multiwire branch circuit question

[Flicker Index]

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

It's dangerous. If they're two separate breakers, it would be assumed that if breaker for pump 1 is turned off, the neutral associated with pump 1 can be lifted when in reality, it would be live with pump 2 in series. If it was handle tied, independent trip, you can not turn one breaker off without turning both off.

 

[tthh]

It is a code violation. It is a safety issue for anyone working on either of those two circuits or maybe even the equipment attached to those circuits.

What can happen is that someone shuts off, say circuit breaker 37. Then they go monkeying around with the whatever is connected to 37 and touch the white neutral. Well, in the case of a MWBC, that white neutral is working for both 37 and 39 and if 39 is operating it's pump and you touch the white neutral bad things happen even if 37 is tripped off.

The 2008 code required handle ties between those two breakers such that each could trip indenpently, but you could only manually shut off both at the same time. This accomplishes what you want. One trips and you want the other one to keep working. Makes sense. But, if you go to the panel and want to work on the cirucit, you manually shut off the breaker and the handle tie will pull them both since the throw is longer than what you get with just a trip.

 

[winnie]

I absolutely agree that this is a safety issue if someone is doing electrical work on the circuits. I specifically mentioned the issue of being shocked by the neutral with only one of the two breakers off.

I absolutely agree that this is a violation of current code. Thanks tthh for providing the date of the change.

But what is the safety issue if electrical work is not being done?

-Jonathan

 

[tthh]

I agree -- if no one is doing work it is safe.

But, does everyone who is qualified remove the panel cover to make sure the breaker they are turning off isn't a MWBC before they go work on it? They should, but do they?

The guy who wired it left out a $2 part.

BTW, I may be overly sensitive since this last house I bought, built in 2001, has more MWBC than not -- almost all of them tandems. And, some were on the same leg and some were on breakers not next to each other. Same leg is a real overload potential and not next to each other is unnecesarily confusing even with the panel cover off. Now, I don't know what the original electrican did and what others did in the meantime that didn't know what they were doing. Nevertheless, it's all cleaned up now.

 

[jeff360]

thanks very much for everyone's help. i'll speak to the electrician about the safety issue.
just to confirm, after reading all the replies:
1) if pump 1 trips their breaker, pump 2 will still operate
2) each pump has a dedicated circuit with the equivalent power of a separate 20 amp circuit?

thanks again everyone

 

[winnie]

thanks very much for everyone's help. i'll speak to the electrician about the safety issue.
just to confirm, after reading all the replies:
1) if pump 1 trips their breaker, pump 2 will still operate

Yes

2) each pump has a dedicated circuit with the equivalent power of a separate 20 amp circuit?

Essentially yes.

 

[don_resqcapt19]

thanks very much for everyone's help. i'll speak to the electrician about the safety issue.
just to confirm, after reading all the replies:
1) if pump 1 trips their breaker, pump 2 will still operate
2) each pump has a dedicated circuit with the equivalent power of a separate 20 amp circuit?

thanks again everyone

Once the installation is made code compliant, that may not be true. The code required handle tie may open the second breaker if the first one trips.

 

[Rick 0920]

I'm hoping that you have a qualified electrician doing this work. The grounded conductors must be pigtailed in a MWBC and can not depend on devices for continuity. An open neutral in a MWBC will create a 240v series circuit and can do serious damage to 120v rated equipment.

 

[PaulMmn]

At a minimum, a nice, red warning label next to 37/39 letting the next electrician know would be helpful.

 

[curt swartz]

I realize that it is now required to have simultaneous disconnection for multi-wire circuits and new installations should comply. This was not always the case.

I've installed thousands of multi-wire circuits prior to the requirement and the world has not come to an end.

This requirement is to protect unqualified workers not qualified electricians.

 

[roger]

I realize that it is now required to have simultaneous disconnection for multi-wire circuits and new installations should comply. This was not always the case.

I've installed thousands of multi-wire circuits prior to the requirement and the world has not come to an end.

This requirement is to protect unqualified workers not qualified electricians.

Same here, thanks Curt.

 

[Rick 0920]

I realize that it is now required to have simultaneous disconnection for multi-wire circuits and new installations should comply. This was not always the case.

I've installed thousands of multi-wire circuits prior to the requirement and the world has not come to an end.

This requirement is to protect unqualified workers not qualified electricians.

Absolutely. These rules would not be required if we educated our employees. I was taught early on that if I ever untwisted neutrals and saw a spark, there was a load on it somewhere!!! Twist them back together and find out where that load is!

 

[roger]

What's the basis for this?

This might help

 

[Flicker Index]

Theoretically, yeah, assuming constant current test load, or a load whose power input changes in proportion to the voltage. What real life load is constant current though? If it's a resistive load, power change is square of voltage change.

But ordinarily, most motors are induction. If the rated speed after slip is 3,450RPM, but it drops to 3,400RPM with a voltage drop, the horsepower demand drops by 4.3% if it's a centrifugal pump due to affinity law.

 

[readydave8]

Personally if I see a red I start looking for the other breaker before grabbing hold of a wire

But I guess that since common sense is so uncommon, we need handle tie code

Unless it's in there to protect the unqualified worker? And if so, why is that guy opening up boxes in the 1st place?

 

[winnie]

Once the installation is made code compliant, that may not be true. The code required handle tie may open the second breaker if the first one trips.

Two single pole breakers with a handle tie do not have a proper common trip mechanism. There is a real chance that the force transmitted by one breaker tripping will turn off (not trip) the other breaker, or it may not.

There are some options for preventing one breaker from turning off the other.

1) Breakers are required to be 'trip free', meaning that they should trip even if the handle is locked in the on position. A mechanical breaker lock can be added to prevent the breaker handles from moving when one trips. This would prevent on breaker's tripping from toggling the other.

2) Handle ties on the outer poles of 'quad' breakers are known to be very loose and do a bad job of transmitting force between the two poles.

Jonathan

 

[don_resqcapt19]

Two single pole breakers with a handle tie do not have a proper common trip mechanism. There is a real chance that the force transmitted by one breaker tripping will turn off (not trip) the other breaker, or it may not.

There are some options for preventing one breaker from turning off the other.

1) Breakers are required to be 'trip free', meaning that they should trip even if the handle is locked in the on position. A mechanical breaker lock can be added to prevent the breaker handles from moving when one trips. This would prevent on breaker's tripping from toggling the other.

2) Handle ties on the outer poles of 'quad' breakers are known to be very loose and do a bad job of transmitting force between the two poles.

Jonathan

That is why I said may cause the other breaker to open. If the goal is to make sure that when one sump pump trips its breaker, that the other pump continues to run, I would not be using a multiwire circuit as it is possible that when one breaker trips the other will open.

 

[charlie b]

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.

Question for the membership. The word "romex" has been interpreted by those who have posted replies as meaning two ungrounded conductors, one grounded ("neutral") conductor, and one EGC. Is that necessarily true? Could this romex run have included two neutral conductors? I just want to pin this detail down before the OP needs to take any corrective measures.

 

[ActionDave]

The install described is 12-3 Romex - two ungrounded and one grounded.

The jacket says 12/3 ..

 

[charlie b]

I would not be using a multiwire circuit as it is possible that when one breaker trips the other will open.

This added comment might not be needed, but to clarify to the benefit of the OP, a breaker trip takes place by way of the mechanism internal to the breaker. But it also causes the breaker's handle to move to a middle position (i.e., between ON and OFF ). That may or may not cause the other breaker's handle to move far enough from it's ON position to turn off the second pump. I agree with Don; I don't like this setup.