When sizing a branch circuit conductor, 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 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 conductor. See the example below and the image for the proper 3-step method to size the branch circuit:

Example (assuming all terminals are rated 75°C): What minimum size copper type THW conductors are required to supply a 60 amp continuous load where there are 10 current carrying conductors (CCC) in the raceway?

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 ÷ 50% (based on 10 CCC in the raceway and Table 310.15(C)(1)).

60 ÷ .50 = 120 amps. Select a THW copper conductor from Table 310.16 that can carry 120 amps from the 75°C column (#1 AWG).

Step 3, compare results from the first two steps and pick the larger wire which is 1 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:

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the larger of 210.19(A)(1)(a) or (A)(1)(b) and comply with 110.14(C) for equipment terminations:

(1) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load in accordance with 310.14.

Exception to (1): If the assembly, including the overcurrent devices protecting the branch circuits, is listed for operation at 100 percent of its rating, the ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load in accordance with 110.14(C).

(2) The minimum branch-circuit 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 310.15.

Exception to (1) and (2): Where a portion of a branch circuit is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), an allowable ampacity in accordance with 310.15 not less than the sum of the continuous load plus the noncontinuous load shall be permitted. No portion of a branch circuit installed under this exception shall extend into an enclosure containing either the branch-circuit supply or the branch-circuit load terminations.