DanielW
Well-known Member
- Location
- Haliburton, Ontario
I'll second what John T and Gene said: Most codes allow it (at least, here in Canada they do), but the breakers must be tied together and on opposite legs/poles. Depending on the panel, two breakers side-by-side may or may not be from the same or separate poles. Some panels have every adjacent single breaker slot opposite (i.e. A, B, A, B, ...) so any double breaker will automatically be out-of-phase (or 240V between). But some panels have a different phase map, with two slots in a row the same, then the next two opposite (i.e. A, A, B, B, A, A, B, B, ...). On this second style of panel if you want 240 (or opposite poles) you need to make sure your double breaker (for 240) or tied breakers (for shared neutral) span one of the spots between A & B (rather than A & A or B & B). It should be pretty obvious what you have by looking at the bus bars in your panel.
As John mentions above, they need to be opposite phases to make sure you're not overloading the neutral. And they need to be tied together, because otherwise if you killed one breaker to work on the circuit, the neutral would still be live via current passing through anything that's running on the other breaker (and also to make sure a short that causes a breaker to trip knocks out both breakers, as both would be tied to the short via the neutral).
Two of the houses on our farm have loads of MWBC's and had been fooled around with by goofballs over the years. At one point someone had clearly done things correctly with tied breakers on opposite poles, but goofballery in later years with half-arsed circuit additions and trying to penny-pinch by saving a few feet of wire meant that everything was all bungled up: Every time you killed a circuit to work on it you'd get a shock due to some backfeeding through the neutral from a non-tied breaker or due to some other goofballery. There were also other backfeeding & voltage drop issues. Both houses are now (finally) wired properly (more-or-less). There's still some shared neutrals which I'm not a fan of, but at least they're all wired correctly as per above: Tied breakers and opposite phases.
For any wiring I do, I wouldn't run any shared neutrals. I'd rather just keep things straight forward and have one neutral for each live leg without any sharing concerns. I can see why it'd be convenient for some large commercial projects with long runs. But for residential circuits, I don't really see any reason to share neutrals. Sometimes it makes things convenient, but for a small amount of extra wire and routing work you can avoid it. One of the reasons I don't like it is because if something ever goes awry with your neutral (if it ever broke at some spot between where it's shared and your panel), you effectively have 240V potential between two sides of your 120 circuit that are tied together via the neutral: All the 120V appliances, lights, wiring, etc. on both circuits would suddenly be subject to 240V, which is going to cause some significant damage/melting/fire-hazards.
Of course, due diligence would say to be cautious of anything you read online, abide by local codes, don't assume what I write here (or anyone else writes) is necessarily correct for your situation, etc., etc.
As John mentions above, they need to be opposite phases to make sure you're not overloading the neutral. And they need to be tied together, because otherwise if you killed one breaker to work on the circuit, the neutral would still be live via current passing through anything that's running on the other breaker (and also to make sure a short that causes a breaker to trip knocks out both breakers, as both would be tied to the short via the neutral).
Two of the houses on our farm have loads of MWBC's and had been fooled around with by goofballs over the years. At one point someone had clearly done things correctly with tied breakers on opposite poles, but goofballery in later years with half-arsed circuit additions and trying to penny-pinch by saving a few feet of wire meant that everything was all bungled up: Every time you killed a circuit to work on it you'd get a shock due to some backfeeding through the neutral from a non-tied breaker or due to some other goofballery. There were also other backfeeding & voltage drop issues. Both houses are now (finally) wired properly (more-or-less). There's still some shared neutrals which I'm not a fan of, but at least they're all wired correctly as per above: Tied breakers and opposite phases.
For any wiring I do, I wouldn't run any shared neutrals. I'd rather just keep things straight forward and have one neutral for each live leg without any sharing concerns. I can see why it'd be convenient for some large commercial projects with long runs. But for residential circuits, I don't really see any reason to share neutrals. Sometimes it makes things convenient, but for a small amount of extra wire and routing work you can avoid it. One of the reasons I don't like it is because if something ever goes awry with your neutral (if it ever broke at some spot between where it's shared and your panel), you effectively have 240V potential between two sides of your 120 circuit that are tied together via the neutral: All the 120V appliances, lights, wiring, etc. on both circuits would suddenly be subject to 240V, which is going to cause some significant damage/melting/fire-hazards.
Of course, due diligence would say to be cautious of anything you read online, abide by local codes, don't assume what I write here (or anyone else writes) is necessarily correct for your situation, etc., etc.
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