Had a customer comment on placing lump motor loads on MCC's for modeling. We do this form small motors less than 50 HP but he had a site where another firm had huge loads added which made the MCC bus Dangerous and higher than the line side of the main breaker. How do others address this? Maybe put them in but turn them off for the arc flash calculations? Thanks

1584 only says, "...include large motors..." or something like that. However, the IEEE Red Book has the break points on where you lump motors and when you model as a single element.

I typically use the "<50hp" option in SKM to exclude motors in the lumped category.

Mike

If it's a new design I generally model everything off each MCC no matter the size as the design progresses and I'm developing the single line. Even though there is an "exception" for motors smaller than 50 hp for arc flash calcs, I don't think there is such a thing for short circuit calcs, but I could be wrong.

If I'm doing an arc flash study on an existing installation I discuss what all will be included and what won't with the client and price accordingly.

I try not to "lump" motors if I can help it.

Here's the SKM check box Mike is referring to: