THHN size to motor termination

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Jpflex

Jpflex

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A question In my book for licensing prep ask:

Determine the MINIMUM size THHN copper conductors required to supply a 3 hp, 240 volt, single phase continuous duty motor when terminations have a rating of 75 degrees Celsius.

Book says answer is (B) 12 AWG

Motor FLC = 17 amperes
17i x 1.25% table 430.22 = 21.25 amperes
(I’m good at this point)



But then study book says, Size 12 AWG THHN is rated at 25 amperes at 75 degree Celsius (Table 310.15 (B)16)



NO IT IS NOT!!!

Nec table 310.16 for not more than 3 current carrying conductors in a raceway shows 12 AWG wire at 75 degree Celsius good for 25 amperes

and 14 AWG!!! THHN wire rated 90 degrees Celsius For 25 AMPERES!! But not 12 AWG THHN as study book suggest

Therefore we should be selecting 14 AWG THHN rated 90 degrees if using THHN at higher temp rating and book is WRONG???

I’m still not getting temperature terminations apparently?
 

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The answer is #12. Looks like they gave the wrong section for the conductor ampacity table unless you're on the 2020 NEC.
 
infinity said:
The answer is #12. Looks like they gave the wrong section for the conductor ampacity table unless you're on the 2020 NEC.
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I could not afford 2020 test prep book so I have 2017 edition cheaper on eBay. However, I do have 2022 NEC code book though. Is this error in year book?
 
Jpflex said:
However book says using THHN conductors which are rated 90 degrees Celsius but not 75 degree Celsius
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That is not correct. THHN conductors have ampacity ratings at 60°C, 75°C and 90°C. The use of either the 60 or 75 degree ratings is based on the temperature rating of the equipment. Other than some very rare cases, the 90°C ampacities are only used for ampacity adjustment and/or ampacity correction.
 
You need to look at 110.14 where the Code addresses termination limitations.
 
I
don_resqcapt19 said:
That is not correct. THHN conductors have ampacity ratings at 60°C, 75°C and 90°C. The use of either the 60 or 75 degree ratings is based on the temperature rating of the equipment. Other than some very rare cases, the 90°C ampacities are only used for ampacity adjustment and/or ampacity correction.
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I looked under table 310.4 (A) conductor applications and insulation’s under 600 volts and THHN does not show other temperature ratings from 90 degrees whether in wet or dry locations.

Some other insulations other than THHN do show a range of terminating temperature values
 

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Jpflex said:
I looked under table 310.4 (A) conductor applications and insulation’s under 600 volts and THHN does not show other temperature ratings from 90 degrees whether in wet or dry locations.

Some other insulations other than THHN do show a range of terminating temperature values
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Temperature ratings of insulation are maximum ratings. The head of the column in Table 310.4(A) is "Maximum Operating Temperature." Some insulation types, such as THHW, show multiple temperatures in that column because the maximum operating temperature depends on environmental conditions, like wet location vs dry location.

So while THHN has a 90C rating on its insulation, the circuit as a whole includes both the wire and its terminations. When the terminations have a 75C limit, that means the overall circuit ampacity is limited to the uncorrected 75C ampacity, even those the THHN itself could possibly carry more current without being damaged.

Cheers, Wayne
 
augie47 said:
wwhitney said:
Temperature ratings of insulation are maximum ratings. The head of the column in Table 310.4(A) is "Maximum Operating Temperature." Some insulation types, such as THHW, show multiple temperatures in that column because the maximum operating temperature depends on environmental conditions, like wet location vs dry location.

So while THHN has a 90C rating on its insulation, the circuit as a whole includes both the wire and its terminations. When the terminations have a 75C limit, that means the overall circuit ampacity is limited to the uncorrected 75C ampacity, even those the THHN itself could possibly carry more current without being damaged.

Cheers, Wayne
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Jpflex said:
I

I looked under table 310.4 (A) conductor applications and insulation’s under 600 volts and THHN does not show other temperature ratings from 90 degrees whether in wet or dry locations.

Some other insulations other than THHN do show a range of terminating temperature values
Click to expand...
The reason some of the other conductors show more than one temp. (such as THHW) is they have a wet & dry rating *90°dey, 75°wet) whereas THHN, without the "W" is not listed for wet. The temperatures in that chart are limiting factors based on insulation. After you check that you need to check 110.14 for limiting factors based on termination.
 
In this example below it is said to use a 1/0 THHN 90 degree Celsius rated at 170 amperes (Table 310.16) For a 150 ampere load terminating at 75 degree Celsius

This goes against NEC 110.14 C

which states a conductor with its temperature rating associated with its ampacity is not to exceed the lowest temperature rating of any termination.

So a 170 ampere 1/0 THHN rated 90 degrees goes over 150 ampere limit at 75 degree rated wire, so IT SHOULDN'T BE Allowed

Instead if a THHN is to be used it should be a 1 AWG at 145 amperes because this is UNDER 150 ampere limit for comparable 75 degree wire. My book tells me that amps are to be equal or under ampacity of 75 degree column but never over as this will cause greater heat build up
 

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You are reading it backasswards. If you need a conductor capable of carrying 150 amps at 75° a #1 rated 130 amps would be insufficient;
it would need to be a 1/0 rated 150 amps. (If you wanted to use 500kcmil, it's permissible.. any conductor having 150 rating or greater)The fact that it has a higher 90° rating is irrelevant as far a termination.
If you were derating due to fill, ambient, etc. then you use the 90° rating. For practical applications, the only time you use the 90° rating ios for derating.
 
Jpflex said:
In this example below it is said to use a 1/0 THHN 90 degree Celsius rated at 170 amperes (Table 310.16) For a 150 ampere load terminating at 75 degree Celsius

This goes against NEC 110.14 C
Click to expand...

#1/0 is the correct conductor size for 150 amps because it's ampacity is 150 amps at 75°C.
 
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