Feeder cable size

[James L]

I understand your argument, but I interpret 310.16 to be a absolute maximum ampacity allowed for conductors based on temperature rating. You're right that it does not specifically state that temperature correction could not potentially increase the ampacity, but I think the intent was for the correction factors to derate conductors under higher than normal operating temperatures.

I could be wrong.. I would put in a request for clar on this in the next code cycle.

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If that's the case, why have adjustments going upward?

Why the multiplier of 1.05 and lower temperatures?

 

[wwhitney]

But at the bottom of that table, it directs us to 310.15(b) to make corrections for ambient temperature.

If you want to be literal, "shall be referenced" doesn't say to apply them, just where to find them if you need them. 110.14(C) doesn't mention using them. And this is standard for terminations, look at the difference between 215.2(A)(1)(a) and (b).

Cheers, Wayne

 

[DrSparks]

If that's the case, why have adjustments going upward?

Why the multiplier of 1.05 and lower temperatures?

It's a good point. I gave you that. I really wouldn't consider it worth the argument with my inspector but definitely worth pursuing with the code panel.

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[don_resqcapt19]

Well for residential 1 fam, it's 400 MCM.

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True, but only because the calculated load for dwelling units in Article 220 results in a service 50% to 100% larger than actually required.

 

[wwhitney]

Terminations are rated by temperature, and are directly determined by conductor temperature relative to ampacity, actual current, and ambient temperature

Here's an argument for why temperature correction shouldn't apply for equipment, like it does for conduits

For conduits, the conduit is in an ambient temperature; the only source of heat not reflected in the ambient temperature is the Joule heating in the wires. So ambient temperature adjustment via tables can reasonably model the actual situation. [Not a perfect model, as the ability of the environment to carry away heat may vary independently of temperature, but a reasonable model.]

Equipment has other sources of internal heat. The ambient temperature outside the equipment is going to influence the internal temperature at various points, but the effect is not so simple. The correction factors in the tables may not be correct. Instead the equipment manufacturer is going to specify allowable conditions for installation of the equipment, including ambient temperature. They have the ability to do the thermal modeling and can set the maximum ambient temperature appropriately. So the NEC just specifies the baseline sizing for terminations; no field ampere rating adjustment is appropriate for that.

Cheers, Wayne

 

[DrSparks]

True, but only because the calculated load for dwelling units in Article 220 results in a service 50% to 100% larger than actually required.

I figured that. Everything is overkill for residential.

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[DrSparks]

True, but only because the calculated load for dwelling units in Article 220 results in a service 50% to 100% larger than actually required.

I figured that. Everything is overkill for residential.

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