Conductor Ampacity

Status
Not open for further replies.
C

Cieba

Member
Hi,

I would like to know what would be the ampacity of the 12AWG THHN in a raceway, 40ft long with 4 current carrying conductors and an ambient temperature of 97* F?

Which column in table 310.16 of the NEC do you use to calculate the ampacity of the conductor?, the reason I ask is that THHN is only shown on the 90*c column, and according to article 110.14 C. you should use the 60*c columb if the conductors are smaller than #1 AWG

Thanks,

Cieba
 
resistance

resistance

Senior Member
Location
WA
Generally speaking, the 90 degree C column should not be used for sizing circuit conductors [Conductors should be sized in accordance with the lowest temperature rating of any terminal, device, or conductor of the circuit]. But, if the insulation has a higher temp rating, it will give you the opportunity of having a larger conductor ampacity. Always use the 60 degree column if taking a test--unless other info is given to do other wise.

If using the 90 degree column (not taking a test): 30-based on 310.16(.91-correction factor, with a 90 degree F temp)(.80 adjusted for four or more conductors)=21.84A or 22A

Taking a test: 25(.91)(.80)= 18.20A
 
C

cloudymacleod

Senior Member
in acticle 240-6d, it says unless specifacally permitted,the overcurrent protction cant exceed 15a for 14 awg, 20a for 12awg,and 30a for 10awg. so wouldnt you use 20aas your amperage rating in the 60c column?
forexample: 20a x .80 x.91=14.56 or 15amps?
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
cloudy,
Artice 240 has nothing to do with the ampacity of a conductor. It only covers overcurrent protection. The question is about the ampacity and you start with the 90?C column. If the result is greater than 20 amps for a #12, then the small conductor rule in 240 will limit you to a 20 amp OCPD.
Don
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
cloudymacleod said:
in acticle 240-6d, it says unless specifacally permitted,the overcurrent protction cant exceed 15a for 14 awg, 20a for 12awg,and 30a for 10awg. so wouldnt you use 20aas your amperage rating in the 60c column?
forexample: 20a x .80 x.91=14.56 or 15amps?
Click to expand...


You would not use 240.4(D) at the beginning of your calculation you would use it at the end. Looking at the OP you would take the conductor and find it's ampacity #12 THHN=30 amp, then find it's derating percentage based on the # of CCC's in the conduit, 4=80%, then adjust for the 97 degree ambient temperature, =.91. Do the math:

30(.80)(.91)=21.84amps

Now 240.4(D) comes into play. Since the derated value is greater than the values listed in 240.4(D) we must use the value of 20 amps (240.4(D))for sizing the OCPD.
 
Status
Not open for further replies.
Top