3-Pole Breaker Use as 2-Pole

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dvcraven0522

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I would like to use a 3-pole breaker but only use 2-poles.

Longer story short - I need GFI protection for welding outlet at 90/2p. Gear guy says he can get 90/3p not 90/2p.

I am guessing there might be some issue with the breaker not performing as it should if only using two of the poles.

I would just like some input from others if this a code violation among other things.

Thanks

Dan Craven
 
Not a code violation.

Not a functional problem as long as the breaker's protection circuitry is powered (which means you probably need to connect the breaker to a neutral and supply the correct phase poles...which itself is not a problem if this is a common breaker pressed onto 3 bus stabs.

The breaker will happily provide GF function with any subset or all of its output poles (4, remember the neutral) used.

Electrical noise or actual leakage caused by function from the welder is another issue.
-Jon
 
It might depend on how that particular breaker is doing the GF monitoring. In a 3 phase breaker where the GF algorithm is summing all three poles looking for a net zero, what’s the difference between one phase going to ground and one phase not being used? I’m not saying I know, I’m just saying we DON’T know. This is something I would ask of the breaker mfr.
 
dvcraven0522 said:
Jon - Last question.

Could it be a violation if I am not using the breaker not as designed. Using 2-poles and not the three?
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No on that part, it’s done all the time (on regular non-GF breakers).

Side note; 3 phase IEC breakers usually have what’s called a “differential trip” that looks for equalized current on all 3 legs, then if one is missing, like in a single phasing condition, it biases the trip curve to trip at lower currents to help protect the load. So to purposely use a 3 pole breaker for single phase, you must loop one pole back though the unused pole so that there is current flowing on all 3. That is not the case on traditional NEMA breakers so if this is in a panelboard that’s a few years old, that will not be the case. But all of the gear makers are now owned by Europeans and over time they have been changing to “world class” breakers, meaning the IEC concept has infiltrated us, making it impossible to use 3 pole breakers in this way in panelboards. That’s still rare, but coming as older product lines get phased out.
 
Jraef said:
It might depend on how that particular breaker is doing the GF monitoring. In a 3 phase breaker where the GF algorithm is summing all three poles looking for a net zero, what’s the difference between one phase going to ground and one phase not being used? I’m not saying I know, I’m just saying we DON’T know. This is something I would ask of the breaker mfr.
Click to expand...

If the breaker is individually measuring each phase, and doing either the instantaneous sum or the vector sum, then with two poles in use with no leakage then the sum will be zero and the breaker will be fine.

However it is very likely that the system uses the simplest approach which is a single toroidal core through which all conductors pass, with the magnetic flux induced in the core proportional to the net current flowing through it. Again with only two phases in use and no leakage there will be no net current and no GF trip.

-Jon
 
Jraef said:
Side note; 3 phase IEC breakers usually have what’s called a “differential trip” that looks for equalized current on all 3 legs, then if one is missing, like in a single phasing condition, it biases the trip curve to trip at lower currents to help protect the load. So to purposely use a 3 pole breaker for single phase, you must loop one pole back though the unused pole so that there is current flowing on all 3. That is not the case on traditional NEMA breakers so if this is in a panelboard that’s a few years old, that will not be the case. But all of the gear makers are now owned by Europeans and over time they have been changing to “world class” breakers, meaning the IEC concept has infiltrated us, making it impossible to use 3 pole breakers in this way in panelboards. That’s still rare, but coming as older product lines get phased out.
Click to expand...

Very interesting. So the overcurrent trip curve gets modified in a 'single phasing' situation. This would still not be a ground fault, but it might make the breaker subject to nuisance tripping (on overload, not ground fault) when only two poles are used.

-Jon
 
texie said:
Just so I'm clear, is this proposed 3 pole breaker going to be in a 3 phase panel?
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And if so is it delta or wye supply? If GFCI logic circuit is 120 volt and only supplied by one of the poles, you don't want that pole to end up on a high leg if you happen to have one.

Otherwise I don't see it would be a problem if installed in a single phase panel, you just have same potential between the two outer poles so don't use both of them for supplying the load.
 
Jraef said:
No on that part, it’s done all the time (on regular non-GF breakers).

Side note; 3 phase IEC breakers usually have what’s called a “differential trip” that looks for equalized current on all 3 legs, then if one is missing, like in a single phasing condition, it biases the trip curve to trip at lower currents to help protect the load. So to purposely use a 3 pole breaker for single phase, you must loop one pole back though the unused pole so that there is current flowing on all 3. That is not the case on traditional NEMA breakers so if this is in a panelboard that’s a few years old, that will not be the case. But all of the gear makers are now owned by Europeans and over time they have been changing to “world class” breakers, meaning the IEC concept has infiltrated us, making it impossible to use 3 pole breakers in this way in panelboards.
Click to expand...

Not true for the standard branch circuit protection breaker listed to UL 489.
What you are describing, using all poles, is commonly found on motor circuit breakers, particularly those used for motor overload protection.
 
winnie said:
If the breaker is individually measuring each phase, and doing either the instantaneous sum or the vector sum, then with two poles in use with no leakage then the sum will be zero and the breaker will be fine.

However it is very likely that the system uses the simplest approach which is a single toroidal core through which all conductors pass, with the magnetic flux induced in the core proportional to the net current flowing through it. Again with only two phases in use and no leakage there will be no net current and no GF trip.

-Jon
Click to expand...
I agree that it's almost certainly using a single toroidal core for measuring the net current. If you needed to resolve this to a 1 mA resolution and each phase current were being measured with separate detection circuits, then their gains would have to match each other within about 0.001% for a 90A breaker, which would not be practical in a commercial device. If you wanted sense how unbalanced the magnitude of the currents are on three phases, as was mentioned is done in some motor circuit breakers, then each phase current would need to be sensed independently. But in that case around 1% mismatch in the detection circuits would probably be acceptable and relatively easy to achieve.
 
dvcraven0522 said:
Well - It all sounds like this could work. But, I am fearful an inspector might not agree.
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Assuming this is a 3 pole GFCI breaker in a 3 phase panel, just explain it this way: that breaker might be supplying another 3 phase panel. That panel might have only a 2 pole breaker in it supplying your equipment. That configuration is obviously fine, and it is electrically equivalent to just omitting any connection to one of the poles of your 3 phase GFCI breaker.

Cheers, Wayne
 
dvcraven0522 said:
Well - Everything just got thrown out the window. Breaker guy say's the 90/3p is only good for equipment protection not personnel.
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Well other than you mentioning the equipment protection thing - I was going to say it makes sense to possibly find a 90 amp three pole but not a 90 amp two pole simply because of wording in 210.8 (B) only requires GFCI for up to 50 amp single phase but up to 100 amp three phase, have no idea why the difference though. For dwellings you just about never will find over a 50 amp receptacle being used so they sort of not needed there either.
 
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