I have to run conduit from a fiberglass well above a diesel tank to a leak detection panel and I'm not sure if or where to install a seal off. Is diesel a flammable gas or vapor? I don't think so.But I look at 501.5 F3 and wonder if this is a service connection.Also, do I need an explosionproof j-box in the well? Any comments appreciated, Thank you.

Seal off goes on the first thread entering the sump or first thread out of the ground.
Diesel fuel shouldn't need Exp Prf stuff although leak detection manufacturer probably wants seal off.