At the plant I work for, they have always used type THHN wire for everything except underground applications. But I see that 110.14 requires referencing wire ampacities at 60 degrees C, for 100A and under, and 75 degrees C, for over 100A (with some motor exceptions). Since there is no ampacity rating for THHN wire below 90 degrees C, would these installations be in violation.
Motor Wire Type
- Thread starter derek22r
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- Chapel Hill, NC
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- Retired Electrical Contractor
derek22r said:
Thhn wire is rated 90C but cannot be terminated at 90C because the terminals and breakers are not rated 90C. Most breakers and terminals are rated 75C. Thus when you use 90C wire the ampacity of it is to used at the 75C rating.
Find the wire size and use the 75C if you are using thhn.
bphgravity
Senior Member
- Location
- Florida
Probably not. The thhn conductors simply cannot be loaded beyond the temperature ratings of the listing and marking of the motor and conductor termination devices/equipment. In short, the thhn conductors cannot be loaded to 90 degrees, but 75 and/or 60 depending on your particular equipment.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
You would use the 90C for derating purposes only.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
You are permitted to operate THHN conductors at 60C. See 310.10; you are not permitted to _exceed_ the temperature rating of the insulation.
THHN has an insulation rating of 90C. But if the terminations mean that you can only use the 60C ampacity, then you simply calculate ampacity at the termination to remain below the 60C conductor temperature limit.
Study 310.15 and 310.16. The temperature values used in table 310.16 are not the temperature rating of the insulation, but instead the design temperature of the conductor. If you use the 60C column, you are saying 'at the amp rating given by the table, the conductor will heat up to 60C'. As long as the conductor temperature is less than both the insulation limit and the terminal limit, then you are fine with regard to ampacity.
-Jon
THHN has an insulation rating of 90C. But if the terminations mean that you can only use the 60C ampacity, then you simply calculate ampacity at the termination to remain below the 60C conductor temperature limit.
Study 310.15 and 310.16. The temperature values used in table 310.16 are not the temperature rating of the insulation, but instead the design temperature of the conductor. If you use the 60C column, you are saying 'at the amp rating given by the table, the conductor will heat up to 60C'. As long as the conductor temperature is less than both the insulation limit and the terminal limit, then you are fine with regard to ampacity.
-Jon
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This article can be a little confusing...it was to me until I read it about ten times.derek22r said:
You only have to do this if the temperature rating of the equipment (terminals, etc) is unknown.
Look up some past posts to clarify. There was one just about a month ago that should clear things up.
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