I'd like to hear the answer too.

But I am thinking that because the two hots are out of phase the neutral won't get overloaded.
As in option 2

 

A double pole handle tied breaker will trip the same as a single 15 amp breaker, but it takes both circuits with it for safety's sake. There is no concern of overloading the neutral if it is set up properly. You can't use a single neutral for two separate non handle tied breakers.

 

The neutral will not overload if the 2 single pole breakers are on both legs. The code requirement for handle ties or a 2 pole breaker is to protect unqualified people from getting hurt.
The two single pole breakers will work the same as a 2 pole breaker if you turn them both off.

 

The thing I don't like about using two seperate breakers for a MWBC is that in the future you or the next owner will do some rearranging in the panel for whatever reason.
And then they could both land on the same phase.

Also from what I understand If you are working on one of the branches the neutral will still be energized if only one breaker is turned off.

 

The math is this-- if the 2 hot legs are on different "phases" then the formula is N = Phase A - Phase B

If the hots are on one phase then the formula is N= A + B

So if you have both circuits that are loaded to 15 amps the neutral load will be 0 amps, or 30 amps.

 

In a single phase panel there are 2 hot conductors coming into the panel and one neutral. The wires are on Phase A and Phase B.
NOTE: Technically there is only one phase but they connection in the transformer is 180 degrees out of phase from each other. But for ease of understanding we talk of it in terms of 2 phases.
The conductors are attached to the busbar where every other breaker on one side of the panel is connected to the opposite phase.
Notice the panels are generally labelled with the odd numbers on the left and even numbers on the right. So if you connect a multiwire branch circuit to circuits 1 and 3 then you hot wires are on different phases and the neutral is safe.
If, however, you connect the multiwire branch circuit to circuit 1 and 5 then you have a problem.

You run into big problems with some of the thin or twin type breakers which are on one phase sometimes depending on how you install it.

 

Here is a good shot of a panel and you can see how each phase is connect in a panel. Not all breakers on one side are on one phase as you might think. This is how you get 240V with 2 breakers next to each other. In fact, if you tested the hot wires on a multiwire branch circuit you should get 240v across them. If you get 0 then you are not wired correctly.

 

Here is a good shot of a panel and you can see how each phase is connect in a panel. Note all breakers on one side are on one phase as you might think. (snip)

Bolded word is a very bad typo. (Should read "Not".)

Thin breakers come in a variety of styles. The most common style is a two breaker tandem unit that fits in one breaker position. It snaps onto just one fin down the middle of the panel. Both halves are powered by the same leg (phase if you insist) and cannot be used together for a MWBC.

 

MWBC has to be powered by both legs, which occurs with a double 240 volt type breaker. Current draw on both sides at the same time cancels itself out, like a 240 volt circuit thet doesn’t need a neutral. For example, if each side has a 10 amp draw, neutral sees 0 amps. Only the difference shows at the neutral. For example, if one side has a 12 amp draw and the other has a 7 amp draw, neutral sees 5 amps. The difference will never overload the neutral.

 

I think I hear an echo in here. hahahaha

 

A MWBC is always ONE circuit no matter whether you tie the breakers or not.

The handle-tie doesn't cause the phasing to be correct. You placing the breakers correctly causes the phasing to be correct. The handle-tie helps a bit by encouraging correct placement in many panels, but not always: in GE you can easily tie two THQPs and land them in the wrong place.

you have the potential for overloading the neutral without actually tripping either breaker. E.g. On two 15 amp breakers, you could have 10 amps running on each circuit, thus bringing back more than 15 amps on a 14 awg neutral wire and possibly causing a fire.

1) Is there truly a concern of overloading if you use a shared neutral across two single pole breakers?
2) Will using a double pole 15 amp breaker fix this potential hazard?

That's only a problem if you phased the hot wires incorrectly (on the same phase).

If they are on opposite phases/poles, then neutral only carries the difference of current, and you're fine. E.G. 12A on one leg and 10A on the other leg will result in 2A on the neutral.

This is well-illustrated here. Even the thumbnail illustrates it pretty well

 

A MWBC is always ONE circuit no matter whether you tie the breakers or not.

The handle-tie doesn't cause the phasing to be correct. You placing the breakers correctly causes the phasing to be correct. The handle-tie helps a bit by encouraging correct placement in many panels, but not always: in GE you can easily tie two THQPs and land them in the wrong place.



That's only a problem if you phased the hot wires incorrectly (on the same phase).

If they are on opposite phases/poles, then neutral only carries the difference of current, and you're fine. E.G. 12A on one leg and 10A on the other leg will result in 2A on the neutral.

This is well-illustrated here. Even the thumbnail illustrates it pretty well

Excellent video and explanation. Thank you

 

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Thanks everyone. I really appreciate all the feedback on this one!

 

Yeah, I hear what you mean, but I just have to say... man, that's confusing language.

It's like the guy who dreamed up "grounded conductor" said "nobody can make more obtuse language than me" and then the MWBC guy said "hold my beer!" Lol

 

""A multiwire circuit shall be permitted to be considered as multiple circuits"" -- One (12 gauge 120/240 volt) MWBC can satisfy the requirement of two 120 volt small appliance branch circuits for the kitchen.

""For the purposes of this section a {MWBC} shall be considered a single circuit"" -- Where/when only one circuit is allowed to feed an outbuilding, one MWBC, both halves of which carry current and are breakered, is allowed.

""{G}rounded conductor"" -- "Neutral" is not an official term in the NEC code book. A circuit requires two conductors, plus and minus respectively if you insist. In some situations with metal all around you such as in an automobile, it has been expedient (saving the cost of and space occupied by extra wire) to use the chassis as one of the two conductors and that conductor, sometimes the positive, is for that make and model said to be the grounded conductor. There are a few situations, notably medium tension primary lines along the road in rural areas, where the dirt (earth, soil) below is the grounded conductor with just one metal conductor (the ungrounded conductor) carried on the utility poles.

Some three phase systems may have one of the 3 phases grounded and are called corner ground systems. These circuits do not have a neutral whose neutral to hot voltage is non-zero for any of the phases but may have a 4'th conductor as an equipment grounding conductor bonded to the phase that is the corner ground.