Derating large conductors in attic?
Looks like the homeowner (not me) who is having her home gutted and remodelled, and extended, is in for a large expense.
She is in a bit of town where all the utilities are underground. I had some of the laborers work two full days digging a 5' hole to find the conduit.
Which now seems to be such antiquated stuff that it is asbestos bearing. Meaning that not an inch of it is going to be useable. We will need to trench back to the power company's distribution hub up the block. 3' deep, 3" schedule 40 except the bends as schedule 80.
Here's the thing: the original duct follows a path all the way around the house. If the new duct were to come down the street from the hub and turn directly onto the property it would reach the garage very quickly. This path would be half as long.
So if I do that, put a 200a service panel on the side of the garage, and then put a 100 or 125a sub where the current plans called for the new 200a panel (right outside the kitchen, grovey) I'd need about 90' possibly 100' of conduit to link the two.
But I'd be going thru the attic. The hot attic.
What sort of derating would you think I'd need to apply to thhn for either 100 or 125a sub panel?
And a #6 ground bond. 1 1/2" conduit, I'm thinking.