Three Phase Breaker Single Phase Power

[A-A-Ron]

I have a design application were I want to use a three phase breaker for a single phase application. I have attached a picture of the application on paper. The reason I want to do this is because of the space saving with the distribution block that attaches to the breaker. I spoke with Rockwell & they said its fine but one of our electricians is saying it wont work. I can't find anything disputing this application in the NEC but I may be missing something. Opinions?

 

[roger]

I agree with Rockwell. What is the electricians reason for saying it won't work?

Roger

 

[PaulMmn]

Is the neutral supposed to be switched?

 

[GoldDigger]

Code allows the neutral to be switched as long as it is opened only when all of the ungrounded conductors are open. The diagram explicitly makes the load circuit an MWBC if it has individual 120V loads, make sure that had no unforseen consequences.

 

[Jraef]

Code allows the neutral to be switched as long as it is opened only when all of the ungrounded conductors are open. The diagram explicitly makes the load circuit an MWBC if it has individual 120V loads, make sure that had no unforseen consequences.

Bingo.

By the way, you (OP) said "breaker", your drawing shows a non-fused disconnect switch (NFDS). Was that just a semantics glitch? The reason I ask is that SOME breakers that use bi-metal current sensors on the thermal trips must have current flowing on all three poles, otherwise they can nuisance trip. So to avoid that, you would feed the neutral WITHOUT going through the breaker, then loop one of the hot lines back through the other pole.

If it is in fact a NFDS, then that is totally irrelevant.

 

[topgone]

Bingo.

By the way, you (OP) said "breaker", your drawing shows a non-fused disconnect switch (NFDS). Was that just a semantics glitch? The reason I ask is that SOME breakers that use bi-metal current sensors on the thermal trips must have current flowing on all three poles, otherwise they can nuisance trip. So to avoid that, you would feed the neutral WITHOUT going through the breaker, then loop one of the hot lines back through the other pole.

If it is in fact a NFDS, then that is totally irrelevant.

Very important catch there! Any imbalance of currents on the breaker will be "seen" as a fault on the load side and three phase breakers will trip (especially with breakers having bi-metal current sensors). Besides, if its an MWBC, the neutral current will be different from the other legs. That won't work, IMO.

 

[don_resqcapt19]

Bingo.

By the way, you (OP) said "breaker", your drawing shows a non-fused disconnect switch (NFDS). Was that just a semantics glitch? The reason I ask is that SOME breakers that use bi-metal current sensors on the thermal trips must have current flowing on all three poles, otherwise they can nuisance trip. So to avoid that, you would feed the neutral WITHOUT going through the breaker, then loop one of the hot lines back through the other pole.

If it is in fact a NFDS, then that is totally irrelevant.

Jeff, What breakers work like that? The UL Guide Information for molded case breakers says :

A 3-pole breaker used in place of a 2-pole breaker on a 3-phase system, such as a 2-pole breaker used in a branch circuit that is actually two legs of a 3-phase system, is acceptable without the 3-pole breaker being specifically marked.

Would not that result in one pole without a load?

 

[hillbilly1]

Very important catch there! Any imbalance of currents on the breaker will be "seen" as a fault on the load side and three phase breakers will trip (especially with breakers having bi-metal current sensors). Besides, if its an MWBC, the neutral current will be different from the other legs. That won't work, IMO.

Am I missing something, but I have never heard of that before. Each pole of the breaker has its own current sensor, so as long as it under the set point of any three, it will not trip no matter how unbalanced the load is. RTU’s come to mind, the compressor and indoor fans are three phase, but the condenser fans are single phase, so the load on the breaker is not balanced.

 

[synchro]

Some comments on the use of 3-pole breakers on single phase from Schneider:

Can a 3-pole breaker be used in a 2-pole application? | Schneider Electric USA

 

[electrofelon]

Very important catch there! Any imbalance of currents on the breaker will be "seen" as a fault on the load side and three phase breakers will trip (especially with breakers having bi-metal current sensors). Besides, if its an MWBC, the neutral current will be different from the other legs. That won't work, IMO.

As others have mentioned, some of us are very confused by this. I can think of a zillion common cases where each pole has different currents. Are you thinking a GFCI or GFPE (which would still allow "most" differential currents)?

 

[mbrooke]

I/m confused, the neutral current will never exceed the phase current- so it should not bother the bi-metal.

