Minimum breaker sizing for a transformer

[Jerry T]

I was wondering what the minimum breaker size is for a 112.5 KVA transformer 480 volt primary 3 phase and 200amps panel at 208/120 3pv4w. The transformer is currently running on a 100 amp three phase ite bolt on breaker. Is there a code that you can reference for me? Thank you

 

[jim dungar]

The NEC does not specify a minimum size breaker for transformers or conductors.

 

[don_resqcapt19]

There is no minimum...only a maximum in the code. See Table 450.3(B).

However I am a bit surprised that the 100 amp holds on the inrush for a transformer with a full load current of 135.3 amps. . The minimum I would use would be a 150, but more likely would be looking at a 175.

How long has it been in service with the 100 amp primary breaker?

 

[Jerry T]

6 months

 

[LarryFine]

However I am a bit surprised that the 100 amp holds on the inrush for a transformer with a full load current of 135.3 amps. . The minimum I would use would be a 150, but more likely would be looking at a 175.

How long has it been in service with the 100 amp primary breaker?

Are you only talking about the primary current?

 

[Jerry T]

Yes

 

[Jerry T]

Sometimes if you don't turn the breaker on fast enough it does trip but not because of load I assume higher in rush when I dragged the breaker handle too slow but the very next time it stayed on and no problems. Thanks

 

[don_resqcapt19]

6 months

How many times has the breaker been off, or the power lost to the transformer feeder circuit?
Obviously, the secondary is not loaded to its capacity, but there is still the inrush on energization issue. If the transformer has been energized a few times without tripping the 100 amp breaker, I would not be concerned.

Of course, adding secondary load may cause the breaker to trip. A 100 amp primary current would be an 83 kVA transformer

 

[don_resqcapt19]

Sometimes if you don't turn the breaker on fast enough it does trip but not because of load I assume higher in rush when I dragged the breaker handle too slow but the very next time it stayed on and no problems. Thanks

The internal spring keeps the actual closing time pretty much identical, no mater what speed the handle is operated at. Inrush varies with the exact point on the sine wave when the transformer is energized.

Since you do have startup/inrush issues, I would change the primary breaker to a 175, but you will also have to install 175 amp wire when you do that.

 

[Jerry T]

Thanks

 

[winnie]

In a pinch I've had to run a couple of small transformers (50kVa or so) on circuits rated about 50% of full load current.

Never had any problems, but they were on high impedance/ low fault current circuits.

IMHO there will be a minimum circuit breaker size based on the calculated circuit load, but that isn't set by the transformer itself but rather by the loads being supplied.

Jon