On Mon, 25 Mar 2013 05:34:07 -0700 (PDT), stryped
wrote:

I need to add a gfi outlet to my wifes bathroom. I want to add it to the current light switch box. This box currently has two switches, one for the fan,light and one for the vanity light. Each switch has a white wire, a black wire, and a ground. When checking with a voltmeter, the white wire on both switches always has power. Each black wire only has power when the switch is turned on. I am assuming the white wire is the power wire.

Can I take these white wires off the switches, connect them to the top and bottom "hot" terminal on the gfi outlet, then feed the switches from both terminals on the other side of the gfi? When I did this messing around with the switches, everything worked properly. I just thought that the black wire was always the constant power wire.

If that's all you have - two runs of Romex jacketed cable coming into
the box, each one with it's Black and White going to one switch - then
No, you can't add anything to that box. You don't have a Neutral
available in the switch box to connect to the outlet, just Hots coming
in on the Whites, and Switchlegs leaving on the Blacks back to the
lights.

The light box at the other end has the Neutral stop there - but the
schmuck that roped the house was too cheap to use a length of
3-conductor Romex to make sure there was a Neutral in the switch-box
for just that purpose. Or for a Motion / Occupancy detector light
switch to meet the next "Energy Efficiency" mandate - the new ones
require a Neutral to operate properly.

And before you put something built like this back together the way it
was, get a roll of Red or Yellow electrical tape (or just Black if
that's all you have) and wind it around the White wires to identify
that they are NOT being used as neutrals in this instance.

The electrician who built the house should have re-colored the wires
like this, but there are a lot of lazy people out there who go by the
motto "Doesn't matter, the wire is colorblind." Yes, but the next
Electrician is NOT colorblind, and will make some dangerous
assumptions based on the color of the wire without checking.

If you try installing a receptacle at the switchbox by connecting the
neutral off one of the bare Ground leads, you've violated one of the
electrical safety Prime Directives - Safety Ground NEVER carries
return current.

That's assuming the house is new enough to have the 2W+Ground Romex -
if you have the old 2-wire and no ground stuff you can't even do that.

Look inside the box and make sure the house doesn't have flex conduit
in the walls and was wired by a moron who only had Black and White
wire. It happens...

(Been The "In Russia, you use whatever you can get, yes, I
understand - But this isn't Russia, it's Los Angeles. And we have
Inspections that this work will never pass, and I'll have to redo the
whole thing - You're fired.")

If so, you can get a fishtape and pull through a different colored
wire up to the ceiling light box for the Switchleg and turn the Black
into the Power feed and White into Neutral.

Otherwise, you're going to have to make a few holes in the wall and
re-run a 3-wire Romex to one of the light boxes, and then you'll have
a Neutral in the switch box to hook up the receptacle.

You might get lucky and find that both hot wires to the switches are
on the same circuit - then you could use one Hot to feed both
switches, and turn one of the White wires back into a real Neutral to
feed the receptacle - but don't count on it.

-- Bruce --