Disconnect sizing

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27hillcrest

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I need to size a disconnect for the following condenser.
FLA 23.5
Max Bkr. size 40amp

Can i size the disco at 30 amp or do I need a 60amp?
Can the wire size be #10 or #8 because of the 40amp bkr?

TIA
 
The disconnect must be sized at no more than 115% of the minimum circuit ampacity marked on the A/C nameplate. Take a look at 440.12(A)(1).

So the you can use a 30 amp disconnect.

As for the branch circuit conductors you can use a conductor rated for 23.5 amps from Table 310.16. So for example if you are using THHN and there is no derating required you could use a #12. #12 THHN is rated at 25 amps at 60 or 75 degrees C, also keep in mind that the small conductor rules of 240.4(D) don't apply to A/C circuits.

Chris
 
raider1 said:
The disconnect must be sized at no more than 115% of the minimum circuit ampacity marked on the A/C nameplate. Take a look at 440.12(A)(1).

So the you can use a 30 amp disconnect.

As for the branch circuit conductors you can use a conductor rated for 23.5 amps from Table 310.16. So for example if you are using THHN and there is no derating required you could use a #12. #12 THHN is rated at 25 amps at 60 or 75 degrees C, also keep in mind that the small conductor rules of 240.4(D) don't apply to A/C circuits.

Chris
Click to expand...

That should be at least 115%. I know that's what you meant. :smile:
 
vinster888 said:
My area says when you use any size breaker, you're disco and wire will match or better. No matter the load
Click to expand...

Welcome to the forum.:)

Unfortunately it appears that the people in your area don't apply the rules for motors and A/C units properly.

Ask them what code rule prohibits the above installation.

Chris
 
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My own opinion on disconnect sizing is reflected by my infrared scanning experience. Motor loads that their full load amperage ratings come close to 80 percent of the rating of their disconnect stands a good chance at failing before their designed lifespan . Especially with loads that exceed that 80 percent due to their services. An example would be an elevator motor. Your motor's rating is within a half amp of the 80 percent rating if you use a 30 amp disconnect switch. The design lifespan of that disconnect is effected by currents that exceed the 80 percent mark. Engineers have told me something like 100 times reaching currents of 10 percent without effecting it's lifespan. And they come up with maybe 400 times at 90 percent. And 1200 times at currents at 80 percent! The labor in your installation is the same and the price difference is minimal between a 30 amp vs a 60 amp switch. The closer you get to that 80 percent and above figure, the more important it is to consider this upgrade! My $.02
 
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