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Is it legal (NEC/Canada) to drill a drain hole in the bottom of a PVC weatherproof electrical box, when used in a wet location?

If so, what size is recommended to drain the water out and keep the wasps out (or avoid other issues)?

For the rest of the seal, is it common to silicone the entire seal between cover and box? Or do you typically seal the top and sides, but not the bottom? Is this a matter of code or good practice?

An instruction manual for a motion sensor reads "If not installed on a weatherproof box or if an adaptor plate was used, caulk the wall plate and mounting surface with silicone." I note, based on an answer below, that the silicone recommendation does not apply, and the opposite is recommended, when mounting on a PVC weatherproof box.

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Yes, and you'll want a 1/8" hole

This is governed by NEC 314.15:

314.15 Damp or Wet Locations. In damp or wet locations, boxes, conduit bodies, outlet box hoods, and fittings shall be placed or equipped so as to prevent moisture from entering or accumulating within the box, conduit body, or fitting. Boxes, conduit bodies, outlet box hoods, and fittings installed in wet locations shall be listed for use in wet locations. Approved drainage openings not smaller than 3 mm ( ⅛ in.) and not larger than 6 mm ( ¼ in.) in diameter shall be permitted to be installed in the field in boxes or conduit bodies listed for use in damp or wet locations. For installation of listed drain fittings, larger openings are permitted to be installed in the field in accordance with manufacturer’s instructions.

I would not seal the rest of the box though; the idea with normal weatherproof electrical enclosures is to provide a rough seal to keep bulk moisture, bugs, and such out, while having enough air movement to prevent any pressure differentials across the seal from driving moisture in through the slightest imperfections. (The drain hole helps with this as well, although you'll want to keep an eye on it as spiders can be particularly pesky hole-pluggers.)

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  • I'm shocked! The NEC uses mm? Anyway, this is clear. Good point on equalizing pressure differentials to prevent moisture intake. – P2000 Aug 15 '20 at 2:38
  • USA has been using an inch of exactly 25.4mm since 1933, some would say as a crutch. – Jasen Aug 15 '20 at 6:19

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