Is it enough to have just two underwater lights? This is a common question among boat and yacht owners. If you have a yacht, a boat with a wide beam, or one with several outboards, you may need three or more lights to get even coverage across the transom.
Underwater lights should light up the entire width of your transom. For most boats, two lights are enough. However, if your boat is wider or has several outboards, you’ll need at least three lights.
A Switch Can Only Handle So Much Power
There are two main challenge when adding underwater lights to you boat.
- For each additional light you add you must run two more wires. One to power and one to ground.
- Each additional light doubles the amount of inrush current your switch must handle. This can be a problem when you add more powerful lights.
What is Inrush Current?
Turning on a light, or any other electrical device, creates a higher than normal surge of current. This surge can be many times the lights normal operating current but lasts no more than a few milliseconds. Over time, this high inrush current will cause a standard switch to overheat and fail.
It’s All About Amperage
A bigger concern is the amperage rating of your switch. Most rocker switches are rated for 20 amps at 12 volts. To handle inrush current, you need a switch rated for twice the continuous current of your lights.
For example, an SCR-16 underwater light uses 5 amps at 12 volts. If you have three of these, they’ll draw 15 amps, so you’d need a switch rated for 30 amps. That’s 10 amps more than a standard 20-amp rocker switch can handle. The issue gets worse with more powerful lights like the SCR-24, which uses 7.8 amps at 12 volts.
To make this work you would need to install a high amperage switch or a solenoid raising both labor and material costs. Each light also needs its own fuse. Inline fuses can be use, but again, installation will be more expensive and complicated. It also makes troubleshooting problems down the road much harder.
Most importantly, this setup only lets you turn the lights on or off, and it only works with single-color and multi-color lights.
| Model | Amps / Light | Inrush Current x 3 Lights | Inrush Current x 4 Lights |
| SC2/SC3/SC3-ALTB | 2 amps | 12 amps | 16 amps |
| SCR-16 | 5 amps | 30 amps | 40 amps |
| SCR-24/ SCR-CC | 7.8 amps | 46.8 amps | 62.4 amps |
Wiring Underwater Boat Lights Has Never Been So Easy
SCM-PD4CH
A better solution is to install a Shadow-Caster SCM-PD4CH power distribution box. This watertight box is mounted close to your underwater lights drawing power from a nearby fuse block or buss. Deutsch style connectors make wiring your lights, and wiring to a switch or any Shadow-Caster LED controller easy and trouble free. This greatly simplifies wiring saving you money in installation and material costs. It includes digital overcurrent protection for each light as well as status indicator lights to make troubleshooting a breeze.
Where to Mount the SCM-PD4CH
To cut down on the number of wires running through your boat, the SCM-PD4CH should be mount near the back of your boat. Look for a dry, easy-to-reach spot near the stern, like under the deck, high in the bilge, or inside a console. Also check for easy access to power and ground. The best location keeps both your light cables and power wires as short as possible saving you money on installation costs, reducing wire size, and eliminating voltage drop.
Connecting Your Lights to Power
- Prepare the cable runs by routing cables from each underwater light to the SCM-PD4CH. Cut the cables to the length leaving an extra six inches to create a drip loop. Slide a piece of adhesive-lined heat shrink tubing over each wire before you strip the insulation.
- Next, match each wire color to the same color pigtail on the supplied Deutsch male plugs. Put the wires into marine-grade butt connectors, crimp them tightly, and use heat to shrink the tubing and seal the connection.
- For main power and grounding, run a ground wire from the box’s negative terminal to your boat’s negative bus bar. Connect the positive terminal to a circuit breaker or fuse block.
Safety tip: If you connect the main positive cable straight to a positive bus bar instead of a fuse block, make sure to install a main inline fuse. The fuse should be rated for at least twice the total continuous amperage of all the lights connected to the box.
Each light has 15 amps of digital over-current protection and a watertight Deutsch connector. The connectors come pre-wired with color-coded pigtails that match the light wires. This makes wiring almost foolproof saving you money on labor and materials.
Finally, you control the lights with a switch at the helm. Since the box gets power from a local source instead of the switch, you can use a low-power switch and wire. The switch can turn single-color underwater lights on and off, and it can also control color-changing lights. You can also use any Shadow-NET light controller to control color-changing lights.
