When sizing a feeder, the larger of two values specified in 215.2(A)(1) or (2) 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 language was modified in the 2014 NEC^®, some electricians were taking 125% of the continuous load and then applying additional adjustment or correction factors which results in way too large of a feeder conductor. See the example below and the image for the 3-step method to properly size a feeder:

Example (assuming all terminals are rated 75°C): 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?

Step 1, take 60 amps X 125% which = 75 amps. Select a THW copper conductor from Table 310.16 that can carry 75 amps from the 75°C column (#4 AWG).

Step 2, take 60 amps ÷ .67 (based on 125°F ambient temperature and Table 310.15(B)(1)(1)).

60 ÷ .67 = 90 amps. Select a THW copper conductor from Table 310.16 that can carry 90 amps from the 75°C 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 their link to free access to NFPA 70.

Code Language:

215.2(A) General. Feeder conductors shall have an ampacity not less than the larger of 215.2(A)(1) or (A)(2) and shall comply with 110.14(C).

(1) Continuous and Noncontinuous Loads. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Exception No. 1: If the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

Exception No. 2: Where a portion of a feeder is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), it shall be permitted to have an ampacity not less than the sum of the continuous load plus the noncontinuous load. No portion of a feeder installed under this exception shall extend into an enclosure containing either the feeder supply or the feeder load terminations, as covered in 110.14(C)(1).

Exception No. 3: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.

(2) Ampacity Adjustment or Correction Factors. The minimum feeder conductor size shall have an ampacity not less than the maximum load to be served after the application of any adjustment or correction factors in accordance with 310.14.