Inrush current

[pgordon]

I have 800 amp breaker feeding a 400 KVA UPS 480/480
I have a 600 amp breaker feeding a 800 amp maint bypass panel.
I have a 500 KVA out put transformer. (K13 all copper )
Can the 600 amp breaker start the transformer with no load ?
Doesnt the load of a transformer determine the inrush current ?

 

[templdl]

I have 800 amp breaker feeding a 400 KVA UPS 480/480
I have a 600 amp breaker feeding a 800 amp maint bypass panel.
I have a 500 KVA out put transformer. (K13 all copper )
Can the 600 amp breaker start the transformer with no load ?
Doesnt the load of a transformer determine the inrush current ?

A 500KVA transformer with a 480v primary is 601 FLA. My concern is that you intend on using a 600a breaker to feed the transformer. The inrush for a K13 transformer may be as high a 7500a where a 600a breaker would have a max mag setting of 6000a and that's where I believe your issue would be. You are allowed to use a pri. OCPD of 1.25x the xmr FLA which would be 800a.
The load on the transformer is of no consequence.

 

[Jraef]

The load on the transformer is of no consequence.

That's right. Transformer inrush is only from the magnetizing current that is drawn because for the first cycle or so, there is no mutual induction between the windings. So the windings look like a short circuit; the only thing limiting current is the magnet wire resistance, which is kept low to maintain efficiency.

If there are other transformers or motors on the load side, THEY then will have their own magnetizing inrush current, then later, motors will have their starting current. But if you have sized the transformer correctly, those levels will ALWAYS be lower than the first transformer's inrush.

 

[kingpb]

The inrush could be a problem using a 600A breaker, as pointed out the max instantaneous setting may limit you to 10x the long time, or thermal rating. This if the transformer inrush is greater than 6000A the breaker would trip.

The recommendation to go to a 800A because the NEC allows it, may or may not work. It depends on the device. An 800A thermal mag breaker may not work because it is very common that 800A breakers are limited to a "High" setting which in many cases is 6000A, unless you jump up to a Frame style breaker that has a trip unit which is adjustable above the high inrush.

I have plotted a 500KVA transformer with 600A and 800A CB's, (HV CB and HV CB2, respectively) and 600A Fuses.

For protection of the transformer, fuses actually work the best on the HV side because they have a curve shape that allows the fuse to ride through the higher inrush current whereas a CB can't. Plus they stay under the thermal damage curve. As far as the 800A, I've always had an issue with the wording in the NEC, because in most cases the higher than rated selection of a CB for protecting a transformer does not typically protect the transformer, as can be seen from the curve which shows the thermal damage limit of the transformer and the 800A breaker is above the thermal damage curve. In most cases these curves are not used to select transformer protection.