Please help me keep from tearing my hair out.
I am installing a subpanel in an existing, small 22 by 8 foot (176 sqft) addition to my home. The original wiring was not done to code and has since been removed. I am dividing this area in half, one half will enclose a Jacuzzi whirlpool tub adjacent to existing bathroom. The subpanel will be located in the other dry partitioned space.
My loads are:
Jacuzzi motor 9.7A 115VAC 1116 VA Jacuzzi heater 12A 115VAC 1380 VA Small 5000 BTU A/C 5.2A 115 VAC 598 VA Lighting 4.6A 115VAC (based on code 3VA*176') 529 VA Branch circuit 12A 115 VAC (derated one branch 80%, this is a very small area) 1380 VA
Assuming the above, I have calculated abou 5282 VA load, 22A per leg, including addl, 25% for the Jacuzzi motor. I have not reduced by a "demand factor" because conceivably all the above might be running, although the AC and Jacuzzi heater will cycle. Are these good assumptions?
Because the distance across the house from the main disconnect/load panel is 124 feet, allowing 3% (3.6V) voltage drop from main to subpanel I have calculated that the conducters need to be 18,187 CMA which works out to be just a bit too much for #8 CU, so I think I need to go with #6 CU. Is #6 CU the correct size?
Now is where I get fuzzy. Most of this cable needs to go through my attic (in Florida) which gets quite hot, and will be insulated soon (hence the push to get this cable run). The cable will lay on rafters and will eventualy be buried in as much as R30 insulation. When reading the Ampacity tables 310-13 and the correction facters, I am led to beleive that I need at least a 75C rated #6 CU cable which would derate to 43A. Off I go to Home Depot and Lowes, where I find a cable labeled "Waterson/Hammock E18679 (UL) ROMEX SIMPull AWG 6 CU 3 with AWG 10 ground Type NM-B 600 volts".
Nowhere does this cable have any marking as to the temperature rating or type of insulation, so off I go to the website where I find this statement in a product sheet:
"Southwire's Romex ® SIMpull TM Type NM-B (nonmetallic-sheathed cable) may be used for both exposed and concealed work in normally dry locations at temperatures not to exceed 90°C (with ampacity limited to that for 60°C conductors) as specified in the National Electrical Code".
Does this mean that the cable is actually rated to 90C? Is it good to
55A under such an installation without derating? If I derate it, per 310-13 the numbers come out to 22A which pose a problem as my loads exceed a 20A breaker size, and as a practical matter, I need at least a 40A breaker to accomodate a #6 CU conductor.Given my loads, how would you do this?
Thanks