Conductor and Breaker

[Timm1]

I have a question about sizing the conductors for a single feeder supplying a continuous load. I understand that there are four derating factors that need to be applied when sizing a conductor. First is the ambient temperature factor (correction factor at the bottom of NEC table 310.16). Second is the raceway factor (NEC table 310.15-B-2-a). Third is the factor that the conductor must have a rating of 125% of continuous load (NEC section 215.2-A-1). And the fourth is the temperature limitations on device terminals (this is the factor that requires that the ampacity should be read from the 60C column of NEC table 310.16 instead of 75C column). Please let me know that I am correct?

Also what about the breaker sizing? Do these four factors need to be considered while sizing the breaker? Or is the sizing of breaker is solely based on NEC section 215.3?

Your help will be highly appreciated.

 

[PHovnanian]

You are pretty close for the factors governing wire ampacities. Not knowing the occupancy type or other factors that may mandate the 60C max conductor temp, it is possible to use a higher maximum for conductor sizing provided that it is compatible with the connected equipment, other conductors, the raceway, etc. There are a few exceptions that allow branch circuit sizing of 100% of the non-continuous plus continuous loads (See NEC 210.19-A-1 and 210.20-A exceptions).

The derating factors for overcurrent protection devices (breakers, etc.) will be specified by the manufacturer for various ambients and other conditions.

Additional NEC requirements apply to special loads (transformers, motors, etc.)