multiwire branch circuit question

[jeff360]

Hi-

I'm a non-professional looking for advice on recent electrical work. I have a sump pit with 2 sump pumps, which previously ran off of the same circuit. I wanted 2 20 amp runs to provide dedicated power to each pump, and as a 2nd layer of protection in case one breaker trips. When I came home, I only noticed a single romex run and was confused. After some googling, it seems they used a multiwire branch circuit to create the 2 circuits.

Does a multiwire branch circuit provide the same amount of power as two 20 amp circuits, or only a single 20 amp circuit?
Is a multiwire branch circuit providing the layer of redundancy/back-up that I had in mind when I requested separate 20 amp circuits for each sump pump?

Thank you!

 

[ActionDave]

Even though the OP is not an electrician topics like this can be open for discussion as long as the thread does not drift into DIY wiring advice.

 

[roger]

Since you're just asking for information and not "how to" advice I will offer this. A Multi Wire Branch Circuit can be two circuits. In theory it is a single circuit however the NEC allows it to be considered multiple and in your case you can consider it two.

 

[infinity]

Your electrician did run 2-20 amp circuits but if you were looking for one circuit as a back up he should not have run a MWBC because a multi-pole CB required for the MWBC will shut off both circuits anyway. A handle-tie and two single poles might work.

 

[jeff360]

Thank you for the replies, I'm certainly not looking for DIY wiring advice!

 

[winnie]

A 120/240V multi wire branch circuit provides slightly more than twice the power of a single 120V circuit of the same amp rating.

The circuit breaker supplying the mwbc is required to simultaneously disconnect both halves of the MWBC when toggled off. It is not required to 'trip' both poles simultaneously. So one question you should ask the electrician is which specific circuit breaker was selected for this circuit.

Finally there are types of nm cable which are designed to contain two completely independent circuits. This is another possibility for the installation you describe.

Jonathan

 

[curt swartz]

If you would have asked me to do this installation I would have used 12/2/2 cable that has independent neutrals so handle ties or common trip breakers would not be required. If you haven't already you should try to read the jacket on the cable to see if it is 12/3 or 12/2/2. They look very similar. The 12/2/2 is slightly larger in diameter.

 

[jeff360]

A 120/240V multi wire branch circuit provides slightly more than twice the power of a single 120V circuit of the same amp rating.

The circuit breaker supplying the mwbc is required to simultaneously disconnect both halves of the MWBC when toggled off. It is not required to 'trip' both poles simultaneously. So one question you should ask the electrician is which specific circuit breaker was selected for this circuit.

Finally there are types of nm cable which are designed to contain two completely independent circuits. This is another possibility for the installation you describe.

Jonathan

Thanks very much. I will clarify whether its a MWBC or 2 independent circuits run in one cable.
If it's the former (MWBC) - I do have the specific breakers for the circuit, if toggling is needed.
But regarding "tripping a breaker", if a pump trips one of the circuits on a MWBC setup, the other pump/circuit will still operate? Is that correct?

 

[jeff360]

If you would have asked me to do this installation I would have used 12/2/2 cable that has independent neutrals so handle ties or common trip breakers would not be required. If you haven't already you should try to read the jacket on the cable to see if it is 12/3 or 12/2/2. They look very similar. The 12/2/2 is slightly larger in diameter.

The jacket says 12/3 ... so I guess that confirms a MWBC set-up?

 

[roger]

The jacket says 12/3 ... so I guess that confirms a MWBC set-up?

Yes it does. It will probably trip both circuits if something happens.

 

[don_resqcapt19]

Thanks very much. I will clarify whether its a MWBC or 2 independent circuits run in one cable.
If it's the former (MWBC) - I do have the specific breakers for the circuit, if toggling is needed.
But regarding "tripping a breaker", if a pump trips one of the circuits on a MWBC setup, the other pump/circuit will still operate? Is that correct?

If the breaker is a double pole breaker, and if the circuit has GFCI protection it would have to be a double pole breaker, both pumps will lose power when the breaker trips.

If, for some reason GFCI protection was not required, and two single pole breakers with a handle tie were used, when one trips it may or may not cause the other breaker to open.

 

[winnie]

But regarding "tripping a breaker", if a pump trips one of the circuits on a MWBC setup, the other pump/circuit will still operate? Is that correct?

Do you have two separate breakers connected by a handle tie, or a single 'double pole' breaker, or something else?

 

[Flicker Index]

A 120/240V multi wire branch circuit provides slightly more than twice the power of a single 120V circuit of the same amp rating.

The circuit breaker supplying the mwbc is required to simultaneously disconnect both halves of the MWBC when toggled off. It is not required to 'trip' both poles simultaneously. So one question you should ask the electrician is which specific circuit breaker was selected for this circuit.

Finally there are types of nm cable which are designed to contain two completely independent circuits. This is another possibility for the installation you describe.

Jonathan

What's the basis for this?

 

[winnie]

What's the basis for this?

The ever so slightly lower losses in the neutral of the MWBC.

Jonathan

 

[tthh]

Probably best to post a photo of the circuit breaker installed in order to tell if this is a common trip or independent trip with handle tie.

 

[jeff360]

Probably best to post a photo of the circuit breaker installed in order to tell if this is a common trip or independent trip with handle tie.

Do you have two separate breakers connected by a handle tie, or a single 'double pole' breaker, or something else?

The MWBC is 37/39 in this pic. They look like all the other breakers on the panel (save for a couple of breaker pairs that actually DO have a handle tie on them)

 

[winnie]

Those breakers will operate completely separately. If one trips the other won't, unless some common fault actually hitting both circuits occurs.

This may be a code violation. This may be permissible in your locality, and was permitted in prior versions of the code. But current code requires requires breakers that are tied together so that if you are working on one half of the circuit you don't get shocked by the neutral. It isn't a problem unless someone is working on the circuits, so as long as you are not doing any DIY work it isn't a safety issue.

-Jonathan

 

[LarryFine]

A. Those are two individual 1-pole breakers.

B. You said Romex, not permitted outdoors.

 

[Little Bill]

A. Those are two individual 1-pole breakers.

B. You said Romex, not permitted outdoors.

Where did the OP say it was outdoors?

 

[LarryFine]

Where did the OP say it was outdoors?

You're right; my bad.