Mod #79: 3-Way Light Switch Circuit

Mod #79: 3-Way Light Switch Circuit

Submitted on: 04/15/09

     Category: electrical, lighting
Mod Rating: 12345

(7 ratings)

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Mod Description:

This mod comes to us courtesy of RV.net senior member professor95, otherwise known as Randy Agee, and is copyright with all rights reserved.

So you turn in for bed and start some nighttime reading, and then find yourself nodding off between pages. You reach over to switch off the bedroom light but realize you left the main lights on. You are sleepy and just can’t bring yourself to get out of bed to turn off the lights. Wouldn’t it be nice to not have to wake up to do the deed? To just reach over and switch off the main lights from the comfort of your bed? Well now you can. This mod details how to install a 3-way switch circuit that let’s you control a light or lights from two different switches.

Mod Difficulty:

RV electrical systems are comprised of two separately derived power systems. The primary system for a conventional RV is a direct current system similar to what you have in your tow vehicle or dingy that uses a 12 volt battery and alternator to recharge the battery. The exception on the RV is that the alternator is replaced by a device know as a converter or converter/charger that modifies 120 volt alternating current (AC house power) to 12 VDC (Volts Direct Current). In reality the 12 VDC system is more like a 13.4 VDC system – but 12 volts is the number commonly used.

Most, if not all of the lighting circuits in a RV operate off of 12 VDC. The conventional North American style electrical plug-in outlets are, of course, 120 VAC. This style of outlet is commonly referred to as a duplex outlet. Some RVs may also include one or more 120 VAC overhead lighting circuits. Check the description on the bulb if you are not sure of what the power source is for the light.

Looking back through the years and all of the RVs I have owned, I can honestly say that all of them were sadly, inadequately wired for both lighting and AC power as built by their respective makers.

Lights were typically located where they produced glare and switches for the lights were either on the light itself or located at some point in an area that was inconvenient or required crossing dark spaces to turn on or off.

One of many modifications I make to my RVs is to reconfigure the electrical system(s) so that they offer both convenience and safety.

Before we go too far, let me point out that I do hold a Master Electrician’s License. While electricity is not my professional trade, I have made a lifetime of teaching subjects related to electricity, electronics and technology applications.

I have one precise rule for anyone making electrical modifications to a RV: “Know what you are doing – and do it right!”

My intent is to provide the background information for this modification so you know what you are doing and then to provide the steps to do it right. It is important to understand the process – and if you do not understand the process, wait and ask questions until you get the answers you need to do it right.

This particular modification is to the switched lighting circuits powered from the 12 VDC side of an RV. First a word of caution: While 12 VDC power will not electrocute you as 120 VAC might if you make a mistake, an improperly wired 12 VDC circuit can overheat and even cause a fire. Always follow good workmanship practices, insulate bare wires and be sure the 12 volt circuit is fused properly for the gauge wire and load. For the project that follows the fuse should be no larger than 15 amps and the wire gauge no smaller than #14 AWG copper. The lighting fuse will be located in the original (OEM) “fuse box” for your 12 VDC circuits. It is a good idea to locate this fuse and pull it out before beginning any modifications on the circuit.

Additionally, be absolutely positive the circuit you are modifying is for 12 VDC lights and not a maverick 120 VAC lighting circuit. If the lights work with the battery charged and the RV disconnected from 120 VAC power, you have a 12 VDC lighting circuit.

When we bought our new Cedar Creek 5th wheel, I was bothered by the lack of a switch to control lighting over the stairway. The switch was beside the bedroom dresser several feet from the top of the stairs. This left us either leaving a downstairs light on or climbing the stairs in the dark. You will also note from the photo of the stairway that I added a handrail beside the stairs – another factory omitted safety feature.

I considered several options; putting a new light and switch at the bottom of the stairs, keeping a flashlight clipped to my belt, adding a battery light or rewiring the current switch at the top of the stairs to a 3-way switch circuit so that the light could be turned on or off from two independent locations.

I decided on the 3-way switch.

