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Code Quiz

Sep 25, 2006 11:37 AM, By Steven Owen

Q. What is the minimum size XHHW insulated feeder conductor permitted for an industrial machine, which consists of the following loads? All terminals are rated at 75°C.

• 10 each – 5 hp, 480VAC, 3-phase, squirrel-cage induction, continuous duty motors,
• 10 each – 3 hp, 480VAC, 3-phase, squirrel-cage induction, continuous duty motors,
• 10 each – 3kW, 480VAC, 1-phase, electric resistance heating loads,
• 10 each – 2kW, 480VAC, 1-phase, electric resistance heating loads.

A) 3/0 AWG XHHW
B) 4/0 AWG XHHW
C) 250kcmil AWG XHHW
D) 300kcmil AWG XHHW

Answer: B

Explanation: Per Table 430.250, each of the 5 hp, 480VAC, 3-phase, squirrel-cage induction, continuous duty motors produces a full load current of 7.6A and each of the 3 hp, 480VAC, 3-phase, squirrel-cage induction, continuous duty motors produces a full load current of 4.8A.

Motors:
Largest motor (one of the 5hp motors) @ 1.25 (125% as per 670.4) = 7.6A X 1.25 = 9.5A
9 each, additional 5 hp motors X 7.6A = 68.4A
10 each, 3 hp motors X 4.8A = 48.0A
Subtotal = 125.9A

Resistance heating loads:
10 X 3kW = 30kW
10 X 2kW = 20kW
Subtotal = 50kW X 1.25 = 62.5kW
Convert 62.5kW to amperes – 62,500 ÷ (480V X 1.732) = 75.17A
Note: Even though the heating loads are single phase, the total heating load is supplied by a three-phase system.

Total = 125.9A + 75.17A = 201.07A; round down to 201A

The feeder conductors will be installed in one EMT raceway. The total number of conductors installed in the raceway is four. Three of the conductors are considered to be current carrying. Determine conductor size using XHHW insulated conductors. With no adjustments to make, (i.e., more than 3 current-carrying conductors in the EMT raceway, or ambient temperature exceeding 30°C), while also considering the 75°C terminals, we look at the 75°C column of Table 310.16 [based on terminal ratings – 110.14(C)]. A 4/0 AWG, XHHW insulated conductor is rated at 230A, from the 75°C column of Table 310.16. A 3/0 AWG, XHHW insulated conductor is rated at 200A, from the 75°C column of Table 310.16. However, it is one ampere short of the required 201A required.

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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