Mike Dolt Enterprises
Mike Dolt Enterprises
User: malachi constant, Location: Minneapolis
Asked on: Wednesday, April 13, 2011 at 2:16 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:09 am EDT
Question: I asked a question about a year ago (http://forums.mikeholt.com/showthread.php/126987-5HP-motor-conductor-easy-question). Got a great answer from David Luchini and others, really helped me understand the code. Now I have a follow up question.To recap (or you can read the first three posts in the above link)...Still working off the 2008 code. Basically the code allows you to size the motor feeder per 430, and size the OCPD per table 310.16. For a 5HP motor, for instance, you can have a #12 feeder on a 25A circuit. We are used to the "small conductor" clause in 240.4(D), but a careful reading of 240.4(D) and (G) clearly shows that the small conductor requirement does not apply to motors.And the follow-up question. What is the ground for this motor? I skimmed 430 and didn't see anything that would direct away from using Table 250.122. This table shows if your breaker is greater than 20A but less than or equal to 30A then a #10 copper ground is required.So I've got a feeder schedule that looks like this: 1/2" conduit, 3#12 CU + #10 CU ground, fed from 25A/3P breaker.Is that right? At this point I know I am going to get a bazillion questions over the years to come and think I am better off reconfiguring my spreadsheets to bump up the feeder to match the ground & OCPD in "small conductor" instances. Thoughts?(I will say, the engineer who created the spreadsheets I use was pretty sharp, and so far most times where I thought he was wrong...well, -I- was wrong. But for a 5HP motor the spreadsheet is spitting out a #12 feeder w/ #12 ground on a 25A breaker, and I think that is clearly wrong.)Thanks!
Yes, that is correct. For a 5 HP motor, you can use a #12 feeder with a #10 copper ground. This is because the small conductor requirement in 240.4(D) does not apply to motors.Table 250.122 shows the required grounding conductor size for a given overcurrent device rating. For a 25A breaker, the required grounding conductor size is #10 copper.Therefore, the feeder schedule you have created is correct.However, it is important to note that the small conductor requirement in 240.4(D) does apply to other types of loads, such as lighting and receptacles. In these cases, the grounding conductor size must be the same as the circuit conductor size, but not smaller than #12 copper.I hope this helps! Flag for review