Autopilot Power Protection: Fuses and Breaker Sizing

Why power protection matters for an autopilot

An autopilot is an electrical system that draws power from your boat's DC supply. When you choose a marine autopilot, you are also choosing its electrical requirements. The fuse or breaker in that supply line has two jobs: protect the wiring and the autopilot from overcurrent, and let the autopilot work without nuisance trips. If the protection is too small, it trips or blows during normal operation. If it is too large, it may not protect the circuit as intended.

Voltage drop is the other side of the same question. A fuse or breaker that is undersized, or wiring that is too thin, can reduce the voltage the autopilot sees. Many autopilot components, including control heads and drive units, are sensitive to low voltage. The maker's manual states the supply voltage range and the recommended protection for each component. For more on how autopilots draw power, see autopilot power consumption.

What the maker specifies in the manual

Every autopilot pack and drive unit comes with a manual that lists the electrical requirements: supply voltage, maximum current draw, and the fuse or breaker rating for the power feed. When you are looking at complete autopilot packs, that rating is the starting point for your choice. The manual also states the wire gauge for the run from the battery or distribution panel to the autopilot.

The maker's page for a product line may give general electrical details, such as the supply voltage range. For example, the Furuno NavPilot 300 page lists "12-24 VDC" as its power usage. The specific fuse rating and wire size are in the installation manual, which is the authority for your boat. For more on voltage systems, see autopilot voltage system.

How fuses and breakers work in a DC circuit

A fuse is a one-time device that melts when current exceeds its rating. A breaker is a resettable switch that opens the circuit on overcurrent. Both are rated in amperes. The rating is the continuous current the device can carry without opening. Above that, it opens after a delay that depends on the amount of overcurrent.

Autopilot drives draw more current when they are actively steering than when they are idle. The drive motor may have a surge on startup or when the rudder moves against a load. The fuse or breaker must handle that surge without opening, while still protecting the wire. The maker's rating accounts for the drive's characteristics. For more on drive types, see autopilot drive types explained.

Avoiding nuisance trips

A nuisance trip happens when the fuse or breaker opens during normal autopilot operation. The most common cause is a protection device rated too close to the autopilot's running current, leaving no margin for surges. Another cause is a weak connection or corroded terminal that adds resistance and heat, which can make a breaker trip early.

To avoid nuisance trips, use the exact rating the maker specifies. If you are replacing a fuse or breaker, match the amperage and the type (slow-blow or fast-acting) that the manual calls for. A slow-blow fuse tolerates short surges, which is often what a drive motor needs. A fast-acting fuse is for circuits that must open quickly. If you are unsure about the installation, see autopilot installation skills.

Voltage drop and wire size

Voltage drop is the loss of voltage along a wire due to its resistance. The longer the wire run, the larger the drop for a given gauge. A drop that is too large can cause the autopilot to operate poorly or shut down. The maker's manual specifies the minimum wire gauge for the distance from the battery to the autopilot. For more on cable length, see autopilot cable length.

The fuse or breaker protects the wire, so the wire gauge and the protection rating must match. A thicker wire can carry more current without overheating, allowing a higher-rated fuse. A thinner wire needs a lower-rated fuse to protect it. The manual's table for wire gauge and fuse rating is the guide for your installation.

Questions to ask your installer

If you are not doing the installation yourself, ask a qualified installer these questions before work begins: What fuse or breaker rating does the manual specify for my autopilot model? What wire gauge is needed for the run from the battery to the autopilot? Is the protection device located close to the battery, as recommended?

Also ask: Does the autopilot have a separate feed for the drive and the control head, or one combined feed? Is there an in-line fuse near the autopilot in addition to the main breaker? The answers will tell you whether the power protection is set up to avoid nuisance trips and voltage drops. For powerboat-specific guidance, see powerboat autopilots.

What the catalogue cannot say

A product listing on this site states the parts in the box, the steering type, and the model line. It does not state the fuse rating for your boat's wiring. That rating depends on the wire length, the wire gauge, and the autopilot model, all of which are in the maker's manual. When you compare core packs, see autopilot core packs.

The listing also cannot tell you whether a particular fuse or breaker will trip on your boat. That depends on the condition of your electrical system and the load. The manual and a qualified installer are the sources for that answer.

What to pick for your situation

Questions

What size fuse do I need for my autopilot?

Use the fuse rating stated in the autopilot's installation manual. That rating is based on the drive's maximum current draw and the wire gauge. Do not guess from the boat's size or the autopilot's model name.

Why does my autopilot fuse keep blowing?

A fuse that blows repeatedly usually means the circuit is overloaded, there is a short, or the fuse is too small for the drive's surge. Check the manual's rating and have a qualified installer inspect the wiring.

Can I use a breaker instead of a fuse?

Yes, if the breaker has the same amperage rating and type (slow-blow or fast-acting) that the manual specifies. A breaker is resettable, which is convenient, but it must protect the wire the same way a fuse would.

What happens if my autopilot wire is too thin?

Thin wire causes voltage drop, which can make the autopilot operate poorly or shut down. The manual specifies the minimum wire gauge for the run length. Use that gauge or larger.

Where should the fuse or breaker be located?

The protection device should be as close to the battery as practical, per the maker's manual and standard marine practice. This protects the entire wire run from the battery to the autopilot.