Transformer inrush (again)

[powersupplyguy@netzero.ne]

Greetings,

I have selected a 6kVA 480v 3 phase transformer for my design. I am now trying to select primary fuses. Transformer manufacture tells me NEC requires 12Amp. Seems reasonable. I believe he is using 167% for a 6 kVA 480v 3 phase unit. Manufacturer claims inrush current to be 300A @0.01 second.

I am looking at time delay fuse that has some current limiting. I think I might be able to squeek by with a RK1 10 amp fuse. The Cooper Bussman Edison LESRK melting time curve suggests I can get over 1000 Amps for 0.01 second. Looks like 350 Amps for 0.10 second which would satisfy rule of thumb posted here a little while back 12x for 0.1 second under post called "transformer inrush" I believe.

I don't think I can coordinate a breaker, so I will use a disconnect to get the power off locally.

Plan Bravo is to use 3 NTC inrush limiters if I start popping fuses. I've used them before when designing charge circuits for drives and rarely have had any problems.

Do you think I am on the right track here?

Thank you.

regards,
Bob

 

[Smart $]

Why not use a primary and secondary protection schema? Use a 15A branch circuit breaker to supply the primary. Fuse the secondary.

 

[powersupplyguy@netzero.ne]

Why not use a primary and secondary protection schema? Use a 15A branch circuit breaker to supply the primary. Fuse the secondary.

This gear will be located in various industrial sites. I do not know the source impedance of power line, but I suspect transformers to be hundreds of kVA into the MVA region. I am concerned that a fault could rupture a circuit breaker package. I am counting on the fuses to limit fault current. Does this make sense?

I am not very familiar with NEC or this kind of gear, so any advice you can offer would be appreciated. Most of my professional life has been spend designing motor drives.

 

[petersonra]

This gear will be located in various industrial sites. I do not know the source impedance of power line, but I suspect transformers to be hundreds of kVA into the MVA region. I am concerned that a fault could rupture a circuit breaker package. I am counting on the fuses to limit fault current. Does this make sense? No.

I am not very familiar with NEC or this kind of gear, so any advice you can offer would be appreciated. Most of my professional life has been spend designing motor drives.

Whatever OCPD you provide, be it CB or fuse, it has to have an interrupting rating that exceeds the available fault current. The only way you can select an appropriate CB is if you know what the available FC is. If you use class J fuses they can interrupt 200kA, which is probably adequate for most cases. Just slap a label on it that says what the SCCR of the assembly is and tell the buyer that up front.

I suspect the NTC inrush limiters do not have a SCCR, so you would have a very hard time justifying using them in an unlisted power circuit.

 

[powersupplyguy@netzero.ne]

Whatever OCPD you provide, be it CB or fuse, it has to have an interrupting rating that exceeds the available fault current. The only way you can select an appropriate CB is if you know what the available FC is. If you use class J fuses they can interrupt 200kA, which is probably adequate for most cases. Just slap a label on it that says what the SCCR of the assembly is and tell the buyer that up front.

I suspect the NTC inrush limiters do not have a SCCR, so you would have a very hard time justifying using them in an unlisted power circuit.

I have chosen fuses to limit fault current to level below disconnect rating. I will 'slap a label' on my portion of machine to indicate maximum short circuit current rating. Thanks for head's up on the limiters.