Determine the minimum feeder conductor size, before you apply any conductor adjustment and/or correction factors, by adding the two following quantities:
- 125 percent of the continuous load [215.2]
- 100 percent of the noncontinuous load.
Once you have the total load, you size the minimum conductor required to carry that load based on the terminal temperature rating ampacities as listed in Table 310.16 [110.14(C)]. You size the OCPDs, based on this same ampacity [215.3 and 240.4].
Here’s a pop quiz to see if you can size a feeder. What size feeder conductor do you need for a 200A continuous load, if the terminals are rated 75ºC?
(a) 2/0 AWG (b) 3/0 AWG (c) 4/0 AWG (d) 250 kcmil
How do we determine the correct answer? Since the continuous load is 200A, the feeder conductors must have an ampacity at least 250A (200A x 1.25). Using the 75ºC column of Table 310.16, we find that 250 kcmil conductors are suitable because they have an ampere rating of 255A at 75ºC. Thus, “d” is the correct answer.
There is an exception to the 125% rule. Where the assembly and the OCPD are both listed for 100 percent continuous load operation, you can size the feeder conductors at 100 percent of the continuous load. Note that equipment suitable for 100 percent continuous loading is rarely available in ratings under 400A.
The feeder grounded (neutral) conductor must not be smaller than the size listed in Table 250.122, based on the rating of the feeder OCPD.
Test your knowledge, with another pop quiz. What size grounded (neutral) conductor do you need for a feeder consisting of 250 kcmil ungrounded conductors and one grounded (neutral) conductor protected by a 250A protection device, where the unbalanced load is only 50A, with 75ºC terminals?
(a) 6 AWG (b) 4 AWG (c) 1/0 AWG (d) 3/0 AWG
Table 310.16 and 220.61 would permit an 8 AWG grounded (neutral) conductor, rated 50A at 75ºC to carry the 50A unbalanced load. But Table 250.122 requires that the grounded (neutral) conductor not be smaller than 4 AWG. So, the correct answer is “b.”
You also have to consider the size of the service conductors, when you size feeder conductors. Feeder conductors for individual dwelling units or mobile homes needn’t be larger than service conductors sized per 310.15(B)(6).
