Electrical – Is it allowed to run NM wire in PVC pipe under an outdoor porch

As has been discussed many times in this forum, back feeding a generator into a double or even a single pole breaker without a transfer switch is risky business. If the main breaker is not disabled while generator is online, you will be back feeding power to the grid and pose a safety hazard to utility workers.

With that said, look at the following link, http://www.southwire.com/support/voltage-drop-calculator.htm The wire size for an 80 run at 30 amps/120vac is 8 AWG. Use this calculator to determine wire size or determine voltage drop. 3% drop is usually considered max for 120/240 runs. Once you start to draw amperage, the drop can become greater as you tax the voltage source with a fixed or max amp output.

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.)