At Home Alterations { old & new | cheap & beautiful | him & her | ups & downs }

11Jan/11Off

Technical Tuesdays – replace a pull chain with a light switch

I think somewhere along the line someone decided that every house needs to have at least one light that's turned on and off by pulling on a chain or string.  They pop up in basements, garages, pantries,  and attic spaces.  I guess the idea is that it's easier than wiring a light switch but really, adding a switch to a light isn't rocket science.  We have a few pull chain lights in our basement that have been driving us nuts, especially since there's no way to get light into the basement before walking through the dark!  I decided to start working on such lights in our basement back when I charted the entire electrical system in our home. I'm learning as I go with the help of The Circuit Detective website and a book I got for Christmas - Wiring a House.

Making sense of the switch

First, lets look at what's happening in a normal light bulb.

Electricity goes into the bulb over the negative (black) wire, travels through the filament creating light, and travels back over the neutral (white) wire.  In this diagram the light would always be on.

Here's what a simple switch looks like for a light.  The negative (black) wire is disconnected so the light will be off.  If the switch is turned on, the negative (black) wire connects the circuit and the light turns on.  In a pull chain bulb this switch is located inside the light housing and is connected or disconnected by pulling on the chain.  In this diagram the switch is located on the wire before it gets to the bulb.  If you have access to the wire on it's way to the bulb (and the bulb is the last thing in the circuit) you could simply cut the wire in half, install a switch and be on your way, but I'm guessing if it were that easy you'd already have a switch there!  I'm going to assume that like the light I was tackling, you also don't have access to the incoming wire and need to add a switch without disturbing the rest of the circuit.  The switch will look like this:

With our switch, the negative (black) wire enters the the light's box but doesn't connect to the bulb.  Instead it runs along a new wire down to a switch, then back along another wire to the light's box.  At this point the electricity runs through the bulb and back along the neutral (white) wire.

  • "So how many wires do we need?"  Here's the part you need to wrap your head around.  We're only adding a single cable to the circuit.  We have two wires running into the box on the right and two wires running into the box on the left.  That's two cables (each cable contains two wires, black and white along with a ground).
  • "But they don't make a cable with two black wires, right?"  Right.  Our cable will have two negatives but who says a negative wire can't be colored white?  Electricity will run on the wire no matter what color it is, we just use the color to make it easier to visually see what's going on.  With that in mind, we'll mark the tips of the white wire with a black marker or black electrical tape so it's clear that this wire is negative.
  • "Doesn't the switch need a neutral wire connected to it?"  No.  Switches only affect the negative wire.  If the switch was before the bulb like the second diagram, the neutral wires from either side of the switch would just be spliced together which basically skips the switch.  It might help if you think of the switch as just an extension of the negative wire.  There is a negative and neutral wire connected to the light, the negative wire is just a lot longer.

If that makes sense then adding the cable should be pretty simple.  We just need to disconnect the original negative wire from the light, splice the original negative to the new black negative, add a switch at the other end that connects the black and white (both negative), then connect the new white negative to the light. Clear as mud?  If so, lets begin.

How To

Disclaimer: As always, be extremely careful when working with electricity, it can kill you.  If you feel like you're in over your head, stop and call an electrician.  Do this project at your own risk.  My guidelines are based on the NEC (National Electrical Code) that most cities have adopted.  It's possible your city has changes to the code so please contact them for their code, to get a permit (if required), and to make sure you're allowed to DIY this without a licensed electrician.

What you'll need

  1. Romex - Check the breaker for this circuit.  If it's 15 amps then use NMB 14-2, the the breaker is 20 Amps use NBM 12-2.  I have a 250ft pack because I'm going to use a lot.  You can get by with 25 feet if this is your only project.  Measure the distance the wire will travel before buying a short length though.
  2. (optional) Conduit.  Romex doesn't need to be run in conduit unless the romex is outside of a wall and in danger of being damaged.  Mine will be running next to a door on a brick chimney so I need it.
  3. A 1-gang electrical box.  Mine is designed to mount outside of a wall.
  4. Light switch (make sure you get a 2-way, not a 3-way
  5. Switch face plate
  6. Fittings to connect the romex or conduit to the electrical boxes
  7. A circuit tester to test if there's electricity running through wires
  8. Screw drivers
  9. A wire stripper
  10. Insulated Needle Nose Pliers
  11. Insulated wire cutter
  12. Wire Nuts, the red ones will work for what we're doing
  13. Romex Staples

Run the wire
Running the wire is probably the hardest part.  You'll most likely need to drill holes in your joists and studs to make room for the wire.  It might be tempting to just staple the wire under the joists but that is not allowed by code- you would have to rewire the room if you ever add a ceiling and it's more likely to get damaged by something hitting the ceiling.  It's a pain, but you need to drill the holes.

  1. Plan your wire run.
  2. If you have to go through joists or studs drill a 1" hole at least 2" from the edge of the wood to protect the wire from future nails.
  3. Start with the light's electrical box and leave about 10" of romex in the electrical box.
  4. Secure the wire with a staple within 8" of the light's electrical box.
  5. Continue to secure the wire at least every 4.5 feet.  If you're running through joists or studs you can consider the hole a secure point but staple if you run parallel to the joist or stud.  It won't hurt anything to add more staples.
  6. Secure the wire within 8" of the switch's electrical box.
  7. Leave about 10" of wire in the switch electrical box

I purposefully left out how to bend and secure conduit because I think that's a whole other can of worms for another day so you'll have to check back in a week or two if you need to use conduit!

