The short answer
To wire a 12 V or 24 V lighting circuit, first add up the power of the fixtures that will operate together, calculate the current, then size the conductor based on its total length (out-and-back) and the allowable voltage drop. The protection is chosen to protect the cable and must be compatible with the DC voltage on board. Each significant branch should have its own identifiable switch and protection. Check the voltage rating of each fixture, the dimmer compatibility, and the quality of connections in wet areas. After installation, measure the voltage directly at the fixtures when they are on: a circuit that works at no load can still drop, flicker, or overheat under real load.
Inventory loads by circuit
Do not start from the rating available at the distribution panel. List each fixture, its rated power or current, and its usage mode. Group loads by zone and function: cabin, circulation, cockpit, deck light, and technical lighting.
For a first-order current estimate in DC, use I = P / U. Four 5 W fixtures total 20 W, about 1.7 A at 12 V or 0.8 A at 24 V, before margins and losses. However, use the manufacturer’s documented consumption when the fixture includes electronics or multiple modes.
The Mantagua catalog lists bulbs, strips, ceiling lights, reading lights, and spotlights. Their power ratings and voltage ranges vary: do not size a circuit based on another fixture in the same family.
Calculate using total length (out-and-back)
Circuit resistance depends on the full current path. Measure the positive wire to the fixture and the negative return to the distribution point. In a mast or large cockpit, this length can be much greater than the straight-line distance.
Voltage drop increases with current, length, and conductor resistance. It decreases as cross-section increases. Victron’s official Wiring Unlimited guide recommends sizing the cable from the current and aiming for low voltage drop; it also notes that connections add their own resistance.
Do not use a universal table copied out of context. Calculate the cross-section for your current, total length, ambient temperature, and installation method, then verify it meets applicable marine installation standards.
Choose protection for the conductor
The primary role of the fuse or circuit breaker is to prevent the cable from overheating in the event of overcurrent or short circuit. Its rating must therefore not exceed what the conductor and connections can safely handle under installation conditions. It must also allow normal operation of the load.
The rated voltage of the protection must be equal to or greater than the maximum DC voltage expected, and the device must be rated for DC. Components from Blue Sea Systems and BEP are available in several formats; selection depends on current, interrupting rating, and system architecture.
Place the protection at the start of the branch, near its source or distribution point, according to the chosen schematic. A fuse added near the fixture does not protect the cable run leading to it.
Separate important functions
A single circuit for all lights seems simple, but a failure then leaves the entire boat without lighting. At minimum, separate safety functions, general lighting, and spotlights or high-load fixtures. Navigation lights must remain identifiable and not depend on a cabin dimmer or non-essential accessory.
This separation simplifies troubleshooting. If one branch trips, the other zones remain available. Label protections, document cable cross-sections, and mark hidden connection points.
Check switches, relays, and dimmers
A switch must handle the DC current of the circuit, not just an AC-like rating. A spotlight or group of strips can exceed the capacity of a small switch; a suitable relay then separates control and power.
For a dimmer, verify:
- input and output voltage;
- total LED power;
- compatibility of the fixture with dimming;
- minimum and maximum load;
- behavior at off and low levels.
Test the entire setup before final installation. Flickering, electrical noise, or overheating indicate incompatibility or poor wiring.
Make durable connections
Use conductors and connections rated for the marine environment. Crimp with the tool matching the terminal, secure the wire, and protect areas exposed to moisture. Heat-shrink tubing does not fix a poor crimp; it should not mask a poorly seated conductor either.
Avoid unnecessary splices, low points where water can pool, and edges that can abrade insulation. Allow a reasonable bend radius and provide access for maintenance without pulling on the fixture.
Commission the circuit under load
Before energizing, check polarity, continuity, and absence of short circuits. Then turn on all loads in the circuit and measure voltage at the source and at the farthest point. Inspect each connection and verify that the switch, dimmer, and protection do not overheat.
Repeat after the first sail: vibration and moisture often reveal loose connections or insufficient cable support.
Frequently asked questions
Does a 24 V circuit always use a smaller cable?
At equal power, current is about half that of 12 V, but the final cross-section still depends on length, voltage drop, installation method, and applicable rules.
Does the fuse protect the fixture?
It primarily protects the conductor from overcurrent. It must be coordinated with the cable cross-section and remain compatible with normal operation of the load.
Can all LEDs share the same dimmer?
Only if the dimmer accepts their voltage, total power, and electronics. A non-dimmable LED or a load outside the range can flicker or fail to turn off cleanly.
Why measure voltage at the lit fixture?
Because voltage drop appears when current flows. A no-load measurement can hide a resistive connection or an undersized cable.
