Flexible metal conduit

I see in 348.12 that FMC is allowed in wet locations if the conductors are approved…

I see a lot of exterior hvac units with the FMC but can not be sure if the correct conductor insulation type is present.

Some have THHN and some have stranded copper with no easily visible markings.

Anyone have any advice on this?


Virtually all THHN is dual rated THWN, stranded or solid

I was told the actual item that turns 60C insulation into 90C insulation is the clear outer jacket. Also the jacket serves the purpose to make it water tight, thus making it THWN as well.

Now don’t quote me on this, just that I was told this. :wink:


Had to edit this, not ‘water tight’ but part of it being ‘liquid resistant’ Had clear up what I was told, since Greg reminded me. :wink:

That is not the way I have heard it. I understand the outer clear jacket is not even necessary to get to the listed rating. It is just there to lower pulling tension and to provide oil resistance. Maybe Pierre has more on this. Logic says all of the insulation would have to be capable of handling the 90c. As for the “W” part, when you use dual rated conductors in a wet location it assumes the THWN rating of 75c. That can become important when you have a lot of wire in the raceway or when you are running over a rooftop.

Here’s your rule of thumb… if you see individual conductors pulled in that FMC, and they have plastic insulation, chances are 99% that they are conductors with a “W” in the nomenclature (wet location). If you see braided conductors or NM cable pulled in that FMC, that would be worthy of calling out.

Dreadlocks or pig-tails?


I guess I did open that door, didn’t I? Braided fabric insulation. :neutral: