Fuses & Circuit Breakers: Overcurrent Protection (OCP) on Boats

Fuses and circuit breakers can save your boat! In this article, BoatHowTo co-founder Nigel Calder explains in detail, why overcurrent protection (OCP) devices are so important and how to install fuses and circuit breakers on your boat.

This article was initially published in Professional BoatBuilder Magazine.

If you want to learn how to install and maintain a DC system on your boat THE RIGHT WAY, check out our Boat Electrics 101 online course.

I have, on occasion, demonstrated what happens when I use a length of 12-gauge (AWG 12/4mm²) cable to put a dead short across a 12V battery. The moment I throw the switch on the circuit, the cable vaporizes, filling the entire room with a cloud of noxious smoke and forcing everyone to hastily evacuate.

I am not suggesting that anyone try this experiment, but I frequently see what is, in effect, a similar test apparatus on boats I inspect. It is done by wiring one or more pieces of equipment directly to the boat’s batteries without installing a fuse or circuit breaker for overcurrent protection (OCP) at the battery’s positive connection, thus setting the stage for a cable meltdown. All it takes to set things in motion is a short to ground, such as may occur if the insulation is damaged or a terminal comes adrift and contacts a grounded object, or someone installing new cabinetry puts a screw through the wiring.

In complex systems, it's hard to check if every conductor is protected. Potential fire risks are a common find even on brand new boats.

Unfortunately, the plethora of electric circuits on modern boats, as compared to boats of even a few years ago, has greatly increased the likelihood of unprotected circuits. Worse yet are older boats upgraded by tacking on additional circuits, rather than by rewiring the boat. And, generally, worst of all are boats on which the owners themselves have done the wiring.

A common culprit is the small-gauge voltage-sensing conductors for systems-monitoring devices: wherever these connect to the positive side of a system, they need the correct fuse (typically 1 or 2 amps).

Insurance statistics from Boat U.S. show that 55% of boat fires are electrical in origin, of which more than half are caused by short circuits, most of them in the DC circuits. By comparison, engine overheating causes 22% of fires, and fuel leaks (almost all on gasoline-engine boats) cause another 8%. So be warned: If you, or anyone else, adds any circuits to a boat, make sure they are properly protected. 

Better yet, make a complete wiring diagram for the boat and ensure that all existing circuits are properly protected.

We teach you in detail how to create such a wiring diagram for your own boat in our Boat Electrics 101 online course.

It also amazes me how many boats with one or more unprotected or inadequately protected circuits come from boatbuilders. (Note that circuits used solely for data transfer between electronic devices, and wiring to temperature sensors, are the only ones that do not require OCP.)

All of which leaves us with three questions:

  • What size fuse or circuit breaker should be used on a given circuit?
  • What is the best practical method for ensuring that all circuits are properly protected?
  • Where in a circuit should OCP be located?

Sizing Fuses or Circuit Breakers

A popular misconception is that fuses and circuit breakers should be sized according to the current rating (amperage) of the equipment in a circuit. This betrays a misunderstanding of the purpose of the overcurrent device: to prevent the wiring in the circuit from melting down in the event of a short circuit. Since the most heat in any circuit will be developed by the section of wiring with the highest resistance (i.e., the smallest conductor), an overcurrent device is sized to protect the smallest conductor in the circuit. This includes the wiring on the negative and the positive sides of the circuit.

Note: This text deals with DC systems where the negative side is connected to ground. Isolated two-pole systems, which you can find on some metal boats, have different requirements, such as two-pole circuit breakers in all conductors except for the main battery cables.

For example, if a GPS, which draws minimal current, is hooked into a circuit wired with 12-gauge (4mm²) conductors, the overcurrent device should be rated to protect the conductors, not the GPS, from melting down. Something on the order of a 20-amp fuse or breaker will do fine. Separate protection (generally a fuse, probably as low as 1 amp) will be needed for the GPS or any other equipment on the circuit.

If, however, a circuit breaker in a distribution panel protects a circuit to a single load, and the protection required by the load is less than that required by the conductors, the breaker can always be sized to protect the load, down to about 5 amps, typically the smallest breaker available. If, on the other hand, the same approach is used for a breaker that protects several circuits, the sizing is more complex. Two factors must be considered: (1) the total load of all the electrical equipment to be served by the breaker, and (2) the current-carrying capability (ampacity) of the smallest conductor being protected. The breaker must be sized according to the lower of the total load or current-carrying capability of the smallest conductor.

If a breaker feeds another subsidiary panel or fuse block, and if all the conductors from the second panel or fuse block to individual appliances are protected by their own breakers or fuses, the only conductor the first breaker has to protect is the feeder cable to the subsidiary panel. In this case, the breaker will be rated according to the lower of the total load on the subsidiary panel, or the current-carrying capability of the conductor to the feeder panel. The same design approach governs the sizing of fuses for battery cables. The fuse generally protects the circuit up to the main breaker in the panel, in which case it is sized to protect the smallest conductor up to the panel.

