Originally Posted By: roconnor
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NEC 334.80 would require 90C insulation on the wires (which would normally be a 75A capacity for #6 wire with say THHN insulation), but then goes on to state that ampacity shall be based on 60C insulation which results in 55A capacity. This is consistent with typical ampacity tables and manufacturer?s data.
You are permitted to derate the 90C value ? and if you look at say an ambient temperature rise of 86F to 104F for unconditioned space, this would result in a 68A cable capacity. But that NEC section says you still cant exceed the 60C capacity of 55A. Why restrict the ampacity like that?
I have heard that the wires may have a 90C rating, but that the jacket is only rated for 60C. Why then allow the higher insulation value to be derated if the jacket is only good for 60C ... and is this subpanel feeder really an issue?
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee
I am absolutely amazed sometimes by how much thought goes into doing things wrong