Motor wire size

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roc1

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Lubbock Tx
One thought I have.The code says you can size conductors by the 90 deg chart for Thhn as long as you derate and for terminations not to exceed the 75 deg chart over 100 amps.I say this because if you size a motor wire,you multiply by 125% to get amp rating of wire and then by NEC you go by 75 deg chart.Why can you not size by 90deg as long as load is not in excess of 75 deg chart?Ex 200hp 460 volt three phase motor FLC 240 amps.Wire size by charts Square D for example is 350mcm.The FLC x 125% is 300amps.By 310.16 chart 300mcm 90deg is rated at 320 amps derate by 75deg is 285 which is more than the 240FLC of motor?I am just curious I know if wire is underground then it is not rated at 90deg but 75deg.I was wondering why you can not derate this like a service when wire ampacity is same no matter what.
Thanks
roc1
 

iwire

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Massachusetts
It may sound like I am being picky but the fact is in your example you are not 'derating' the conductors.

The wire terminals will be 75 C so the ampacity of the conductor is that shown in the 75 C column regardless of the fact you may have 90 C condutors...it's not a 'de-rate' it is the rating.

The 125% required for motors is not a 'de-rate' it simply requires that the conductors be sized 25% larger than the FLA for the motor.
 

roc1

Member
Location
Lubbock Tx
Thanks for answer I guess what I am saying is a motor wire size by 90 deg chart should be ok even though the termination is limited to 75deg?The 300mcm is still large enough by either? Why is chart showing 350mcm even on short run?
Thanks
roc1
 

infinity

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Location
New Jersey
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Journeyman Electrician
The 300 kcmil conductor is rated for 285 amps. The motor requires a conductor rated for a minimum of 300 amps. Your 300 kcmil conductor is too small. 350 kcmil would have an ampacity of 320 amps which is OK since it's larger than the required minimum of 300 amps. Forget about the 90 degree conductor ampacity. In this application it has no use. The conductors are sized according to their 75 degree ampacity.
 

gwpowell

Member
I have never seen an industrial termination rated at 90 degrees C, therefore the 75 degrees C rated ampacity must be used even if THHN or equivalent wire is used. The only use for the 90 degrees C rating in this example is for temperature correction if the wire is in an ambient above 30 degrees C.

There was an excellent wire sizing article published in the March 1998 issue of EC&M titled The Middle Of The Wire Isn?t Its End. Obviously the code references are outdated, however the basic procedure is still valid.
 
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