Portable Dimmer Cord for Incandescent/Halogen Lighting

The halogen lights for my trade show booth are way too bright and hot at full power, so I need to dim them.

Now, there are already affordable UL-listed "dimmer switch plug" devices on the market, but they all use 2-prong plug/outlets only.  In certain applications it is desirable (or even required) to use grounded 3-prong plugs and outlets for all electrical connections.  For example, some trade show regulations mandate that all wiring used in booth displays must use this type of grounded plug.

There IS one kind of device designed for this, called a Variac, or auto-transformer.  You can buy one that is all pre-wired with plug, outlet, fuse, and volt meter.  In fact, I used one just like that for my booth lights in the past.  Variacs are expensive and heavy, but very awesome.  Unfortunately, due to their combination of mass and fragile wires, variacs are susceptible to damage from being dropped and landing hard, which is guaranteed to happen at least three hundred times during freight shipping to the trade show.  So, rather than purchasing another expensive, heavy, fragile variac, I will build a dimmer control cord that should survive years of use and (alas) punishment in my show crates.

So ironically, the 3-prong rule (which has been designed to encourage safe electrical connections) has led to the need for me to hack together cables and switches in a most un-UL-listed fashion!  Amusement aside, I shall endeavor here to make proper, safe connections that will result in a safe device.  Also I will try to point out things you should check to ensure safety along the path of constructing a similar device.

WARNING #1!  This kind of dimmer control can ONLY be used for incandescent/halogen bulbs, or other equivalent simple resistance loads.  Just because you see a 3-prong outlet doesn't mean you can plug in your kitchen appliances or fans or something.  If you do, Bad Things can happen.

WARNING #2!  Pay attention to ratings!  In my project I am using a dimmer control rated for 600 watts, which (for 120 volts) means 5 amperes.  This is sufficient for the lights I want to plug in.  Also, the sacrificial extension cord I use is rated to 13 amperes at this voltage, which is plenty.

WARNING #3!  YOUR OWN RISK.  Cutting up a cord and wiring your own switch like this definitely voids all warranties and makes you liable for whatever Horrible Accidents may occur based on your work.  ELECTRICITY CAN KILL YOU and start fires and other terrible consequences. 

Parts used in this project:
* USA 3-prong 120v 13 ampere rated extension cord. Flat ribbon type optional but neat looking.
* 120v 5 ampere single pole dimmer switch control. You better know how many amperes your lights will draw!
* Plastic switch/outlet gang box.  Metal ones exist, but plastic is nicer looking for this exposed project, and safer.
* Switch cover plate, for that professional touch.

Tools used in this project:
* Knife to help split apart the plastic between the 3 wires. Maybe your fingernails are that sharp, but not me.
* Wire strippers+cutter.  Or are you the kind of person who uses scissors and teeth?
* Flat head screwdriver for switch plate screws
* Philips head screwdriver for dimmer switch screws
* Electrical tape, used to mark the neutral wire.  You could use a permanent marker.
* Multimeter, continuity tester, or other means of determining which is the neutral wire. IMPORTANT for safety.

Clip the cord somewhere that leaves enough to work with on both sides.  Lacking a reason to choose another place, I just picked the mid-point.

For a flat cord like mine, you also have to split the plastic insulator between the three conductors on each side, because each wire needs to be wired up separately.  For a round extension cord, you'd have to split open and peel back the outer sheath.

For safety, it's important to figure out which wire is hot and which is neutral.  Switching the hot and neutral wire between plug and outlet, or wiring the switch to the neutral wire, could pose shock hazards!  Use a continuity tester to determine which wire is electrically connected to the "left", wider slot of the cord.  THIS is the "neutral" wire.  I have marked BOTH sides of the neutral wire with electrical tape, so I don't accidentally mix them up with the "hot" wire ends.  DON'T SKIP THIS STEP unless you want to meet the nice fire inspector.

Pull the cut ends of the cord into the gang box, and then pull them up out of the center so you have an easy length to work with.

