1.25x + adjustment factors for more multiple conductors in cable tray?

[majesus]

This is a cable sizing question. This really has to do for a design that must meet CEC, but I wouldn't mind feedback from the NEC folks as well, I do like to understand their methods too...

I have a bunch of continuous motor feeds that will go in a cable tray. These are for motors sized from 1HP up to 75HP. However, I do not know how many cables will be placed in this cable tray. I'm assuming it will be random filled and I'll derate the cables using the adjustment factors for more than 41 current carrying conductors . In the NEC this would be an adjustment of 35% (Table 310.15(B)(2)(a)), in the CEC this would be 50% (Table 5C).

If I had a continuous 50HP motor at 575V with a FLA at 50A. Will I pick the cable based as follows:
50Ax1.25x(adjustment factor) OR can I omit the 1.25% at this point because I'm sizing the cable larger than the 1.25 factor?

I would say no because the 1.25 factor is not really a de-rating but compensation for an overload motor condition and must be considered when adjusting the ampacity for the random filled cable-tray.

However, I'm not sure the logic/reasoning behind sizing conductors for transformers. Say I have a lighting transformer located somewhere in the field and again I have to feed it with a cable via the same cable-tray routing. Would I size the primary conductor as follows:
FLA*1.25*(adjustment factor), or can I omit the 1.25x?
In this case, I'm not really sure. The 1.25x was due to the fact that the primary breaker had to be derated for 80% for continuous use, so I'm in doubt if you need to keep the 1.25%.

Much appreciated,
Majesus

 

[majesus]

I want to bump this.

 

[rbulsara]

If you mean 'remove, by bump, you can red flag your own post.


Rafiq Bulsara

http://www.srengineersct.com

 

[majesus]

No, after 5 days, I wanted to return my question back up to the top.

 

[TurbineGen]

Bump means bring it up to top post.

I believe the NEC does not allow you to disregard the 25% adjustment factor when you have already given it a cable tray derating of 35%. The reason is from heating. If you are running the motor at the continuous rating with the 35% adjustment factor for the heating of other wires in the cable tray, what happens in an overload condition? You cannot assume the motor will fail before the cable. Thus the cable must be sized FLA X 1.25 X 1.35 = cable Ampere capacity.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[majesus]

TurbineGen, That's my logic too. But I can't explain a reason why we whould keep the 25% adjustment factor for transformer feeds (relatively small transformers typically used for lighting and misc 120V loads... 30kVA upto 75kVA with 120/240V secondary.)

Thanks for the feedback.
Maj

 

[rbulsara]

ah..thanks for clearing the bump. I hate use of slangs.

majesus:

The 125% sizing and derating are independent code requirements. You have to meet 125% requirement, but it also helps while determining the derating.

The derating applies after you have selected cables for individual circuits (it is an iterative process until you get the right ampacity). While calculating the derating you can use the listed ampacity of the conductors. Meaning, say the calculated load is 80A and you have selected cables rated 100A, they are already 20% derated.

The key factor with multiple circuit conductors is the diversity factor. For example, if there are five circuits, individually rated 100A each and only 3 of them may run simultaneously (or all of them may not run at full load all the time), the effective loading is only ((3/5)*80*5)/100*5=0.48 or 48%.

Do not forget to check for the voltage drop, another independent criteria. Any up sizing of individual circuit conductors will also help when considering the derating of the group of conductors.

Hope this helps.

I am not sure why you question the sizing of transformer cables only, when they are no different than sizing cables for other continuous loads. Also if you use a larger breaker to accommodate a transformer's inrush, the conductor ampacity must meet the rating of the breaker as well.




Rafiq Bulsara

http://www.srengineersct.com