MULTI-WIRE BRANCH CIRCUITS WITH DIFFERENT SIZE CIRCUIT BREAKERS

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Normally, a multiwire branch circuit consists of two ungrounded conductors from two circuit breakers that are the same amps (say 10 amps) and they'll share one neutral.

Is it permissible to use a 15 amp circuit breaker and a 20 amp circuit breaker as long as the neutral wire is sized for the 20 amp circuit. Why or why not?

Let's say this multiwire branch circuit had a neutral that was sized for the 15 amp breaker. What would happen?

I could not find an answer in the code or Mike Holt's illustrated guide to the NEC.

Thanks for your help.
 

ActionDave

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Normally, a multiwire branch circuit consists of two ungrounded conductors from two circuit breakers that are the same amps (say 10 amps) and they'll share one neutral.

Is it permissible to use a 15 amp circuit breaker and a 20 amp circuit breaker as long as the neutral wire is sized for the 20 amp circuit. Why or why not?
Yep, it's allowed.

Weird? Unusual? Not done in most cases in the real world because there is no real reason to? Yes.

Let's say this multiwire branch circuit had a neutral that was sized for the 15 amp breaker. What would happen?
There is a chance that the neutral could be overloaded. Very small chance, but still possible.
I could not find an answer in the code or Mike Holt's illustrated guide to the NEC.

Thanks for your help.
Because it's a one in a million chance that it will ever happen.
 

infinity

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Normally, a multiwire branch circuit consists of two ungrounded conductors from two circuit breakers that are the same amps (say 10 amps) and they'll share one neutral.

Is it permissible to use a 15 amp circuit breaker and a 20 amp circuit breaker as long as the neutral wire is sized for the 20 amp circuit. Why or why not?

Let's say this multiwire branch circuit had a neutral that was sized for the 15 amp breaker. What would happen?

I could not find an answer in the code or Mike Holt's illustrated guide to the NEC.

Thanks for your help.

In your example, if the 20 amp circuit had 20 amps of current and the 15 amp circuit had 0 amps the neutral current would be 20 amps which exceeds it's 15 amp value. The NEC wouldn't allow the reduced size neutral in your example.
 

goldstar

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New Jersey
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Electrical Contractor
Before (I think) the 2011 Code cycle you were permitted to use s/p breakers for MWBC's in new construction. That changed and you are now required to use 2-pole breakers. However, with the advent of AFCI requirements for most rooms in a residence using 2-pole breakers for MWBC's is a thing of the past.

Usually what constitutes a MWBC is the cable assembly (unless you're in Illinois and using EMT with individual conductors). So, let's say 14/3 or 12/3 NMC as an example and let's say you're upgrading an electric service. If you wanted to use one s/p 15 and one s/p 20 with a 12/3 NM cable that would be permissible. The only thing you'd have to be concerned with is the neutral overload as Infinity pointed out. On the other hand using a s/p 15 and a s/p 20 with 14/3 NM cable would not be permissible because you would be over-protecting one of those conductors with the 20 amp breaker.
 

kwired

Electron manager
Location
NE Nebraska
Before (I think) the 2011 Code cycle you were permitted to use s/p breakers for MWBC's in new construction. That changed and you are now required to use 2-pole breakers. However, with the advent of AFCI requirements for most rooms in a residence using 2-pole breakers for MWBC's is a thing of the past.

Usually what constitutes a MWBC is the cable assembly (unless you're in Illinois and using EMT with individual conductors). So, let's say 14/3 or 12/3 NMC as an example and let's say you're upgrading an electric service. If you wanted to use one s/p 15 and one s/p 20 with a 12/3 NM cable that would be permissible. The only thing you'd have to be concerned with is the neutral overload as Infinity pointed out. On the other hand using a s/p 15 and a s/p 20 with 14/3 NM cable would not be permissible because you would be over-protecting one of those conductors with the 20 amp breaker.
You can still use single pole breakers, but they must have approved handle ties installed on them. Same ties work on both 15 and 20 amp breakers, so you can easily mix a 15 and 20 amp in a MWBC.

Add: in many cases it cost the same to use the handle tie as it cost to just use a two pole breaker, or the two pole breaker is more readily available then a handle tie.

Handle ties allow for "independent trip", but you still end up turning off the second circuit when you attempt to reset the one that is tripped.
 
Seatramp

Seatramp

You can still use single pole breakers, but they must have approved handle ties installed on them. Same ties work on both 15 and 20 amp breakers, so you can easily mix a 15 and 20 amp in a MWBC.

Add: in many cases it cost the same to use the handle tie as it cost to just use a two pole breaker, or the two pole breaker is more readily available then a handle tie.

Handle ties allow for "independent trip", but you still end up turning off the second circuit when you attempt to reset the one that is tripped.

I think you to be exactly right. A multi-wire circuit is still "one circuit", and applies to one neutral if needed. If you separate the breakers, this implies two separate circuits requiring a neutral for each one.
 

al hildenbrand

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Location
Minnesota
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Electrical Contractor, Electrical Consultant, Electrical Engineer
Let's say this multiwire branch circuit had a neutral that was sized for the 15 amp breaker. What would happen?
The "worst case" happens when there is no current drawn by the load on the 15 Amp breaker side of the multiwire branch circuit, and, at the same time, the maximum current is drawn by the load on the 20 Amp breaker side. When the load on the 20 Amp breaker draws 20 Amps, that 20 Amp current must return to the source on the neutral. . . a neutral which is only rated for 15 Amps. The longer 20 Amps runs in the 15 Amp rated wire, the longer the insulation of the 15 Amp wire is heated.
 

templdl

Senior Member
Location
Wisconsin
Normally, a multiwire branch circuit consists of two ungrounded conductors from two circuit breakers that are the same amps (say 10 amps) and they'll share one neutral.

Is it permissible to use a 15 amp circuit breaker and a 20 amp circuit breaker as long as the neutral wire is sized for the 20 amp circuit. Why or why not?

Let's say this multiwire branch circuit had a neutral that was sized for the 15 amp breaker. What would happen?

I could not find an answer in the code or Mike Holt's illustrated guide to the NEC.

Thanks for your help.
What wire gauge is terminated on the breaker? Is there any derating factors that would apply?
Since breasker's for the most part protect wire knowing this would go a long way to addressing your we question. Whether or not is allowed by the NEC is one thing but provided ding protection is more important.
 

kwired

Electron manager
Location
NE Nebraska
I think you to be exactly right. A multi-wire circuit is still "one circuit", and applies to one neutral if needed. If you separate the breakers, this implies two separate circuits requiring a neutral for each one.
That is what it has become the past 10 years or so. Go back to maybe 1999 NEC and handle ties were only required if both ungrounded conductors were landed on the same device. Otherwise a MWBC was still considered one branch circuit, yet in some cases could meet the requirements of multiple circuits. Example: one MWBC could fulfill the requirement to have at least two small appliance branch circuits.

The handle tie requirements that have been added over the years were not so much required for the purpose of making it a single circuit as much as they have been required to protect the unqualified when working on such circuits - they don't know any better that they will have back feed on the neutral if they don't turn off all the ungrounded conductors of the circuit.
 
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