Derating question.

[electricalist]

Not trying to sidetrack anything so if its ok with the op. I did resi since 98 comm since 2010. How long has derating been applied and in what instance has not doing it proven to be bad. I ve been in many bldgs and seen as many as 27 #12 circuits in one conduit. Whats my sign that lack of derating was the problem

 

[ActionDave]

Not trying to sidetrack anything so if its ok with the op. I did resi since 98 comm since 2010. How long has derating been applied and in what instance has not doing it proven to be bad. I ve been in many bldgs and seen as many as 27 #12 circuits in one conduit. Whats my sign that lack of derating was the problem

I think it goes back to some time in the late '80's when thhn started to take over from tw and more conductors could fit in a pipe and not go over forty percent fill.

As far as any damage from lack of derating, I haven't seen it. I don't claim to have seen it all. I am sure there is some insulation failures out there somewhere.

 

[infinity]

313. (Chapter 10) Correction Factors for More Than Three Conductors in One Raceway or Cable appears in the 1951 NEC and states that 4,5, or 6 conductors in a raceway need to be adjusted to 80%, 7,8 or 9 need to be adjusted to 70%.

 

[kwired]

Not trying to sidetrack anything so if its ok with the op. I did resi since 98 comm since 2010. How long has derating been applied and in what instance has not doing it proven to be bad. I ve been in many bldgs and seen as many as 27 #12 circuits in one conduit. Whats my sign that lack of derating was the problem

What was being supplied by your 27 - 12 AWG conductors? I have done that myself, but not for general lighting and receptacles. You can easily have that many small conductors with motor circuits if you have all low amperage motors involved. @ 480 volts and 7.5 hp or less - 14 AWG is good for 25 amps @ 90C, derate that to 50% and 14 AWG is still acceptable for any motor under 12.5 amps. Current in a motor circuit is limited by overload protection so you can go with motor amps as the minimum ampacity unlike most other load types where you need to look specifically at the branch circuit overcurrent device.

 

[electricalist]

I cant say I know. It was a lease space in a strip mall. We were doing a new sports store. .the space was empty . Except the conduit with the curcuits. They could have been for motors imo I dont think so

 

[kwired]

I cant say I know. It was a lease space in a strip mall. We were doing a new sports store. .the space was empty . Except the conduit with the curcuits. They could have been for motors imo I dont think so

Well deration factor would be to high for 27 conductors, unless there are "non CCC" neutrals in there but if you have 20 or less CCC and 12 AWG 50% is 15 amps @90C so they coul have been feeding 15 amp lighting circuits or something like that with enough non CCC neutrals to keep it below 20 CCC.

 

[ActionDave]

313. (Chapter 10) Correction Factors for More Than Three Conductors in One Raceway or Cable appears in the 1951 NEC and states that 4,5, or 6 conductors in a raceway need to be adjusted to 80%, 7,8 or 9 need to be adjusted to 70%.

I had no idea the requirement went back that far. I do have some faded memories of four full boats of 12AWG THHN in 3/4" EMT home run pipes.