Help with understanding 240.4(D), 240.4(G)7

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tim89s

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I have a chiller that has a MCA of 32 amps. I want to use 10 gauge wire 240.4(D) refers me to 240.4(G)7 which then refers me to 440 parts III, VI. I can't find anywhere in 440 parts III and VI that specifically states I can use 10 gauge wire for up to 35 amps.
Please help to clarify.

Thank you
Tim
 
Look at the tables of 310.15(B)(16). 10 awg at 75C is rated 35 amps. Art. 240.4(D) limits us to 30 amps with #10 however (G) allows us to ignore the limit of 35 amps if you wire motors, etc list in (G)
 
As I see it, 32 + 25% = 40a. Your wire must be rated for 40a at the temperature rating for its insulation type and termination ratings.

To me, that means you can use #10 copper only if you can use the 90-degree column of Table 310.16; otherwise, you need to use #8.

Having said that, the 25% may be built into the 32a MCA, in which case, you can ​use the #10 at 75 degrees, as Dennis stated above.
 
LarryFine said:
As I see it, 32 + 25% = 40a. Your wire must be rated for 40a at the temperature rating for its insulation type and termination ratings.
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As I understand it, when a manufacturer calculates MCA it already has the extra 25% as required. So 32A really is the Minimum Circuit Ampacity.

Cheers, Wayne
 
tim89s said:
I have a chiller that has a MCA of 32 amps. I want to use 10 gauge wire 240.4(D) refers me to 240.4(G)7 which then refers me to 440 parts III, VI. I can't find anywhere in 440 parts III and VI that specifically states I can use 10 gauge wire for up to 35 amps.
Please help to clarify.

Thank you
Tim
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Assuming your terminals are rated 75 degree (likely) and you are not using NM cable you are good to go. The MCA already includes the 125% so you just need ampacity of 32 which #10 will meet. Use whatever max OCPD is listed on the data plate. I'm guessing it will be 40 or 50 amp.
 
wwhitney said:
As I understand it, when a manufacturer calculates MCA it already has the extra 25% as required. So 32A really is the Minimum Circuit Ampacity.

Cheers, Wayne
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Yes that is the minimum based on the manufacturer's calculation which includes 125% of the largest motor load (typically the compressor) and as Texie pointed out you need to check the wiring method, some have a 60° C ampacity limitation which for #10 conductors is 30 amps.
 
electrofelon said:
Yes I agree. This comes up fairly often, but what I am still unclear of is this stated in the code anywhere?
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It would be a whole lot easier if they just put some bold type in art. 240 that said Motors Are Different.

Instead what you have to do is read 240.(4)(D) which refers you to 240.(4)(G) which tells you to go to art. 430 and 440. In there you find permissions to use bigger breakers on smaller wires.

It's a long journey from your finger around your elbow just to get to your thumb. It's really a pain when you are dealing with an inspector who doesn't get it.
 
ActionDave said:
It would be a whole lot easier if they just put some bold type in art. 240 that said Motors Are Different.

Instead what you have to do is read 240.(4)(D) which refers you to 240.(4)(G) which tells you to go to art. 430 and 440. In there you find permissions to use bigger breakers on smaller wires.

It's a long journey from your finger around your elbow just to get to your thumb. It's really a pain when you are dealing with an inspector who doesn't get it.
Click to expand...

I didnt mean the "small conductor" requirements, meant that MCA already includes the extra 25%....?
 
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