Motor feed wire heating

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John H. Gutman's picture
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Joined: 2013-10-28

What are the requirements for connecting a motor feed wire (250 MCM THHN) to the 2/0 class F motor leads? Is a section of 250 MCM 1800C wire required between the 250 MCM THHN feeder wire and the 2/0 motor leads? We are seeing signs of heating on the THHN insulation near the connection. The 2/0 motor leads extend out from the motor about 6 inches.  The motor is rated 460V, 3ph, 250 hp with 6 leads.

sparky377's picture
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Joined: 2013-10-07

John:

Based on the information, it is my opinion you are experiencing overheating of the motor feeder conductors because they are undersized.

According to Table 430.250, a 250 hp 460V motor has a full load current of 302A. This is the required value to use in sizing the motor feeder per 430.22 and 430.6, resulting in an ampacity of: 302A x 1.25 = 378A.

The installed 250kcmil THHN feeder, assuming Copper, has an ampacity of 290A at 90 deg-C. I have not run across too many pieces of equipment, other than low-voltage power circuit breakers, that have terminals rated for use at 90 deg-C. That implies that the 250kcmil THHN feeder can only be used at its 75 deg-C rating of 255A. In either case, the required ampacity exceeds that of the 250kcmil feeder.

The correct size feeder according to Table 310.15(B)(16) and 75 deg-C terminations, such as at the motor controller, would be 500kcmil with an ampacity of 380A. The 90 deg-C ampacity of 430A may be used if any deratings for temperature or conductor fill are required.

John H. Gutman's picture
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Joined: 2013-10-28

Sparky 337:
While I concur in your calculations and comments, there is one major error. The fact that the motor is a 6 lead machine indicating it is a Y/Delta start. Per NEC Sec. 430.22(C), the size of the feeder conductor should be 0.72 X motor full load A (per Table 430.250) (302A) =217.44A. A 250MCM THHN cable is rated 290A at 90 degree C per Table 310.15(B)(16). Use 90 degree C cable rating for derating if calculation results are equal to or above the 75 degree C rating (255A). For this condition the 750C rating must be used (Para 310.15(B). Derated 0.80 for 6 conductors / conduit (NEC Table 310.15.(B)(6)) and 0.95 for an ambulant temperature of 35 degree C (Table 310.15(B)(2)(a) 290 x 0.8 x 0.95 = 229A. 229<255 so the 75 degree C rating may be used as the motor load is rated at 217.44A, 217.44<255A. I believe the 250 MCM THHN cable is large enough for the load. The question remains how to connect the 2/0 180 degree C motor leads to the 75/90 degree C feeder wire.
John H Gutman, P.E.

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