The NEC requires the ampacity of a feeder shall be equal to or greater than ___.

The NEC requires the ampacity of a feeder shall be equal to or greater than ___.

• A. 125% of the overcurrent protection device
• B. the sum of all branch circuit overcurrent protection devices
• C. the calculated load after demand factors have been applied
• D. the connected load

Answer: (C)

Sizing a feeder comes from the expected load it must carry, not from the maximum possible load of every device or the protection devices themselves. The NEC directs that the feeder’s ampacity be at least the calculated load for that feeder, after applying the appropriate demand factors to reflect that not all loads peak at once. This means you estimate each branch circuit’s load, apply the demand factors, sum them to get a total calculated load, and size the feeder to carry at least that amount. Using the sum of branch OCPD ratings would overstate what the feeder must carry, and using the connected load assumes every device runs at full capacity simultaneously, which isn’t realistic. The 125% figure relates to continuous loads and protection sizing, not the feeder’s required ampacity based on the calculated load.

Sizing a feeder comes from the expected load it must carry, not from the maximum possible load of every device or the protection devices themselves. The NEC directs that the feeder’s ampacity be at least the calculated load for that feeder, after applying the appropriate demand factors to reflect that not all loads peak at once. This means you estimate each branch circuit’s load, apply the demand factors, sum them to get a total calculated load, and size the feeder to carry at least that amount. Using the sum of branch OCPD ratings would overstate what the feeder must carry, and using the connected load assumes every device runs at full capacity simultaneously, which isn’t realistic. The 125% figure relates to continuous loads and protection sizing, not the feeder’s required ampacity based on the calculated load.