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?
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?
