Code Quiz

Nov 20, 2008 12:01 PM, By Steven Owen

Q. What is the minimum ampacity and minimum size conductor required for a circuit that connects a capacitor to the terminals of a 25 hp, 460VAC, 3-phase, air conditioner induction motor? The rated current of the capacitor is 20A. The motor branch circuit conductors are 8 AWG THHN/THWN. The terminal ratings of the capacitor are 60°C. The motor terminals are rated at 75°C.

18.15A, 14 AWG THHN.
25A, 12 AWG THHN.
27A, 10 AWG THHN.
42.5A, 8 AWG THHN.

Answer: C

As per the requirements noted in 460.8(A), we can make the following observations and calculations.

As stated in the first sentence of 460.8(A), "the ampacity of capacitor circuit conductors shall not be less than 135% of the rated current of the capacitor." Since the capacitor has a rating of 20A we can make the following calculation.

20A x 1.35 = 27A

As stated in the first part of the second sentence of 460.8(A), "the ampacity of conductors that connect the capacitor to the terminals of a motor or to motor circuit conductors shall not be less than one-third the ampacity of the motor circuit conductors…". These are the motor branch circuit conductors. A quick check of Table 310.16 reveals an allowable ampacity of 55A for 8 AWG conductors (90°C column).

55A x 0.33 = 18.15A

Note that this value (18.5A) is less than the value we calculated above (27A).

Since the terminals are not marked, we consider them to be rated at 60°C, as per 110.14(C)(1)(a). This means that we need to pick our conductor from the 60°C column of Table 310.16. From the 60°C column we can see that 10 AWG THHN conductor is rated at 30A.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.