Minimum motor circuit conductor size for 20 HP 460V 3 phase motor

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bozo said:
Maybe it would be helpful if I stepped through my methodology for sizing conductors and then someone can tell me where I messed up.
The code says to size motor circuit conductors at 125% of FLA shown in Table 430.250, so for 20HP, 460V FLA is 27. 125% of 27=33.75A. I then go to ampacity table 310.15(B)(16), look at ampacities in 75 deg. column, see that #10 is rated 35A, but notice double asterisks next to 10, so proceed to 240.4(D), which ultimately takes me to 240.4(G), which makes it permissable to provide overcurrent protection per Article 430.
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Stop! You're done. You've sized the conductor correctly at #10awg.

Now lets look at overcurrent protection per Article 430.


bozo said:
In Article 430 I find no information which states that 240.4(D) does not apply (at least for #10s). So I apply the requirements of 240.4(D), and since the overload protection (sized at 125% of nameplate FLA) is likely to be greater than 30 and is providing the conductor overcurrent protection as allowed in 430, I bump the size to #8. So apparently everyone is saying this is not required because 240.4(G) permits you to protect per 430? Having trouble connecting those dots.
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240.4(D) says that #10 conductors shall be protected at not more than 30A, unless specifically permitted in 240.4(E) or (G).

240.4(G) says that "overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G). Looking at Table 240.4(G), you see that "Motor and motor-control circuit conductors" are a listed "specific conductor." The Table sends you to Article 430, Parts III and IV specifically.

Art 430 Part III tells you how to protect the motor circuit conductors against overload."

Art 430 Part IV tells you how to protect the motor circuit conductors against short-circuit and ground-fault. Part IV allows an Inverse Time circuit breaker to be sized at 250% of the motor FLA, using the next standard size up. For the 20HP, 460V motor, this allows a maximum c/b size of 70A.

So the Code permits #10awg motor circuit conductors supplied from a 70A c/b with overload protection as required by Art 430 Part III.

bozo said:
Now if anywhere in the code it stated that 240.4(D) didn't apply to motor circuits, would make complete sense. But to just say, well because you're allowed to refer to 430, then 240.4(D) doesn't apply? Huh?
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240.4(D) ITSELF says that it doesn't apply to motor circuits.

Unless SPECIFICALLY PERMITTED in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7).
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240.4(G) SPECIFCALLY PERMITS overcurrent protection for motor circuit conductors to be other than required by (D)(1) through (D)(7).
 
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augie47 said:
D) Small Conductors.Unless specifically permitted in 40.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors

Once last attempt.
The facts are above.
This certainly looks like a case of "my mind is made up, don't confuse me with the facts"
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If you are saying 240.4(G) is in reality stating "The provisions in 240.4(D) do not apply to the specific conductor applications referenced in Table 240.4(G)." then I will accept that because you all certainly appear to be more knowledgeable than I. I do not understand why the code makers didn't just state that instead of being so cryptic. Was not trying to be difficult, just wanted to understand. In this case, that's apparently beyond my comprehension. Appreciate your efforts. Thanks.
 
david luchini said:
Stop! You're done. You've sized the conductor correctly at #10awg.

Now lets look at overcurrent protection per Article 430.




240.4(D) says that #10 conductors shall be protected at not more than 30A, unless specifically permitted in 240.4(E) or (G).

240.4(G) says that "overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G). Looking at Table 240.4(G), you see that "Motor and motor-control circuit conductors" are a listed "specific conductor." The Table sends you to Article 430, Parts III and IV specifically.

Art 430 Part III tells you how to protect the motor circuit conductors against overload."

Art 430 Part IV tells you how to protect the motor circuit conductors against short-circuit and ground-fault. Part IV allows an Inverse Time circuit breaker to be sized at 250% of the motor FLA, using the next standard size up. For the 20HP, 460V motor, this allows a maximum c/b size of 70A.

So the Code permits #10awg motor circuit conductors supplied from a 70A c/b with overload protection as required by Art 430 Part III.



240.4(D) ITSELF says that it doesn't apply to motor circuits.



