Conductor adjustment factor and voltage drop

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Grouch1980

Grouch1980

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New York, NY
Hi all,
How do you approach this? let's say i have a raceway with 4 current carrying conductors (two 20 amp, 277 volt lighting circuits, each with one hot and one neutral)... so i have to apply the 80% adjustment factor. so the #12 AWG wire is now rated for 20 amps, rather than 25 amps. Now, i have to upsize the wires because the distance is over 1,000 feet. so i'll use #6 AWG wires. .... how does one still take into consideration the 80% adjustment factor with the #6 wires? does it apply at all?

Thanks!
 
Take your voltage drop and figure out your cable such as #6 as you suggest. (you must be well under 10 amps for #6 to be good)
Then click up one wire size per your derating suggestion. Use #4 for example. #4 is good for 65 amps. Use the inverse of 80% or 1.25.
1.25 * 65A = 81 amps or #4 wire.
 
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You do not have to apply one on top of the other in this case. You look at the minimum requirement for each and go by whichever requires the larger wire. AFAIK voltage drop allowances are not even an NEC code requirement so unless you're subject to an energy code or a job spec requirement then you can upsize whatever you want, or nothing, from the 12awg wire.
 
Grouch1980 said:
Hi all,
How do you approach this? let's say i have a raceway with 4 current carrying conductors (two 20 amp, 277 volt lighting circuits, each with one hot and one neutral)... so i have to apply the 80% adjustment factor. so the #12 AWG wire is now rated for 20 amps, rather than 25 amps. Now, i have to upsize the wires because the distance is over 1,000 feet. so i'll use #6 AWG wires. .... how does one still take into consideration the 80% adjustment factor with the #6 wires? does it apply at all?

Thanks!
Click to expand...
You start with the minimum conductor size you can use by ampacity, all in, which includes your derating/ampacity adjustment for conduit fill. Then you upsize for voltage drop and upsize the minimum sized grounding conductor by the same ratio of cross sectional area, using the next size up from the calculated value.
 
Thanks guys. so it looks like you don't have to consider the derating once you get the larger wire size with the voltage drop calcs. @jaggedben... that is correct, voltage drop is not an NEC requirement, only energy code.
 
Grouch1980 said:
Thanks guys. so it looks like you don't have to consider the derating once you get the larger wire size with the voltage drop calcs. @jaggedben... that is correct, voltage drop is not an NEC requirement, only energy code.
Click to expand...
If you are saying that ampacity derating is not a player in voltage drop calculations, that is correct. It does, however, sometimes enable you to start with a larger CCC size before you calculate the ratio between Vd compensated conductors and those that are not, so you might not have to increase the size of your ground conductor as much.
 
the wires are based on the 90C insulation the code permits for adjustments to be based on this 310.15 (B) tables, (Ampacities must remain with in the confines of the terminal temperature rating), if you reverse the math #12@90C=30A Then 30x.80%=24 if you were to look the table for 75C says 25A. if you have 4-6 CCC's you are in the confines of that 80% once you go over 6 then you really start to notice a change in the allowable ampacities. your table for this is going to be your 310.15(B)(3)(a). *Also the corrected ampacities must able to be protected by a corresponding OCPD ie. if your corrected number is 12.3Amps that wire can no longer be protected, it is not corresponding to a common breaker size.
then you have to refer to 240.4 (D) then it will shoot you over to table 240.4(G) essentially states, the larger OCPD on the 75C column can be used in these circumstances only.

as for your voltage drop I always do it, equipment can burn up if not done correctly, BTW the NEC REQUIRES voltage drop for fire pumps and some other things but not in the normal rules

hope it helps!
 
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ggunn said:
If you are saying that ampacity derating is not a player in voltage drop calculations, that is correct. It does, however, sometimes enable you to start with a larger CCC size before you calculate the ratio between Vd compensated conductors and those that are not, so you might not have to increase the size of your ground conductor as much.
Click to expand...

I couldn't follow what you wrote in red... could you please rephrase that?
 
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