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I want to run grey PVC schedule 40 conduit down the wall from my breaker panel outside and through an 18 inch deep trench back to my hot tub. I'll terminate the conduit at a Cantex box at ground level where we will hook in the disconnect and whip for power.

Is that OK by NEC code?

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In most jurisdictions the "down the wall outside" part needs to be schedule 80 as being "exposed to damage." So does the "up to the disconnect box" part. In-between, schedule 40 is fine, just do a careful job of backfilling with fine material (sand not rocks) all around the conduits - don't dump big rocks on them and break them.

The trench needs to be deep enough for the TOP of the conduit to be 18" below finished grade.

Other than that, you may need (or want) expansion joints so that soil movement with frost does not break the conduit at the house and disconnect - or you may live nearer the Equator and say "what's frost movement?"

Also, since you didn't say what you were planning on running in the conduit - and many people lacking experience with conduit make errors - you'll need wet rated wires or cable, and with conduit, you should virtually ALWAYS choose wires, not cable. You absoluely cannot use NM/B cable ("romex") becasue it's not wet rated. THWN wires are commonly available and usually the right choice. Every outside conduit is defined as wet, and almost all of them meet the definition.

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