Wires at the positive pole of a used boat, badly installed and no OCP in sight. There are many such installations out there and all of them are a fire risk!

Ampere Interrupting Capacity

The primary rating for fuses and circuit breakers is the amperage at which they are designed to blow or trip. Buried in the small print is another amperage known as the Ampere Interrupting Capacity (AIC). This is typically thousands of amps, and tells us the maximum current the device can safely interrupt if it gets hit by an enormous spike, such as might occur with a dead short across a battery or in the event of a lightning strike. If the current goes above the AIC level, the device may arc across or form some other conductive path, and fail to do its job.

The ABYC requires AC and DC circuit breakers to have a minimum AIC rating of 3,000 amps in most circumstances, and 5,000 amps in some. The ABYC’s E-11: AC and DC Electrical Systems on Boats has detailed prescriptions. The primary fuse on any relatively large battery bank needs to have an AIC rating of at least 5,000 amps and, in some cases, much more than this. (Lithium-ion batteries, for example, may be capable of creating short-circuit currents of tens of thousands of amps.) E-11 provides a table and procedure for determining the necessary AIC rating for that all-important first fuse in a DC system, and for the main breaker in an AC system. If a battery system requires a very high AIC rating, Class-T fuses will do the job. These are widely available and not that expensive.

Defining Limits

The current-carrying capability of conductors is given in various tables. These are used to determine the maximum circuit breaker or fuse rating (you can always go lower). In the American Boat & Yacht Council (ABYC) table, for a given conductor size (e.g., AWG 12/4mm²) there are various columns with different temperature ratings (e.g., 90°C/194°F; 105°C/221°F), with each temperature rating column divided in two (“Inside Engine Spaces” and “Outside Engine Spaces”).

The ABYC ampacity table for unbundled conductors. You can download this table as a PDF here.

The conductor size (e.g., AWG 12/4mm²) defines the diameter of the copper core in the conductor. When current (amperage) is run through this copper, its internal resistance causes some heating. The higher the melting point of the insulation wrapped around the copper, the more amps it can carry, hence the various columns with insulation temperature ratings. The higher the insulation temperature rating, the more amps we can safely put through a given piece of copper.

Cable Sizing: ISO versus ABYC, and AWG versus SAE

U.S. conductor sizes are counterintuitive—the bigger the conductor, the smaller the number. Eventually, the system gets down to 0 and runs out of numbers, at which point as conductors get even bigger the zeros are stacked up (e.g., 00, commonly written as 2/0, or 000, written as 3/0, and so on). The European system simply measures the diameter, or cross-sectional area, of the copper in a conductor and uses that.

Not only are U.S. cable sizes counterintuitive, but there are also two different conductor-sizing formulas using the same numbering system. One is American Wire Gauge (AWG); the other has been developed by the Society of Automotive Engineers (SAE). For a given conductor size (e.g., 16-gauge), SAE-rated cables have approximately 10% less copper than AWG-rated cables. SAE sizing is acceptable in DC circuits, but is not acceptable in AC circuits.

To avoid confusion, the best practice is to use AWG-sized cables in all applications in the U.S.

In the rest of the world, a conductor's cross-sectional area is typically given in mm² which makes calculations more intuitive.

For cabling in an engineroom, it is assumed that engine operation will raise the ambient temperature (to 50°C/122°F in the case of the ABYC standard in the U.S., and 60°C/140°F for the ISO norm in Europe), in which case the conductor is already warmed up before any amps are run through it. This preheating means the conductor can carry fewer amps before the insulation reaches its temperature limit, which explains the “Inside” and “Outside” engineroom sub-columns for each insulation temperature rating.

This text deals mostly with sizing conductors according to the American ABYC standards. If you live outside the US, where cable sizes given in mm² are more common, you can use the ampacity tables according to the ISO standard.

A conductor can also be preheated if it is installed in a bundle with other cables that are under load and have warmed up. To cope with this situation, the higher the number of conductors in a bundle, the more the current-carrying capability (ampacity) of the individual conductors is de-rated. This is done by applying a de-rating factor. Until recently the ABYC applied this de-rating of bundled conductors only to circuits carrying more than 60V, which in practical boating applications means AC circuits, but the ABYC now also partially applies de-rating to DC circuits.

Derating factors for bundled conductors for AC and DC circuits. You can find a PDF version of these tables and an explanation on how to use them here.

A coiled shore-power cord sitting in the sun is one instance not covered by bundling and other rules for which a cable should still be derated because of high temperature. The combined effects of the coiling (in effect, a form of bundling) and solar heating can effectively derate a shore-power cord by as much as 50%. Another example would be conductors run against insulated surfaces, or inside a conduit surrounded by insulation. A conductor should be derated any time there is any obstruction to dissipating the heat generated within it.

Use high-quality cables, preferably with a high insulation temperature rating.

