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

[david luchini]

mine says #8. It also says copyright 1987 on it.

I have gotten used to using the UL508a charts. They have no provision in them for #10 conductors to exceed 30 A.

Interesting...Mine says #10 minimum copper wire size (75degC)...Copyright 1988 based on 1987 NEC.

in what I would consider "good company" (Cutler Hammer, Square D and others who have published minimum wire sizing tables for motors.

Also, an online search found interactive charts from both Square D and Cutler Hammer both of which say #10 for a 20HP, 460V, 3ph motor.

 

[bozo]

240.4(D) itself tells you to ignore 240.4(D) for motor conductor protection. The Code doesn't need to tell you in 430 section II what it has already told you in 240.4(D). In addition, 430 section II is the wrong place to look for motor circuit conductor protection. 240.4(G) directs you to 430 sections III, IV, V, VI and VII, not to section II.

Check out the 2014 NEC, they added Section II.

The special requirements for #18 and #16 modify 310.106(A) "Minimum Size of Conductors" which says the minimum size of conductors rated 0-2000V shall be #14awg Cu, except as permitted elsewhere in the Code. 430.22(G) "Conductors for Small Motors" is one of the sections that permits smaller than #14.

The provisions in these sections also provide load and protection requirements that differ from those in 240.4(D) [I know, 240.4(D) supposedly doesn't apply]

My Square D Motor Data Calculator list #10awg is the minimum copper wire size for a 20HP, 460V, 3ph motor.

Interesting. I have a Square D Motor Data Calculator right in front of me (right now) that quite clearly shows #8 (for 460V). However, it does show #10 for 575V. (Might have some collector value since it apparently has incorrect data).

It appears that this discussion all boils down to the fact that everyone else posting interprets 240.4(G) "Overcurrent Protection for Specific Conductor Applications.
Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)." to mean "The provisions in 240.4(D) do not apply to the specific conductor applications referenced in Table 240.4(G)." Apparently, my brain lacks the complex algorithm required to make that translation, because I interpret 240.4(G) to mean "Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)." (Oops, that's what it actually says.)

Thanks for the dialog.

 

[david luchini]

Interesting. I have a Square D Motor Data Calculator right in front of me (right now) that quite clearly shows #8 (for 460V). However, it does show #10 for 575V. (Might have some collector value since it apparently has incorrect data).

I wonder if the copyright on your calculator is 1987 or earlier, like Bob's.

It appears that this discussion all boils down to the fact that everyone else posting interprets 240.4(G) "Overcurrent Protection for Specific Conductor Applications.
Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)." to mean "The provisions in 240.4(D) do not apply to the specific conductor applications referenced in Table 240.4(G)." Apparently, my brain lacks the complex algorithm required to make that translation, because I interpret 240.4(G) to mean "Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)." (Oops, that's what it actually says.)

Don't those two interpretations say the same thing, just worded differently?

 

[kwired]

Interesting. I have a Square D Motor Data Calculator right in front of me (right now) that quite clearly shows #8 (for 460V). However, it does show #10 for 575V. (Might have some collector value since it apparently has incorrect data).

How old is it? Which NEC cycle is motor full load current and conductor ampacity taken from - though those haven't changed since at least 1984 maybe even longer then that. Only change was recently to a couple 60C values and doesn't effect most of us very much at all, unless dealing with NM cable maybe, which must still apply 60 values.

Does it mention whether it is for 60 or 75 deg C. - those details will make the difference - chances are it is correct and you are missing some details. It used to be a device was assumed to be 60 C unless marked otherwise. Today you just won't find anything not marked 75C, those charts have changed a long time ago because of this. An older chart based on 60C may not mention it, but a chart based on 75C will likely mention that it is based on 75C.

 

[GoldDigger]

I think that the distinction he is straining to find is that allowing a higher OCPD is not the same as allowing the conductor to carry higher current.

 

[kwired]

I think that the distinction he is straining to find is that allowing a higher OCPD is not the same as allowing the conductor to carry higher current.

And/or factoring in 240.4(D)

bozo how do feel about something that gets away from 14 - 10 AWG but seems to push conductor limits at first glance, say a 40 hp 460V motor - minimum conductor 6AWG but could easily be on a 125 amp thrermal magnetic breaker and would be code compliant?

