NEC Art.700 & 701. Selective Coordination

[mzeleny]

NEC 2005 Art.700 & 701 requires selective coordination for circuit breakers of emergency/standby systems.
In our case there are three circuit breakers (wired in parallel) fed from the generator: two existing (150A C.B. and 400A C.B.) feeding emergency loads of different buildings, and we are adding one new (with 250A C.B.) - standby load for the new Data Center. Stanby load shoud be disconnected first in case of any short circuit.
How in this case selective coordination should be provided?

 

[don_resqcapt19]

Re: NEC Art.700 & 701. Selective Coordination

It is almost impossible to selectively coordinate breakers of that size if the available fault current is very high. The instantaneous trip curves for smaller (less than 400A) have a large amount of overlap. There is no way to know if the branch or main will trip, and no way to provide selective coordination. Note that this change was submitted by someone from Bussmann and that it is relatively easy to coordinate fuses for this application.
Don

 

[iwire]

Re: NEC Art.700 & 701. Selective Coordination

Originally posted by don_resqcapt19:
Note that this change was submitted by someone from Bussmann and that it is relatively easy to coordinate fuses for this application.
Don

I am sure that is just a coincidence. :roll:

 

[ron]

Re: NEC Art.700 & 701. Selective Coordination

If the three breakers are acting as output breakers from the generator, essentially in parallel, you want to disconnect the 702 loads first, not necessarily provide selective coordination.
Selective coordination is to permit the OCPD closest to a fault/overload, to clear it before an upstream device attempts to clear it.
In your case you don't have an upstream/downstream OCPD situation.
You need some type of load shedding scheme or SCADA system that will shunt the 702 loads until the 700 and 701 loads are energized (assuming the generator can handle all the loads at the same time).