Annex D3(a) of the 2011 NEC provides a good example of feeder conductor sizing. It's more detailed and complex than the simple feeder example in Annex D3.
In Annex D3, much of the work is done when you size your OCPD:
- Subtotal noncontinuous loads
- Subtotal continuous loads
- Total all loads
- Divide VA by V to get amps
- Select the next size up breaker
To determine feeder size, start with the total load calculated from step No. 3 for OCPD sizing. Because it's 120/240V (single phase loads), step No. 4 is the same. You always divide by the square root of the number of phases. So if you had three-phase loads, you'd divide by the square root of three (1.732).
In Annex D3, there's really only one step (because you did the first four when sizing the OCPD). That is you find your ampacity in the correct ampacity table and see what minimum size conductor you need.
In Annex D3, the feeder sizing takes up four lines on the page. In Annex D3(a), it's not nearly that simple. Why the difference?
The inclusion of motor loads surely adds a layer of complication to the load calculations. But again, you do those calculations to size the OCPD and so when you get to the feeders they’re already done.