"easy question" revisited - using #12 on a 25A breaker for motors

Status
Not open for further replies.

malachi constant

Senior Member
Location
Minneapolis
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!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
For the NEC answer, take a look at 250.122(D) while keeping in mind 250.122(A) states in part "no case shall they (EGC) be required to be larger than the circuit conductors...."
 
Last edited:

malachi constant

Senior Member
Location
Minneapolis
In light of my example, I can't make sense of 250.122(A), and am confused what 250.122(D) does to (A).

To paraphrase the code...
250.122(A) Equipment grounding conductors shall not be smaller than shown in Table 250.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment.

So:
* "EGCs shall not be smaller than shown in Table 250.122." This table sizes my EGC at #10 CU.
* "But in no case shall EGCs be required to be larger than the circuit conductors supplying the equipment." The circuit conductors supplying the equipment are #12 CU. #10 is larger than #12. That is prohibited by this sentence.

So you are expressly prohibited from using a #10 ground on a #12 feeder. And you are expressly prohibited from using anything smaller than a #10 ground on a 25A breaker. The only solution I see is to upsize the feeder to a #10. And that, my friends, takes all the fun out of what you taught me 10 months ago. :( Nor does it seem like the intent of the code. Am I missing something?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Malachi, it states in no case shall you be required to have an egc larger than the circuit conductors. You certainly can use #10 egc with #12 circuit conductor but you are not required to.
 
Status
Not open for further replies.
Top