Wire size

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Tcouillard

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I do not have my codebook with me at the moment but I was hoping someone could help me out I have a mini split max breaker size is 50 I know I probably do not have to run #6 Trying to find out what gauge wire I should run thank you.
 
You need to find the minimum circuit ampacity of the unit on the nameplate or specs and size your wire to that.

Remember if you're using NM cable, it's limited to 60°, so you would have to use #6 if the minimum ampacity is 50A.
 
Max overcurrent protection size and conductor size typically do not have to match. Your MCA of 50 amps might be fine with #10 or #8 conductors.
 
Electromatic said:
#8 NM at 60° is only good for 40A.
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That needs to be qualified, and might not be exactly right. There are ambient temperature adjustment that can be made. If your ambient temperature is 69- 77° F you can add 8% to the ampacity of the 60° column.

That would be 43.2 amps for #8
 

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James L said:
That needs to be qualified, and might not be exactly right. There are ambient temperature adjustment that can be made. If your ambient temperature is 69- 77° F you can add 8% to the ampacity of the 60° column.

That would be 43.2 amps for #8
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I don't see how what you're proposing gets around 334.80.

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final calculated ampacity does not exceed that of a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable trays shall be determined in accordance with 392.80(A).
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david luchini said:
I think he was just saying that the #8NM might not be limited to 40A, based on ambient temperature, not that 43.2A would be enough for the MCA.
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Not the MCA in this case but in general you can forget trying to get the ampacity of #8 NM cable above its 60° C ampacity of 40 amps by using any ambient adjustment factor.
 
infinity said:
He's saying that the ampacity is 43.2 amps, next standard size up is 45 amps. That's greater than the 60° C ampacity.
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That 60° ampacity is based on a certain ambient temperature - 30° C or 86° F

It tells us that right in the chart.

It also says - right there in the chart - "FOR AMBIENT TEMPERATURES OTHER THAN 30° C" you multiply by the provided correction factor.

And it lists a whole host of temperatures higher AND lower than 30° C

It is real, whether anyone wants to follow it or not.
I, for one, rarely (if ever) install NM cable in a house that's 86° 🥵🥵
 
infinity said:
Not the MCA in this case but in general you can forget trying to get the ampacity of #8 NM cable above its 60° C ampacity of 40 amps by using any ambient adjustment factor.
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In a 75°F ambient, for instance, the ampacity of #8NM cable is 43.2. This isn't ABOVE it's 60°C ampacity...it IS it's 60°C ampacity.
 
david luchini said:
Yes. Maybe I don't understand your point. Nothing in the Code says the ampacity of #8NM is limited to 40. It says that the ampacity of #8NM is 40 for an ambient temperature of 78-86F.
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I'm referring to post #6 where it was stated that you can have an ampacity of 43.2 amps. It's limited to the 60° C ampacity in T310.15(B)(16) which is 40 amps so I don't see how 43.2 is relevant.
 
infinity said:
I'm referring to post #6 where it was stated that you can have an ampacity of 43.2 amps. It's limited to the 60° C ampacity in T310.15(B)(16) which is 40 amps so I don't see how 43.2 is relevant.
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What is limited to the 60C ampacity in T310.15(B)(16)?

310.15(B) says the ampacities for conductors shall be as specified in Table 310.15(B)(16) AS MODIFIED by 310.15(B)(1) through (B)(7).
 
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Ah, I had limited my thought process to only reducing the ampacity based on temperature correction but in actuality if you used an ambient lower than 78-86° you could in effect re-write the 60° C column of 310.15(B)(16).
 
infinity said:
Ah, I had limited my thought process to only reducing the ampacity based on temperature correction but in actuality if you used an ambient lower than 78-86° you could in effect re-write the 60° C column of 310.15(B)(16).
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Yes. For ambients greater than 86F the ampacities go down and for ambients lower than 78F the ampacities go up.
 
david luchini said:
Yes. For ambients greater than 86F the ampacities go down and for ambients lower than 78F the ampacities go up.
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This is one of the great overlooks in residential wiring, for sure. How many run NM through an attic with an ambient temp as high as 130°F and don't make any correction? That's only good for 6.15 amps on a #14
 
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