motor calculation question

[joe j]

What minimum size THWN cu conductors are required to supply a continuous duty 25 hp, 208 v, 3 phase motor? The motor is on the end of a short conduit run that contains only 3 conductors at an ambient temp of 50 deg. C. Consider the motor terminations to be rated for 75 deg.C. Full load current rating of the motor is 74.5 amps from table 410.250 and as per 410.22 multiply by 125% =93.5 amps. The temp correction factor found in 310.15(B)(2)(a)is 0.75 -------MY question is do you divide by the temp correction factor and why?

 

[Smart $]

What minimum size THWN cu conductors are required to supply a continuous duty 25 hp, 208 v, 3 phase motor? The motor is on the end of a short conduit run that contains only 3 conductors at an ambient temp of 50 deg. C. Consider the motor terminations to be rated for 75 deg.C. Full load current rating of the motor is 74.5 amps from table 410.250 and as per 410.22 multiply by 125% =93.5 amps. The temp correction factor found in 310.15(B)(2)(a)is 0.75 -------MY question is do you divide by the temp correction factor and why?

You would divide 93.5 (circuit ampacity, CA below) by 0.75.

The ambient correction factors (ABF below) apply to table ampacities (TA, below), so its simply a matter of which side of the equation it is on.

TA ? ABF ≥ CA
TA ≥ CA ? ABF

 

[joe j]

So if I understand this????

So if I understand this????

When looking to find the allowable amps of a wire size with an ambient temp change you would look for total amperage in table310.15(B)(16) then multiply that by the ambient correction factor in table 310.15(B)(2)(A) to find the corrected circuit amps of the wire size TA X ABF = CA When looking to find the wire size for a circuit with an ambient temp change when you know the circuit amperage you would divide that amperage by the ambient correction factor in table 310.15(B)(2)(A) to find corrected amperage CA ? ABF = TA

 

[Smart $]

When looking to find the allowable amps of a wire size with an ambient temp change you would look for total amperage in table310.15(B)(16) then multiply that by the ambient correction factor in table 310.15(B)(2)(A) to find the corrected circuit amps of the wire size TA X ABF = CA When looking to find the wire size for a circuit with an ambient temp change when you know the circuit amperage you would divide that amperage by the ambient correction factor in table 310.15(B)(2)(A) to find corrected amperage CA ? ABF = TA

There are some caveats that enter the picture when using the term "circuit ampacity", but you have the gist for this part of it.