flexible cable questions (NEC Article 400)

[antigfk]

I have a few NEC code questions that I have not been able to have answered to my satisfaction yet, and I am hoping some of you code experts can help me out.

Background:

My company routinely uses #4/0 AWG single-conductor type-W cables and smaller (#6, #4, #2 AWG) multi-conductor type-SOW cables for our portable, temporary power distribution setups. This may include temporary generator(s) powering our own portable distribution systems which in turn power our portable utilization equipment such as air conditioners, air compressors, heaters, cooling towers, etc. Many times we provide power to our customer’s distribution systems in the event of an outage (such as after hurricanes, or in the event of a main transformer failure at a plant), and many times we tie in our portable generators directly to the electric utility to in the event of heavy power demand, or in the event that utility peaking generators fail. All of the installations are performed by qualified personnel.

Questions:

In the above cases, does the NEC permit the use of flexible cables (addressed in article 400) without the use of attachment plugs? For example: Using multiple runs of type-W cable to tie a portable, temporary 1000kW generator to a MCC main breaker at a refinery… with the type-W cable being terminated on 75 degree C lugs on both ends

If yes to question #1, would we still need to apply the requirements of 110.14(c), and hence the ampacity of the circuit must not exceed the rating of the 75 degree C lug based on table 310.16 (of the 2008 NEC), even though the rating of the flexible cord may be much higher? For example: Using a type-W #4/0AWG cable with a 75 degree C lug on both ends and assuming no correction factors for ambient temperature and # of current-carrying conductors in a raceway… the cable ampacity would be 405A, but the termination ampacity would be 230A, so the ampacity of the circuit would be limited to 230A?

I would be extremely happy if you could answer these questions or even point me in the right direction.

 

[EricAXEWC]

As I'm sure your qualified personnel are aware NEC Article 590 Temporary Installations would cover most of these situations.

I'm not sure where you are getting your numbers. Looking at Table 400.5(A)(2) in the 2011 NEC for single conductor 4/0 Type W the 90C ampacity is 405A. The 75C ampacity is 360A.

Anyways, per 110.14(C)(1)(b) you can only use the 90C temperature rating for adjustment or correction and can only use this calculated ampacity if it does not exceed the 75C rating (or "up to their ampacity if the equipment is listed and identified for use with such conductors" - I've found this is usually not applicable, typically people want to use 90C ampacity on 75C equipment).

You also need to confirm the temperature rating of whatever you are connecting to, it's possible it is only rated 60C and probably is not listed to 90C. RTFM

See the attached bulletin from Square D as well, it explains all this nicely.

 

[antigfk]

So article 590 allows the terminating of flexible cable using lugs instead of using an attachment plug.

Assuming using a type-W #4/0awg cable and terminated at a 75 degree lug on a breaker: The 90 degree C rating of the cable is 405A, but per 110.14(c) the ampacity of the termination must be based on the 75 degree C column which would be 230A. This is my understanding of the NEC and the square-D article you posted seems to confirm it. I just don't see how you could actually use the 405A or 360A ratings of the #4/0awg type-W if terminating at a standard 90 degree or 75 degree C lug. Am I missing something?

 

[EricAXEWC]

I don't see any indication in 590 that you aren't allowed to terminate using lugs as long as the disconnection and overcurrent protection is all in line. As I read it you should be allowed to terminate with lugs. That said it could come down to what the AHJ is comfortable with.

Now I understand the rest of your question more clearly.
You need to know if an inspector will allow you to use Table 400.5(A)(2) instead of Table 310.15(B)(16) (formerly 310.16).

From a safety standpoint I think you should be allowed to do this, although I do see the issue here. I'm assuming there is language in the code that allows this but a brief search didn't turn up anything concrete.

I've asked an inspector I know both of these questions to see if he could offer some explanation, waiting on a response.

 

[antigfk]

Eric,

In the mean time I have asked several professional engineers and some inspectors, and they told me that nothing escapes the requirement of 110.14(c) in that the ampacity of the circuit is limited by the ampacity of the termination which is based on the rating for the particular wire size of table 310.16 (2008 NEC).

When I asked these questions, I didn't have the 2008 NEC Handbook. The handbook specifically answers my question and gives an example. If you have the handbook, read the commentary at the end of 110.14 and at the beginning of 400.5. In the example, they use a type-W 3-conductor #3/0awg and use the ampacity of table 400.5 for ampacity correction, but that corrected ampacity was greater than the ampacity of a #3/0 cable terminated at a 75 degree C lug, so the circuit is limited to 200A from table 310.16 of the 2008 NEC.