Rocker Switch Guide: Hole Sizes, Ratings and 12V Wiring

A rocker switch is the snap-in ON/OFF switch on heaters, power boards, dash panels and project boxes. Choosing one comes down to three things: the hole it fits, what it is rated to switch, and how many pins it has. This guide covers the KCD11, KCD1, round KCD1-105 and KCD4 rockers we stock, then shows how to wire one into a 12V car, boat or caravan circuit with an indicator light.

In this guide

1. Choose by panel hole size
2. Ratings: what each switch can handle
3. 2 pins, 4 pins and lit switches
4. Cutting the hole and getting a snap fit
5. Wiring a rocker on 12V
6. Adding a light on 12V
7. Using a 4-pin DPST rocker
8. Test it before it goes in the panel
9. Troubleshooting
10. Frequently asked questions

Choose by panel hole size

Rocker switches are sold by the size of their front face, but what matters when you fit one is the hole behind it. The model name only tells you the family: a KCD1 and a KCD11 look alike and need different holes. If you are replacing a switch, pull the old one out and measure the hole, not the face.

The round KCD1-105 is the easy one to fit: a 20mm step drill or hole saw cuts its hole in one go. The rectangular switches need the hole marked out, drilled in the corners and filed square.

Ratings: what each switch can handle

Every rocker we sell is marked with AC ratings only, such as 6A 250VAC on the KCD1. Those numbers tell you the most current the contacts are made to switch on 230V mains. None of these switches carries a DC rating, and DC is harder on contacts than AC, because a DC spark does not stop by itself the way AC does 100 times a second.

For 12V and 24V DC we use a simple rule of thumb: keep the load to half the 250VAC marking. To find a load's current, divide its watts by the voltage. A 24W LED light bar on 12V draws 24 / 12 = 2A.

Motors, pumps and halogen lamps draw several times their running current for a moment when they start, so give them extra room. For a 12V load bigger than these, use the switch to turn on a 12V relay and let the relay carry the load, or use a switch with a real DC rating, such as our KN3C-101 toggle switch, marked 30A at 12VDC.

The KCD4 40A is a replacement part for equipment that already uses a 40A KCD4 rocker, such as a welder. Match it to the switch you are taking out.

2 pins, 4 pins and lit switches

The number of pins tells you what the switch does inside.

On a 12V lit rocker with three pins, the light only works when its negative pin is connected. Which pin that is varies between makes, so go by the markings on the switch body and check with a meter, never by the pin's position.

Our KCD4 16A lit colours (green, yellow, blue and red) are a different design. The light and a resistor sit between the two output terminals, the arrangement used on 230V switches where the light shows the appliance has power. We haven't confirmed the lamp voltage, so we don't sell them as 12V lit switches. On 12V, treat them as unlit DPST switches and add a separate light as shown below. The black KCD4 16A is the unlit version of the same switch.

Cutting the hole and getting a snap fit

Snap-in rockers are held by spring clips on the sides of the body. You push the switch in from the front, the clips fold in as they pass through the hole, then spring out behind the panel. That only works when the hole is the right size and the panel is thin enough for the clips to clear it.

1. Start small. Cut the hole 0.5mm under size, then file it out until the switch slides in with a firm push. Plastic is much easier to remove than to put back.
2. Keep the corners square. A rounded corner from a drill bit stops a rectangular body going in straight. Drill inside each corner, then file to the line.
3. Check the panel thickness. The KCD4 16A suits panels from 0.7 to 3mm, and its hole grows slightly with thickness: 28.9 to 29.1mm wide for a 0.7 to 1.25mm panel, up to 29.2 to 29.3mm for a 2 to 3mm panel. The height stays 21.9 to 22.1mm.
4. Thin a thick wall. If the clips can't spring out behind a thick panel, file a shallow recess around the back of the hole so the wall is thinner just where the switch sits.
5. 3D-printed boxes: add about 0.2mm clearance to the hole in your design and print a small test plate with the hole before printing the whole box.