240.4(G) SPECIFCALLY PERMITS overcurrent protection for motor circuit conductors to be other than required by (D)(1) through (D)(7).
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Thanks David, I certainly appreciate your efforts and time.
 
bozo said:
If you are saying 240.4(G) is in reality stating "The provisions in 240.4(D) do not apply to the specific conductor applications referenced in Table 240.4(G)." then I will accept that because you all certainly appear to be more knowledgeable than I. I do not understand why the code makers didn't just state that instead of being so cryptic. Was not trying to be difficult, just wanted to understand. In this case, that's apparently beyond my comprehension. Appreciate your efforts. Thanks.
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That is the style of the NEC, not just this one section, but it does this in many places. It lays down the general rules, then follows up with the exceptions to the general rules.
 
kwired said:
That is the style of the NEC, not just this one section, but it does this in many places. It lays down the general rules, then follows up with the exceptions to the general rules.
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I understand that. What I don't understand is if the intent was to state 240.4(D) does not apply, why not just state that. Because they did not say that, my interpretation was that they were referring to the specific conductor sections because those sections might have requirements that differ and supercede 240.4(D) in only some instances. In that situation, the wording would make complete sense to me. And apparently I'm not alone in that interpretation, as witnessed by the documents, calculators that still specify #8 as minimum in this situation. But if 240.4(D) absolutely does not apply at any time to the specific conductor sections, just say so. I guess I need to find myself an NEC secret decoder ring.

Thanks for the input.
 
bozo said:
But if 240.4(D) absolutely does not apply at any time to the specific conductor sections, just say so. I guess I need to find myself an NEC secret decoder ring.
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I'm still confused as to what exactly is tripping you up.

The Code says exactly what you want it to say...The small conductor overcurrent protection requirements do not apply specifically to motor circuit conductors (or electric welder circuit conductors, or air-conditioning and refrigeration equipment conductors, or motor operated appliance circuit conductors, or tap conductors, etc.)
 
david luchini said:
I'm still confused as to what exactly is tripping you up.

The Code says exactly what you want it to say...The small conductor overcurrent protection requirements do not apply specifically to motor circuit conductors (or electric welder circuit conductors, or air-conditioning and refrigeration equipment conductors, or motor operated appliance circuit conductors, or tap conductors, etc.)
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Sorry, it doesn't say that, although that might be the intent. You and others have interpreted what they actually did say to mean that, and quite possibly correctly so.
 
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david luchini said:
Is it the word "specifically" that is causing you the difficulty?
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My contention is that since 240.4(G) does not state that 240.4(D) does not apply at all to the specific conductor applications then the specific conductor sections might only be superceding 240.4(D) in certain instances or conditions and that it's possible that 240.4(D) might apply under certain conditions. You have to read the specific sections to make that determination.

I am willing to admit you are likely correct, but why can't the code just state that (using those exact words). If that's what you mean, just say it. Had they done that, you wouldn't have had to spend your time trying to understand why I find the NEC confusing.

Thanks.
 
bozo said:
My contention is that since 240.4(G) does not state that 240.4(D) does not apply at all to the specific conductor applications then the specific conductor sections might only be superceding 240.4(D) in certain instances or conditions and that it's possible that 240.4(D) might apply under certain conditions. You have to read the specific sections to make that determination.
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Why would 240.4(G) have to state anything regarding 240.4(D). It was 240.4(D) that directed you to 240.4(G) in the first place. You would just be going in circles...taking something straight forward and making it convoluted.

bozo said:
I am willing to admit you are likely correct, but why can't the code just state that (using those exact words). If that's what you mean, just say it. Had they done that, you wouldn't have had to spend your time trying to understand why I find the NEC confusing.
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If we all wanted to Code to be written in the "exact words" that each of us understand, it would be about one billion pages long.

240.4(D) says to protect #10 at not greater than 30A, unless permitted in 240.4(E) or 240.4(G).

240.4(E) directs you to six different locations where #10 tap conductors can be protected at greater than 30A.

240.4(G) directs you to ten different specific conductor applications where #10 conductors can be protected at greater than 30A.

It's no more complicated than that.
 
david luchini said:
Why would 240.4(G) have to state anything regarding 240.4(D). It was 240.4(D) that directed you to 240.4(G) in the first place. You would just be going in circles...taking something straight forward and making it convoluted.



If we all wanted to Code to be written in the "exact words" that each of us understand, it would be about one billion pages long.

240.4(D) says to protect #10 at not greater than 30A, unless permitted in 240.4(E) or 240.4(G).

240.4(E) directs you to six different locations where #10 tap conductors can be protected at greater than 30A.

240.4(G) directs you to ten different specific conductor applications where #10 conductors can be protected at greater than 30A.

It's no more complicated than that.
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Maybe you should have written the code. Your synopsis is short and concise (and probably even fewer words than in the code).

Thanks.
 
bozo said:
Maybe you should have written the code. Your synopsis is short and concise (and probably even fewer words than in the code).

Thanks.
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It is the "style" I was mentioning earlier. State a general rule, then follow with any exceptions to that general rule. The exceptions may or may not be titled as "exceptions" but the general rules are still followed up with any allowed variations from the general rules. The NEC has many places where things happen in this way.

