NEC cable rating vs terminal

ventilator · Mar 5, 2013

I want to get some opinions on this because I'm confused. Here is the situation:

550 HP motor at 480v, using 650FLC for calculations

Feeder breaker is 100% rated 800A CB

motor feeder cable is rated for 90deg C

Feeder breaker and motor terminations rated at 75deg C

Customer spec is use 35deg C for ambient temp correction

Neglecting voltage drop (distance unknown at this point) and assuming only 3 conductors

What size wire is needed for feeding the motor?

First, by NEC 430.22 we need 125% of motor FLC for the conductor which is 650*1.25=812a uncorrected cable minimum

With our cable at 90deg C, a 500MCM cable is good for 430a. Now correcting for 35deg ambient, 430*.96=412a per cable rating.

So using the corrected cable ampacity of 412a, a paralled run of 500mcm is enough ampacity to carry the 812a required.

Now the limiting factor of the terminations only being rated for 75deg C is where I am getting confused.

The site engineer who is a PE is saying we can use the 500mcm cables because even though we have to use the 380a rating of 75deg rated cable, the 125% of NEC 430.22 doesnt apply at 75dg since he already accounted for it in the sizing at 90deg values which doesnt make any sense to me.

I think we need to use parallel 600mcm in order to achive the min uncorrected ampacity requirement of 812 amps. NEC 110.14© allows for 90deg C to be used for correction but also states it cannot exceed the rating of the lowest rated termination. To me this means that 500mcm is good for 760a only, regardless of cable type used.

Sorry for the long post, any help/clarity is appreciated.

pgm44 · Mar 5, 2013

The way I read the code is as follows. Sorry there is not a simplier way that I see to answer the question.

NEC 110.14© allows the 90deg ampacity rating to be used for adjustment of the cable ampacity provided that the resulting ampacity after adjustment is not greater than the rating of the termination rating for the cable. In this case the 75deg rating of the cable is 380A so after all adjustments the rated current through the cable cannot exceed 380A because of the 75deg terminal ratings. This allows use of the 90 degree rating for temperature adjustment, or adjusting to derate for multiple conductors, but per NEC 110.14(1)(b)(2) the resulting ampacity after the calculations are performed is not allowed to exceed the ampacity at 75 degrees.

In the case of this example, NEC 430.22 requires the cable to be sized at 125% of the full load current which equals 812.5A at 480V. The NEC 430.22 handbook note clarifies that the 125% conductor size requirement is to accommodate for a sustained running amperage in overload conditions and is not an amperage associated with conductor derating. Assuming a parallel run the cable must at minimum be sized to carry 406.25A after derating per NEC 430.22. Since the maximum amperage after derating for a 500kCM cable is 380A at 75 degrees per NEC 110.14(1)(b)(2) it is not large enough to serve the load the way that I read the code. A 600kCM cable in parallel would is rated at 420A at 75degrees and 475A at 90degress prior to derating. After derating for 35degrees the 600kCM cable is rated for 385A at 75degrees which is not large enough to meet code for this application. Since the 600kCM cable is 90degree rated when the temperature factor is applied the result is a 456A rating at 90 degrees. Per NEC 110.14(1)(b)(2) since the 90 degree rating is above the 75 degree rating for the cable the adjusting rating is equal to the maximum rating at 75 degrees which is 420A. Since 420A is greater than the needed 406.25amps, a 90 degree rated 600kCM cable would meet the code requirement for this example as I read the code.

Hope this helps. If the site engineer interprets this differently, please let me know the reasoning. There are many different ways to interpret the code so I do like to hear other perspectives. This one looks clear to me.

ventilator · Mar 6, 2013

Thanks for the reply, that's the exact same thought process and answer I came up with. The only reasoning the other eng gives for being able to use 500s is that he builds his 125% into the calculation for the 90deg C cable and the 125% doesn't apply to the terminations at 75deg because table 310.15(B)(16) is only for cable rating, not terminations which sounds like a pretty big gray area to me.

pgm44 · Mar 6, 2013

NEC 110.14(1)(b)(2) and NEC 110.14 specifically require that the calculation limit the ampacity to the lowest rated component of the connection, including the termination. Any sustained amperage that is above the maximum rated amperage of the termination (cable-lug combination) will exceed its rating and violates this section of the NEC as I read it per this section. If you put in a 1200 amp cable but installed it with a 500 amp splice in the middle to extend the cable length the run would be rated for 500 amps not 2,000A just like the termination at the lug limits the rating.

I agree with your interpretation and your reasoning. I would not view this as a gray area. If asked to review a failure of this example cable run which resulted in injury or equipment damage I would expect all segments of the run to be rated for the sustained current as described by code and would view components or connections not meeting this rating as a design deficiency. Items such as this can have significant liability attached if a problem were to ever happen. I think your thought process on this one is good and probably worth another tactful discussion with the site engineer.

smkh · Sep 15, 2014

pgm44 said:
NEC 110.14(1)(b)(2) and NEC 110.14 specifically require that the calculation limit the ampacity to the lowest rated component of the connection, including the termination. Any sustained amperage that is above the maximum rated amperage of the termination (cable-lug combination) will exceed its rating and violates this section of the NEC as I read it per this section. If you put in a 1200 amp cable but installed it with a 500 amp splice in the middle to extend the cable length the run would be rated for 500 amps not 2,000A just like the termination at the lug limits the rating. I agree with your interpretation and your reasoning. I would not view this as a gray area. If asked to review a failure of this example cable run which resulted in injury or equipment damage I would expect all segments of the run to be rated for the sustained current as described by code and would view components or connections not meeting this rating as a design deficiency. Items such as this can have significant liability attached if a problem were to ever happen. I think your thought process on this one is good and probably worth another tactful discussion with the site engineer.

Hi,

I'm going to get the PE exam on next Oct, and I'm wondering if I can ask you some questions regarding the exam?

if you don't mind, let us communicate via emails,

here is mine, [email protected]

if you have any advices, it will be highly appreciated,

regards