Kind of woodworking, but mostly a question about electricity...

[thatCharlieDude]

23 minutes ago, Tom King said:

In some of those old systems, the metal box may be grounded, either by the metal sheathing on the cable, or by a ground wire being attached to the box but not the receptacle itself.  

Gotcha. Would the screw by in the outside back of the box? I've seen the ground attached to the inside of the box.

[wtnhighlander]

I know its probably more than you planned to do, but this sounds like a perfect excuse to hire a local, licensed electrician to add another receptacle to the room. And reduce the risk of burning your house down with any sort of inexpert DIY work.

Having covered that, I'll chime in that using surface mount raceway to route a commercial extension cord around the baseboards is probably the safest way to go DIY. 

Something like this https://m.lowes.com/pd/Wiremold-500-1-Piece-120-in-L-Ivory-Raceway/3345398 should be available from most home centers. I prefer the plastic type that has self-adhesive backing (but not if you plan to remove it later).

Also note that using that 3-prong receptacle expander with USB power may not function correctly with a 2-wire feed. Often, those accessories (USB charger) require a seperate ground, in addition to the neutral. But not always.

[Bankstick]

Quote

Hidden wires in a baseboard could be dangerous.

Especially if someone down the road drives a nail in it.  I used to help a fellow do small jobs.  He put a nail into a copper water line once.

I ran Wire Mold from my panel to my shop as the yo-yos who built the house didn't put enough outlets in the basement- four outlets for it.  Plus the guy I bought the house from didn't rewire the basement when he finished part of the basement for home-schooling.  I used flex armor under the foyer and conduit into the shop.  The basement was unfinished and had four outlets in the whole basement.  If I had to do it over,  I would tear out the sheetrock ceiling and start from scratch.

[Tom King]

1 hour ago, thatCharlieDude said:

Gotcha. Would the screw by in the outside back of the box? I've seen the ground attached to the inside of the box.

I've seen some really odd stuff in old wiring-it could be.   I would test instead of count wires.  With the plug in tester, if you plug it in and the LED for the ground lights up, shake it around, and if it blinks , it's probably grounded through the receptacle screw to the box.

A GFCI will offer protection on a 2 wire circuit, but not as much protection as on a grounded circuit.  It measures a difference between amount of current on the hot leg, and compares it to the amount on the neutral. If there is a difference, it trips  There could be a current leak, say to the outside of a metal motor case, and there not be a difference from hot to the neutral until someone touches it, and provides a course to ground.  Theoretically, it would only be milliseconds, but I don't want it to be me, or a small child,  providing the tripping path to ground.  If that circuit had been grounded, whether GFCI or not, it would have tripped the breaker.