Code Change Summary: New code language was added to clarify how a feeder is sized.

The code language added to this section in the 2014 NEC^® is not a new concept. It is just a clarification. Feeder conductors are still calculated the same way as before but the revised code language is a bit more user friendly.

Now, it is clear than when sizing a feeder, the larger of two values is used for the final selection of the conductor:

• Either 125% of the continuous load without any additional adjustment or correction factors,

OR

• 100% of the load (not 125% of it) after applying adjustment and correction factors.

Before this code change, some electrician were taking 125% of the continuous load and then applying additional adjustment or correction factors which results in an oversized feeder conductor. See the example below for the 3-step method to properly size a feeder:

Example: What minimum size copper type THW feeder conductors are required to supply a 60-amp continuous load where the feeders are installed in an area where the ambient temperature is 125°F? (Assume all terminations are rated 75°C for simplicity)

Step 1, take 60 amps X 125% which = 75 amps. Select a THW copper conductor from Table 310.15(B)(16) that can carry 75 amps from the 75° column (#4 AWG).

Step 2, take 60 amps ÷ .67 (based on 125°F ambient temperature and Table 310.15(B)(2)(a)).

60 ÷ .67 = 90 amps. Select a THW copper conductor from Table 310.15(B)(16) that can carry 90 amps from the 75° column (#3 AWG).

Step 3, compare results from the first two steps and pick the larger wire which is 3 AWG copper.

Below is a preview of the NEC^®.  See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).