Ambient air temps for LV and MV cables

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diakonos1984

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I posted this back in January, and didn't get a response. The question has come up again, so I'm posting again...

Can anyone tell me why the base level ambient air temperature for LV cables is 30 degrees C (Table 310.16,17) but the base level ambient air temperature for MV cables is 40 degrees C (Table 310.67-76)?

I know there are also 40 degree C tables for LV cables, and I know you can derate either one, but does anyone have a logical explanation for why we have different bases?

:-?
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Additionally, what is your typical practice in an installation with BOTH LV cables and MV cables? Do you just stick with the base ratings in the tables, or do you standardize on one ambient temperature for both?

Thanks!
 
Wow... denied again! Only question in three days with no replies.

Is this a dumb question, a hard question, an inappropriate question, or am I just the only one who cares?
 
Thanks Derek, that makes me feel a little bit better. And maybe I need to post this in the Electrical Calculations/Engineering forum, but I'm really more interested in what others do for rating cables in an installation with both LV and MV cables, than I am in the history/science of why it's that way in the NEC (though history and science are always good).
 
Just a guess, but maybe it has something to do with insulation thickness. MV cables have a lot more insulation than LV.

As for which base to use when using both types, I believe you would need to derate each one individually on its own base.
 
@wirenut1980

I follow you, and that's what I expected. I just don't like it...

So let's say I'm here in Oklahoma, where today's high is 111F :eek:hmy:, and the ASHRAE tables list 98F as the design temperature, which is pretty close to 40C.

I am installing MV and LV cables in the same facility. I can use Table 310.73 with no de-rating for my MV cables, but I've got to de-rate all my LV cables by 88% from Table 310.16! Do you all really do that?!?!

It seems like when they wrote the code they should have used the same base for everything. :slaphead: Then I either de-rate ALL my cables, or none! Maybe this is where the history/science lesson comes in.

What I'm really hoping for is someone to say,
Naw, you're making this too complicated! Unless there are special circumstances (such as these are being installed in a volcano), just use 40C for MV cables and 30C for LV cables and here's the science why that's okay: (insert impressive science lesson)
Click to expand...

I will wait patiently in the corner. :dunce:
 
I can offer a hint from Annex B, Section B.310.15 (B)(2); it references the original Neher-McGrath paper and several subsequent Standards. While the ultimate physics are still the same, various product standards play to the strengths/weaknesses of the products involved. For example conductor voltage stress still exists on low voltage conductors but the insulation?s mechanical strength is adequate to overcome it ? so you don?t see a section on shielding for common building wires in the product standard.

LV cable standards just ain?t the same as MV for reasons that you would need to have the various cable standard making bodies address. NOTE: sometimes the original reasons are so buried in history that even the current standard making committees can no longer tell why something is ?standard.? However, while the NEC may definitely affect a product standard, it is just as often the other way around.
 
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