Wire sizing

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tonyjz

Member
Good day. I have completed a ICC webinar and the instructor was doing wires calculations. He stated that
you may decrease your wire size by using 90c conductor on 75c equipment. I was taught and checked in the code book this can't be done. If all equipment is rated 75c you stay with that column. Have i missed somethig??

Example; using a #8 not a #6 as the 8 is rated higher at 90c.
 

G._S._Ohm

Senior Member
Location
DC area
Good day. I have completed a ICC webinar and the instructor was doing wires calculations. He stated that
you may decrease your wire size by using 90c conductor on 75c equipment. I was taught and checked in the code book this can't be done. If all equipment is rated 75c you stay with that column. Have i missed somethig??

Example; using a #8 not a #6 as the 8 is rated higher at 90c.

It may not be code compliant, but

for a T ambient of, e.g., 40 C
75-40 = 35
so
if I-squared-R [= watts dissipated in the wire] causes a 35 C temp rise in a conductor,
then for a 90-40 = 50 C temp rise it would take a larger I-squared-R.

If I and therefore I-squared is the same in both cases, then the R can be higher [smaller wire size].
The new R can be 50/35 ~ 1.43x the old R.

BTW, the R per foot of #8 and of #6 exceeds this ratio.
 
Last edited:

iwire

Moderator
Staff member
Location
Massachusetts
Good day. I have completed a ICC webinar and the instructor was doing wires calculations. He stated that
you may decrease your wire size by using 90c conductor on 75c equipment. I was taught and checked in the code book this can't be done. If all equipment is rated 75c you stay with that column. Have i missed somethig??

Example; using a #8 not a #6 as the 8 is rated higher at 90c.

Is it possible you misunderstood them?

The reason I ask is because for purposes of derating you can often start in the 90C column.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
I agree with Iwire and think there may be some missunderstanding. I think your instructor may be refering to 110.14(C)

C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

Roger
 
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