAC circuits. From a practical standpoint, you would have to go to extra trouble to run both circuits from the same side of the transformer.

If you have a common neutral w/ both lines on the same leg, YOU WILL fry the neutral if the load is high enough, you misread what I said. Multiwire circuits are fine, it's just that GFCI requirements make it harder to use multiwire circuits, if AFCI's are expanded to garages & shops then that just adds to the issues. I don't refer to the legs as phases unless it is a 3 phase panel.

[Rollie Meyers]

I was under the impression that we were all talking about using 240VAC plus neutral and ground to energize two 120VAC circuits. From a practical standpoint, you would have to go to extra trouble to run both circuits from the same side of the transformer.

If someone is careless or does not know what they are doing it's pretty easy to miswire, all it would take was putting both legs on a twin breaker, at least the common ones, GE, Challenger, Zinsco/Sylvania, & Crouse-Hinds, had twins that 240V was possible, & GE is the only one of those that is not obsolete. There is nothing wrong with multiwire circuits, it's just getting harder to use them and be code compliant (not impossible though).

[Mike Henderson]

This is just my opinion but I don't like shared neutral circuits. You aren't saving a lot of money doing it and you never know when that "saving" will come back and bite you. Just run two circuits.

Mike

[Art Mann]

Adjacent breaker positions are on opposite poles on all the breaker boxes I have ever wired.

If someone is careless or does not know what they are doing it's pretty easy to miswire, all it would take was putting both legs on a twin breaker, at least the common ones, GE, Challenger, Zinsco/Sylvania, & Crouse-Hinds, had twins that 240V was possible, & GE is the only one of those that is not obsolete. There is nothing wrong with multiwire circuits, it's just getting harder to use them and be code compliant (not impossible though).

[Frank Pratt]

Adjacent breaker positions are on opposite poles on all the breaker boxes I have ever wired.

Adjacent breaker positions are on different poles, but there are tandem, of duplex breakers that have 2 circuit breakers in a single space case. Connection a 3 wire circuit to one of those is where there's the danger.

[Rod Sheridan]

I inadvertently missed that...no way would I do a shared neutral for two high loads like that.

Actually Jim, that's an ideal application for a shared neutral as the neutral will only carry the unbalanced current.

So if the table saw draws 10 amperes and the dust collector 12 amperes, the neutral will carry 2 amperes.

regards, Rod.

[Rod Sheridan]

Adjacent breaker positions are on opposite poles on all the breaker boxes I have ever wired.

Art, there are many panel/breaker combinations where that isn't true.

I've had to correct 3 where the circuit had no line to line voltage because they were both on the same pole.............Regards, Rod.

[Jim Becker]

Actually Jim, that's an ideal application for a shared neutral as the neutral will only carry the unbalanced current.

So if the table saw draws 10 amperes and the dust collector 12 amperes, the neutral will carry 2 amperes.

regards, Rod.

Yes, I realized that after a bit considering they are on opposite poles (if wired correctly), but I honestly would still do discrete separate circuit wiring...I'm just like that!

[Kevin Beitz]

Yep, a split/multi wire circuit sort of functions like a 240 volt circuit. Basically where the neutrals tie together between the two loads, the current goes to the other circuit, and the neutral only carries the difference in load back to the breaker box.

This also happens in the main breaker box in your house and in sub panels. If the loads are exactly balanced, there theoretically would be no current traveling back to the breaker box in the neutral, so again in theory, you have less resistance from wire length. Whether this has any practical impact in a regular house or shop circuit is kind of debatable though. Myself, I don't like them because they are kind of a pain, and I have too many memories of getting zapped from current backfed from the neutral of unexpected split wire circuits in old plug fuse fuseboxes.

Note that the opposite is true if you are on the same leg of the breaker box, then the neutral would be carrying both loads, which is bad.

Could someone put this in a drawing. I can't see where no current is traveling back to the breaker box in the neutral when using a 120v circuit ...

[Julie Moriarty]

... but I honestly would still do discrete separate circuit wiring...I'm just like that!

Being "just like that" is a good thing. Keeps out the poltergeist.

[brent stanley]

Could someone put this in a drawing. I can't see where no current is traveling back to the breaker box in the neutral when using a 120v circuit ...

In a split phase system, think of the electricity in one line as coming while the other line is going...180 degrees opposite to each other. So they kinda cancel each other out with respect to each other, but with respect to ground they are as you'd expect for the load being run. And yes (sparkies!) I know the analogy isn't perfect, but it gives the jist.

B

[Jason Roehl]

Could someone put this in a drawing. I can't see where no current is traveling back to the breaker box in the neutral when using a 120v circuit ...

If it was just one 120V circuit, the neutral would carry current, but the discussion here is about a multi-wire circuit. In a multi-wire circuit, you would have, for example, two hot legs (red and black) and a neutral. The two hot legs are out of phase with each other (if wired properly to a double breaker). So if you monitored their voltage with an oscilloscope, you would see that each has a 60 Hz sine wave going, but opposite of each other. In other words, as one is peaking positive, the other is peaking the same amount negative, so the sum is zero, but there is 240V between them (RMS, or “root-means-squared”—actual peak is about +/-170V). While the black is “pushing” 120V (call it positive, perhaps), the red is “pulling” (call that negative) 120V and vice versa. If one of them is off, then the neutral allows the pushing and pulling to continue, but without contributing an opposite pull or push, respectively.