The major outline of the code is this way. Chapter 1 - 4 are more less basic general requirements covering a pretty large range of applications, then chapters 5,6 and 7 follow up with specific applications where the rules in 1-4 may get some modification for specific applications.
 
iwire said:
No

It says that period.

You are simply mistaken and refuse to admit it but that's fine. It will not be my money you are spending or my test you will be taking.
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Had 240.4(G) actually SAID: "The provisions in 240.4(D) do not apply to the specific conductor applications referenced in Table 240.4(G)." this thread would never have existed. No, Really.
After the preponderonce of responses by obviously intelligent and knowledgeable people such as yourself I have no other alternative (no matter how badly I might wish I did) but to admit I was mistaken (I'm not that stupid. No, Really). However, I am disappointed that the NEC isn't made more easily comprehensible for the more intellectually challenged individuals such as myself. Maybe that is by design. Possibly a form a intellectual Darwinism is at work here (obviously I have no idea).

I do sincerely want to thank all (well most, anyway) of those who posted. I did learn something (actually several things). Man, if 240.4(G) had just said ..................

Hey, just curious. What do the five gold stars next to this thread on the list mean? Is it code for "Hey, check out this idiotic post!"?
 
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bozo said:
Had 240.4(G) actually SAID: "The provisions in 240.4(D) do not apply to the specific conductor applications referenced in Table 240.4(G)." this thread would never have existed. No, Really.
After the preponderonce of responses by obviously intelligent and knowledgeable people such as yourself I have no other alternative (no matter how badly I might wish I did) but to admit I was mistaken (I'm not that stupid. No, Really). However, I am disappointed that the NEC isn't made more easily comprehensible for the more intellectually challenged individuals such as myself. Maybe that is by design. Possibly a form a intellectual Darwinism is at work here (obviously I have no idea).

I do sincerely want to thank all (well most, anyway) of those who posted. I did learn something (actually several things). Man, if 240.4(G) had just said ..................
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NEC is open for anyone to make proposals for changes - if you feel that strongly about how it should be written... submit a proposal for the next code, I think it is still possible to submit proposals for 2017. With 2014 they did away with calling it proposals BTW and it is now "public input". Information on how to submit your "public input" is all in the front matter of the NEC (before art 90)

Looks like you have until October 3 for paper submittals and November of this year for online submittals. After that you will have to submit changes for 2020 instead.
 
kwired said:
NEC is open for anyone to make proposals for changes - if you feel that strongly about how it should be written... submit a proposal for the next code, I think it is still possible to submit proposals for 2017. With 2014 they did away with calling it proposals BTW and it is now "public input". Information on how to submit your "public input" is all in the front matter of the NEC (before art 90)

Looks like you have until October 3 for paper submittals and November of this year for online submittals. After that you will have to submit changes for 2020 instead.
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That's a great idea, however I could probably spend a lifetime submitting the proposals required to make the code comprehensible by me (have you delved into cable sealing in a div. 2 area in Article 501? WOW!)

Thanks for your help.
 
bozo said:
That's a great idea, however I could probably spend a lifetime submitting the proposals required to make the code comprehensible by me (have you delved into cable sealing in a div. 2 area in Article 501? WOW!)

Thanks for your help.
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Look in the Proposal Forum, I think fmtjfw has beat you to the lifetime job :D
 
iceworm said:
Not necessarily. 99.99% of everything I work is sf1.15 and I tend to set the overloads to 140%. I've never upsized the conductors because the OVLD is set up. Would you think one would need to?

ice
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Good question (for me anyway). Never considered that. I really don't know. Intuitively I would think that it shouldn't be allowed, as it seems that the 140% of nameplate could approach (or maybe even surpass?) the 75 deg. ampacity of a conductor sized at 125% of table FLA. However, I could find nothing in a brief code search that says what you did isn't code compliant and the mere fact that you posted and no-one jumped you for it would indicate that it is code compliant, correct?

Thanks. Learned something else new.
 
The reason that the CB can be oversized wrt the wire ampacity is that the CB provides only short circuit and ground fault protection while the OL provides sustained small overcurrent protection.
On that basis, if the OL no longer protects the wires I would say that you had a problem.
Making use of the SF should also change the rated MCA and FLA proportionally. Which is another question....
There. You have been jumped on. :)
 
GoldDigger said:
............................................
Making use of the SF should also change the rated MCA and FLA proportionally. Which is another question....
There. You have been jumped on. :)
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One would think.... but, so far, I don't see that addressed in the Code.
 
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