Built-in Troubleshooting & Diagnostics
Instant Diagnostics with status LEDs: The SCM-PD4CH uses 15-amp digital overcurrent protection for each channel instead of physical inline fuses. Each output has its own red and green diagnostic LEDs. A green light means everything is working, while a red light shows a tripped circuit. Troubleshooting a non-working light is quick—no need for a multimeter or fuse replacement..
Operating Modes (Toggle vs. Momentary vs. Controller)
How different switches control color-changing lights: If you don’t have a Shadow-NET digital controller, the SCM-PD4CH still lets you control all the colors right from a switch at the helm.
- With a standard toggle switch, quickly flipping the switch off and on lets you cycle through 12 preset color modes.
- A momentary rocker switch gives you more options. The regular side turns the lights on and off, while holding the momentary side lets you smoothly fade between colors or cycle through all 12 colors. You can also use it for single-color strobe or flashing modes.
- With a Shadow-NET controller or MFD integration, you can connect to a Shadow-Caster lighting controller, such as the SCM-ZC-KIT or MFD interface. This gives you unlimited color choices, zone control, and even music sync.
Does A Power Distribution Box Really Matter? Short Answer: Yes!
If you’re installing several underwater lights, the SCM-PD4CH power distribution box is a must-have. It keeps your wiring tidy and safe from moisture, makes installation easier, and helps you save on labor and materials. You can control single-color lights with a switch, and color-changing lights with either a switch or a controller.
SCM-PD4CH vs. SCM-PWR6
The SCM-PD4CH isn’t the only choice for boat light power distribution. Shadow-Caster also offers the SCM-PWR6 power distribution box, which adds Shadow-NET control for single-color and third-party lights.
| Feature | SCM-PD4CH | SCM-PWR6 |
| Primary Purpose | Underwater transom lights | Underwater, deck, courtesy, & accent lights |
| Channel Count | Up to 4 lights | Up to 6 lights/zones |
| Connections | Pre-wired Deutsch connectors | Terminal block wiring |
| Compatibility | Shadow-Caster underwater lights | Shadow-Caster & 3rd-party marine lights |
| Digital Protection | 15A digital overcurrent per channel | Individual ATO fused outputs |
Why do I need a power distribution box when wiring 3 or 4 underwater boat lights?
Wiring multiple high-output underwater lights through a single helm switch often exceeds standard switch amperage limits (typically 20A at 12V) due to inrush current. A power distribution box like the SCM-PD4CH draws power from a local bus bar or fuse block while using a low-amp switch signal, eliminating complex wire runs to the helm, preventing voltage drop, and providing individual digital overcurrent protection.
Where is the best place to mount the SCM-PD4CH power distribution box?
Mount the SCM-PD4CH in a dry, accessible location near the stern, such as under the deck, high in the bilge, or inside a console. Keeping the box close to the transom keeps light cable runs short, which minimizes voltage drop and allows your lights to run at peak output efficiency.
How do you control color-changing underwater lights using a helm switch?
Color-changing lights can be operated via standard toggle or momentary switches. Toggling a standard ON/OFF switch rapidly cycles through 12 preset color modes. A momentary switch uses the standard side for switching colors and the momentary push side to initiate smooth color fading (such as white-to-blue transitions) or single-color strobe modes.
What fuse size is required when connecting a power box to a positive bus bar?
If connecting the main positive power wire to a positive bus bar instead of a dedicated fuse block, you must install an inline fuse rated to at least twice the total continuous amperage draw of all connected lights to safely handle startup inrush current.
How do the diagnostic LEDs on the SCM-PD4CH work for troubleshooting?
Each channel on the SCM-PD4CH includes an individual LED indicator. A green light indicates normal operation, while a red light indicates a tripped circuit from the digital overcurrent protection. This allows instant visual troubleshooting without needing physical fuse replacements or a multimeter.
What is the difference between the Shadow-Caster SCM-PD4CH and SCM-PWR6?
The SCM-PD4CH is a 4-channel watertight power relay box with pre-wired Deutsch connectors and 15A digital protection designed specifically for underwater lights. The SCM-PWR6 provides 6 individual fuse-protected channels using terminal block wiring, making it ideal for controlling a combination of underwater lights, deck lights, courtesy lights, or third-party marine lighting via Shadow-NET.
Buy your SCM-PD4CH today!