Multiwire branch circuit question

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olc

Senior Member
I ran into a situation where the two lines of a multiwire branch circuit are connected the two connections of a tandem circuit breaker.

Correct me if I say anything wrong:

The two lines, that share a neutral, should be connected to different phases/legs.

The connection should be to a two pole circuit breaker so both circuits disconnect/open together.

Suppose if the two circuits served loads in a dwelling unit and assuming the required number of circuits were still met and the total connected load were OK could the two lines be connected to one circuit breaker (one circuit). This one bothers me because there would be two hot wires connected to the same phase and one neutral in the same cable. On the other hand the sum of the load on the two lines would equal the load on the neutral.
 
olc said:
I ran into a situation where the two lines of a multiwire branch circuit are connected the two connections of a tandem circuit breaker.

Correct me if I say anything wrong:

The two lines, that share a neutral, should be connected to different phases/legs.

The connection should be to a two pole circuit breaker so both circuits disconnect/open together.

Suppose if the two circuits served loads in a dwelling unit and assuming the required number of circuits were still met and the total connected load were OK could the two lines be connected to one circuit breaker (one circuit). This one bothers me because there would be two hot wires connected to the same phase and one neutral in the same cable. On the other hand the sum of the load on the two lines would equal the load on the neutral.
Click to expand...

It depends on the tandem breaker. Does it connect to two different busbars in the panel, if so you are OK. If it connects to the same busbar for each output, you have the problem of the potentially overloaded neutral as you described.
 
olc said:
The two lines, that share a neutral, should be connected to different phases/legs.
Click to expand...
That is part of the very definition of multi-wire branch circuit. Nothing in the code allows a neutral to be shared with two ungrounded conductors from the same phase.
olc said:
This one bothers me because there would be two hot wires connected to the same phase and one neutral in the same cable. On the other hand the sum of the load on the two lines would equal the load on the neutral.
Click to expand...
That is probably why this is not allowed.

 
charlie b said:
Nothing in the code allows a neutral to be shared with two ungrounded conductors from the same phase.

That is probably why this is not allowed.
Click to expand...
I can switch a ceiling fan and light sepparatlly, two switch legs one neutral.
I can switch half the lights in an office the same way. I can pigtail the two wires together at the switch and have one switch turn all the lights on. Same thing if I move the switch to right beside the panel. Why can I not do the same thing at the breaker?
 
ActionDave said:
I can switch a ceiling fan and light sepparatlly, two switch legs one neutral.
I can switch half the lights in an office the same way. I can pigtail the two wires together at the switch and have one switch turn all the lights on. Same thing if I move the switch to right beside the panel. Why can I not do the same thing at the breaker?
Click to expand...

In my first point, the two legs are connected to different circuit breakers on the same phase. So (to simplify) the neutral could see 40A (assuming the circuit breakers are 20A).

In my third point the two legs are connected to the same circuit breaker - which is more or less the situation you describe. In this case (again to simplify) the neutral would only see 20A.
 
fmtjfw said:
It depends on the tandem breaker. Does it connect to two different busbars in the panel, if so you are OK. If it connects to the same busbar for each output, you have the problem of the potentially overloaded neutral as you described.
Click to expand...

Don't tandem circuit breakers typically connect to the same bus (one connection)?
 
olc said:
In my first point, the two legs are connected to different circuit breakers on the same phase. So (to simplify) the neutral could see 40A (assuming the circuit breakers are 20A).
Click to expand...
Correct. In this case in order to share a neutral they need to be on different phases.

In my third point the two legs are connected to the same circuit breaker - which is more or less the situation you describe. In this case (again to simplify) the neutral would only see 20A.
Click to expand...
Correct.

olc said:
Don't tandem circuit breakers typically connect to the same bus (one connection)?
Click to expand...
Depends on the brand of breaker.
 
I think the real issue is that we are using slang and not clearly defined terms.

A tandem breaker is different things to different people. Some call a double pole breaker a tandem, others call any space saving breaker a tandem and then there are double pole space saving breakers as well.
 
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