MC Cable/Free Air

[Guanac]

Hi! A contractor installed MC cable, enclosed in ceilings, walls and vertical ducts. They sized the wires according to Annex B Table B310.3, for MC cable on free air. Is that correct? Can those locations be considered free air? It is no placed on a cable tray, just BLine supports every 6ft.
Also, since that table is based on a 40C ambient temperature, they say can use the multipliers on the bottom of that table to increase the ampacity of the MC cables.

For example, they say average temperature is 21-25C, so they multiplied the Table B310.3 by 1.2.

I have the following concerns:

An MC Cable indoors/enclosed can be sized according to free air table?
Why use the tables on the Annex B and not the ones on 310?
If they use the 21-25C multipliers, what happens if actual ambient temperature is 33?

Thanks!

 

[Dennis Alwon]

Annex B is not to be used unless under engineers supervision. If the mc is in the walls, ceiling etc then it cannot use a free air table

 

[Guanac]

Thanks Dennis. What's your opinion on using the adjusted ampacities for ambient temperature?

 

[ActionDave]

What is the contractor using to justify walls and attics as "free air"? No insulation in the building perhaps?

 

[Cavie]

That's why they make A/C contractors and Electricial contractors. Is the A/C contractor a licensed electrician?

 

[Dennis Alwon]

The question of ambient temp can be difficult especially if there is a big difference in temperatures. The NEC allows using the ambient temperature as long as it is accurate. In most case the ambient temp may vary but if it is consistent then that should not be an issue.

You stated that suppose the ambient temp was 33 instead of 21-25-- what makes you think that is the case. They should know the ambient temp before applying the value. If there is a chance it will be higher then I would use the higher value.

 

[Guanac]

Dennis, what I meant is that their argument is that the average ambient temperature for the whole year is 22C, but there are a couple of months during the year where the average is 33C. In my opinion they should use that higher temperature.

Regarding free air, there are exposed MC cables, supported by hangers inside the electrical room, they consider that free air. I dont really consider that to be free air, but since there is no definition, how to convince the contractor?

Regards.

Alberto