I'd like to just comment on the CB or OCPD rating of the feeder. The ampacity is determined by the method described, 125% of the anticipated full load. The OCPD then can be selected base on that. For feeders, the standard size value for OCPD can be rounded down from that ampacity rating, not up as required for branch circuits.
The 125% is not always needed as it would depend on the loads served. For instance, a sub panel with a calculated load of 50 amps would not need to have 125% added to it unless the load was continuous. So you could have a non-continuous load calculated at 60 amps and still use a 60 amp overcurrent protective device.
Section 240.4(B) allows us to use the next size overcurrent protective device if the load is not a standard cb size as long as the load isn't larger than the conductors serving the load.
240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The
next higher standard overcurrent device rating (above the
ampacity of the conductors being protected) shall be permitted
to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cord-andplug-
connected portable loads.
(2) The ampacity of the conductors does not correspond with
the standard ampere rating of a fuse or a circuit breaker
without overload trip adjustments above its rating (but
that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes.
215.2(A)(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts III,
IV, and V of Article 220. Conductors shall be sized to carry not
less than the larger of 215.2(A)(1)(a) or (b).
(a) Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
minimum feeder conductor size shall have an allowable ampacity
not less than the noncontinuous load plus 125 percent of
the continuous load.
Stacy Littleton said:
I'd like to just comment on the CB or OCPD rating of the feeder. The ampacity is determined by the method described, 125% of the anticipated full load. The OCPD then can be selected base on that. For feeders, the standard size value for OCPD can be rounded down from that ampacity rating, not up as required for branch circuits.
You are correct the feeder must be sized to have an ampacity at least (minimum) 125% of the load, if it is a continuous load, as stated in 215.2(A)(1)(a). The overcurrent device must also be sized to at least 125% of a continuous load as stated in 215.3. You might be able to round down from the conductor ampacity but it cannot be rated less than 125% of the load, whether or not it is a standard size. These rules are the same for branch circuits in 210.19 and 210.20.
Article 215 of the NEC covers Feeders. The method you described will always result in a safe installation, but not always the smallest OCPD & conductors.
Where a feeder supplies loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. This is after any adjustments or correction factors.
As already stated you can round up until you reach 800A if the ampacity of the conductors does not correspond to the standard size. You can round up, but you are not required to round up.