Wiring a rocker on 12V

An on/off rocker goes in the positive wire between the battery and the load. The load's negative goes straight back to the battery negative or to the vehicle's chassis earth.

1. Fuse first. Fit an inline fuse in the positive wire as close to the battery as you can. Size the fuse to protect the wire, so the wire can never carry more than it is made for.
2. Positive to the switch. Run the fused positive to one switch pin. On a 2-pin rocker either pin works.
3. Switch to the load. Run the other pin to the load's positive. This wire is now live only when the switch is on.
4. Load negative to earth. Connect the load's negative to battery negative or a clean chassis earth point.
5. Fit the connectors. The KCD4 16A has 6.3mm blades, which take our fully insulated 6.3mm female spades: red for 0.5 to 1.5mm² wire, blue for 1.5 to 2.5mm². The KCD1 and KCD1-105 have smaller 4.8mm tabs, so solder those or use 4.8mm female connectors. The KCD11's lugs are for soldering.

Our crimp terminal guide covers matching wire size to terminal colour and making a crimp that won't pull off.

Adding a light on 12V

The simplest reliable way to get a 12V on-light is an unlit rocker plus a 12V indicator LED, wired as in the diagram above. Our 5mm pre-wired 12V LED has its resistor already fitted and 20cm leads, in red, green, yellow, orange, white or blue. To use a bare LED instead, our LED wiring guide gives the resistor value for 12V and for a vehicle with the engine running.

1. Red lead to the switched wire. Join it to the wire between the switch and the load, or to the switch's output pin.
2. Black lead to negative. Take it to battery negative or the same earth point as the load.
3. Mount the LED. Drill a 5mm hole beside the switch and push the LED through from the front.

With the switch on, the LED and the load both have power. With it off, both are dark, so a glance at the LED tells you whether the load has power.

Using a 4-pin DPST rocker

A DPST switch is two on/off switches worked by one rocker. On 12V it is handy for turning two separate circuits on and off together, such as the cabin lights and a water pump in a caravan, each on its own fuse.

1. Find the pairs with a meter. Set it to continuity. With the rocker on O, no two pins should beep. Press I and find the two pairs that now beep. Each pair is one contact.
2. Write the pairs down. A quick sketch of the back of the switch saves rewiring.
3. Wire each circuit through its own pair. Fused positive to one pin of the pair, load positive to the other. Don't join the two pairs to share one big load: the two contacts never close at exactly the same moment, so one of them takes the whole current.

On the lit colours, the light connects the two output pins through a resistor. If your meter shows a high resistance reading rather than a beep between two pins, that is the light, not a contact.

Test it before it goes in the panel

1. Meter check. With nothing connected, put your meter on continuity across the two pins of a 2-pin rocker, or one pair of a KCD4. Press O: silence. Press I: a beep.
2. Bench check. Connect a 12V supply through the switch to one of our 12V LEDs. Press I and the LED lights. Press O and it goes out.
3. Install. Wire it into the real circuit with the fuse in place, then snap the switch into the panel.

Troubleshooting

Frequently asked questions

Which side of the rocker is on?

The side marked I. Pressing I closes the contact and pressing O opens it, on every rocker in this guide.

Is a KCD1 the same as a KCD11?

No. The KCD1 has a 21 x 15mm face and needs an 18.8 x 12.7mm hole. The KCD11 is the mini version with a 15 x 10.5mm face, and it is marked for half the current.

Should I use a rocker or a toggle switch?

Both make a good on/off switch. A rocker needs a rectangular or 20mm round hole and sits nearly flush. A toggle fits a small round drilled hole and is easy to find by feel. Our toggle switch guide covers SPST, SPDT and DPDT toggles, and you can see the ones we stock under toggle switches.

Can I get a 12V push button instead?

These rockers stay where you put them. For a 12V button that is only on while you press it, or one that clicks on and off, see our momentary vs latching switch guide. To have a moving lid or hatch switch a 12V circuit by itself, see our micro limit switch guide.

Are these switches waterproof?

No. None of them is sealed, so mount them where they stay dry, or under a splash cover.