Using a 3 pole circuit breaker in a 2 pole application

Spike6245

Member
Location
York, PA.
Occupation
Electrical Designer
I have a 240 volt 3 phase system/service with a high leg. They are using 3 pole circuit breakers (in the Main Distribution Panel) to feed various single phase panelboards. Leaving one phase of the circuit breaker open (high leg). Is this acceptable?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
All breakers tested to UL 489 have been tested using all pole combinations.
The practice in the OP is actually quite common.
 

kwired

Electron manager
Location
NE Nebraska
The breaker doesn't care.

Had you done everything else the same but run the third phase to say a three phase sub panel but never connected any branch circuits to that third phase.... the breaker still sees the same thing and will still function the same.

A current based phase loss monitor (like some electronic motor overload protectors have) need equal loading on all three phases, but a standard thermal mag breaker won't care if you have 100% rating on just one pole and nothing on the other two, or any other combination that never exceeds 100% on any pole.
 

balatnv08

New User
Location
India
Occupation
Electrical Design Engineer
Hi, I have a 415V 3Ph 4 wire Distribution Board in which one feeder is connected to a single phase industrial socket outlet. This feeder has 4 Pole circuit breaker converted into a 2 Pole circuit breaker 250V through wiring. Please show standard stating this procedure.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I like to explain it this way...If you have a short on one phase of the single phase hookup on a 3 phase breaker, then what will happen? The breaker will trip because it is common trip. The same holds true for over current. If one phase is overloaded then the 3 pole breaker will trip so there is no reason that you can't do it.

We had one inspector tell the electrician that the breaker wouldn't function correctly in that situation.... He was incorrect.
 

kwired

Electron manager
Location
NE Nebraska
Hi, I have a 415V 3Ph 4 wire Distribution Board in which one feeder is connected to a single phase industrial socket outlet. This feeder has 4 Pole circuit breaker converted into a 2 Pole circuit breaker 250V through wiring. Please show standard stating this procedure.
Is it field constructed thing or a listed assembly that happens to look like four poles (even if it sort of is)?

Please also note that 415 volts isn't really a NEC or NEMA standard voltage so the IEC standards that likely apply might not be quite the same as NEC and NEMA standards either.
 

AdrianWint

Senior Member
Location
Midlands, UK
Is it field constructed thing or a listed assembly that happens to look like four poles (even if it sort of is)?

Please also note that 415 volts isn't really a NEC or NEMA standard voltage so the IEC standards that likely apply might not be quite the same as NEC and NEMA standards either.
The IEC or BS doesn't care either. No problem with a single phase load thru 1 pole of a three phase breaker.... as stated by others, it'll still work the same.
 

paulengr

Senior Member
The IEC or BS doesn't care either. No problem with a single phase load thru 1 pole of a three phase breaker.... as stated by others, it'll still work the same.

It’s not as important with circuit breakers as contactors but you can run into uneven contact tip wear problems. If it’s plug in not much you can do but if it’s standalone a common trick is to series all 3 poles together. Don’t parallel obviously because then the trip unit won’t read the correct value (unless it’s switchgear with external CTs).
 

kwired

Electron manager
Location
NE Nebraska
The IEC or BS doesn't care either. No problem with a single phase load thru 1 pole of a three phase breaker.... as stated by others, it'll still work the same.
I understand that, but isn't the question about paralleling overcurrent devices to effectively make a higher setting device?
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Or in some cases I have seen two pole breakers in a three pole frame.
In a few cases I have seen three pole breakers with two poles with two poles looped on to the third pole of the breaker.
 

kwired

Electron manager
Location
NE Nebraska
In a few cases I have seen three pole breakers with two poles with two poles looped on to the third pole of the breaker.
Mostly only see that sort of thing here in the US with motor overload relays that offer phase loss protection of some sort but they are only supplying a single phase motor. All three poles will need to see a certain amount of current balance or it will trip those devices.
 
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