Motor circuit sizing

Status
Not open for further replies.
Strahan

Strahan

Senior Member
Location
Watsontown, PA
All check my calculations. 30HP motor 480V design A. From 430.250 40amps. 40amps X 125%= 50amps. 110.14(c)(1) unless marked for 75 use 60 column in 310.15. Only other exception that I saw that would apply is the design which is not B, C or D. I sized the wire at 6AWG. My next question is the disconnect. The current disconnect is a 20HP but is rated for 60amps. 430.110 115% X 40amps = 46amps. I believe the disconnect is ok. I always sized my disconnects to the HP of the motor but the 115% in this case is ok compared to the disconnect amperage rating.

Manufacturer ran 10awg from the controller (which is sized at 30HP) to the disconnect and motor. I say big problem here. My only area of concern is that there is a soft start after the contactor. Does the soft start allow for reduced wire size? I don't think so but any info would be helpful. Thanks!
 
Strahan said:
All check my calculations. 30HP motor 480V design A. From 430.250 40amps. 40amps X 125%= 50amps. 110.14(c)(1) unless marked for 75 use 60 column in 310.15. Only other exception that I saw that would apply is the design which is not B, C or D. I sized the wire at 6AWG.
Click to expand...
Most new equipment is marked for use with 75? C conductors, so #8 would be fine.
My next question is the disconnect. The current disconnect is a 20HP but is rated for 60amps. 430.110 115% X 40amps = 46amps. I believe the disconnect is ok. I always sized my disconnects to the HP of the motor but the 115% in this case is ok compared to the disconnect amperage rating.
Click to expand...
If the disconnect is actually marked 20 hp you cannot use it with a 30 hp motor no matter what the amp rating is.

Manufacturer ran 10awg from the controller (which is sized at 30HP) to the disconnect and motor. I say big problem here. My only area of concern is that there is a soft start after the contactor. Does the soft start allow for reduced wire size? I don't think so but any info would be helpful. Thanks!
Click to expand...
It the wiring is internal to manufactured equipment, the NEC does not apply.
 
don_resqcapt19 said:
Most new equipment is marked for use with 75? C conductors, so #8 would be fine.

Don I know where to find this info on the disconnect but where on the motor? Existing motor no paperwork. I'm sure I can find specs online just didn't know if this was something that I can find on the nameplate.

If the disconnect is actually marked 20 hp you cannot use it with a 30 hp motor no matter what the amp rating is.

I'm not doubting you for one second but where does it clearly state this in NEC? I've always matched according to HP but got the idea that it didn't need to be based on 430.110.
I will be questioned on this one and will need to show my reason.

It the wiring is internal to manufactured equipment, the NEC does not apply.
Click to expand...

I understand this. Thank you.
 
I thought there was a section in the NEC that said the motor leads are rated at 75?C, but I am not finding it now.

As far as the disconnect, if the disconnect is rated for 20 hp and you are using it for a 30 hp motor, I would cite 110.3(B).
 
Last edited:
don_resqcapt19 said:
I thought there was a section in the NEC that said the motor leads are rated at 75?C, but I am not finding it now.

As far as the disconnect, if the disconnect in rated for 20 hp and you are using it for a 30 hp motor, I would cite 110.3(B).
Click to expand...

110.14(C)(1)(a)(4) allows for 75 deg for Design B, C, D motors.
 
Make sure your disconnect is not rated 20 HP std duty and higher for a max duty. Most heavy duty safety switches will likely be rated for this motor, but a standard duty switch may not be.
 
Status
Not open for further replies.
Top