Derated wiring solutions?

S

savage-sparky

Member
Location
Maine
Occupation
Electrician
I have a customer who had a union electrician friend of his help him with his wiring in his new house up here in maine. For starters not positive if his friend is licensed or not or just has his helpers, however he ran all circuits to a 12x12 junction box in the ceiling and feom the junction box he ran a 2 1/2" emt to the main panel. My question is once you account for derating calculations of approximately 20 circuits give or take this is reqyiring me to run a #10 for 15a circuits and #8 for 20a circuits is there a simplied and way less expensive solution to runing these circuits? I've considered just running feed wires through the conduit to a sub panel but even in thay case the entire place is already drywalled and finished off? Never ran into this situation so just trying to brainstorm thanks.
 
The 2.5" raceway might be usable if you have an accurate count of the CCC's (current carrying conductors). If you have 20 2-wire circuits that would be 40 CCC's. #10 THHN=40 amps*50%=20 amps derated ampacity so #10 conductors will work with up to 40 CCC's and still can be used for 15 or 20 amp circuits.

Welcome to the Forum. :)
 
Hey thanks for the reply, the only problem i run into is the cb's are only rated at 75°C so i have to adjust for that column because of the lug rating and that gives me a maximum ampacity rating of 17.5A if im understanding that correctly, i very well may be wrong lol, just want to make sure before i start collecting material and pulling in circuits
 
savage-sparky said:
Hey thanks for the reply, the only problem i run into is the cb's are only rated at 75°C so i have to adjust for that column because of the lug rating and that gives me a maximum ampacity rating of 17.5A if im understanding that correctly, i very well may be wrong lol, just want to make sure before i start collecting material and pulling in circuits
Click to expand...
Derating begins with the temperature rating of the conductor not the terminal so for THHN you use the 90°C conductor ampacity in your calculations.
 
infinity said:
The 2.5" raceway might be usable if you have an accurate count of the CCC's (current carrying conductors). If you have 20 2-wire circuits that would be 40 CCC's. #10 THHN=40 amps*50%=20 amps derated ampacity so #10 conductors will work with up to 40 CCC's and still can be used for 15 or 20 amp circuits.

Welcome to the Forum. :)
Click to expand...
Also if there are 20 circuits that gives you 40 ccc's wouldnt I have to derate at 40% leaving me a 16A rating on #10 and yea the cb rating issue makes sense wasnt really thinking about the fact that start with the conductor rating thank you
infinity said:
Derating begins with the temperature rating of the conductor not the terminal so for THHN you use the 90°C conductor ampacity in your calculations.
Click to expand...
 
savage-sparky said:
Also if there are 20 circuits that gives you 40 ccc's wouldnt I have to derate at 40% leaving me a 16A rating on #10 and yea the cb rating issue makes sense wasnt really thinking about the fact that start with the conductor rating thank you
Click to expand...
I'm going from memory but isn't 20-40 CCC's 50%?
 
winnie said:
1 What is the distance from the junction box to the panel?

2 Are MWBCs a possibility here to reduce CCCs?
Click to expand...
Distance to maine panel is about 30ft and he only brought in 3 mwbc's. Thats why im wondering if id would better to let the customer know it would be a lot cheaper to cut open a 3 ft section of drywall in the ceiling from the junction box and open a stud slot on the wall to mount a sub panel and just bring my feers through that conduit to the sub panel and then from the junction box run standard romex to the sub panel really odd set up
 
savage-sparky said:
he only brought in 3 mwbc's.
Click to expand...
You can run circuits from the panel to the junction box as MWBCs, and then split each MWBC into two 2-wire circuits as necessary. So for 20 2-wire circuits, that would let you run 10 MWBCs with bumping the wire size up one more than typical (#10 for 20A circuits, #12 for 14A circuits).

Or if there's some way you can replace the 2-1/2" EMT with a bundle of 12/3 MC cables, derating does not apply up to 20 CCCs.

Cheers, Wayne
 
You must log in or register to reply here.
Top