Electrical – How to extend a circuit from an outlet in a wall to the surface of concrete with conduit

I have had need to seal up a few places like that in basements where the conduits came in and had a potential leak risk. On occasion I would use a Sika product, 2c-sl is the name Sika give it. A 2 part, self leveling, pourable material that can be painted on. I even made a form and poured it into that to ensure it saturated the crevices, since it was well below grade. This is good stuff. WR Grace also makes a 2 part liquid applied membrane that comes in 2 consistencies, trowel or knife grade. One is thick, the other thinner. I think either one will work for you. It is called Bituthene Liquid applied membrane. If you can get to Tremco products, they make a one part waterproofing solution, Tuff and Dri is all that I can find on that. They make another product that works really well, to bad I cannot find not more info on, other than I used it, sorry.

All these products are super flexible, and if you clean the wall real good you can paint that and the end of the conduit that the wire goes in to wrap it all in waterproofing.

Disclaimer: I do not work for or represent any of the above mentioned companies, I only know how to recommend products that do the job, and do it well.

The first problem can be dealt with using a Bell 5403 or equivalent weatherproof extension box to provide a place to hook the conduit to and some extra cubic inches for the additional wire fill. You'll need to hook a ground pigtail to it, by the way -- there do not seem to be non-metallic weatherproof extension boxes.

Since you are running conduit under the deck -- you can run individual THWN wires inside it instead of a UF cable (although running UF aboveground is permitted when protected from physical damage and direct sunlight if it isn't IDed as sunlight resistant), and running THWN will make it easier to pull even with an oversized conduit -- 3/4" inch will provide ample space for a single circuit extension like yours. Don't forget to put in expansion fittings, lest your conduit run do the snake on you!

Last but not least -- simply use an ordinary outlet for the new outlet, and run it from the LOAD terminals on the existing GFCI. (Although putting two GFCIs in series won't break anything, it could confuse you as there is no guarantee as to which one will trip when something happens.)