When considering wiring a dimmer, it is useful to review what are High-Side and Low-Side connections, as the wiring scheme will dictate which dimmer you must use.

Back in the day, when Bi-Polar Junction Transistors (BJT) were the norm, a "Low Side" switch would be typically called an "Open Collector". You may hear this term occasionally - but it's just another term for Low Side.

In a typical High-Side connection, the switch is on the high side (or positive) side of the circuit. This is very typical of RV wiring (or for that matter - most wiring). This is done simply by convention sake, and high-side is no more or no less effective than low-side wiring.

As shown here, low side wiring switches the low side (or negative) side of the circuit. This is rather unusual in RV wiring. However, you must be aware that dimmers are available in both High-Side and Low-Side configurations. Since Low-Side dimmers are slightly cheaper to produce, many of the low-cost dimmers are of this type. But as you will see, this can present a challenge when attempting to wire them into your RV.

Wiring a High-Side dimmer is fairly straight-forward. Most High-Side dimmers are 3 wire, with an Input (on the positive side), and Output (also on the positive side) and a Ground (only required to power the Dimmer itself - not the light fixture). There are no standards, however by convention you will often see a RED, WHITE, and BLACK wire as shown. However, check with the instructions that come with the dimmer as they may differ.

It must be noted that the BLACK wire is only used for the dimmer electronics to work, and is not required by the light. This may be a bit confusing; just remember that the actual "DIMMER" function is on the positive side.

In fact, some High-Side dimmers may only have two wires - an Input and Output. This is not common, but they do exist. If you have this type of dimmer, they will typically have a RED and WHITE wire - the BLACK ground wire is eliminated (but always check the dimmer's instructions).

With a Low-Side dimmer, things get a bit more complicated. In this first scheme, the "DIMMER" function is on the negative side, and both Input and Output wires are typically BLACK. They are often marked IN and OUT or some other differentiating marking. The Positive wire is only required to power the dimmer itself - it does not connect to the light fixture in any way. A few 3 wire low side dimmers are on the market, but they are not common.

The last scenario shows a 4 wire Low-Side dimmer, which is more common. Oddly enough, when inspecting the various dimmers, the two positive wires are typically just connnected together internal to the dimmer, simply as a convenient method of connecting the positive side. In that regard, the 3-wire and 4-wire low side dimmers are identical.

It can be a bit confusing I suppose when we have 3 and 4 wires poking out of a dimmer. but perhaps it will be a bit more clear if we peek inside a dimmer and see how it is constructed. In the High-Side dimmer shown above, you can clearly see that the Dimmer Module is on the positive side, while the only negative connection is to the control circuitry.

And in a similar fashion, you can see that in a Low-Side dimmer, the Dimmer Module is on the negative side, and the positive side wiring functions as a "pass-through" (for a 4-wire dimmer) and supplies the positive side of the LED, as well as powering the dimmer circuitry.

If you look at the orientation of the Dimmer Module only, it should become clear which one is a High-Side vs. Low-Side dimmer.

So now that we know the basic configuration of a dimmer, what are some scenarios you are likely to encounter in a RV?

In this first scenario, we will be replacing a switch in the control panel that goes to a puck light. The light could also be a LED strip or any other similar fixture. Note that the ground wire is not in the control panel, but only goes to the LED. However, another ground - not associated with the switch - runs to the control panel. Also, the switch itself is on the High-Side of the circuit.

In the first solution, the dimmer is connected to both Light and 12V power (see dimmer instructions for wire color), and the ground is connected to the ground on the light. This may be problematic in some wiring situations as the ground may not be available at the panel. However, this is the preferred solution. In some cases, this may require running a ground wire back to the fuse panel. This is safe as the ground wire is not sharing the circuit with any others.

If the light's ground is not available, it may be tempting to connect the dimmer to another ground. This has it's own risks though. First, even though the dimmer's load on the ground wire may only be a few mA, you are powering two circuits with a single ground, so it may be possible to overload the ground wire.

For example, if the wiring is 16AWG, we can expect a maximum of around 12Amps of current carrying capability. However, if we share two circuits, each with a 16AWG positive wire, fused at 8Amps, then the ground could conceiveably be forced to carry 16A before a fuse would blow - which would be a dangerous situation as the ground wire can only handle 12A.

Occasionally though, you may find a larger AWG ground wire going to the control panel, which is designed to be a common ground. Connecting to such a wire would be safer.

I cannot recommend using such a ground as there are risks involved. If you desire to do so, you do it at your own risk. See the disclaimer at the top of the page.

If you desire an On-Off switch in addition to a dimmer, simply wire it in series. I will leave it up to you where to put the switch.

If you use a Low-Side dimmer, you can see that it may be quite difficult to wire, especially if the light's ground wire is not available at the control panel. You MUST remove the ground connection from the light and connect it to the dimmer. Essentially you are interrupting the ground wire from the light to the fuse panel. For this reason, Low-Side dimmers are often not recommended.

In this next scenario, we will be replacing a wall switch with a dimmer. In the typical RV, a circuit consists of a "daisy-chain" or "string" from one device to the next. As shown here, the wall switch taps into the circuit at mid-point (although it could also be at the end of the string.

Fortunately both Positive and Negative wires are available, so connecting a High-Side dimmer is pretty easy, with only the need to ensure the wire colors match up. Note though that some dimmer manufacturers use different color wires, so check the documentation that came with the dimmer to be sure.

And as luck would have it, connecting a Low-Side dimmer is also pretty easy. This is because the ground side of the light fixture is readily available.

In this last scenario, we will add a dimmer to a 3-Way switch; for example, a switch in the headboard of the bedroom so the lights can be turned on or off from either side of the bed. The best approach here is to add a dimmer either before or after the switches. If the dimmer has it's own On-Off switch, then you may place the dimmer in either location. However, if the dimmer itself has no switch, it is best to place the dimmer after the switches.

A High-Side dimmer is located before the two switches, which preserves the 3-way action. This dimmer has it's own On-Off switch. Again, wire colors may differ so check with your dimmer's instructions.

And a Low-Side dimmer works here as well. This dimmer has no On-Off switch so it would be appropriate to locate it after the 3-way switches.

Low-Side dimmer used in a High-Side situation. Some RV manufacturers have been wiring the PCA EPL101x Low-Side dimmer into a High-Side configuration as shown above. However, I have not independantly confirmed the suitability of this configuration, so you assume all risks if you wire the dimmer in this fashion.