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Aug 8, 2005 10:32 AM, By Steven Owen

What is the minimum size THHN insulated conductor required for the power conversion equipment, included as part of an adjustable speed drive system that uses a bypass device. The rated input of the power conversion equipment is 36A. The motor is rated 30 hp, 480VAC, 3-phase. The terminals at the power conversion equipment are rated 75°C.

A) 10 AWG THHN
B) 8 AWG THHN
C) 6 AWG THHN
D) 4 AWG THHN

Answer: B

Explanation: A new Part X was added to Art. 430 of the 2005 NEC. This portion of the Code specifically focuses on the installation of adjustable speed drive (ASD) systems. Section 430.122(B) addresses conductor sizing for bypass devices.

The requirements of 430.122(B) stipulate the conductor size to be based on the larger of the following:
• 125% of the rated input to the power conversion equipment
• 125% of the motor full-load current rating as determined by 430.6

Power conversion equipment:
36A X 1.25 = 45A
Per 110.14(C)(1)(a)(3), the temperature rating of the conductor must not exceed the lowest temperature rating of its connections (i.e. 75°C terminals). Therefore, in reviewing data from the 75°C column of Table 310.16, we can see that a 8 AWG THHN conductor is rated 50A.

Motor:
Per Table 430.250, the full load current for a 30 hp, 480VAC, 3-phase motor is 40A.
40A X 1.25 = 50A
Taking into account the 75°C terminal rating again, a review of Table 310.16 reveals that a 8 AWG THHN conductor is rated 50A.

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.

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