PDA

View Full Version : Adding A GFCI to old Bathroom Wiring



Rick Hamblen
04-27-2005, 7:38 PM
Okay, here's a tricky one that I have yet to figure out... :eek:

Existing 1940's rubber cloth (Knob & Tube) wiring. Currently, there is a light switch that switches two old vanity mirror lights. I would like to convert the switch to a GFCI using a new GFCI/Switch combo commonly purchased at your major home supply warehouse. This gives you the GFCI and Switch in the same footprint (single gang box)

The tricky part is that the switch is downstream from the supply (only two wires in the switch box) and I was hoping to avoid fishing wires in the lathe/plaster walls.

Any help out there amongst you pros?

Thanks:) :)
Rick

John Hart
04-27-2005, 9:13 PM
I fear that you're in trouble Rick. The best thing that those two wires are going to do for you is help you feed new wires in. Ground fault Interrupters require three dedicated wires to work properly.

I feel your pain though...my house is full of it.

Jim Becker
04-27-2005, 9:23 PM
You can use a GFCI on a two-wire, non grounded circuit, but it is still not grounded. In fact, it's recommended as a "fix" for replacing a 2-prong outlet where no ground is available in my home wiring guide. Personally, I'd re-pull the circuit for a bathroom if at all possible if you really need an outlet. Otherwise, leave the lights alone.

John Hart
04-27-2005, 9:32 PM
Yup...but I guess I was interpreting this switch as not having a hot lead at all. The two wires are probably just the return going through the switch(or vice-versa). You know, that old style way of feeding return on one side of a joist or stud and then the hot being on the other side. They'd run the hot up to the light and then run a return over to a switch and then up to the light.

So I guess I was imagining those two wires as not being very useful in a new box. Unless I'm wrong! I've lost count how many times that's happened! :D

Jim Becker
04-27-2005, 9:43 PM
Doh! John, you're correct...the switch is likely just switching the hot. Nothing there for an outlet. Did I happen to say, Doh!!

John Hart
04-27-2005, 10:53 PM
Oh yeah! You're right Jim...It would have to be switching hot....that way, you don't kill yourself at the light sockets with the switch off! :eek:

Rob Russell
04-28-2005, 8:04 AM
John,

I'd like to explain why your post about a GFI requiring 3 wires to work is incorrect.

First – a definition. The proper term for the “neutral” is the “grounded” conductor, as in the “intentionally grounded” conductor. This conductor is a current-carrying conductor and, on a 120 volt circuit, carries just as much amperage as the black or red “hot” conductor. The neutral is just carrying the current back to the panel. Taking that explanation a bit futher, if you have a microwave that's running and drawing 10 amps, there is 10 amps of current flowing from the breaker/fuse through the "hot" conductor to the microwave. There is also 10 amps of current flowing back through the grounded/neutral conductor to the panel.

The green or bare wire, commonly known as the equipment grounding conductor or EGC, is properly called the “grounding” conductor. In the U.S., there have been attempts to get that EGC renamed in the NEC to the “bonding” conductor. This renaming attempt is because of the confusion that can arise between the terms “grounded”(neutral) and “grounding”(equipment ground) conductor.

A “ground fault” occurs when current that is supposed to flow back through the grounded/neutral conductor doesn’t – which means it’s flowing somewhere else (like through the EGC or through you). Accordingly, a GFI breaker or GFI receptacle has a sensor that compares the current flowing out through the hot conductor(s) to the current flowing back through the grounded/neutral conductor. If the two current loads are the same, then no current is leaking out and everything is hunky-dory. If there is an imbalance between current flowing out and back, the sensor trips. For example, if the current flowing back through the neutral is low because you’re being electrocuted, the GFI sensor detects that lower return current and trips. This situation is called a ground fault, in that there is an alternate path for the current to flow to ground.

If you install a GFI receptacle and only have 2 wires to feed it (eg. (1) hot and (1) neutral/grounded, but no EGC/grounding conductor), the NEC requires that you label the receptacle as "No Equipment Grounding Conductor". If you look in the box a GFI receptacle comes in, you'll find stickers for that.

Rob

Rick Hamblen
04-28-2005, 10:10 AM
Hey, you guys are great! Thanks for the quick responses!

I have also been told by a local electrician that the right thing to do is connect the Ground to the Neutral Line side to provide the GFCI with a ground sense ability. Since the new GFCI's can detect downstream grounded neutral situations it is important that the ground does not touch the Load side terminals.

This of course, does not address the trick of how to wire the whole thing to make the light switcheable and provide a GFCI outlet with only the two wires. It is perfectly fine by local code here to allow the GFCI to be switched on/off with the light, as long as it is powered when the lights are on (who really needs to run a hairdryer in the dark?)

BTW, one of the incoming wires reads as a Hot with my current sensor.

Thanks!
Rick

John Hart
04-28-2005, 10:44 AM
John,

I'd like to explain why your post about a GFI requiring 3 wires to work is incorrect....


Now there's a shocker :eek: Get it? Shocker?
Thanks Rob...It needed better explaining.

Good luck Rick! A lot of wiring in my house is the "Old" way. Sometimes I think it would be easier just to replace the house. :)

Rob Russell
04-28-2005, 2:48 PM
Hey, you guys are great! Thanks for the quick responses!

I have also been told by a local electrician that the right thing to do is connect the Ground to the Neutral Line side to provide the GFCI with a ground sense ability. Since the new GFCI's can detect downstream grounded neutral situations it is important that the ground does not touch the Load side terminals.

This of course, does not address the trick of how to wire the whole thing to make the light switcheable and provide a GFCI outlet with only the two wires. It is perfectly fine by local code here to allow the GFCI to be switched on/off with the light, as long as it is powered when the lights are on (who really needs to run a hairdryer in the dark?)

BTW, one of the incoming wires reads as a Hot with my current sensor.

Thanks!
Rick

Rick,

Forgot to mention earlier - welcome to SMC!

If you're suggesting tying the Equipment Grounding Conductor (aka "grounding" conductor) to the Line side Neutral, that violates the NEC. That creates a parallel current return path, potentially through uninsulated wires and you could also make the grounded case of an appliance electrically hot.

As far as your GFI and switch question, you need 3 wires at the switch/receptacle:

Hot wire in that supplies both the switch and receptacle
Switched hot from switch to lights
Grounded ("Neutral") conductor for receptacle

That's sorta tough to do with 2 wires.

Rob