#12 THHN Adjustment Factor

[machronister]

Hi everyone,

On table 310.15(B) (16) THHN on the 75 degree column #12 is rated at 25A**. The ** stands for the maximum overcurrent rating 240.4D which #12 would be 20A. Okay. Table 310.15(B) (3) (a) the adjustment factor of 7-9 current caring conductors is 70% assuming the ambient temperature is 30 Degrees Celsius. In article 100 the neutral is defined as a current caring conductor.

I have a 3/4" EMT conduit with 6 circuits and 2 neutrals and a ground all circuits have a load of 16A. Using the adjustment factor of 70% would I adjust the wire rating using 25A or 20A?

Thank you for your responses.

 

[Lectricbota]

THHN is rated 90 deg. You would start your adjustment at 30 amps.

 

[raider1]

310.15(B)(5)(a). A neutral that only carries the unbalanced current in a multiwire branch circuit is not counted for the ampacity adjustments in 310.15(B)(3)(a).

So you have 6 current carrying conductors for application of 310.15(B)(3)(a) (Unless you have a major non-linear load with harmonics).

Also as already stated you can start your adjustments from 90 degree column when using THHN.

So with that said you would take 30 amps multiply that by 80% and you would have an adjusted ampacity of 24 amps.

240.4(D) Limits the overcurrent protection device for #12 wire for most installations to 20 amps so you are OK.

Chris

 

[charlie b]

As it turns out, you would also be OK if there were a significant percentage of non-linear loads. If you have to count your two neutral wires as being current-carrying, that gives you a total of 8 CCCs. You then derate from 30 amps by 70%, and end up with 21 amps. That still allows you to use a 20 amp OCPD. The rule of thumb is that if you are using 90C rated wire, you can have up to 9 #12 CCCs in a conduit and still use a 20 amp breaker. The similar rule for #10 wire and a 30 amp breaker is 6 CCCs.