Bulk of my work involves relocating circuits out of MSP into a backup LC. Strictly THHN, no NM. When doing a back-to-back equipment install, I connect boxes with metallic running thread conduit cut to length instead of having to manufacture a PVC nipple; when doing side-by-side equipment mounting I typically use a gutter box for circuits. To plumb into MSP I use a malleable iron bushed elbow in wall to connect to surface mount gutter box. Elbow has male threads one end, female threads on other end. My question is, why can't I find running thread conduit or elbows constructed out of PVC? Both are in a wall where not subject to physical damage. Surely PVC is cheaper. Electrical PVC has high temp rating. Using a PVC fitting would negate the need for a ground bushing between MSP and gutter box. Any ideas?