3 phase motor question

[Stevenfyeager]

Did I do this correctly ? Air Compressor with a 3 phase induction motor, 10 hp, 230 v, S.F.A 30.8, Duty Cont., EFF 90.2 %. I ran #6 on a 60 amp breaker, then to a fusible 60 amp disconnect. I asked an electrical contr. friend who does a lot of commercial work, he said the machine will have a thermal protector on it. He said what I have is good. I asked if I needed to downsize the fuses. Your advice ? Thank you.

 

[infinity]

What voltage are you applying, 240 or 208?

 

[ptonsparky]

You could but not needed.
See NEC Table 430.250 Nameplate SFA is not included in your Calculations
You should already know if the compressor has a starter with overloads just, by doing the work. The mfg would have installed the correct overloads and thermal if one was used.

 

[augie47]

A little more investigation is warranted but I lean toward Tom's answer (#3). Most 3 phase 10HP compressor I have seen have a factory starter with overloads but you do need to verify.

 

[Stevenfyeager]

What voltage are you applying, 240 or 208?

240v delta. (with phase B being 208)

 

[ptonsparky]

240v delta. (with phase B being 208)

That tells you, via the tables, you use 28 amps to size the conductors. The minimum amp rating of disconnect @ 1.15 * 28 and with a HP rating >= to 10.

 

[kwired]

What voltage are you applying, 240 or 208?

That tells you, via the tables, you use 28 amps to size the conductors. The minimum amp rating of disconnect @ 1.15 * 28 and with a HP rating >= to 10.

And 175% for time delay fuses gives you 49 amps, so a 50 amp fuse is supposed to be the max, unless it won't hold during starting then you are allowed to increase.

 

[infinity]

And 175% for time delay fuses gives you 49 amps, so a 50 amp fuse is supposed to be the max, unless it won't hold during starting then you are allowed to increase.

Since he has a 60 amp circuit breaker (up to 250%) the fuse size seems to be irrelevant.

 

[kwired]

Since he has a 60 amp circuit breaker (up to 250%) the fuse size seems to be irrelevant.

Guess so. Kind of makes it pointless to use a fused disconnect unless some sort of supplemental protection is going on or if there were say a feeder tap situation.