5HP motor conductor - easy question

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malachi constant

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At least I think it's easy...

5HP 208V 3P motor - typical pump, exhaust fan, etc type motor that you'd find in a commercial building. That's about all the information the mechanical engineer and shop drawings will tell you. What's the conductor size?

430.22(A) states that for single motor continuous duty applications the conductor shall have an ampacity of not less than 125% the motor's FLC. This refers you to 430.6(A)(1) which refers you to Table 430.250, which states FLC is 16.7A. 16.7A x 1.25 = 20.875A. From Table 310.16 and 240.4(D) #12 is only good for 20A, so #10 is the correct size.

Right???

I ask because our motor schedule is an automated Excel spreadsheet that an engineer wrote who is no longer around, and it spits out #12s for some motors with a FLAx1.25 that is greater than 20A - which based on my understanding of the code is wrong. And another "cheat sheet" I have, this one being used for the last 20 years or so at a good-sized engineering firm, has on their motor tables a 5HP motor fed with a #12. I think #10 is correct for a 5HP motor but before I redo the spreadsheet/tables I figured I'd better do a little more research.

Thanks!
 

david luchini

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No, #10 is not required.

The ampacity of #12 is 25A (from Table 310.16.) Therefore, it is large enough for your 20.875A.

See 240.4(G) instead of 240.4(D).
 

malachi constant

Senior Member
Location
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So the code trail goes like this?:
240.4(D) references 240.4(G) as superseding it. 240.4(G) for motor circuit conductors references 430 Parts III through VII. These do not make any mention of the "small conductor" restrictions found in 240.4(D), so Table 310.16 reads as-is, and #12 is OK up to 25A for copper conductors.
 

david luchini

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So the code trail goes like this?:
240.4(D) references 240.4(G) as superseding it. 240.4(G) for motor circuit conductors references 430 Parts III through VII. These do not make any mention of the "small conductor" restrictions found in 240.4(D), so Table 310.16 reads as-is, and #12 is OK up to 25A for copper conductors.

That is correct.
 

kingpb

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No, #10 is not required.

The ampacity of #12 is 25A (from Table 310.16.) Therefore, it is large enough for your 20.875A.

See 240.4(G) instead of 240.4(D).

Disagree, in theory.

240.4 relates to overcurrent protection not current carrying capacity of conductors. The conductor ampacity is determined in 310.15.

So, in general application, according to 240.4(D) you can only use a maximum 20A protection on a #12 AWG copper unless trumped by 240.4(E), (F), or (G).

Since 240.4(G) and corresponding Table applies, you must go to 430 Part III, IV, V, VI, and VII. Again, Part III deals with overload protection, not current carrying capacity of conductors. For this example we can assume 430.32 (A) (1) applies and the motor has a S.F of 1.15, so overload protection can be 125%. So, the overload is selected to trip at 16.7A x 125% = 20.875A.
This is thermal protection of the motor (long time) and not instantaneous, that is in Part IV, and where 240.4(G) relates is because the breaker on the #12 conductor can actually be much higher to carry the starting current. We'll say inverse time CB, so 250%. Which means 41.75A, select next size 45-50A breaker.

Parts V, VI, and VII do not pertain to this discussion.

So, I am still not seeing where the justifcation is for using the #12 based on 240.4?

Actually, the statement that we live by is in Part II of 430, 430.22, whereby the ampacity of the conductor must be 125% of the FLC rating from 430.6(A)(1), which sends you to the 430 Tables.

So, based on 310.15 and 430.22, the ampacity of the #12 AWG is 25A at 75 degree C. Which is greater than the 16.7A x 125% = 20.875A, therefore you cna use the #12 on this motor and put a 50A CB, and OL set to trip at 20.875A.

Therefore, I agree that the #12 AWG is correct, but disagree on the fact that 240.4 allows it.

Just gotta love the NEC!:roll:
 

kingpb

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No, #10 is not required.

The ampacity of #12 is 25A (from Table 310.16.) Therefore, it is large enough for your 20.875A.

See 240.4(G) instead of 240.4(D).

You state that it is large enough because of 310.16, but 310.16 is simply a Table of values. The reason that it is OK is due to 430.22 not because of 240.4.

So, you got the answer right, but not the right code sections.
 

david luchini

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You state that it is large enough because of 310.16, but 310.16 is simply a Table of values. The reason that it is OK is due to 430.22 not because of 240.4.

So, you got the answer right, but not the right code sections.

No, you have it backwards. The required minimum ampacity for the motor circuit conductors is determined by Article 430 (20.875A in this case.)

Article 430 doesn't give any information as to whether a certain conductor size is OK. Article 310.15 (and by extension Table 310.16 et. al.) gives you the "Allowable Amapacity" for the conductors. #12Awg is big enough for the 20.875 minimum ampacity because T310.16 tells us the "allowable ampacity" for the #12 is 25A.

The reason the #12 is OK for the 20.875 minimum ampacity is due to T310.16, not 430.22.

240.4 allows the use of an OCPD greater than 20A with the #12.

Let me repeat myself: 240.4 has nothing to do with the conductor size. It allows an OCPD greater than 20A with the #12.
 
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kingpb

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Huh?!?!:-?



Nobody said that 240.4 allows use of the #12, T310.16 does.

240.4 allows the use of an OCPD greater than 20A with the #12.

Sorry, but this statement is wrong. T310.16 lists current carrying capability of conductors, it is not tied to any specific circuit or piece of equipment.

430.22 requires the motor circuit to be 125% of the FLC of the motor, which turns out to be 20.875A.

Then going to the T310.16 you can find the conductor that most closely matches your need.

