Motor conductors

[tomito]

Hello guys, when you are sizing motor conductors... you size them like 215.2(A)(1)? continuous load (1.25) "or" conditions of use?
or... continuous load (1.25) "and" conditions of use?

Thanks
Tom

 

[don_resqcapt19]

The conditions of use determine the ampacity of the conductors. Those always apply and where the conditions of use are outside those specified in 310.16, the ampacity must be adjusted and/or corrected to match the actual conditions of use. The adjusted and corrected ampacity is the actual ampacity of the conductor.

In general the motor conductors must have an ampacity at least 125% of the full load current found in Tables 430.247 through 430.250.

 

[tomito]

The conditions of use determine the ampacity of the conductors. Those always apply and where the conditions of use are outside those specified in 310.16, the ampacity must be adjusted and/or corrected to match the actual conditions of use. The adjusted and corrected ampacity is the actual ampacity of the conductor.

In general the motor conductors must have an ampacity at least 125% of the full load current found in Tables 430.247 through 430.250.

Yes, you are right, but my doubt came because 1.25 of all motor loads must be considered continuous loads by the note on 430.22(E) and 215.2(A)(1) (for feeders) permits the largest of continuous loads "or" correction or adjustment factors.
So, in motors, you have to consider the continuous load of the motor "and" the conditions of use?

 

[wwhitney]

Motor conductors sized under the basic rule in 430.22 already have a 1.25 factor applied, so there is no need to add an additional 1.25 factor for "continuous use".

Then once you come up with the 430.22 size, you need an unadjusted/uncorrected 75C ampacity (assuming 75C terminations) of at least that value. And you also need an adjusted/corrected 90C ampacity (assuming 90C conductor insulation) of at least that value. Two separate checks; use whichever size is larger.

Cheers, Wayne

 

[tomito]

Motor conductors sized under the basic rule in 430.22 already have a 1.25 factor applied, so there is no need to add an additional 1.25 factor for "continuous use".

Then once you come up with the 430.22 size, you need an unadjusted/uncorrected 75C ampacity (assuming 75C terminations) of at least that value. And you also need an adjusted/corrected 90C ampacity (assuming 90C conductor insulation) of at least that value. Two separate checks; use whichever size is larger.

Cheers, Wayne

Ok, lets assume I need to wire a 100 hp motor, 480 V, table 430.250 shows 124 A.
It's going to be installed on a place with a 105–113 °F temperature (table 310.15(B)(2)(a) = 0.87) and I will use THW-2 copper wire (90 °C)
124 A (1.25 (continuous load)) = 155 A, this the minimum ampacity of my wire
I choose #2/0 wire that has an ampacity of 195 A on the 90 °C column
195 A (0.87) = 169.65 A > 155 A so the 2/0 wire is adequate.
Is this calculation ok?

 

[wwhitney]

Ok, lets assume I need to wire a 100 hp motor, 480 V, table 430.250 shows 124 A.
It's going to be installed on a place with a 105–113 °F temperature (table 310.15(B)(2)(a) = 0.87) and I will use THW-2 copper wire (90 °C)
124 A (1.25 (continuous load)) = 155 A, this the minimum ampacity of my wire
I choose #2/0 wire that has an ampacity of 195 A on the 90 °C column
195 A (0.87) = 169.65 A > 155 A so the 2/0 wire is adequate.
Is this calculation ok?

Yes, although incomplete. The other check is that the 75C table ampacity of 2/0 Cu is 175A, which is greater than 155A, so that's OK. Which of course is how you got to 2/0 Cu in the first place, as 1/0 Cu has a 75C table ampacity of 150A. So just spelling this out for thoroughness.

Cheers, Wayne