Breaker Loading Consideration

[tin2779]

I have a 225 A molded case thermal magnetic breaker. I beleive it is 80% rated that means it is 185 A continous rating. I have three fans 75HP, 75 HP and 65 HP at 600 V. The total ampacity FLA is 210 A. That means I am overloading this breaker. I am not sure, how this will effect the thermal trip unit.

My concern is when two 75 HP are already running and 65 HP fan comes in to picture. Six times 65 A+ 150 A= 450 A for the total inrush cycle which will be around 2 secs for a typical NEMA design B motor.

My question is, Is there a possibility that brining this third fan coming online might drop the voltage to 80% and might open the contactors of already working fans and if eventually if the continous current remains 210 A, it might trip the therma unit of the breaker.

Any suggestions, if this will work.

Thanks

 

[jraef]

First, check your math. (6 x 65) + 150 = 540, not 450.

Second, that breaker is 80% rated unless there is a very clear indicator on the from stating it is 100% rated. If you can't see it, it isn't there. So that breaker is overloaded.

Voltage drop has nothing to do with the breaker size. That breaker may drop out on startup of the 3rd motor for magnetic reasons however; it's going to be a crap shoot.

A 225A thermal trip SHOULD hold in at 210A, but the 80% rule has more to do with accuracy and thermal dissipation than anything else. Trip settings on 80% rated breakers are plus or minus a tolerance of about 10%, you don't have that much. Also, a 100% rated breaker is typically the same frame with beefier power components and lugs. You have virtually no room for error, i.e. a 5% voltage drop from your utility, although perfectly acceptable to them, may cause the breaker to overheat even if it doesn't trip out.

Did you check out your wire size as well? that's another issue to be concerned about here.