Sizing feeders when load is unknown (NEC code question)

[baddriver]

I'm looking for help determining the answer to the following in accordance with the NEC.

A 480v 3-phase 400A panel board with a 400A 100% rated main breaker is being installed. It is fed by a 400A 100% rated breaker in an upstream distribution board. The question is about required ampacity of the panel board feeder.

From 215.2(A)(1): The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity of not less than the noncontinuous load plus 125 of the continuous load.

215.2(A)(1) Exception No 1 allows for sizing the feeders at 100% cont+non cont load since we have 100% rated breakers.

What happens if we don't know what the actual connected load will be? Does this allow for a smaller feeder to be installed?

For example: If we assumed the connected load was 375A continuous, we could select 500MCM cable rated for 380A and satisfy 215.2(A)(1). Then we could size the overcurrent protective device at 400A 100% rated to satisfy 240.4(B). If we do not know the connected load, could we not use the same sizing as this and select a 500MCM cable rather than a 600MCM and satisfy code? Does the code have any language that states this is not allowed?

 

[cranky108]

The utility always seems to find out the load before the customer recieves service. So why don't you know?

Some where, someone has an estimated load.

On the other hand, when customers exceed the utility service size, they have a sudden outage.
We have seen several of these cases lately. And the customers are then asked to provide an estimate of there peak load. There electriction usually provides that estimate, based on the new service panel size.

 

[dpc]

If you are protecting the feeder with a 400 A breaker, the ampacity must be at least 400 A.

If the load is unknown, you make your best guess, but if you go with a smaller feeder, the upstream breaker must be down-sized as well.

BTW, I believe the "next larger size" rule for conductor sizing has been eliminated in the 2011 NEC. You might want to check this.



David Castor

http://www.cvoes.com

 

[baddriver]

David, I disagree with your assertion that 'If you are protecting the feeder with a 400 A breaker, the ampacity must be at least 400 A.'

Section 240.4(B) clearly states for less than 800A protective devices we are allowed to go up to the next higher standard overcurrent device rating. In this case we go up from 380A cable ampacity to 400A breaker size.

Further, this section has not changed on this issue in the 2011 NEC.

 

[davidbeach]

If you do an NEC load calc and come out with a value of 351-380A, you can put in a single run of 500kcm and protect with a 400A breaker. If you don't know the load, but figure that 400A should cover all future contingencies and put in a 400A breaker you then have to have at least 400A of conductor ampacity, that single run of 500kcm won't cut it.

 

[dpc]

You're right regarding the next-larger-size exception - I must have dreamed that. However, the conductor size is still tied to the upstream breaker size - 500 kcmil copper would be OK for a 400 A breaker assuming your calculated or estimated load was 380 A or less. Anything smaller would not. Also, I believe a 100%-rated breaker requires 90 deg C wire (applied at 75 deg C ampacity), per UL.

David Castor

http://www.cvoes.com