Connect the Switch
Now that you've run your wire it can be connected to the light switch.

  1. Strip the cover off the Romex an inch or two from it's entrance to the electrical box.  Be careful not to cut any wires inside.
  2. Cut off the strip of paper
  3. Strip 3/4" from the white and black wires
  4. Bend the black and white wires 180 degrees
  5. Slide the black and white wires behind the screws on the side of the switch so the wire is on the left and the hook faces right.  That way when you tighten the screws clock-wise it will go with the bend in the wire.  It doesn't matter which screw is connected to which wire because a switch just connects the two wires, there isn't a direction.
  6. Tighten the screws
  7. Connect the bare ground wire to the green screw on the side of the switch the way you connected the black and white wires.  If your box is plastic that's all  you need to do.  If it's metal like mine you need to ground the electrical box as well as the switch.
  8. Bend the wires accordion style so they'll fit in the box.
  9. Wrap black electrical tape around the white wire to mark it as hot since it's carrying a negative charge, not a neutral charge like white usually carries.
  10. Screw the switch to the box and screw the face plate over the switch

Connect the Light
The final step is to make connections in the light's electrical box.  Remember from the diagram that we don't need to touch the neutral side of the circuit, just extend the negative side.  That actually makes for a pretty simple change!

  1. Double check that power is off in the box.  Use your circuit detector to see if any wires are live.  There can be more than one circuit in a box so just because you flipped the breaker doesn't mean there isn't a live wire in there.
  2. Strip the Romex, cut the paper, and strip the black and white wire to 3/4" like you did for the switch.
  3. Disconnect the black negative wire of the light from the black negative wire in the box.  It's most likely connected with a wire nut so just unscrew the nut and separate the wires.
  4. Connect the black wire from the new Romex to the negative wire that was in the box.  Not the negative wire connect to the light but the negative wire that comes from the circuit.  Connect these wires with the wire nut that you just unscrewed in step 3.
  5. Connect the white wire from the new Romex to the negative wire coming from the light.  Connect these with a wire nut as well.
  6. Use black electrical tape to mark the new wire as negative.
  7. Push all the wires back into the electrical box and screw it back together.

I didn't photograph each step because my set up is a bit nasty.  Several branch circuits start at this light box and it can get confusing.  Lets look at the before and after shots to try to make sense of everything.

Oh where to start!  First off, the two blue things are wire nuts covered in old rubber covers.  The rubber had gotten super hard and was impossible to work with.  They can be replaced with regular wire nuts and this is a good time to mention that there's no need to cover wire nuts in electrical tape like people sometimes do.  The next thing to notice is the group of yellow wires which are the neutrals.  They were probably white at some point but the old wires have transformed into a yellow.  What's really going on here is one circuit (one black and one yellow) wire is entering the box and 3 new circuits (3 black and 3yellow) are connected to that one incoming circuit by wire nuts.  Along with that, the light has a negative (black) and neutral (unfortunately also black) wire connected to those same bundles of negative and neutral wires.  Whether the other 3 circuits were present or not, I'd have 1 negative and 1 neutral entering the box and connecting to the light's wires by a wire nut.  In my light there are just some "extras" to ignore.

This picture is after my handwork.  My new wires are on the right.  The black (outgoing negative) is connected to the incoming negative bundle in the wire nut on the upper left.  My new white (which is the returning negative) is connected to the negative of the light by the lower wire nut.  Nothing changed on the neutral side.  10 points to anyone who can spot something wrong here........... no black tape showing the white wire is negative!  Don't worry, I added it before I closed it up, I just forgot to do it before the photo.

Here's what it looks like now.

If none of that made sense you can join Katrina in the "you lost me..." club, but come back tomorrow for some fashion instead of function!

Filed under: electrical Comments Off
Comments (5) Trackbacks (0)
  1. I need to do this in my basement. I wish you were down the street from me so I can offer you drinks and food. Just to bribe you into doing it for me. Haha! Thx for the tutorial.

  2. I am with Katrina on the "you lost me…" BUT if it's something I needed to do, I'd be SO happy to come across your post with very detailed instructions and that is what matters. :)

    Great job! I had a pull chain light in my bedroom growing up and of course it was in the middle of the room. Lots of stubbed toes. ;)

  3. Just what I needed – makes perfect sense. Did you run metal conduit the entier run?
    If not, is there something to put in the end of the conduit so the rough edge doesn’t cut into the romex eventually?

    • I just used conduit on the wall down to the switch. There are plastic covers for the end of the conduit that get rid of the sharp end. You should be able to get them where you buy Conduit.

  4. This makes more sense to me than any literature I have ever read on this subject. I get confused every time I try to remember how to add a switch at the end of a run. Thank you so much for clarifying this process for me in elementary language, it is much appreciated!!


Trackbacks are disabled.