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 electrician 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 310.15(B)(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(B)(3)(a)).

60 ÷ .50 = 120 amps. Select a THW copper conductor from Table 310.15(B)(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.

2017 Code Language:

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).

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

(b) The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.

For complete code sections, refer to the actual NEC^® text at NFPA.ORG. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.