Feeder Conductor for Multi-motor Panel

[Jeff_H]

Want to run this by you and see what you say. An OEM is supplying a piece of equipment. The equipment will consist of (2) 75 HP, 460 V, 3 phase induction motors. These are to be fed by the OEM supplied motor control panel. The OEM panel does not have a main OCPD, but does contain branch OCPD?s for each motor.

I am sizing the OCPD and feeder to this OEM panel. Both motors can be started at the same time and do run at the same time. The OEM panel includes wye-delta starters for the motors.

Step 1: Full load current = 96 amps (NEC 430.250)
Step 2: Standard overload = 96 amps x 1.15 = 110 amp (NEC 430.32(A)(1))
Step 3: Branch circuit conductor: 96 amps x 1.25 = 120 amps => #1 AWG conductor to each motor (NEC 430.22 and 310.16)
Step 4: Branch over current protection: 96 amps x 2.5 = 240 amps => next size up = 250 amp breaker MAX. (NEC 430.52)
Step 5: Feeder conductor: 96 amps (largest motor) x 1.25 + 96 amps (other motor) = 216 amps => 4/0 AWG conductor. (NEC 430.24 and 310.16)
Step 6: Feeder over current protection: 250 amp breaker + 96 amps motor = 346 amps => next size down = 300 amp breaker MAX. (NEC 430.62)

Steps 1 through 4 relate to devices that are inside the OEM panel. Steps 5 & 6 determine the size of the OCPD in my service entrance switchboard and the conductor size from the switchboard to the OEM panel.

My question is?The 4/0 feeder conductor in step 5 is only good for 230 amps. Should this be increased to 300 MCM to account for the 300 amp breaker in the switchboard or is this OK?

 

[david luchini]

I think this installation may fall under Art 409 - Industrial Control Panels.

Using 409.20, the feeder conductor size would be the same as your calculation.

Using 409.21(C) for the rating of the OCPD, the c/b would be based on the size of the c/b's in the panel. If the OEM provided 250A c/b's for each motor, the 300A feeder c/b would be OK. But if the OEM provided 200A c/b's, then you would need a 250A feeder c/b, per 409.21(C).

In either case, the 4/0 would be acceptable for the 300A or 250A feeder c/b.

 

[Cow]

I'm surprised the OEM didn't have a nameplate spec'ing minimum MCA and maximum OCPD?? Or at least a main OCPD in the control panel?

 

[Jeff_H]

Thanks for the replies. We haven't received the equipment yet, just getting ready for it. The OEM doesn't specify the minimum MCA or maximum OCPD. The equipment that this panel controls has two "sides". The feeder will hit the OEM's junction box that contains a distribution block. The leads then go from the distribution block to the branch OCPD for each side. This is to allow one side to be locked out for mechanical work, while the other side continues to operate.