feeder size for moters

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bob52

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Location
pittsfield ma
having a little disagreement in the office in regards to the feeder size for a 150 hp motor with FLA of 183 amps, I say 2/0 my coworker says 4/0 hope someone agrees with me there is a large wager of coffee on the line. Thanks for your help
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Smart$ is correct. Go get the coffee.
For reference so that you don't loose the whole pot, 430.6 notes that for general applications you use the values in Tables 430.247-250 rather than the nameplate value for conductor sizing.
That can make a difference.
Your reference for the 4/0 requirement is 430.22
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
having a little disagreement in the office in regards to the feeder size for a 150 hp motor with FLA of 183 amps, I say 2/0 my coworker says 4/0 hope someone agrees with me there is a large wager of coffee on the line. Thanks for your help

125% of 183 Amps is 229. I don't see how 2/0 is even close. In any case, you have to use the charts and not the NP FLC values as another poster mentioned.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
If the cables- three single-core cables-run in cable tray as per art. 392.80 Ampacity of Conductors. (A) Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays. (2) Single-Conductor Cables.
(c) Where single conductors are installed in a single layer in uncovered cable trays, with a maintained space of not less than one cable diameter between individual conductors,
the ampacity of 1/0 AWG and larger cables shall not exceed the allowable ampacities in Table 310.15(B)(17).
Table 310.15.(B)(17) 2/0- 265 A 75oC- or 300 A 90oC insulation rated.
Where is my coffee?
:bye:
 
First things first

First things first

A number of questions to answer before even considering using the ampacity tables:

What is the ambient temperature of the area where the starter or drive is located, where the motor is located, and along the entire route the installed cable will be run?

Does the cable installation route include a run across a roof with sun exposure?

Is the cable spec for single or multi-conductors?

Are the cable(s) going to be installed in a raceway and/or underground? If not, what is the wiring method (e.g. Uncovered ladder cable tray)?

If the ambient temp is 40degC and you are using multi-conductors, you would both be incorrect, and might have to flip a coin to determine who buys the coffee.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
A number of questions to answer before even considering using the ampacity tables:

What is the ambient temperature of the area where the starter or drive is located, where the motor is located, and along the entire route the installed cable will be run?

Does the cable installation route include a run across a roof with sun exposure?

Is the cable spec for single or multi-conductors?

Are the cable(s) going to be installed in a raceway and/or underground? If not, what is the wiring method (e.g. Uncovered ladder cable tray)?

If the ambient temp is 40degC and you are using multi-conductors, you would both be incorrect, and might have to flip a coin to determine who buys the coffee.
Correct.I agree!
 

Smart $

Esteemed Member
Location
Ohio
If the cables- three single-core cables-run in cable tray as per art. 392.80 Ampacity of Conductors. (A) Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays. (2) Single-Conductor Cables.
(c) Where single conductors are installed in a single layer in uncovered cable trays, with a maintained space of not less than one cable diameter between individual conductors,
the ampacity of 1/0 AWG and larger cables shall not exceed the allowable ampacities in Table 310.15(B)(17).
Table 310.15.(B)(17) 2/0- 265 A 75oC- or 300 A 90oC insulation rated.
Where is my coffee?
:bye:

A number of questions to answer before even considering using the ampacity tables:

W
hat is the ambient temperature of the area where the starter or drive is located, where the motor is located, and along the entire route the installed cable will be run?

Does the cable installation route include a run across a roof with sun exposure?

Is the cable spec for single or multi-conductors?

Are the cable(s) going to be installed in a raceway and/or underground? If not, what is the wiring method (e.g. Uncovered ladder cable tray)?

If the ambient temp is 40degC and you are using multi-conductors, you would both be incorrect, and might have to flip a coin to determine who buys the coffee.
Now coordinate all that with 110.14(C) and tell me the minimum size conductor... :D
 
Now coordinate all that with 110.14(C) and tell me the minimum size conductor... :D

Ambient temp. and cable spec are always my first questions. They dictate the ampacity table selection. Other factors (e.g. raceway, wiring method) become multipliers.

The use of the 90degC rated cable column should ONLY be used if the motor AND motor control equipment are rated for 90degC, which is not standard.

Above 100A, 110.14(C) allows for the cable ampacity selection column for 75degC rated cables.

At 40degC, using multi-conductor cable, and without a cause for any additional derating, I would size the cable for 3/C 350kcmil + #3AWG GND (.88 x 285A = 250.8A @ 75degC), which is greater than 180A x 1.25 = 225. Unfortunately, 250kcmil = .88 x 255 = 224.4A, not that the AHJ would catch that. It (0.6A) would also not be a cause for concern for me regarding fire prevention, unless the motor was an industrial motor than runs continuously (like a paper machine headbox fan pump). But to design 'by the book' ....

If it was only one calculation, and one table lookup, I'd be out of a job.;)
 

Smart $

Esteemed Member
Location
Ohio
And the list goes on....:). The duty cycle should also be considered, although continuous duty should be the default assumption.

Ambient temp. and cable spec are always my first questions. They dictate the ampacity table selection. Other factors (e.g. raceway, wiring method) become multipliers.

The use of the 90degC rated cable column should ONLY be used if the motor AND motor control equipment are rated for 90degC, which is not standard.

Above 100A, 110.14(C) allows for the cable ampacity selection column for 75degC rated cables.

At 40degC, using multi-conductor cable, and without a cause for any additional derating, I would size the cable for 3/C 350kcmil + #3AWG GND (.88 x 285A = 250.8A @ 75degC), which is greater than 180A x 1.25 = 225. Unfortunately, 250kcmil = .88 x 255 = 224.4A, not that the AHJ would catch that. It (0.6A) would also not be a cause for concern for me regarding fire prevention, unless the motor was an industrial motor than runs continuously (like a paper machine headbox fan pump). But to design 'by the book' ....

If it was only one calculation, and one table lookup, I'd be out of a job.;)
I just noticed you are new here... so let me first welcome you to the forum. :thumbsup:

That said, asking you to coordinate with 110.14(C) was more a hint than a directive... :happyyes:

What it amounts to is, under 110.14(C) you determine the minimum SIZE conductor. Other factors enter the determination of AMPACITY. With just the information given in the OP (original post; first post of this thread), coordination with 110.14(C) clearly eliminates 2/0 as a contender, as 4/0 copper is the minimum SIZE. Your posts clearly indicate not enough information was provided to determine AMPACITY.

Stick around. We'll help you streamline your wire-sizing paradigm. :D
 
I just noticed you are new here... so let me first welcome you to the forum. :thumbsup:

That said, asking you to coordinate with 110.14(C) was more a hint than a directive... :happyyes:

What it amounts to is, under 110.14(C) you determine the minimum SIZE conductor. Other factors enter the determination of AMPACITY. With just the information given in the OP (original post; first post of this thread), coordination with 110.14(C) clearly eliminates 2/0 as a contender, as 4/0 copper is the minimum SIZE. Your posts clearly indicate not enough information was provided to determine AMPACITY.

Stick around. We'll help you streamline your wire-sizing paradigm. :D

Hey There. I understood that it was not a directive:). I'm not too busy today, so I thought I would respond in more detail. Thanks for the welcoming words.
 
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