MCC Design Question

[adamscb]

Forum,

I'm designing an MCC lineup for one of our project engineers, and I've run into a quagmire of sorts. The load is 3 x 200hp and 3 x 15hp motors. I was thinking of ordering an 800A horizontal bus MCC, but a coworker reminded me of the 80% loading rule. Does this apply for MCC horizontal ratings as well? I also learned that only two of the three 200hp motors will be running at one time, so does this mean I'm in the clear? Or does the 80% rule apply to potential load, and not actual load?

Also the fuses and upstream breaker that are feeding this MCC will be rated for 800A, does the 80% rule apply only to branch circuits, or feeders as well?

 

[Ingenieur]

The mcc bus bar should be fine
the mcc feeder should be sized for 1.25 x largest load plus sum of others
the individual motor feeders at 1.25
if I'm wrong others will clarify

 

[Ingenieur]

Assumed 480/3 based on the hp and amperage
what starting method? Fvnr, ss ss, vfd, etc
obviously v drop will need accounted for

 

[adamscb]

Starting method is across the line.

 

[Jraef]

There is no "80% rule" that applies to bus bars or MCCs. What frequently gets crossed up with this is that conductor WIRES in feeder circuits must be sized for 100% of the continuous load plus 125% of the non-continuous load, but since most feeder loads are non-continuous, most people just default to sizing at 125% of the "load". So once you size the conductors, you size the OCPD accordingly and since the inverse of that 125% is 80%, you end up with the OCPD sized at 80%. All MCC bus bars are rated for 100% of their rating continuously.

So in your case if you are feeding this MCC with 800A fuses and (presumably) conductors sized for 800A, the highest load you can run on that feeder circuit is 640A. MCC bus bars come in 600A and 800A, so 600A is too small, 800A is the next size up, so that's appropriate. Then, you can factor in the fact that only 2 of the 200HP motors will run at any time, so assuming 240A each, that's 480A, plus 3 x 15HP motors at 21A each = 63A so your total continuous load is 543A on a circuit that is sized for 640A in an MCC sized for 800A. I don't see an issue here.