Code Quiz

Jun 23, 2004 1:56 PM, By Steven Owen

1. According to the 2002 NEC, including all appropriate sections, what's the minimum size THHN feeder conductor required for a 200A continuous load if the terminals at the equipment aren't marked with temperature ratings but the terminals at the circuit breaker are marked 75°C?

A) 4/0 AWG
B) 250 kCMIL
C) 300 kCMIL
D) 350 kCMIL

2. What size overcurrent device, which in this case is an inverse time circuit breaker, is required by the 2002 NEC.

A) 225A
B) 250A
C) 300A
D) 350A

Question 1 Answer: B

Explanation: Step 1: According to 215.2(A)(1), size the feeder conductor by applying a 125% multiplier to the 200A continuous load.

200 X 1.25 = 250A, minimum size conductor.

Step 2: Without making adjustments of any kind, we can proceed to Table 310.16 and check the 75°C column to find the minimum size conductor rated at least 250A. Even though THHN is found in the 90°C column, the terminals are rated at 75°C. Sec. 110.14(C) Temperature Limitations at Terminations requires that when determining the (allowable) ampacity of a conductor, “the temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor or device. Conductors with temperature ratings higher than specified for terminations shall be permitted for ampacity adjustment, correction, or both.” From the 75°C column, the minimum size conductor rated at least 250A is a 250 kcmil.

Step 3: Check the terminal ratings. As long as the maximum current flowing in this circuit doesn't exceed the value from the 75°C column (255A), we're in compliance with 110.14(C)(1).

Question 2 Answer: B

Explanation: Step 1: According to 215.3, size the overcurrent device by applying a 125% multiplier to the continuous load.

200 X 1.25 = 250.

Based on 240.6(A), 250A is a standard size. Overcurrent rating of this circuit breaker is 250A.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states.