Note that I DID NOT do this at this point, but you should: Knot the wires nearby the ends, so that if you tried to pull the wire all the way back out of the gang box it would hang up on the knot.  This is a stress-relief knot.  It's called that because you are putting the force of the pull onto the knot and the wall of the gang box, instead of the connections inside.  And this will relieve your stress :)

Strip the ends of the cord's neutral wires, and twist the strands together.  Twist a wire nut over the ends until it is firmly binding the connection and anchoring both ends together.  (Wire nuts should be included in your switch package)  MAKE SURE there is no exposed copper below the wire nut.  If there is, then your stripped wires are too long and you should trim the leads shorter and start over.

Strip the ends of the cord's green ground wires, and twist together these two strands AND the green ground wire of your dimmer control.  Add a wire nut to keep the connection strong and secure.  In other words, you should have a wire nut with 3 green ground wires coming down out of it.  MAKE SURE there is no exposed copper below the wire nut. 

Now strip the ends of the cord's remaining (hot) wire ends, and twist each one together with one of the black wires from the dimmer control switch.  Add a wire nut on each, and as before, make sure there is no exposed copper when you are done.

If you have done all the connections carefully and securely, and made sure everything is tight and there's no exposed copper, then your project is electrically complete already.  Wow, because I mean it sure doesn't LOOK done yet, right?  Anyway, since there's no wires exposed, it is now (in theory) safe enough to plug in.  Test before closing the box up, just to make sure everything is IN FACT wired correctly and securely.  Open-case testing is an offering to the gods.  If you skip this step, then the universe will punish your hubris by causing your connections to be wrong.

Start with the dimmer switch in the "off" position. Plug in the cord to a live outlet and a nice low wattage incandescent light.  Now, turn the dimmer switch on and vary the setting.  The light bulb should switch on and vary in brightness directly corresponding to the dimmer switch.  Eureka!

Now it's time to pull the slack cords through and draw the dimmer control down to the gang box.  Carefully bend and tuck the wire nut connections down and align the dimmer control to the front of the gang box.  It's OK if there's a little push-back pressure from the wires, just make sure you don't do anything that will bite into the insulation.

See how everything is all jammed together now?  That's why it's SO IMPORTANT to make sure there's no exposed copper under your wire nuts.  That plus the plastic gang box add layers of safety against dangerous short circuits and other electrical gremlins.

Bind down the metal front of the dimmer control using the two screws included with it in its package.  These will fit into two corresponding posts in your gang box.  Make sure it lines up nicely or else the face plate will end up at a weird angle and people will be like "Hey man are you sure this thing is safe to use?" and then they will start to back away slowly when you explain that it's perfectly safe since you made it yourself from instructions on the internats.

Bind down the face plate to the metal front of the dimmer control using the two screws included with the face plate.  These will fit into two corresponding holes in the metal plate.  Man that lines up so nicely.  At this point I wish once again that I could have found matching colors.. but the dimmer switch knob only comes in white and putty color, and the gang boxes came in only gray and bring construction-colored blue.  So all things considered, I think I did pretty well with a gray back, and a white cord and front.  Shut up, I bet your socks don't even match your pants.

Attach the dimmer knob onto the exposed post and make sure it can slide freely.

Now for the thing i forgot:  Since I'll be using my cord during trade shows I wanted to add extra insurance against the cords pulling on the wire nut connections.  So I made knots in the cord that were slightly larger than the gang box openings, and then carefully forced them in.  Now, pulling on the cord will exert pressure on the knot, but the wire nuts beyond will be safe.  Note that IF I WAS SMART I would have prepared these knots back at step 2 where I was initially drawing the wires into the gang box.. Instead, I have to cram them in at the end like an idiot. 

Finally, I used a red permanent marker to very visibly label the outlets with the words "DIMMED OUTLETS!" so that someone who just sees the outlets will notice they are not normal plugs that carry 120v, and understand that something unusual is going on.

Now, ideally, you will have a mostly safe device to dim your lights. 

NOTE: To make it even safer, I should have put a 5 ampere inline fuse into the circuit, so that if you plug in too much load, the fuse snaps before the switch catches fire.  Since I already know exactly what lights my switch will be used for, I am confident and lazy enough to have skipped this step.