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multiwire branch circuit question

J

jeff360

Member
Location
US
Occupation
Non-electrician
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!
 
roger

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
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

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
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.
 
J

jeff360

Member
Location
US
Occupation
Non-electrician
Thank you for the replies, I'm certainly not looking for DIY wiring advice!
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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
 
C

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
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.
 
J

jeff360

Member
Location
US
Occupation
Non-electrician
winnie said:
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
Click to expand...
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?
 
J

jeff360

Member
Location
US
Occupation
Non-electrician
curt swartz said:
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.
Click to expand...
The jacket says 12/3 ... so I guess that confirms a MWBC set-up?
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
jeff360 said:
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?
Click to expand...
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.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
jeff360 said:
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?
Click to expand...

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

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
winnie said:
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
Click to expand...
What's the basis for this?
 
T

tthh

Senior Member
Location
Denver
Occupation
Retired Engineer
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.
 
J

jeff360

Member
Location
US
Occupation
Non-electrician
tthh said:
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.
Click to expand...
winnie said:
Do you have two separate breakers connected by a handle tie, or a single 'double pole' breaker, or something else?
Click to expand...

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)

image0.jpeg
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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
 
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