Any mention of 240.4 convolutes the issue, because the OP was trying to determine current of the cable, not OCPD. Therefore 240.4 is inconsequential to the discussion and should not have been referenced.
 

david luchini

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Sorry, but this statement is wrong. T310.16 lists current carrying capability of conductors, it is not tied to any specific circuit or piece of equipment.

I'm sorry but my statement is correct. It issue has nothing to do with any specific circuit or piece of equipment, it has everything to do with the "allowable ampacity" of a specific size conductor.

430.22 requires the motor circuit to be 125% of the FLC of the motor, which turns out to be 20.875A.

Then going to the T310.16 you can find the conductor that most closely matches your need.

So, lets try this. Rip section 310 out of your code. No imagine a 15HP, 460V motor with an fla of 21. The minimum required ampacity of the conductors will be 26.25.

Where does 430.22 tell me if #14 Awg conductors are large enough? Or #12 Awg conductors? Where does 430.22 tell me if #8 Awg or #4 Awg conductors are large enough for the 25.25 minimum required ampacity?

The clear answer is that it doesn't. I don't see anything in 430.22 that tells me that any conductor of a particular size is large enough to carry a particular current. Article 310 does that, not Article 430.

Any mention of 240.4 convolutes the issue, because the OP was trying to determine current of the cable, not OCPD. Therefore 240.4 is inconsequential to the discussion and should not have been referenced.

Did you not read the Original Post?

From Table 310.16 and 240.4(D) #12 is only good for 20A, so #10 is the correct size.

The OP mentions 240.4(D), and had confused that section as not permitting #12 conductors on a greater than 20A motor circuit. He wasn't "trying to determine the current of the cable," he had already done that.

FLC is 16.7A. 16.7A x 1.25 = 20.875A.

He was trying to figure out how a #12 Awg conductor could be used with a 5HP-208V motor, when 240.4(D) limits #12 Awg to 20A circuits.
 

kingpb

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I'm sorry but my statement is correct. It issue has nothing to do with any specific circuit or piece of equipment, it has everything to do with the "allowable ampacity" of a specific size conductor.



So, lets try this. Rip section 310 out of your code. No imagine a 15HP, 460V motor with an fla of 21. The minimum required ampacity of the conductors will be 26.25.

Where does 430.22 tell me if #14 Awg conductors are large enough? Or #12 Awg conductors? Where does 430.22 tell me if #8 Awg or #4 Awg conductors are large enough for the 25.25 minimum required ampacity?

The clear answer is that it doesn't. I don't see anything in 430.22 that tells me that any conductor of a particular size is large enough to carry a particular current. Article 310 does that, not Article 430.



Did you not read the Original Post?



The OP mentions 240.4(D), and had confused that section as not permitting #12 conductors on a greater than 20A motor circuit. He wasn't "trying to determine the current of the cable," he had already done that.

He was trying to figure out how a #12 Awg conductor could be used with a 5HP-208V motor, when 240.4(D) limits #12 Awg to 20A circuits.
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My impression is your using the NEC as a design tool. I would recommend more time spent in reading IEEE/ANSI, NEMA and other engineering documents. The NEC is merely a conglomeration of over simplified, overly conservative assumptions developed for the unwary to be used as minimum safety requirements.

BTW: I would have installed a #10 AWG, because I won't use anything smaller on a 3-phase motor. Is that CODE, of course not; it's better and my experience over the years has taught me it's a better design, overall.
 

don_resqcapt19

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...
So, based on 310.15 and 430.22, the ampacity of the #12 AWG is 25A at 75 degree C. Which is greater than the 16.7A x 125% = 20.875A, therefore you cna use the #12 on this motor and put a 50A CB, and OL set to trip at 20.875A.

Therefore, I agree that the #12 AWG is correct, but disagree on the fact that 240.4 allows it.

Just gotta love the NEC!:roll:
First you are correct that 240.4 has nothing to do with the ampacity of a conductor. It only covers the size of the required OCPD. However in this case, without 240.4(G) the maximum permitted OCPD for a #12 is 20 amps and that would be too small for this application as that size OCPD would be likely to open with the starting current.
 

david luchini

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My impression is your using the NEC as a design tool.

Wow, I'm (almost) speechless. Where do you see that I've designed anything here. Maybe you missed the title of the post: "5HP motor conductor - easy question."

There's nothing here that is rocket science. All the OP wanted to know is why it was OK to use #12 (instead of #10) with a 5HP-208V motor, when 240.4(D) limits #12 to 20A circuits. It really was an easy question.

You're overcomplicating a simple question way too much.
 

topgone

Senior Member
I ask because our motor schedule is an automated Excel spreadsheet that an engineer wrote who is no longer around, and it spits out #12s for some motors with a FLAx1.25 that is greater than 20A - which based on my understanding of the code is wrong. And another "cheat sheet" I have, this one being used for the last 20 years or so at a good-sized engineering firm, has on their motor tables a 5HP motor fed with a #12. I think #10 is correct for a 5HP motor but before I redo the spreadsheet/tables I figured I'd better do a little more research.

Thanks!

Whoever that engineer who made your motor schedule could have included or missed some, who knows. As posted by others, NEC provisions are basically putting those who are not designers to make a quick pick on the safest conductor size. Having said those, #12 can be correct. #10 is a lot better. Just make sure you don't get too much VD at the motor peckerheads or you will have longer starting times. Another problem with using the smallest allowable size is when the specific installation is far from the source and the starter is located at that far end! The magnetic starters sometimes can't pull-in when starting and you wished you installed a bigger conductor.
 
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