# 3 phase motor question

#### Stevenfyeager

##### Senior Member
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

##### Moderator
Staff member
What voltage are you applying, 240 or 208?

#### ptonsparky

##### Tom
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

##### Moderator
Staff member
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

##### Senior Member
What voltage are you applying, 240 or 208?
240v delta. (with phase B being 208)

#### ptonsparky

##### Tom
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

##### Electron manager
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

##### Moderator
Staff member
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

##### Electron manager
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.