Maintenance

Electrical Installations, Part 2: Fuses and Breakers

Article by Andrew McDonald courtesy of Canadian Yachting magazine

boat fuse panel, boat fuses, boat wiring connections

A dash should have connections properly labeled and the correct gauge wiring.

In part one of this series, we looked at making proper electrical connections — the tools, supplies and methods needed to make connections between components and wiring. When planning out electrical work, one of the more common questions I address is on the setup, installation and sizing of breakers and fuses.



Fuses and breakers are collectively called “overcurrent protection” — and they come in many different shapes, styles and sizes. Their purpose is to prevent a situation where a larger-than-intended electrical current runs through the circuit. This puts the circuit at risk of overheating, fire and damage to equipment. A fuse is a thin metal strip that melts or breaks when too much current flows through it. A breaker is an automatic switch that opens when too much current flows through. A fuse is designed to be replaced each time it breaks (it can’t be re-used), while a circuit breaker can be re-set and re-used repeatedly.

blown fuse, barrel fuse, blown barrel fuse

An example of a fuse that has blown — note the break in the metal inside the tube.

How Breakers and Fuses Work

Excess current running through a circuit can be caused by a faulty battery, a ground fault, a short circuit or a broken/faulty appliance. In each circumstances, the fuse breaks (or the breaker trips), which stops current from running through the circuit (the circuit is now incomplete), preventing additional damage, overheat, or fire. Fuses and breakers are essential components of each electrical circuit. The most recent data suggests that electrical shorts are the Number One cause of fires on boats and that 55% of boat fires are electrical in origin (mostly in the DC circuits.)

I regularly see the following problems with overcurrent protection aboard power and sail boats:

  1. No fuses or breakers at all.
  2. Not every appliance has a separate fuse or breaker.
  3. Improperly sized fuses or breakers.
  4. When fuses or breakers trip regularly, the fault isn’t investigated further.

boat distribution panel, main distribution panel, MDP

An example of a distribution or breaker panel, with breakers of varying amperage installed. Note on the AC panel, the breakers have yet to be installed. The tech is waiting to determine the amperage of the appliances and the ampacity of the wiring

ABYC Standards

The American Boat and Yacht Council (ABYC) sets out a list of standards that marine mechanics, technicians, and manufacturers should follow with electrical installations and repair. These standards (broken down and paraphrased) recommend the following:

  • Every electrical circuit on a boat must be protected in some way — usually using a fuse or breaker.
  • The one exception are the circuits used solely for data transfer between electrical devices.
  • A fuse or breaker should not be chosen based on the current rating of the appliance (example: a light that draws 2 amps), but rather based on the size of the smallest wire in the circuit (as the main concern is the protect the wiring in the circuit from melting down). This is determined by checking the ampacity of the smallest cable, using an ampacity chart.
  • Ensure that the fuse or breaker capacity is no higher than the cable ampacity. At least as high as the continuous load of the appliance (as long as the load doesn’t exceed the ampacity of the cable.)

ampacity chart, ampacity ratings

An example of an ampacity table published by ABYC.

A final note on repairs to electrical systems: if a fuse blows regularly or if a breaker trips continuously, there is a problem to be identified (a short circuit, faulty appliance, or ground fault) that should be investigated and repaired.



NOTE: Installing a larger size fuse is not an appropriate solution. The fuse/breaker is meant to be the first part of the circuit to fail. If a larger size is installed, this may mean that the wiring becomes the first part of the circuit to fail — which can cause melted wiring or a fire. The most appropriate action to take is to determine the cause of the circuit overload, repair it, replace or upgrade any wiring or connections (as necessary) and maintain the fuse size to protect the circuit correctly. If in doubt, check ampacity tables, ABYC electrical standards, and/or a qualified marine technician.

Article courtesy of Canadian Yachting magazine

Andrew McDonald is the owner of Lakeside Marine Services — a boat repair/maintenance firm based in Toronto. Andrew has worked in the marine industry for 12 years and is a graduate of the Georgian College Mechanical Techniques — Marine Engine Mechanic program.