210.19(A)(1) Conductors Minimum Ampacity and Size.

Conductor ampacity Table 310.15(B)(16) and Table 310.15(B)(3)(a) for adjusting the ampacity of a conductor when there are more than 3 current carrying conductors (CCC).
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Conductor ampacity Table 310.15(B)(16) and Table 310.15(B)(3)(a) for adjusting the ampacity of a conductor when there are more than 3 current carrying conductors (CCC).

Code Change Summary: New code language was added to clarify how to properly size branch circuit conductors.

The code language added to this section in the 2014 NEC is not a new concept. It is just a clarification. Branch circuit 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 branch circuit conductor, the larger of two values is used for the final selection of the conductor:

OR

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 conductor. See the example below for the proper 3-step method to size the branch circuit:

Example: 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? (For simplicity, assume all terminals are rated 75°C)

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 ÷ 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° 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 the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2011 Code Language:

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

2014 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.

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210.19(A)(1) Conductors Minimum Ampacity and Size.

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

What minimum size copper type THW conductors are required to supply a 40 amp continuous load where there are 10 current carrying conductors (CCC) in the raceway? (Assume all terminals are rated 75°C)

A: 8 AWG.
B: 6 AWG.
C: 4 AWG.
D: 3 AWG.
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