derating, adjusment

Toros

Senior Member
Location
Tujunga, CA
Hi, lets says,I have to run (4) #12 cu., thhn conductor 1n a emt conduit. do I have to look in 10-15B3a and 90 deg. column in 310.15B16 and do this calc; Ampacity of #12 wire is 30A @ 90 degree than it becomes 30A x .8 (Adj. fact.)= 24A OR #12 thhn,cu @ 86degr: than becomes 20A x .8 (Adj. fact.)=16A ????? thank you
 

infinity

Moderator
Staff member
Location
New Jersey
Start with the conductor ampacity based on it's size and insulation type. For example #12 THHN is a 90? C conductor so use it's 90? C ampacity of 30 amps and derate from there. Then find the appropriate derating value based on the number of CCC's in the raceway and multiply.
 

Smart $

Esteemed Member
Location
Ohio
Hi, lets says,I have to run (4) #12 cu., thhn conductor 1n a emt conduit. do I have to look in 10-15B3a and 90 deg. column in 310.15B16 and do this calc; Ampacity of #12 wire is 30A @ 90 degree than it becomes 30A x .8 (Adj. fact.)= 24A OR #12 thhn,cu @ 86degr: than becomes 20A x .8 (Adj. fact.)=16A ????? thank you
Not sure where you came up with the 86degr or the 20A... but it don't matter because the first method is correct. However, that only covers adjustment for the number of current-carrying conductors, assuming all (4) are counted as such. Final ampacity also depends in part on correction for ambient temperature, terminal temperature limitations, and distance of adjacent portions at higher ampacity. Other considerations may also apply for different wiring methods and conditions of use.
 

Toros

Senior Member
Location
Tujunga, CA
de-rating

de-rating

Start with the conductor ampacity based on it's size and insulation type. For example #12 THHN is a 90? C conductor so use it's 90? C ampacity of 30 amps and derate from there. Then find the appropriate derating value based on the number of CCC's in the raceway and multiply.
what about if the conductors were THWN , than I would look in 75 deg. for ampacity frankly adjust them based on thier quanity in the raceway. right????
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Correct. For four current-carrying #12 THWN conductors in the same conduit, you start with 25 amps and multiply by 0.8 to get an ampacity of 20. Please note that even for the THHN conductors, for which the ampacity turned out to be 24 amps, you still cannot protect them with a breaker rated higher than 20 amps.
 

MichaelK103

New member
Location
Georgia
Multi conductor cable derate in a covered tray

Multi conductor cable derate in a covered tray

Hello forum:

I have an issue in derating a four (4) conductor, 3 power + 1 Gnd, motor cable in an existing cable tray without separation. My question is whether Table 310.15(B)(2) applies because this cable is installed in a tray and the definition indicates -

"... multiconductor cables are installed without maintaining spacing for a continuious length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity ..."

Our installation have over ten (10), motor cables (e.g. 3 + gnd) existing in the existing tray. So would I use the derate of 50%?

Thank you in advance.
 

Smart $

Esteemed Member
Location
Ohio
Hello forum:

I have an issue in derating a four (4) conductor, 3 power + 1 Gnd, motor cable in an existing cable tray without separation. My question is whether Table 310.15(B)(2) applies because this cable is installed in a tray and the definition indicates -

"... multiconductor cables are installed without maintaining spacing for a continuious length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity ..."

Our installation have over ten (10), motor cables (e.g. 3 + gnd) existing in the existing tray. So would I use the derate of 50%?

Thank you in advance.
No.

Your ampacity will likely have little or no derating at all (cable tray portion only if other exists beyond tray). Refer to 392.80(A)(1) & (A)(3).

Note about 392.80(A)(3)... it does not say what to do if the 2 conditions do not apply. :blink:
 

Toros

Senior Member
Location
Tujunga, CA
Correct. For four current-carrying #12 THWN conductors in the same conduit, you start with 25 amps and multiply by 0.8 to get an ampacity of 20. Please note that even for the THHN conductors, for which the ampacity turned out to be 24 amps, you still cannot protect them with a breaker rated higher than 20 amps.
Thank you
 

mjmike

Senior Member
Correct. For four current-carrying #12 THWN conductors in the same conduit, you start with 25 amps and multiply by 0.8 to get an ampacity of 20. Please note that even for the THHN conductors, for which the ampacity turned out to be 24 amps, you still cannot protect them with a breaker rated higher than 20 amps.
If it is a wet location and the insulation is THWN-2, then the derating would start with 30A from the 90 degree column correct? Meaning there could be up to four (4) 20A-1P circuits (1-#12 phase, 1-#12 neutral, 1-#12 ground) in the same conduit because there would be 8 CCC, 30A*70% = 21A rated conductors. Conduit would obviously be sized for 12 #12 conductors; 3/4" minimum if sch 40.
 

mjmike

Senior Member
Thanks for the confirmation. I have been finding most conductors are THWN-2 anymore and rarely see it without the "-2".
 

augie47

Moderator
Staff member
Location
Tennessee
Correct. For four current-carrying #12 THWN conductors in the same conduit, you start with 25 amps and multiply by 0.8 to get an ampacity of 20. Please note that even for the THHN conductors, for which the ampacity turned out to be 24 amps, you still cannot protect them with a breaker rated higher than 20 amps.
What if I'm supplying a motor or HVAC unit :D
 

Smart $

Esteemed Member
Location
Ohio
What if I'm supplying a motor or HVAC unit :D
OCPD rating could be higher. 20A is the limit imposed by 240.4(D)(5). However 240.4(D) says unless specifically permitted in 240.4(E) or (G). Motors and A/C are specifically permitted through 240.4(G).
 
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