Good heavens ... a little secret here ... the linesmans pliers and wire strippers you use are NOT UL listed.
The presence of a UL lable on the can could even be misleading. There's no UL standard for conduit sealing compounds, so any UL listing would relate to its' use as a fire stop, flame spread, or some other use not related to this thread.
Code just says you need to seal the conduit - and is silent as to how. There's no listing requirement, or even an 'approved method' required. You could use old bubble-gum and still be legal.
Still, we are expected by Article 110 to protect the wires from damaging chenicals, etc. So, maybe it's not so crazy to ask 'will this stuff hurt my wires?'
To answer that, we can look to the NEMA studies that explored expanding foams and non-metacallic cable (Romex). Tere were no issues found; foam and the PVC jacket of the cable got along just fine. Since the in sulating part of THHN is also a PVC compound, it's safe to say the foam won't hurt anything.
Please note that we're still expected to arrange the conduit to drain, etc. This might even mean doing things like installing drain holes, and other actions not considered by UL when they rated the various components.
