Motor disconnect question

[guitarsarge]

I'm confused with the answer to this one:

What size disconnecting means is required for a single 5 HP 115V motor on a separate branch circuit?

a. 30A
b. 60A
c. none of these
d. 100A

I came up with 64.4A, so I chose (c) thinking the next size up would be 70A.

Answer is 100A, but I don't understand why. Can someone shed some light for me?

Thanks

 

[augie47]

I can only assume that since to many, especially us old timers, motor disconnect" means switch and the next size switch above a 60 is a 100.
The question was probably designed to see if you knew the 60 amp switch on a motor with a FLA of 56 was not sufficient.

 

[guitarsarge]

Lol, ok. I don't work with motors at all and I've never seen that in print. Where can I find a list of switches for motors and the standard Ampere ratings?

 

[RonPecinaJr]

Lol, ok. I don't work with motors at all and I've never seen that in print. Where can I find a list of switches for motors and the standard Ampere ratings?

Single phase and three phase motor ampere ratings -- tables 430.247-430.251

NEC 430.110

Disconnect ampere ratings requires the disconnect be rated 115% of the the full-load current rating of the motor.

56 amps x 1.15 = 64.4 amps

That's why you need to go with the 100 amp disconnect.

 

[guitarsarge]

Single phase and three phase motor ampere ratings -- tables 430.247-430.251

NEC 430.110

Disconnect ampere ratings requires the disconnect be rated 115% of the the full-load current rating of the motor.

56 amps x 1.15 = 64.4 amps

That's why you need to go with the 100 amp disconnect.

Thanks, I understand how to do the calculation though. I'm looking for the standard ampere ratings for motor switches. So far I know there is a 60 A and 100A. I'm looking for something similar to 240.6 standard ampere ratings, but for motor switches.
Any help appreciated.

 

[augie47]

Check any gear catalog (GE, SqD, Cutler-Hammer etc)
but basically 0,60,100,200,400,600,800,1200 amps

 

[guitarsarge]

Ok, thank you.