When I removed the switch at the top of the stairs, there was a hole punched in the wall with two wires attached. The lights in the ceiling that were controlled by the switch comprised what is called a “switch loop” where the hot wire to the lights is broken by a single pole switch. When the original single pole switch was removed the two wires sticking out of the hole in the wall comprised what is know as the feed (positive + in 12 VDC systems) and the other is the return (also positive with switch closed) which goes to the light(s).

I have a personal rule that prohibits me from drilling new holes in the walls to fish wires. I do not like to open what may be closed air spaces in the exterior walls. Therefore, when I add wiring such as this switching circuit I will use surface mount raceway and boxes. These are available at most any big box home store in the electrical section. While surface raceway may not be the best looking stuff to put on the walls, it does no harm to the wall surface and can later be removed if needed. It can also be repainted to a matching color if one wants it to be less obvious. It is made of plastic and has a self adhesive tape on the back. Preformed matching turns and splice caps are also available.

The wiring circuit for these 3-way switches is the same as used for 120 volt household wiring. The switches themselves are common residential 3-way switches. The wire is #14-3 Romex cable normally used for residential wiring. All of the hardware can be purchased in any big box home store or even at many local hardware stores.

I carefully removed the outer plastic jacket with a razor knife so that I had just the 3 insulated wires (white, black and red). It is a lot easier to put the wire in the raceway without the jacket. The bare copper grounding wire is not needed on the 12 VDC systems and is set aside for some sort of use later.

A 3-way switch will not have on and off embossed on the handle. There will be three connection screws on the switch (excluding the green grounding screw which is not used). Two of the three screws will be the same color – usually gold or brass color. These two connections are called the travelers. The travelers go from one switch to the other and are not broken or attached to any other device. The remaining screw on the switch will usually be black or bronze in color and is called the pivot. The pivot on one of the switches will connect to the power source, in this case either one of the 12 VDC wires removed from the OEM switch. The pivot on the second switch will connect to the second wire that came off of the OEM single pole switch.

It is important to note that if you connect the wires from the OEM switch to a DC volt meter you will not get a voltage reading. You must have both a negative and positive wire to get a DC voltage reading. For purposes of installing the 3-way switch, the polarity or where the wires from the original switch go is unimportant. The wires may be connected to either one of the pivots at random. The wiring for a 3-way switch is shown above.

Just to recap. One of the wires removed from the original single pole switch will go to a pivot (dark screw) on either switch 1 or switch 2. It really makes no difference which switch’s pivot is used.

The other switch’s pivot will connect to the remaining wire removed from the original single pole switch. The original switch may then be discarded or stored away for another project.

The two travelers (gold or brass color) on switch 1 travel to switch 2. They are not broken (unless you install a 4-way switch – which we are not doing.)

It is really an easy project that can enhance personal safety when entering areas normally not illuminated. In our case, we can now safely climb the stairs to the bedroom in our fifth wheel trailer without worry of stumbling in the dark!

ModMyRV recommends these parts for this mod:
COOPER WIRING DEVICES CS315LA-BOX 3-WAY COMMERCIAL TOGGLE SWITCH

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4 Comments For This Mod

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  1. Robrv

    Are You using 12 volt switches or Standard 115 Volt switches? The illustration looks like standard 115 !

  2. John Wallace

    The RV 3 way switch article was helpful ans well written. Thanks.
    Do you know of source for small size (not household) 3 way switches for use with 12 volt lights?
    Thanks

  3. ModMyRV

    Try here: http://www.wiringproducts.com/contents/en-us/d35.html

  4. Bowser

    I’m not sure I understand this. A three-way switch for either 12VDC or 110VAC is not a complex problem. I have a 112VAC circuit in my house which switches on or off from 5 different locations. Using three-way switches can make like far more convenient, but I think the directions on the box are simple and sufficient.

    Unless there is something unique to 12VDC I don’t understand. And that’s a real possiblility, though were I to do what you are proposing, and now that you have me thinking of It I will come up with three of them. One by the door, and one at either end of the bed.

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