 

[topgone]

Am I missing something, but I have never heard of that before. Each pole of the breaker has its own current sensor, so as long as it under the set point of any three, it will not trip no matter how unbalanced the load is. RTU’s come to mind, the compressor and indoor fans are three phase, but the condenser fans are single phase, so the load on the breaker is not balanced.

There are breakers with a compensating bar on the thermal

As others have mentioned, some of us are very confused by this. I can think of a zillion common cases where each pole has different currents. Are you thinking a GFCI or GFPE (which would still allow "most" differential currents)?

Please see @ synchro's post above. There are cbs that allow 1 phase loading. Others do not.

 

[A-A-Ron]

Thank you to everyone for the comments. I have reviewed them all and will take them into consideration. For clarity the Breaker I have selected for this application is a Rockwell (Allen Bradley) part # 140G-G2C3-C50 ( Molded Case Circuit Breakers Bulletin Numbers 140G, 140MG spec link https://literature.rockwellautomation.com/idc/groups/literature/documents/sg/140g-sg001_-en-p.pdf ).
According to Rockwell's Technical support this breaker will allow single phase loading. The Neutral will be 100% load rated.

 

[jim dungar]

There are breakers with a compensating bar on the thermal.

The only breakers, I have seen, like this are designed/intended for motor circuit protection, where they are used in lieu of 'thermal overload relays'.

As mentioned, by others, breakers listed under UL489 do not care about the unbalanced loading of each pole (e.g. a 3 pole breaker has been tested with only 1 pole loaded).

 

[powerpete69]

Random questions or thoughts:

1. We assume this "breaker" does not attach to bus bars? If it did, how would you get the neutral to it?
2. Why not just use a two pole breaker and get rid of the power distribution block?
3. Why do you need a power distribution block at all? Remove block and run wire all the way to the breaker.
4. Why are we switching the neutral?
5. A thermal magnetic breaker reacts to heat. When it gets hot, it "knows" the wire is getting hot, bends and trips off. The magnetizing portion of it works like a magnetic in short circuit and trips instantaneously. I don't believe it senses differences like a GFI.

Way Confused. Clearly.

Wait, perhaps that is the inside of a Rockwell power/control panel. STill confusing.

 

[hillbilly1]

There are breakers with a compensating bar on the thermal

Please see @ synchro's post above. There are cbs that allow 1 phase loading. Others do not.

I would think the compensating bar if so equipped, would be for compensating for ambient temperatures? Not a breaker engineer, but that’s what I think it would be used for?

 

[hillbilly1]

Random questions or thoughts:

1. We assume this "breaker" does not attach to bus bars? If it did, how would you get the neutral to it?
2. Why not just use a two pole breaker and get rid of the power distribution block?
3. Why do you need a power distribution block at all? Remove block and run wire all the way to the breaker.
4. Why are we switching the neutral?
5. A thermal magnetic breaker reacts to heat. When it gets hot, it "knows" the wire is getting hot, bends and trips off. The magnetizing portion of it works like a magnetic in short circuit and trips instantaneously. I don't believe it senses differences like a GFI.

Way Confused. Clearly.

Wait, perhaps that is the inside of a Rockwell power/control panel. STill confusing.

Lug-lug breakers are used quite often in control panels, dimmer cabinets, etc.

 

[jim dungar]

Lug-lug breakers are used quite often in control panels, dimmer cabinets, etc.

Especially when the distribution block can be replaced by lugs on the breaker that can accept multiple conductors.

 

[bwat]

Especially when the distribution block can be replaced by lugs on the breaker that can accept multiple conductors.

Yes this is quite useful for the pseudo-mini-MCC type motor control panel.

 

[Jraef]

I would think the compensating bar if so equipped, would be for compensating for ambient temperatures? Not a breaker engineer, but that’s what I think it would be used for?

The purpose is to help prevent the effects of negative sequence currents in severely unbalanced 3 phase motor loads. The differential bar is going to skew the trip point of the breaker to be lower when current is not causing equal deflection of the bi-metal strips on all three poles. Yes, it is designed for motor loads, but many IEC designed molded case breakers, especially smaller sizes, are sold (mostly overseas) as “Motor Protection Circuit Breakers” (MPCB) that have adjustable thermal trips, just like an IEC overload relay. That type of breaker cannot be sold here as a UL489 breaker because of the adjustable trips, but take away the adjustment dial and they can be. So some of the mfrs don’t make two versions of the breakers, they just cover the dial on the fixed trip versions, meaning a 3 phase breakers will need to see the same current in all 3 poles, which is where you can run into trouble if the single phase load is 120/240 here.