Circuit Protection for Your DC Electrical System

Too much amperage flowing through a wire can cause wire conductors to overheat, burn and melt, possibly
resulting in fire. To prevent this, a "weak link" is intentionally installed in an electrical circuit. The weak
link is called a circuit protection device, or CPD. CPDs are also installed to protect electrical or electronic
equipment from damage.

How much is too much current? That depends on the size of the wire. To calculate proper wire size, use the Wire Sizing Chart below.

1. Determine the maximum sustained amperage of the circuit.
2. Measure the length of the circuit from the power source to the load and back. Then multiply the maximum
current by the length of the circuit to calculate Famps (Feet x amps).
3. Determine the voltage drop. In general, items which affect the safe operation of the boat and passengers
(running lights, bilge blowers, electronics, and distribution panel supply circuits) use a 3% voltage drop. All other loads, such as cabin lights and bait pumps, use 10%.
4. Determine whether the circuit runs inside or outside an engine space. Engine spaces are assumed to be at 122°F (50°C); non-engine spaces are assumed to be at 86°F (30°C).

Select the column showing the correct voltage and voltage drop for your circuit. Look down the column until you find a value greater than your calculated Famps. Move right to the ampacity columns (engine and non-engine) to verify that the total amperage of the circuit does not exceed the maximum allowable amperage of the wire size. If it does, move down until the wire ampacity exceeds the circuit amperage. Finally, move right to select the wire size.

Examples:

A 12-volt system at 10% drop with a 40' circuit x 45 amps=1,800 Famps. Use size 8 wire.

A 24-volt system at 3% drop with a 10' circuit x 100 amps=1,000 Famps. Use size 6 wire.

Once the correct sized wire is installed, circuit protection should also be installed. A circuit may be accidentally
grounded, allowing a dangerous amount of amperage to flow. A circuit can be grounded by a wire's insulation chafing through and contacting a grounded conductor, by the failure of equipment in the circuit, or by accidental grounding of the circuit during maintenance.

CPDs stop amp flow by one of two primary methods. Thermal devices open to break the circuit and stop the
current flow in response to heat generated by excess amperage. Magnetic devices react to a magnetic field
created by excess amperage.

Fuses are thermal devices that open the circuit by utilizing a "fusible link" that melts at a known amperage.
Circuit breakers can be either thermal or magnetic devices or a combination of the two.

Every wire in the boat should be protected by a fuse or a circuit breaker. The CPD must be correctly sized to
the wire it protects. As wires branch away from batteries or other power sources and become progressively
lighter, smaller CPDs must be installed at the beginning of each wiring run if the wire ampacity is lower than the
rating of the last CPD.

The CPD should be rated to open at an amperage greater than the maximum load the circuit will carry and
smaller than the rated amperage capacity of the wire. We recommend choosing a size as close to, but not
greater than, the amperage capacity of the wire. The amperage rating is the amperage on which the opening
speed of the fuse is based. This is the number printed on the fuse and the most common way of referencing
fuses. Most fuses will operate indefinitely at their amperage rating. Only when the amperage rating is exceeded by some significant percentage (usually at least 20%) will the fuse open or "blow."

Opening speed defines the relationship between the percentage by which the fuse is operating over its
amperage rating and the length of time it takes to open. When the fuse "blows" or the circuit breaker "trips"
depends on both the quantity and the duration of the excess amps flowing though the circuit. Other factors
influence a fuse's opening, but are not significant to this discussion.

The interrupt rating specifies how much current the fuse can safely handle in short circuit situations. The voltage rating specifies the maximum voltage for the circuit in which the fuse is used.

How to Select Primary Circuit Fuses and Circuit Breakers

1. Determine whether a fuse or a circuit breaker best suits your needs.

Fuse Advantages:
• generally lower cost
• available in higher amperage ratings
• available in higher interrupt ratings
• generally available in greater size ranges
• can be changed without replacing the fuse block or disturbing the connected wires

Circuit Breaker Advantages:
• resettable after opening
• can be used as a switch
• can be waterproof and vaporproof
• available with a wide range of opening speeds

Note that if the CPD is in an explosive area such as engine room, battery compartment, or propane locker, a vaporproof circuit breaker such as the Blue Seas 7000 or 7100 Series is required.

2. Consult the ABYC Interrupt Rating table above to determine the minimum rating for your application.

3. Determine the maximum continuous current that will flow in the circuit.

4. Determine the amperage capacity of the smallest wire or other conductive element in the circuit.

5. Select a fuse or circuit breaker amperage rating that is larger than the maximum continuous current that will flow in the circuit (3, above) and smaller than the amperage capacity of the smallest wire (4, above). We recommend that the CPD amperage rating be at the upper end of the range to allow for surge currents and for growth in the number of devices on the circuit.

6. Verify that the voltage rating of the selected CPD meets or exceed the circuit voltage.

Information Courtesy of Blue Sea Systems