d-rateing

[liquidtite]

can someone help me understand how todrate conductors .when applying the adjustment factors for more than three current carrying conductors in a raceway {d-rateing}. i belive first you must go to 70-140 table310.15{b}{2}{a} witch shows a graph and percents you can take the conductors at depending on how many are in the conduit.4-6 is 80% 7-9 is 70% .so lets say i have 6 current carrying condutors i know i have to take them at 80% so then i think i have to go to the ampacity chat and i belive drateing is in the 90* coluoum. what i dont get is why does it say 12awg is good for 30 ampsisnt the purpose of drating the conductors is bc heat would build up in the conduit so the conductors can not be taken at the normal ampacity...am i on the rite track and are my steps rite thank you for your time

 

[infinity]

First you need to determine a couple of things:
1-How many current carrying conductors (CCC's) are in the raceway
2-What is the insualtion type and the size of the conductors
3-Is there any ambient temparature correction required

For example a raceway with 6-#12 THHN CCC's would require a derating of 80%. The ampacity of THHN at 90?C is 30 amps. You can use this number as your basis for derating so:

30 amps * 80% = 24 amps adjusted ampacity @ 86? F ambient temperature

 

[liquidtite]

thats what i got 24 amps but i dont get why 12 awg thhns ampacity is greater when you drate the conductor i thought the calc would make it so its ampacity is less do to heat build up.

 

[infinity]

thats what i got 24 amps but i dont get why 12 awg thhns ampacity is greater when you drate the conductor i thought the calc would make it so its ampacity is less do to heat build up.

You're thinking in terms of the #12 THHN being only a 20 amp conductor when in fact it's a 30 amp conductor. There are other parts of the code that limit it's ampacity to 20 amps such as 240.4(D).