This is the second in a three part series of technical briefs on circuit protection. Part 1 showed how to choose the correct wire size for DC circuits, and Part 3 will detail how to select a circuit breaker when installing an AC or DC product.
U.S. Coast Guard regulations, and ABYC E-11.10.1.1.1, require every ungrounded wire except those in the engine starting circuit to have circuit protection. Many DC installations employ fuses rather than circuit breakers. Choosing correct fuse amperage and the best fuse holder or fuse block will help protect your boat and your safety.
Always select a fuse size to protect the wire according to its rating. In some cases, a product manufacturer will specify a fuse value and it will usually be lower than that required for protecting the wire. If this fuse value is too high to protect the wire, use a bigger wire in the circuit. Review Part 1 for more information on choosing wire size for a DC circuit.
Follow the steps to choose fuse amperage and a fuse holder or fuse block.
Choosing the Fuse Amperage
In this step, find appropriate fuse amperage within a range of minimum and maximum
A Find the MAXIMUM FUSE AMPERAGE by following your AWG wire size (from Part 1 of this series) across the chart. Maximum fuse amperage reduces nuisance blows but offers less protection for the wire. Select:
- Single Wire or Bundled Wire column, and
- Outside Engine Room or Inside Engine Room grey bar
Example: For a 4 AWG single wire outside an engine room, maximum fuse amperage is 150A.
B Calculate MINIMUM FUSE AMPERAGE by multiplying product amperage rating by 125%. Minimum fuse amperage provides more protection for the wire but may result in nuisance blows.
Example: 80A x 125% = 100A.
Example: 80A x 125% = 100A.
C Choose FUSE AMPERAGE near the middle of this range. Mid-range values are usually acceptable. Consider the requirements for each individual circuit.
Example: 125A is between 100A minimum and 150A maximum.
Example: 125A is between 100A minimum and 150A maximum.
D Find AVAILABLE FUSES by using the chart.
Example: Fuses available at or near 125A include MIDI®/AMI®, MRBF, MEGA®/AMG®, and ANL fuses.
Example: Fuses available at or near 125A include MIDI®/AMI®, MRBF, MEGA®/AMG®, and ANL fuses.
Choose a Fuse Holder or Block
With fuse amperage and available types in mind, find the right fuse holder or block
A Using the same colored headings as in the steps abovefollow the columns down to find fuse holders or blocks that meet your specific requirements.
B Consider environmental factors
- Insulating covers protect fuses from accidental shorting
- Ignition protection is required where flammable vapors may accumulate
- Ingress protection protects fuses from spray, washdown, or humidity
C Decide between an in-line fuse holder or block
- Fuse holders are compact and hold a single low-amperage fuse
- Fuse blocks mount to a solid s
urface and may hold a single fuse or multiple fuses
