Sizing feeders for multiple motor loads

[clausb]

I understand the code associated with sizing feeders for multiple motor loads and feeds to industrial control panels (430.62 and 409.21), but what if we wanted to oversize those feeders for future loads? We can't per code, because we are limited to 125% of the largest motor OCP size plus all other connected loads (plus 125% of any continuous loads that might be connected to the feeder). How far do we take this?

Case in point, if an industrial control panel is serving several motors, each with its own properly sized branch circuit over-current protection and properly sized distribution within the control panel, then why can't the feeder to the industrial control panel be upsized based on a possible future load?

Am I missing something here?

 

[Dennis Alwon]

I would install a sub panel large enough for the loads to be served and whatever future loads you anticipate. The panel breakers can perhaps be the disconnects for the motors.

 

[pete m.]

Perhaps I'm missing something but I see no restriction on the size of the feeder conductors. Both of the code sections mentioned in the OP deal with sizing of the OCP. The conductors, IMHO, could have an ampacity that exceeds the OCP.

Pete

 

[david luchini]

You've got three issues. The first is the feeder size. You are NOT limited to a feeder size by 430 or 409. These section specify the MINIMUM size of a feeder, not the maximum. The feeder can be as large as you want.

The second is the rating of the feeder overcurrent protection. 430.62 would give you a maximum size for the feeder overcurrent protection WHEN the conductors are based on 430.24. If the feeder conductor size is based on the overcurrent protection size, then the feeder/OCPD can be any size you want.

For example, if you had a feeder for (1) 15HP, (1) 10HP and (5) 2HP motors at 480V, the minimum feeder size would be 1.25x21A + 14A + 5*3.4A = 57.25A (#6 awg.) The max OCPD would be 2.50x21A + 14A + 5*3.4A = 83.5A (80A.) If you wanted to include future capacity, you could use a larger OCPD with a feeder sized for the OCPD rather than for motor loads. For instance if the feeder was #2 awg with a 110a c/b, it would be OK for both the 7 motors listed above and for future loads.

The third issue is that of the industrial control panel. This would be an ASSEMBLY of components such as controllers, c/b's, etc. The size of the feeder and the rating of the ocpd will be based on the assembled items. The assembly might be set up for two motors active and one future, but it is still one assembly.

 

[yired29]

For example, if you had a feeder for (1) 15HP, (1) 10HP and (5) 2HP motors at 480V, the minimum feeder size would be 1.25x21A + 14A + 5*3.4A = 57.25A (#6 awg.) The max OCPD would be 2.50x21A + 14A + 5*3.4A = 83.5A (80A.)QUOTE]

I always under stood that it would be 21 A x 2.5 = 52.5 Amps 430.52 exception 1 allows next larger standard size 60 Amps.
60 amps + 14 amps + (5 x 3.4 amps) = 91 amps 430.62 shall not exceed 91 amps so 90 amp OCPD (inverse time breaker).

Look at Annex D example D8.

Please correct me if I missed something.