Derating process

[Thepeelsessions]

How do you derate? I'm not taking a test, I'm just curious. I'm an apprentice and we had to get 8 THHN #12's in a 1/2 flex and it led to a derating conversation. It was hard to follow. Derating using the NEC is hard to grasp without instructions.

 

[LarryFine]

Peeler, welcome to the forum!

Simply put, derating really means the conductors may need to be upsized, which may includes resizing theie conduit, to maintain the required circuit ampacity.

In your case, the #12's start out with a 30a rating per Table 310.16, and after the 70% adjustment, are still rated at 21a, so they can still be on 20a breakers.

 

[infinity]

For derating you first need to determine how many of the conductors in the raceway are current carrying conductors (CCC's). {Look at 310.15(B)(4) for neutral conductors}. Saying that you 8 THHN conductors in a 1/2" flex is almost meaningless. You could have 8 CCC's or 4 CCC's or some other combination. So first you need to determine how many CCC's that you actually have. From there you'll need to find their ampacity from T310.16 (you may need to add in the temperature correction factors at the bottom ot T310.16) and then you can apply the provisions of T310.15(B)(2). I know it's complicated.

Welcome to the forum.

 

[augie47]

If I might add a bit to what Rob stated. Note that for derating, you can use the ampacity of the insulation such as the 90? column in 310.16 for THHN. You can then use the derived ampacity in accordance with 110.14 and 240.4.
Some folks think that they can use the 90? column only and want to use say #12 THHN at 30 amps without taking into account the limiting factors of those other sections.
It takes a while to grasp but we appreciate that you are interested enough as to why that you came here.

 

[jumper]

I will also add one more bit.

Some conductors such as THHW and XHHW have two different ampacities in T310.16. They are rated according to whether they are used in a wet or a dry location.

I will let some else explain NM and 334.80.

 

[Ponchik]

"The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor"
The same rules of table 310.15(b)(2)(a) apply to cables (Romex). if you have more than 2 cables that total more than 4 CCC and are bundled together for more than 24" then derating applies.

Here is an example:

(5) 12-2 romex running along a wall the ampacity of each conductor is_________?

(step#1) 5 x 2 (CCC each cable) = 10 CCC

(step#2) table 310.15(b)(2)(a) shows derating of 50%

(step#3) 310.16 shows ampacity of #12 @ 30A

(step#4) 30A x 50% = 15Amps

(step#5) the ampacity of each #12 conductor for this installation is 15A

(step#6) max OCPD is 15A

 

[charlie b]

I?ll echo the welcome, and take you back a level or two towards the basics. In order to ?derate,? you need to know what the ?rate? is all about. Start with the article 100 definition of ?ampacity.? It has to do with the amount of current a particular wire can handle, under its ?conditions of use.? A wire?s current rating is essentially based on the ability of its insulation to withstand heat. Heat comes from three places: (1) The surrounding ambient conditions, (2) Heat generated within the wire due to current passing through the wire, and (3) Heat generated by other wires in the same raceway. Some wires can handle more heat than others. That is why Table 310.16 has three columns, for insulation systems rated 60C, 75C, and 90C.

Please note that at the top of that table, it says that it applies to an ambient temperature of 30C, and to a maximum of three current-carrying conductors in the same raceway. You have to derate (i.e., reduce the ampacity of) a wire, if you don?t meet those two conditions. Others have already described the process of performing the derate calculations.