I have noticed differing minimum motor circuit conductor sizes (#10 & #8) from various sources for a 20HP 460V 3 phase motor. I would size at #8, assuming the overcurrent protection limitations of 240.4(D) [as noted in table 310.15(B)(16)] apply for motors also. Apparently, some are either unaware of these limitations or believe they do not apply to motors. I cannot find any evidence to support the notion that 240.4(D) does not apply for motors when using #10 AWG. Appreciate any help.
Minimum motor circuit conductor size for 20 HP 460V 3 phase motor
- Thread starter bozo
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petersonra
Senior Member
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- Northern illinois
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- engineer
my chart says 27 FLA. 125% of this gives you 33.8 A ampacity required.
I don't see where you get 33.8A in a #10.
Actually the 75? rating of #10 according to 310.16 is 35 amps thus it would meet the 430.22 requirement.
As far as 240.4(D) and the 30 ampere limit on #10, 240.4 in the 1srt paragraph states "unless otherwise permitted ..in 240(G) which includes Art 430.
The temperature limitations in 110.14 could be a problem but usually are not.
Bottom line, there may be valid reasons for not using a #10 THHN on the motor, but doing so would be Code complaint in most situations.
As far as 240.4(D) and the 30 ampere limit on #10, 240.4 in the 1srt paragraph states "unless otherwise permitted ..in 240(G) which includes Art 430.
The temperature limitations in 110.14 could be a problem but usually are not.
Bottom line, there may be valid reasons for not using a #10 THHN on the motor, but doing so would be Code complaint in most situations.
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- Connecticut
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- Engineer
I agree. I see no Code issue with using #10 awg for 20HP, 460V motor circuit.
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- Massachusetts
The limitations of 240.4(D) do not apply to motors.
Motors are one on the many items in table 240.4(G) that 240.4(D) does not apply to.
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I can't imagine the number of arguments and false accusations that have happened over 240.4(G).
The limitations of 240.4(D) do not apply to motors.
Motors are one on the many items in table 240.4(G) that 240.4(D) does not apply to.
Thanks for the reply. As I stated in my original post, I couldn't find any data in Article 430 that specifies allowable conductor ampacities for motor applications (except in 430.22(G) which specifies requirements for #18 & #16 AWG). In the absence of such data, aren't you left referring back to Table 310.15(B)(16) for conductor sizing (including applicable footnotes)?
You are, indeed, left with 310.15(B)(16) which shows a 75? amapcity of 35 amps for #10. The footnotes reference you to 240.4(D), HOWEVER, note AGAIN, the wording there:"Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed................................" 240,4(G) allows you, on Art 430 applications, to use the Table ampacities without the (D) restrictions.
240.4(G), by that wording, trumps 240.4(D) allowing you to exceed the (D) 30 amp limitation and use the 310.15(B)(16) ampacity which would be effected by 110.14 or any ambient/fill adjustments.
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- Placerville, CA, USA
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- Retired PV System Designer
And since that gives you 35A if you use a 75 degree wire type, what problem do you see with using #10? Is there something in a footnote that you see as restricting it more?
The "**" that reminds you about 240.4(D) does not restrict you in this case since 240.4(D) itself releases you from its requirements by forwarding you to (G).
The table says, in effect, to read 240.4(G) not to take its limitations out of context and apply them blindly.
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- Location
- Connecticut
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- Engineer
Art. 430 tells you you must have a conductor that has an ampacity at least 125% of the motor FLA...you refer back to Table 310.15 (B)(16) for the conductor sizing. In your original example a #10 conductor meets the ampacity requirements.
Edit: Or what Gus and golddigger said
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I agree that 240.4(G) is telling you that the requirements in 430 supercede 240.4(D). To me that means to look at 430 and size conductors accordingly. Now where in 430 section II does it tell you to size conductors based on Table 310.15(B)(16) but ignore the references to 240.4(D). If that was the intent it would have been extremely easy to just state in 430. Before you make the statement that that conclusion is implicit in the reference to 430 and hence not even necessary to state to begin with, consider this. What if they are referring you to 430 only due to the special modifying requirements for #18 & #16 as mentioned earlier. I might be incorrect in my interpretation, but if so, am in what I would consider "good company" (Cutler Hammer, Square D and others who have published minimum wire sizing tables for motors. At the very least I'm not the only "blind" idiot out here. Appreciate the comments.
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Because that is not what it says, and if you read the words with no preconceived bias it's not hard to understand.
I think you are only in the company of those that can't get their head around the idea that the code allows something that flies in the face of what they have always thought was true about sizing conductors.
Sure it would be easier if 240.4(D) just said "Motors Are Different", but it doesn't. Instead it refers you to 240.4(G) and relies on your ability to comprehend technical information.
Like it or not 310.15 says that 14AWG is good for 20A, 12 is good for 25A, and 10 is good for 35A.
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- Placerville, CA, USA
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- Retired PV System Designer
And it also says that before you go with thodr numbers, for wire sizes #10 and smaller you should look at 240.4(D) to see if it limits you more. In the case of a motor, we did look at it and it did not limit us. 'Nuff said.
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- Licensed Electrician
Somebody thought an asterisk was needed for Roger Maris too.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Somebody thought an asterisk was needed for Roger Maris too.
And since he was not a motor....
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- Licensed Electrician
He too was plagued by an asterisk. Nuff said.
- Location
- Connecticut
- Occupation
- Engineer
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.
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.
My Square D Motor Data Calculator list #10awg is the minimum copper wire size for a 20HP, 460V, 3ph motor.
kwired
Electron manager
- Location
- NE Nebraska
GD already replied with essentially what I was going to reply with here.
So does mine. 10 hp @ 240 volts gives you basicially the same situation. 5 hp single phase motors are right in that same current range and can often have similar confusion. Another one that sometimes baffles people is that you can have a 10hp 460V 3ph on a 14 AWG with possibly up to a 35 amp breaker.
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
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.
kwired
Electron manager
- Location
- NE Nebraska
Does it say 60 or 75 deg conductors? You are getting far enough back it is likely to be 60C .
75C conductor was not the problem at that time, many devices were still only rated 60C or were not marked and had to be assumed to be 60C if not marked.
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