 

[bozo]

How old is it? Which NEC cycle is motor full load current and conductor ampacity taken from - though those haven't changed since at least 1984 maybe even longer then that. Only change was recently to a couple 60C values and doesn't effect most of us very much at all, unless dealing with NM cable maybe, which must still apply 60 values.

Thanks. You had to ask and I'm too dumb to lie. The calculator is based on 1981 Code.

Does it mention whether it is for 60 or 75 deg C. - those details will make the difference - chances are it is correct and you are missing some details. It used to be a device was assumed to be 60 C unless marked otherwise. Today you just won't find anything not marked 75C, those charts have changed a long time ago because of this. An older chart based on 60C may not mention it, but a chart based on 75C will likely mention that it is based on 75C.

Quote "MINIMUM COPPER WIRE SIZE - (75 DEG. C) THW, THWN, XHHW"

 

[augie47]

I have found over the years that the manufacturers motor slide rules, and even the UGLYS book are at times conservative on their wire sizes and conduit fill.
Take into account voltage drop, ambient and other factors it would not take much to make the #10 undersized for the 10HP motor.
They seem to CYA.

I was once taught that the provisions of 240.4(D) were included in the Code due to fact that many devices that are present on those "small conductors" are 60? rated and 110.14 would only allow that termination rating.

 

[augie47]

And/or factoring in 240.4(D)

bozo how do feel about something that gets away from 14 - 10 AWG but seems to push conductor limits at first glance, say a 40 hp 460V motor - minimum conductor 6AWG but could easily be on a 125 amp thrermal magnetic breaker and would be code compliant?

excellent point !

 

[bozo]

bozo how do feel about something that gets away from 14 - 10 AWG but seems to push conductor limits at first glance, say a 40 hp 460V motor - minimum conductor 6AWG but could easily be on a 125 amp thrermal magnetic breaker and would be code compliant?[/QUOTE]

Gotta have overload protection not more than 125% of nameplate FLA, right? Would appear code compliant to me.

 

[david luchini]

Thanks. You had to ask and I'm too dumb to lie. The calculator is based on 1981 Code.

This makes me wonder when 110.14(C)(1)(a)(4) was introduced in the Code.

 

[augie47]

bozo how do feel about something that gets away from 14 - 10 AWG but seems to push conductor limits at first glance, say a 40 hp 460V motor - minimum conductor 6AWG but could easily be on a 125 amp thrermal magnetic breaker and would be code compliant?

The only difference is that we have a 240.4(D) for the #10, but (D) tells us for (E)&(G) the (D)(4) is not applicable.

Out of curiosity, what does your slide rule call for on the 40 hp motor ?

 

[iwire]

I know I tend to be blunt but who cares what a manafacturer says. Those references are not and never have been code.

Even the NFPA produced handbook is not code.

240.4 is clear on its own.

 

[iceworm]

...Gotta have overload protection not more than 125% of nameplate FLA, right? Would appear code compliant to me.

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

 

[augie47]

I know I tend to be blunt but who cares what a manafacturer says. Those references are not and never have been code.

Even the NFPA produced handbook is not code.

240.4 is clear on its own.

I agree to a point* i asked about the sliderule for the 40HP only out of curiosity as I have found those critters and the NEC to be out of sync previously.
* we do care what the manufacturer says when it's part of the listing or labeling.

 

[iceworm]

I know I tend to be blunt but who cares what a manafacturer says. Those references are not and never have been code.

Even the NFPA produced handbook is not code.

240.4 is clear on its own.

Are you saying:

The referenced cardboard "slide rules" are useless.
There is an excellent published reference detailing required wire size.​

If so, I absolutely agree.

However, by the same token, I don't use internet voltage drop calculators either nor subscribe to rule of thumb K factors. There is an excellent published reference for these as well.

ice

 

[bozo]

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. 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. 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?

 

[iwire]

Maybe it would be helpful if I stepped through my methodology for sizing conductors and then someone can tell me where I messed up.

Again I tend to be blunt.

At this point engineers, inspectors, long experienced electricians have all explained it to you and yet you keep asking the same question.

With a user name of bozo and this line of questioning I am really starting to think you are just pulling peoples leg. AKA Trolling.

 

[bozo]

The only difference is that we have a 240.4(D) for the #10, but (D) tells us for (E)&(G) the (D)(4) is not applicable.

Out of curiosity, what does your slide rule call for on the 40 hp motor ?

#4s

 

[augie47]

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"