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Thread: Derating large conductors in attic?

  1. #1
    DIY Senior Member Homeownerinburb's Avatar
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    Default 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.

  2. #2
    Electrical Contractor/Instructor jwelectric's Avatar
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    The first question that comes to mind is where is the meter? I know that different parts of the country does things different than other parts of the country but here the power company brings the power to the meter. Do they do that on the soon to be island of Ca.?

    Soon to be island is referring to something someone told me about some sort of fault out there and the ground shaking all the time. They told me that Ca. would one day be an island, is that true?

    To do the calculation for the attic one would need to know what the ambient temperature of the attic. Once you know this go to Table 310.15(B)(2)(a) of the 2011 cycle or the bottom of 310.16 of older cycles and do the math.

  3. #3
    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by jwelectric View Post
    The first question that comes to mind is where is the meter? I know that different parts of the country does things different than other parts of the country but here the power company brings the power to the meter. Do they do that on the soon to be island of Ca.?

    Soon to be island is referring to something someone told me about some sort of fault out there and the ground shaking all the time. They told me that Ca. would one day be an island, is that true?

    To do the calculation for the attic one would need to know what the ambient temperature of the attic. Once you know this go to Table 310.15(B)(2)(a) of the 2011 cycle or the bottom of 310.16 of older cycles and do the math.
    Well, I'm not enough of a geologist to offer a definitive answer on the San Andres Fault, but I think it is all about the Pacific plate climbing up over the North American plate, so if that is true, no, CA will not become an island.

    Funny joke: California is perfect. If you love it you appreciate everything it offers, and if you have a condescending contempt for it, it delivers all the stimulants you need for your distaste.

    The meter on this house is about as far from the underground hub that serves it as it could be, but that was all installed 50 years ago.

    A much more direct line would have the meter coming up the side of the garage, but that would be as far from the kitchen as possible. Hence the idea of a new main panel (and move all the circuits supplying the bedrooms and bathrooms to the main) and have the kitchen and addition on the sub.

    Actually, there is so much margin on 90degree 12 and 14ga, and the kitchen needs to be utterly re-wired anyway, I am rapidly coming to the conclusion that the efficient thing to do is to string all the circuits to the garage.

    What is a typical attic termp in Carolina in the summer?

  4. #4
    Electrical Contractor/Instructor jwelectric's Avatar
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    We have a fault somewhere around here but it doesn’t shake the ground to much. I do worry about Yellowstone and the ash fall out though. I heard someone say something about it being as bad as having a dinosaur in the back yard or was it about it passing away in the back yard. Well any how it was something like that.

    For doing a rooftop calculation we use 91 degree F but I have never done a calculation for an attic. I always installed under insulation so bundling was the only worry.
    It would have to be above 150 degrees F before I would worry about very much. 30 times .65 equals 19.5 and I would venture that the circuit would never see that amount and if it did it would be a short duration of time.

    What you think?

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    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by jwelectric View Post
    We have a fault somewhere around here but it doesn’t shake the ground to much. I do worry about Yellowstone and the ash fall out though. I heard someone say something about it being as bad as having a dinosaur in the back yard or was it about it passing away in the back yard. Well any how it was something like that.

    For doing a rooftop calculation we use 91 degree F but I have never done a calculation for an attic. I always installed under insulation so bundling was the only worry.
    It would have to be above 150 degrees F before I would worry about very much. 30 times .65 equals 19.5 and I would venture that the circuit would never see that amount and if it did it would be a short duration of time.

    What you think?
    I agree that 150 is the rare day in South Pasadena's attics. So current production NM rated to 90 Celsius ought to bear the load. I am sure the local inspector would agree.

    And I am now of the opinion that all the breakers should be in the garage.

    An odd practice in the South West is that we mount our panels on the outside of the house. I am not wild about it. I prefer a main breaker panel in the stud bay with a meter socket on the outside of the house, with a 2" hub tying them together. But that is when the power is coming off a pole and down from overhead.

    Here I am seeing the 3" conduit coming up thru the base of the wall to a meter socket that flush mounts the meter, then across the top to the panel that faces into the garage.

    But the cleanest installation is just to put in a combination unit. Murray has an excellent unit that costs next to nothing.

    And before you ask, the thinking here is that it is handy for the fire department to be able to shut of the power with the flick of one switch as they pour water on the house.
    Last edited by Homeownerinburb; 07-29-2012 at 08:38 PM.

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    Electrician ActionDave's Avatar
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    Quote Originally Posted by Homeownerinburb View Post
    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.
    Given your description I would go with the meter on the garage and the sub-panel. The hot attic would not concern me in the least. Make the feeder wire size according to whatever code cycle you are under and call it good.

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