ROOF TOP COMPRESSOR UNITS

[Greenboy]

Question about derating.

I have 4 units mounted on the roof, and are all rated at 40 amp MAX Fuse size, with a MCA of 32 A.

There is an existing 1-1/4" conduit that I want to use to bring up these circuits, which I can legally fit (12) #6 . When derating #6 THWN wire (12 CCC in a conduit is 50% derating), I Come up with 37.5 amps. Just want to confirm that I am basing this off of of the MCA, and not the max fuse size?

For connecting these equipment, where in the code allows us to use the 75 degree column for connections. or is this a equipment rating?

 

[Carultch]

Question about derating.

I have 4 units mounted on the roof, and are all rated at 40 amp MAX Fuse size, with a MCA of 32 A.

There is an existing 1-1/4" conduit that I want to use to bring up these circuits, which I can legally fit (12) #6 . When derating #6 THWN wire (12 CCC in a conduit is 50% derating), I Come up with 37.5 amps. Just want to confirm that I am basing this off of of the MCA, and not the max fuse size?

For connecting these equipment, where in the code allows us to use the 75 degree column for connections. or is this a equipment rating?

It is correct that you base it off the MCA, for all equipment that comes with an MCA rating. MCA = minimum circuit amps, which is the ampere capacity of the circuit at every point throughout it.

Derate factors are based off the temperature rating of the wire, usually 90C. And prior to applying derate factors, you also need to meet termination ampacities per 110.14(C).

For 100A and less, the default equipment termination rating is the 60C column ampacities, unless listed and labeled otherwise on both ends, which most modern producs are. For over 100A, 75C is the default. In practice, it is usually the 75C, since the 60C rule is mostly academic.

 

[infinity]

Question about derating.

I have 4 units mounted on the roof, and are all rated at 40 amp MAX Fuse size, with a MCA of 32 A.

There is an existing 1-1/4" conduit that I want to use to bring up these circuits, which I can legally fit (12) #6 . When derating #6 THWN wire (12 CCC in a conduit is 50% derating), I Come up with 37.5 amps. Just want to confirm that I am basing this off of of the MCA, and not the max fuse size?

For connecting these equipment, where in the code allows us to use the 75 degree column for connections. or is this a equipment rating?

Yes, the MCA is just that the minimum conductor size, so after you perform your adjustments the final ampacity has to meet or exceed the MCA. The MaxOCPD is irrelevant for the calculations.

 

[rlundsrud]

You might want to look at Table 310.15 (B)(3)(c), it talks about ambient temperature correction for conduit above a roof.

 

[kwired]

You might want to look at Table 310.15 (B)(3)(c), it talks about ambient temperature correction for conduit above a roof.

I was going to mention that if no one else did. Depending on how things are done/where you may be located roof top ampacity adjustments can increase conductor size pretty dramatically compared to what most are used to seeing in other areas.

 

[iwire]

We come up inside the RTUs from underneath so temp adders are not an issue.

 

[Fitzdrew516]

You might want to look at Table 310.15 (B)(3)(c), it talks about ambient temperature correction for conduit above a roof.

Or use xhhw-2 if you're in the 2011 or newer code and you don't have to. Or just do what iwire suggests