60A non-fused disconnect downstream of 100A Breaker allowed?

Status
Not open for further replies.
E

Engineer1

Member
Is there anything wrong with this design scenario?

30 hp, 480V, 3 ph motor fed from a 100A circuit breaker with a 60A local non-fused disconnect. Wire size is #6 & #8G.

Thanks
 
Welcome to the forum.:)

A 60 amp disconnect would be fine in the scenario that you have.

430.110 requires the disconnecting means for a motor to be rated at least 115% of the motor full load current rating.

The full load current rating of a 30 HP 460 volt 3 phase motor in accordance with Table 430.250 is 40 amps, so the disconnecting means must be rated at least 46 amps.

Do make sure that the disconnect has an SCCR at least as high as the available fault current at the motor.

Chris
 
Engineer1 said:
Is there anything wrong with this design scenario?

30 hp, 480V, 3 ph motor fed from a 100A circuit breaker with a 60A local non-fused disconnect. Wire size is #6 & #8G.

Thanks
Click to expand...



430.150 has that motor at 40 amps

40x1.15= 46,,,,,so a 60 amp disco meets the requirement 0f 430.110 and the 6's assuming they are copper are ok.




EDIT :sorry chris,,,you're too fast
 
Engineer1 said:
Is there anything wrong with this design scenario?

30 hp, 480V, 3 ph motor fed from a 100A circuit breaker with a 60A local non-fused disconnect. Wire size is #6 & #8G.

Thanks
Click to expand...
Can we assume there is a motor starter (with overloads) in there somewhere? You aren't planning on using the disconnect to turn the motor on and off are you?
 
HP rated

HP rated

I think the switch must be also be HP rated for the motor it's serving, which if it were a Sqaure D Heavy Duty 60 amp switch it would just meet the 30 HP limit..
JM
 
Last edited:
Siemens 60 amp fused disco's are rated for 30HP I believe. Their knife switches are 40HP? or so. I know I used a 60 amp knife switch as a local disconnect on a 40HP pump but had to use a 100 amp fused disconect to feed it.
 
Status
Not open for further replies.
Top