# Cable sizing for 3 Phase Induction Motor



## backstay (Feb 3, 2011)

I calculated your FLA at 28. #10 would make it, but depending on wire length and the application , I'd go with #8.


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## splatz (May 23, 2015)

backstay said:


> I calculated your FLA at 28. #10 would make it, but depending on wire length and the application , I'd go with #8.


Does that 28A include the power factor correction?


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## Electrical-EE (May 4, 2017)

*Formula & procedure*



backstay said:


> I calculated your FLA at 28. #10 would make it, but depending on wire length and the application , I'd go with #8.


Could you please share the formula and procedure?


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## splatz (May 23, 2015)

I'll take a stab, someone else can check this ... 

P = EI 
1 hp = 745.7 watts 
watts/VA = 0.8 

15 hp * 745.7 watts / hp = 15 hp * 745.7 watts / hp * 1 VA / .8 watts = 400 * I 
(15 * 745.7 / .8 ) / 400 = 34.9546875 = I 

rounds to 35 amps.


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## DesignerMan (Jun 13, 2008)

No FLA on the motor nameplate? That's what should be used...


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## Dennis Alwon (May 9, 2009)

I believe you would need to divide by 400 *sq. rt 3= 692 because it's 3 phase


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## Dennis Alwon (May 9, 2009)

For conductor size you nrrd to multiply by 1.25

I get 20 amps * 1.25= 25 amps

15 * 745.7= 11185 Watts

11185/692 = 16 amps

16 / PF (.8) = 20 amps

20 amps *1.25 = 25 amps..


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## micromind (Aug 11, 2007)

A Baldor #EM2333T-58 15HP 400 volt 50HZ 1470RPM motor has a full-load current of 21 amps. 

A Leeson #LM32691 15HP 200/400 volt 60HZ 1760RPM motor has a full-load current of 21.8 amps @ 400 volts.


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## Dennis Alwon (May 9, 2009)

DesignerMan said:


> No FLA on the motor nameplate? That's what should be used...


The Nec states that the tables at the end of Article 430 shall be used to determine conductor size.

So for a 15 HP 3 phase 460 Volts we get 21 amps -- now you have to interpolate to get what it would be for 400V . I would be somewhere around 26 amps then you would multiple by 1.25 for conductor size = 33 amps


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## JRaef (Mar 23, 2009)

The thing is, if dealing with 400V, that means it is NOT going to be somewhere that applies to the NEC, so you would need to defer to whatever rules apply to the destination. Most likely 400V is going to be a country that adheres to IEC 60364 rules, which then will not use AWG wire sizes, they will be in millimeters squared (cross sectional area). There are various tables available to give you equivalent AWG sizes to mm2 sizes, but I'm not sure what their standards say about sizing with regard to motor power ratings (which would be in kW, not HP anyway). In this document there is a table that gives you the equivalencies, but this is done as a guide for people in IEC countries sending equipment to US, not the other way around. So it's not going to discuss IEC 60364 rules for conductor sizing.
http://literature.rockwellautomation.com/idc/groups/literature/documents/at/ic-at001_-en-p.pdf

IEC 60364 is way too complicated for me to digest at first glance and since I have never had to, I haven't tried digging into it.


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## Dennis Alwon (May 9, 2009)

Well, you may be correct about the NEC and Turkey but the op did post in an NEC forum. BTW, I believe there are some countries which use the NEC. I know that Japan and Korea use their version of the NEC but I am not sure about Turkey


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## telsa (May 22, 2015)

Dennis Alwon said:


> Well, you may be correct about the NEC and Turkey but the op did post in an NEC forum. BTW, I believe there are some countries which use the NEC. I know that Japan and Korea use their version of the NEC but I am not sure about Turkey


Turkey adopted virtually every American code standard -- going way back.

However, strangely, Turkey hires women to enforce its building codes. Yup, It's deemed strictly women's work.

And there are VERY few inspectors for the entire nation. Last I read, not even 25 !

So, for all practical purposes, Turkey is the Wild Middle East.


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## Dennis Alwon (May 9, 2009)

Thanks for the update on the NEC probably being used in Turkey.

IMO, It is not a matter of who inspects it but rather a matter of what the code states. I also wanted to make a point to the rest of the members that for conductor sizing we use the tables not the nameplate as one member suggested.



> 430.22 Single Motor. Conductors that supply a single motor
> used in a continuous duty application shall have an ampacity of
> not less than 125 percent of the motor full-load current rating,
> as determined by 430.6(A)(1), or not less than specified in
> 430.22(A) through (G).





> 430.6(A) General Motor Applications. For general motor applications,
> current ratings shall be determined based on (A)(1) and
> (A)(2).
> (1) Table Values. Other than for motors built for low speeds
> ...


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## backstay (Feb 3, 2011)

Electrical-EE said:


> Could you please share the formula and procedure?


Using this site https://www.engineeringtoolbox.com/electrical-motor-calculator-d_832.html and the numbers you gave. It comes up with RLA 22.4 and FLA of 28

Then you can plug that number (28 ) into this calculator http://electrician2.com/calculators/motor_ver_2.html And you get the NEC wire size of #10.


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## paulengr (Oct 8, 2017)

You do NOT use name plate FLA for cable sizing. By Code everywhere you use a table which is a worst case value that works for all motors of a certain size. Since it is 400 v and probably 50 Hz NEC or CEC Part [ can’t be used. Use the local Code. The 125% factor is an NEC/CEC “fudge factor” that oversizes the wire to account for starting current.

Finally you can’t size the wire without knowing where it goes. If for instance it is in a cord to the motor that gets buried underground or runs into a conduit with a bunch of other energized wires producing heat, or you are starting and stopping the motor a lot, the wire size has to be increased for thermal reasons. Wire sizing can potentially be very complicated so without a lot more information it can’t be done.


Sent from my iPhone using Tapatalk


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## Electrical-EE (May 4, 2017)

*Local codes*



paulengr said:


> You do NOT use name plate FLA for cable sizing. By Code everywhere you use a table which is a worst case value that works for all motors of a certain size. Since it is 400 v and probably 50 Hz NEC or CEC Part [ can’t be used. Use the local Code. The 125% factor is an NEC/CEC “fudge factor” that oversizes the wire to account for starting current.
> 
> Finally you can’t size the wire without knowing where it goes. If for instance it is in a cord to the motor that gets buried underground or runs into a conduit with a bunch of other energized wires producing heat, or you are starting and stopping the motor a lot, the wire size has to be increased for thermal reasons. Wire sizing can potentially be very complicated so without a lot more information it can’t be done.
> 
> ...


Yes my friend I see every manufacturer has made his local codes. It is fun part bu dangerous as well.:jester:


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