Derating different size conductors.

FlyWhale

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USA
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Electrician
I have 2 #10 THHN for a 30 amp unit and 4 # 12 THHN for a 20 amp unit. I want to put all of them in a 3/4 EMT raceway.I understand derating conductors of the same size but I'm not sure how the formula works with different size conductors. Would someone be so kind as to help me?
 

Dennis Alwon

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Chapel Hill, NC
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Electrical Contractor
You need to use Chapter 9 Tables 4 and 5 and get the area in sq. in for the wires and the conduit for more than 2 wires.
 

Dennis Alwon

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Location
Chapel Hill, NC
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Electrical Contractor
I see... so you would have to de-rate the same percentage for each wire size... Say you have 6 current carrying conductor. That means 80% so your #10 at 90C is de-rated 80%... Same for the #12
 

FlyWhale

Member
Location
USA
Occupation
Electrician
Is this about derating or conduit fill? You apply the derating percentage from the table for the current carrying conductors to each conductor regardless of size.
It's about derating. Ok, I get the formula now. I have a follow up question. Where in the world did you find that out? Is it in the NEC book? Maybe I'm blind but I've looked everywhere and I can't find that explanation. I'm not saying I don't believe you...I just like to read things for myself. I appreciate the response btw.
 

infinity

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Location
New Jersey
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Journeyman Electrician
It's about derating. Ok, I get the formula now. I have a follow up question. Where in the world did you find that out? Is it in the NEC book? Maybe I'm blind but I've looked everywhere and I can't find that explanation. I'm not saying I don't believe you...I just like to read things for myself. I appreciate the response btw.
If you look at Augie's code section and Table 310.15(B)(3)(a) it references Note 1 which states:
"Number of conductors is the total number of conductors in the raceway or cable, including spare conductors. The count shall be adjusted in accordance with 310.15(B)(5) and (6). The count shall not include conductors that are connected to electrical components that cannot be simultaneously energized."
It's good to question responses, we encourage people to dive deeper into the code wording instead of just taking someone's word for it.

Welcome to the forum. :)
 

Carultch

Senior Member
Location
Massachusetts
I have 2 #10 THHN for a 30 amp unit and 4 # 12 THHN for a 20 amp unit. I want to put all of them in a 3/4 EMT raceway.I understand derating conductors of the same size but I'm not sure how the formula works with different size conductors. Would someone be so kind as to help me?
When calculating bundling derates, we don't care what the mix of sizes of conductors will be. Ultimately, all that governs the derate factor, is the quantity of wires deemed current-carrying, that can simultaneously carry current. You then apply the same adjustment factor to each size's ampacity.

The theory behind this was most likely developed assuming identical conductors carrying identical currents, but for lack of further information that specifies how to calculate it otherwise, the wording alone tells us that it is just the number of CCC's that matter.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
It's about derating. Ok, I get the formula now. I have a follow up question. Where in the world did you find that out? Is it in the NEC book? Maybe I'm blind but I've looked everywhere and I can't find that explanation. I'm not saying I don't believe you...I just like to read things for myself. I appreciate the response btw.
Also don’t forget you can start your ampacity adjustments from the wires (THHN)90degree temp ampacity (which is greater) if you are using it in the correct installation. Dry/damp locations. See 310.15(c)(1)
 
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