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Old 09-22-2010, 11:04 AM   #1
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Off Peak Panel Size


I ran into a stumbling block. I talked to my electrical inspector about my electric heat panel. In our discussion he told me that 2-2-4 AL between the meter (point of connection) and the panel can only be figured to support 90 amps and I would have to change the main breaker from 100 to 90 amps in this panel. Then I apply the 80% factor for continuous load. I have plenty of power for the electric heat but I won't be able to connect the water heater to this panel.
He said the main service panel at 100 amps is fine with 2-2-4 AL but the second panel, even though it is on its own meter, can only be at 90 amps. Anyone have a simple explanation for this?

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Old 09-22-2010, 11:12 AM   #2
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I've heard the same thing, not sure why
Never heard a specific code reference
But that some Inspectors do not like a 100a main feeding a 100a sub

I have a 100a sub off a 200a main
The 100a sub feeds a 100a sub for my hot tub & 2 other circuits
Inspector didn't have any problem with the setup
The 2nd sub only has 50a GFCI & (2) 20a...all there is room for

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Old 09-22-2010, 11:19 AM   #3
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i will take a stab at this. 310.15 allows certain conductor sizing if it feeds the entire load of the dwelling. If only a partial load is served the sizing of 310.16 must be used.
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Old 09-22-2010, 01:10 PM   #4
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Off Peak Panel Size


This sounds very similar to what the inspector said, I think
The cable is USE-2 AL 2-2-4. I don't have anything with 310.15 but I see in 310.16 that USE-2 is good for 100 amps in the 90 deg column but then there is something about termination rules. Would it be the 100 amp breaker that is is limiting this to the 75deg column?
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Old 09-22-2010, 01:56 PM   #5
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The terminals are most likely rated for 75 degree max.

Table 310.15(B)(6) lists #2 AL for 100 amps for a service and feeder.

Table 310.16 lists #2 Al at 75 degrees for 90 amps or 75 amps with 60 degree ratings.
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