Loading of Power Cables 1

[ThePunisher]

HI all, I understand that when cable sizing (in our case, based on CEC) is being used, the cables are sized based on 125% of rated load current. In our case, it is a 15 kV VFD power cable, 350 MCM, with published manufacturer ampacity of 435A and the indoor ambient temperature of 30 deg C, the ampacity remains 435 A.

The motor drawn continuous current was originally 317A. Hence the cable was originally 73% loaded (lower than 80%) or the cable ampacity is 137%.

The motor operating point changed which required it to draw 380A. Now the cable is already loaded to 380/435 = 87.4%. Technically it should be ok but do I need CEC variance as I already exceeded 80% loading? Since, this is medium voltage application, the supply equipment and protection should have the capability to be 100% load rated.

Your comments is appreciated.

 

[dpc]

In the US at least, the input feeder to a VFD should be sized per the VFD input rating, not the motor amps.

 

[ThePunisher]

Thanks dpc, I am actually referring more to the VFD output cable connected to the motor itself.

 

[ThePunisher]

The cable is already existing and I am not intending as a base case to replace it to maintain 80% loading on the motor cable

 

[waross]

28-106 Conductors — Individual motors
(1) The conductors of a branch circuit supplying a motor for use on continuous duty service shall have an
ampacity not less than 125% of the full load current rating of the motor.
Note: The criteria is the full load current rating of the motor, not the measured running current.

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

 

[jraef]

Yes, same here with the NEC.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[7anoter4]

In my opinion, you are speaking about NEC 2014 Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable.
Since you have 6 supply conductors you have to apply 80% adjustment factor indeed.

 

[ThePunisher]

HI Zanoter4, I am referring to this cable:

 

[waross]

Check Table 2 or table D17N for the ampacity of your cable.

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

 

[ThePunisher]

The manufacturer provided my with a guaranteed ampacity (based on air, well ventilated cable (no touching of cables) and at 40 Deg C ambient.

Our design ambient is also 40 Deg C and the cable is on ventilated tray indoors with one cable diameter spacing. There will be no de-rating (Derating Factor = 1).

My motor will be operated CONTINUOUSLY at full load (4,000 HP) at 380 A. Hence, my "new full load current" based on continuous operation at 35Hz (not 43Hz) now is 380A. This will equate to 380/435 = 87.4% loading on the cable.

But hey, the cable WAS originally compliant to CEC which maintains 80% loading based on the original 317A at 43 Hz operation. Since this is a EXISTING cable, I would think I do not have to necessarily maintain 80% (equating to 125% x FLA, where the new FLA now is 380A).

 

[EdStainless]

Your cables were undersized to begin with, note the post by Waross, the sizing is based on MAX motor draw, not operating conditions.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[dpc]

OK, sorry I misunderstood which cable you were referring to. For the motor feeder cable, I agree with waross - 125% of the motor full load amps. The original cable was too small per the code.

 

[7anoter4]

In OP you said:
"indoor ambient temperature of 30 deg C, the ampacity remains 435 A."
If it is 30 deg.C indeed you may use a 1.1 factor and so you'll get 478.5 A [more than 1.25*380=475 A]

 

[ThePunisher]

DPC, I would not think the original cable was small as per code. The motor FLA is only 317 A and the cable ampacity at 40 Deg C ambient, conductor rating of 90 Deg C is 435 A (125% x 317A = 396.25). If my operating ambient is 40 Deg C, then the temperature derating is 1.0. We maintained one conductor spacing between the MV cables in the cable tray.

Operations is thinking of operating the motor CONTINUOUSLY at 350 RPM at 5.6 kV at 4,000 HP (full load output) and hence, we got it tested and obtained 380A as the operating current.

The motor nameplate FLA at 4000 HP, 6.9 kV, 43 Hz....however, I would think that this is now the new FLA at 35 HZ.

So I would think that I have to verify the cable ampacity against this 380 A current.

If I am evaluating the continuous current capability of the EXISTING CABLE, do I need to maintain 80% loading on the cable after deration? I think the NEC applies 125% x FLA...then selects the appropriate cable size based on ampacity....and then after the cable size and ampacity is selected, the derating factor is applied and THEN COMPARED IN SUCH A WAY THAT IT SHOULD BE GREATER THAN THE FLA...not necessarily GREATER THAN THE 125% OF FLA. In such case, you may not need to maintain the derated ampacity to be > 125% FLA after deration.

 

[wroggent]

In the NEC I do not think the 125% factor applies to medium voltage motors. See 430.224. Not sure about CEC.

 

[waross]

So the motor is rated at 317 Amps and you are going to run it at 380 Amps, about a 20% overload. Don't worry about the cable. You have bigger potential problems.
The motor limit is the current, not the HP.
4000 HP at 43 Hz = 3256 Hp at 35 Hz.

28-306 Rating or trip selection of overload devices (see Appendix B)
(1) Overload devices responsive to motor current, if of the fixed type, shall be selected or rated or, if of the
adjustable type, shall be set to trip at not more than the following:
(a) 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater; or
(b) 115% of the full load current rating of a motor that does not have a marked service factor or where
the marked service factor is less than 1.15.

So the overloads must be set at 115% of 317 Amps or 365 Amps.

You will have an overloaded, overheated motor. The lifespan of the motor will be reduced.
The overload setting needed to run 4000 HP at 35 Hz will be in violation of the code.
You may have worse problems than cable size.


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

 

[VTer]

If you are operating the motor at constant flux (V/Hz) how is your current going up that much for the same load? Also, this is a VFD operation so THD plays a role in addition to just the fundamental current. NEC also has section for motors over 600V nominal. In short, you have to look at the thermal damage curves of conductors and motor and size your system and protection appropriately for both overload and short-circuit conditions.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[7anoter4]

I agree with wroggent: NEC art. 430.224 Size of Conductors- states :
Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device(s) is selected to trip.
430.225 Motor-Circuit Overcurrent Protection. (B) Overload Protection says:
"Protective device settings for each motor circuit shall be determined under engineering supervision”. That means no NEC requirement.
In my opinion, at first the rated current has to be stated.
Let's say the rated voltage is 15 kV[delta connection] and the rated current 317 A.
If the rated voltage will be 15/sqrt(3)=8.66 kV [star instead of delta] efficiency 0.973 and 0.9 pf then the current [at 60 Hz] will be 227.17 A and at 43 Hz will be 227.2*60/43=317 A.
That means- as waross already said-317 A is actually the rated current.
If the motor operation regime is not continuous you may reach 380 A. For example:
Let's say the load period is 60 sec where 40 sec 380 A and 20sec 104.6 A-33%of 317[no-load motor current] then allowable current will be 316 A:
I=sqrt((40*380^2+20*104.6^2)/60)=316.1 A
That means 317 A is still actually the rated current.
Now you have to now what is the start current [may be 1.5] then 1.25 is still good.
Let's say the stall time is 10 sec and start time is 5 sec then -I say-1.25*317 A and 7 sec may be the overload setting.

 

[HCBFlash]

Rating. Rating. Rating. 125% Full load rating.

Ratings are used to stop these snowballs. If you're having trouble with things meeting"the letter of the law" you've got to really stick to all those "letters". Ratings and definitions create more heat than locked rotors!

.


Me wrong? I'm just fine-tuning my sarcasm!

 

[waross]

VTer and 7anoter4. Thank you for the information. I am always interested to learn more about the NEC and this is new to me. (I don't normally work with or to the NEC.)
The OP stated that he was working under the CEC. I can't find any special treatment for high voltage motors in the CEC.
Some motors may be overloaded at any speed by feeding more product into the driven machine.

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