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Sizing Cable for motors #102090 01/04/05 02:11 PM
Joined: Dec 2004
Posts: 2
J
jaw1 Offline OP
Junior Member
For a 460V 10HP Class B TEFC motor. The 90 degree branch cable to installed in cable tray random fill and over 43 cables in the tray for other motors.
1. The cable in tray will be de-rated 50%, do we need to de-rate further to achieve 125% of full load current?
2. Do we need to de-rate cable even more because of the requirements the ampcity of the conductor entering connection box must be based on 75 degree C?

[This message has been edited by jaw1 (edited 01-05-2005).]

[This message has been edited by jaw1 (edited 01-05-2005).]

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Re: Sizing Cable for motors #102091 01/05/05 08:50 PM
Joined: Feb 2003
Posts: 231
R
RobbieD Offline
Member
460V 10HP 3phase motor
CEC Table 44 FLC-14Amps I went with this because I can't see the nameplate.

Supply conductor ampacity must be not less than 125% FLC of motor.
14 A x1.25= 17.5 A
I am not sure about your description of the tray “Random” so I will provide two answers.
#1-In ventilated and ladder-type cable trays, where the air space between conductors, cables, or both is maintained at not less than 25% nor more than 100% of the largest conductor or cable diameter
I will assume more than 6 cables horizontal and 2 layers.
CEC 12-2210(2) Common multiplier from Table 5D is 0.74
17.5A / 0.74 = 23.6 A
So you need a conductor that holds at least 23.6 A so #10 awg
*This does not take into account ambient temperature or voltage drop.
#2-In ventilated and ladder-type cable trays, where the air space between the conductors, cables, or both is less than 25% of the largest conductor or cable diameter
CEC 12-2210(3) Common multiplier from Table 5C is 0.50
17.5A / 0.5 = 35A
So you need a conductor that holds at least 35 A so #8 awg
*This does not take into account ambient temperature or voltage drop.

I am not sure what you mean about the conductor ampacity must be based on 75deg? These conductors in the cable tray are going to a controller for this motor I am assuming. Please explain.(give code rule) Maybe I am missing something.
Thanks

[This message has been edited by RobbieD (edited 01-05-2005).]

[This message has been edited by RobbieD (edited 01-05-2005).]

Re: Sizing Cable for motors #102092 01/06/05 02:25 PM
Joined: Dec 2004
Posts: 2
J
jaw1 Offline OP
Junior Member
Random fill means two layers or more and no spacing maintained.
Regarding 75 Degree C
CEC rule 28-104(1). Because motor is class B & TEFC, cable rating has to be 90 Degree C and ampcity based on 75 Degree column of table 2

Re: Sizing Cable for motors #102093 01/06/05 05:57 PM
Joined: Feb 2003
Posts: 231
R
RobbieD Offline
Member
Okay,CEC 28-104 is talking about the actual wire comming from the controller to the motor connection box. That I understand, I thought that the wire in your question was going to a controller and not the motor so its class wouldn't matter and you wouldn't even have to look at table 37.
Okay, your talking about the wire going to the motor connection box.

Class B- Table 37 says 90deg, you said you are using that. Where does it state that you have to use the ampacity for 75deg on table 2??

[This message has been edited by RobbieD (edited 01-06-2005).]

Re: Sizing Cable for motors #102094 01/07/05 11:28 AM
Joined: Jan 2003
Posts: 174
B
bigrockk Offline
Member
Robbie D,
If you read 28-104 carefully it states the wire ampacity (Supply conductors to a motor connection box) is to be based on 75deg C conductor insulation except for Class A rated motors.
Its really not much of an issue though, if you check out table 2 based on the 2 scenarios you posted above, conductor size for either 75deg or 90 deg are the same. There is no difference in the two columns until you get to #3 or larger conductors

Re: Sizing Cable for motors #102095 01/07/05 08:00 PM
Joined: Feb 2003
Posts: 231
R
RobbieD Offline
Member
Thanks bigrockk! I now see it.

Re: Sizing Cable for motors #102096 04/05/05 10:01 PM
Joined: Apr 2005
Posts: 5
M
Malcolm Brown Offline
Junior Member
Page 468 Appendix has a great picture of what rule to apply where, I had some spare time on my hands so I wrote a little excel program that will calculate all the condutors for you. (you can change all the stuff in yellow). Let me know if you find any bugs. Download it from the following site. http://www.raylan.com/malcolm/MotorCalculator.xls

Malcolm


Can't find your shorts call me.
Re: Sizing Cable for motors #102097 04/06/05 10:17 AM
Joined: Jan 2003
Posts: 174
B
bigrockk Offline
Member
Welcome to the forum Malcolm!
Nice job on the calculator, will defiantly "play" with it and report any bugs to you.

Re: Sizing Cable for motors [Re: jaw1] #214709 01/14/15 02:35 PM
Joined: Jan 2015
Posts: 1
R
robair78 Offline
New Member
For Canada, you should generally use 75 degree C wire column in table 2 according to rule 4-006 if the equipment is only rated for that. This is the common rating for starters, so must use 75deg C column for motors even when installing widely used 90 C wire. This is also addressed in section 28-104.

I agree with the sizing of cables using Table 2 and 5c for de-rating when installed in tray with other cables to determine de-rated copper cable ampacity. Then using 1.25 x FLA for sizing.

Example: A motor FLA of 25A x 1.25 = 31.25A and if installed in a tray with more than 14 other cables that have 3c each, you need to de-rate cable from 75degC column by 0.5 so 6AWG is 65 x 0.5 = 32.5A is adequate. This does not account for voltage drop or ambient temperature above 30C.

Anyone care to comment on this - I'd appreciate any feedback if I've overlooked something. Sizing cables this way will often be a challenge to terminate the conductors but most starters should handle the cable and with the correct compression lugs or ring terminals it should be ok at the motor too - worth a look before you pull a cable.


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