20 & 30hp 460V wire sizing

[MgTricon]

I need a little help with my wire and conduit sizing. I have a 30 & 20 hp 460V motors close to each other and wish to run the wires in the same conduit. Both motors are less than 30ft from the starter (20hp) and VFD (30hp). Would a #8 and #10 wire be the correct size for the 30hp and 20hp, and they can be ran together in a 1 1/2” conduit?

 

[dpc]

If both motors can run at the same time you need to de-rate the conductors if they are run in the same conduit.

If you are in the US: Check NEC Article 310 for the required derating, and size each conductor for 125% of the motor full load amps in NEC Article 430, after applying the de-rating factor.

Then size your conduit for maximum fill of 40%.

Don't forget the ground wires.

 

[MgTricon]

Thanks dpc, but the motors must run independently of each other.

 

[dpc]

Are you saying the motors CAN run at the same time or CANNOT run at the same time?

 

[MgTricon]

the motors are running two different conveyors, although the motors are side by side. They are controlled by two independent controls. They may or may not run at the same time it is according when they are needed.

 

[dpc]

Then you must de-rate the conductors as I mentioned in my first response.

 

[MgTricon]

So then I will be at #6 for the 30hp and #10 for the 20hp. I am I correct on that.

 

[dpc]

For the 20 hp: Take 125% of the Full load amps in the NEC table in Article 430. Divide that by 0.8 to account for the de-rating required in Article 310 for up to 6 current-carrying conductors in a conduit. The ampacity of the conductor must be at least this high. I believe the #10 is too small.

If you want to post how you came up with your numbers, we can give you some additional help.

Cheers,

Dave

 

[MgTricon]

30hp at 40FLA so 125% gives 50Amps
20hp at 27FLA so 125% gives 34Amps
AWG #3 THHN is 110 Amps
AWG #8 THHN is 40 Amps (but according to the astric it can only be 30 Amps)
AWG #6 THHN is 55 Amps

So looking back over everything you are right the 10 would be to small, so it looks like #3 and #8. And if I pull (4) #3 and (4) #8 @ 40% fill then I will need 1 1/4" Conduit.

 

[jraef]

Unless you use cable that is significantly shielded for use in VFD circuits, I would NEVER run cables from a VFD to a motor in the same conduit as anything else.