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nightfox1925 (Electrical) (OP)
28 Apr 08 11:57
Table 44 of CEC 2006 Part I and the NEC Motor FLA Table are similar and they indicate Full Load Amps for various HP and voltage ratings. Note 4 of these tables indicates that for 90% and 80% power factor, 1.1 and 1.25 are to be multiplied to the FLA values in the tables. My question is, at what %PF BASE or %EFFICIENCY BASE are the values on the table derived from. I am trying to create a typical motor FLA database and hoping to derive them from Code tables as part of my conductor sizing calculations...Any comments?

I would appreciate if somebody could share me a typical FLA table for various voltages which I can conservatively use aside from the Code tables as an option. Thanks

Helpful Member!  waross (Electrical)
28 Apr 08 13:29
Read the fine print. Note 4 applies to synchronous motors.
This would be for a synchronous motor rated for use at a power factor other than unity to supply VARs to the system.
That is, a motor may be rated at 500 HP, unity PF, or rated at 400 HP, 0.8 PF. (Or the 500 HP motor may be used at 0.8 PF provided that the mechanical load does not exceed 400 HP.
Note, in the CEC this table is a guide, and the motor nameplate data is used for conductor sizing.
In the NEC, the table is used rather than the motor nameplate.

Bill
--------------------
"Why not the best?"
Jimmy Carter

nightfox1925 (Electrical) (OP)
28 Apr 08 13:45
Thank you for the explanation waross. What if the project is at its design stage and we have not ordered any motor and what we have are just the motor HP and voltage (460VAC, 3phase). For estimating the cable size for individual motors, is this CEC table FLA conservative enough? I am trying to estimate the Power Factor based from a published client minimum motor efficiency. I was intending to utilize the FLAs stated in the CEC Table 44 and work on backwards with the voltage and client published efficiency to get the power factor...for purpose of design, is this going to be technically acceptable on the design stage?

The power factor calculated will also going to be used for the voltage drop calculation which is:

Vdrop = 1.732 x FLA x L x (RcosY + XsinY) where Y = angle

I just noticed using the published client efficiencies and CEC FLAs, I am getting low power factor values as low as 78% for a 60HP at an efficiency of 92.4% to get a CEC FLA of 77A @ 460VAC.

nightfox1925 (Electrical) (OP)
28 Apr 08 15:26
By the way, I got a G.E. motor TEFC, GE value line motor with published FLA and Nom Efficiency wherein the efficiencies published are similar to the ones indicated in our client's standards. The link is:

http://www.geindustrial.com/publibrary/checkout/Brochures|DEAM-0377|generic

(see section 1.9)

The motor application is mostly Class 1, Div 2, NEMA Design B and TEFC.

Any further comments and suggestions are higly appreciated.

controlsdude (Electrical)
28 Apr 08 16:56
conveyor industry typical, worked at diff manu, same sop

Usually the OEM selects typical motor brands (non VFD, and VFD rated) , motor brand manufacturer gives up the lists giving FLA eff and HP.  Use this lists for the FLAs, eff, and hp.  

If customer requests different, mechanical will usually pass this info during turnover.

we usually have two or three different lists of FLA motor brands.

I do not start on a project until mechanical is done

Why would you worry about PF in calculating wire sizes?   
waross (Electrical)
28 Apr 08 18:01
I understand that the NEC lists the current for typical motors.
By using typical current values, you avoid the possibility of a high efficiency motor being replaced with a low efficiency motor on conductors that may be undersized.
I would suggest looking at some motor catalogs and picking a current value for each size of motor that is in the upper part of the current range. In many instances the wire size won't change and you will have adequate capacity in the event that a motor is changed at a later date and the only motor available has a lower efficiency than the original design motor.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

DickDV (Electrical)
28 Apr 08 19:00
Further, the NEC charts seem to me to be for standard efficiency motors, such as those that were common before the EPACT rules came out.

With today's high efficiency and especially the premium efficiency motors, actual FLA's are a bit and sometimes more than a bit lower than the NEC amps for the same motor.
Helpful Member!  dpc (Electrical)
28 Apr 08 19:06
The NEC requires that conductors be sized per the table values rather than the actual motor nameplate amps, unless the nameplate amps are higher (which is rare for motors covered in the table).  It wasn't always this way, but it is now.

So in NEC land, sizing motor conductors is generally cut and dried.   
Helpful Member!  rbulsara (Electrical)
28 Apr 08 21:26
Why reinvent the wheel when Code books have done the hardwork? What guarantee does alternate method would provide that it leads to some sort of a "foolproof" table?

There are only two alternatives, either use Code tables or use acual value on the nameplate. Anything else is conjecture and waste of time.  Plus using anything less than what Code suggests would only be a liability as dpc pointed out.
nightfox1925 (Electrical) (OP)
29 Apr 08 9:40
Thanks for all the comments. Rbulsara, I understand that the Code tables are conservative. However, it does not indicate at what motor efficiency and power factor those amps where dervived from. In my voltage drop calculation, the power factor is part of the calculation. Moreover, for consistency, I will also be using the same motor FLA, efficiency and power factor for my load calculation to derive kW, kVAR and kVA separately. I place typical motor efficiencies to these Code FLAs but it was giving me an unrealistic power factor (i.e. 78.9% for 60HP motor running at 92.4% efficiency at FLA=77A@460V???). Any suggestions for this?  

dpc (Electrical)
29 Apr 08 10:50
The point is that the conductors must be sized per the table regardless of the actual motor full load current.   So if the purpose of your calculation is to determine the motor conductor size and you must meet NEC, there is really nothing to be gained.  

If you are trying to compute motor kW, kVA, etc, then I would use typical data from two or three manufacturers and take a worst case or an average.  


 
nightfox1925 (Electrical) (OP)
29 Apr 08 11:24
Thank you for the point dpc. Conductor sizing based from code FLA values and load sizing based from typical manufacturer motor data.

I will revise my calculation and provide notes of the basis of the FLA and other motor parameters. For ampacity check, the Code FLA values will be in use.

Now, going to the voltage drop. If I have both resistance and reactance values for the conductors in ohms per 1000', the the motor operating power factor is required and in this case I will assume 80%. However, I also see Table 44 of the CEC code note 4 (which might be a similar note in the NEC)indicates that a multiplying factor of 1.1 and 1.25 will be used on the code table FLAs for 90% and 80% power factor respectively, hence a 10HP motor at 460VAC with a Code FLA of 14A should be 17.5A for a power factor of 80%. If I am going to assume PF=80% for my Vdrop calculation, then I ahve to use 17.5A as FLA. Is this going to be way over-rated?

waross (Electrical)
29 Apr 08 14:07
Are you using synchronous motors?
Note 4 applies to synchronous motors only. 60 HP seems small for a synchronous motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

nightfox1925 (Electrical) (OP)
29 Apr 08 14:41
Oh well, I think my caffeine this morning wasn't working right! Thanks for the reminder waross.

I will use the code FLA for my induction machines regardless of power factor and efficiency. For the Vdrop calculation, I will assume a conservative PF=80%.

Thank you very much for all the supports and comments.

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