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Old 12-23-2009, 12:32 PM   #91
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Older air compressor wiring help


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Originally Posted by electures View Post
110.14(1)(a)(4) is not a code section. I believe you meant 110.14(C)(1)(a)(4) which only applies if the motor is marked with design letters B,C, or D. Otherwise 110.14 (C)(1)(a)(1) applies which requires the use of the 60 degree column.

In my previous post I wrote out the procedure for sizing the conductors for this motor. #8/2 romex will work on a 70A breaker.

The welder is going to require a different size conductor and OC device.

Good luck!
Design letters generally do not show up on anything but 3 phase motors. Nema requires that all three phase motors show the design letter on the nameplate ...this will be either B,C or D in the majority of cases. However this section of 110.114 is simply stating for those particular motors you are allowed to use 75 C ampacities. Other wise any motor showing terminations that may be 75 C can use the 75 C column unless limited by restrictions for certain cables like NM-b or SE. Almost all motors have that specification for terminals and nema requires that any motor leads when terminals are not used be 125C rated.

I agree that a welder may require a branch circuit different than the air compressor, however arc welders have a wide range of OCPD if using the NEC to calculate the branch circuit requirements. The manufacturer often states the OCPD and minimum conductor ampacity. Unless we know the welder he is getting we cannot state that it will be required to have a different branch circuit. It is unlikely that #8 is going to have any effect on whether or not he can use the welder. The limiting factor will be the OCPD for the air compressor and the power cord on his welder. In other words he cannot change the plug to a 6 -50 on a power cord with a 6 -20 or 6 -30 on it and he cannot use a breaker larger than what is specified by the manufacturer.

I would not install the maximum ocpd on this air compressor as calculated by the NEC. I would probably start with a 40 and see if it would hold. I would highly doubt that a 70 amp breaker would need to be required.

I do agree with the information given that the conductor size should be #8 using 60C ampacities but could be #10 if thhn in conduit...however you would need defined information to use 75 C on this motor . You probably cannot get that unless you call baldor.

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Last edited by Stubbie; 12-23-2009 at 12:38 PM.
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Old 12-23-2009, 12:46 PM   #92
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Older air compressor wiring help


Some pretty good info here. Did you get the compressor running?

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