Finally, note that most cabling used in U.S.-built boats has an insulation temperature rating of 105°C/221°F, as opposed to the 90°C/194°F or even 60°C/140°F cabling found in many European-built boats. The insulation temperature rating should be printed on the cable, and if it is not, the cable does not comply with ABYC standards. Most devices attached to this cabling are rated at 90°C/194°F or lower. If the cabling is run continuously at its maximum rated ampacity, heat generated in the conductors will be transmitted to attached devices, potentially exceeding their temperature ratings. In general, even with 105°C/221°F conductors, it is best to use the 90°C/194°F or lower insulation temperature rating columns for determining the maximum allowable ampacity and OCP rating for a given circuit.

Determining Ampacity

When using the ABYC table, for any given conductor size the maximum allowable OCP rating is found by looking up the conductor size in the left-hand column and then tracing across the row to the appropriate insulation temperature rating column. As noted above, in some circumstances a lower conductor insulation rating may be selected. If any part of the circuit being protected is inside an engineroom, the Inside Engine Spaces sub-column is used to determine the maximum allowable OCP, but otherwise, the Outside Engine Spaces sub-column is relevant. If the conductor is in a bundle, apply the appropriate de-rating factor based on the number of conductors in that bundle. The result is a maximum allowable OCP rating for this conductor.

If there is no exact match between the resulting conductor ampacity and available fuse or circuit breaker ratings, the ABYC standards permit the next highest rated fuse or breaker to be used on all but main distribution panel fuses and breakers (these cannot exceed 100% of the circuit rating), as long as the fuse or breaker’s rating does not exceed 150% of the ampacity of the conductor it is protecting. For example, if a conductor is rated at 17 amps in a given environment, but the only fuses and breakers available are rated at 15 and 20 amps, the 20 amp breaker, at 117% of the conductor rating, is acceptable.

Circuit breakers need to match the ampacity of  the cables they are protecting!

Simplified Selection Procedure for Overcurrent Protection Devices

  1. Determine whether a fuse or a circuit breaker is more suitable for the task.
  2. Determine the ampacity of the smallest conductor to be protected by the overcurrent device.
  3. Determine the maximum continuous current that will flow on the circuit.
  4. Check the ABYC AIC ratings tables to find the minimum acceptable AIC for the circuit.
  5. Select a fuse or circuit breaker to meet the AIC rating and that has an amp rating no higher than the conductor ampacity, and at least as high as the continuous load (as long as this load does not exceed the ampacity of the conductor). 6. If this is a circuit to an electric motor, check that the fuse or circuit breaker will respond slowly enough to an overload condition to handle the start-up (inrush) current without nuisance blowing or tripping.

(Adapted from a procedure courtesy Blue Sea Systems)

Electric motors commonly have their own OCP. If this is not the case, the overcurrent protection on circuits with motors should be rated at 100% of the load, assuming the conductor has at least this ampacity. OCP devices for motor loads have to be designed to handle the start-up, or inrush, current, which may be up to six times the running load. This requires a fuse or circuit breaker with a slow response to overload conditions—generally a slow-blow fuse or a thermally rather than magnetically operated breaker.

In our Boat Electrics 101 online course, we talk in detail about tripping times for circuit breakers and when to choose a slow-blow vs. a fast-blow fuse.

One situation needs particular attention. If a motor gets into what is known as a locked-rotor state (meaning the motor is jammed—for example, a centrifugal bilge pump with a piece of trash in the impeller), its current draw can rise sharply, potentially overloading the circuit and/or melting down the pump housing. If the rise in current is high enough and the circuit or motor is properly breakered or fused, the breaker or fuse will trip or blow.

But sometimes, particularly with bilge pumps, there is a long run of marginally sized wiring to the motor. If a locked-rotor condition develops, the total resistance in the circuit may be sufficient to limit the current flow to a level that will not trip the breaker or blow the fuse but which nevertheless is high enough over time to start a fire at the pump. Using a lower-rated fuse or breaker is not the answer, because it may lead to nuisance blowing/tripping with normal inrush currents. The conductor size must be increased so that the voltage drop on the circuit is lowered to a level at which sufficient current to trip the breaker or blow the fuse can flow. In any event, the ABYC requires that all motors and motor circuits be designed and protected so that they can withstand a locked-rotor condition for seven hours without creating a fire hazard. (Typically, if the motor might melt down, it is given an internal heat-sensitive trip device; some of these reset themselves once the motor has cooled off, while others require manual resetting.)

High-Current Circuits

Historically, high-current DC circuits (100 amps and higher) have been the hardest to protect simply because of the lack of affordable OCP devices. Relatively cheap and compact breakers rated at up to 150 amps are now widely available, together with cheap, compact fuses up to 800 amps. There is simply no excuse for not protecting all high-current circuits, including cranking circuits.

Typical amperage ratings for common marine fuse types. Other sizes might be available but hard to find.

High current circuits have large, awkward-to-handle conductors. The neatest way to deal with them, and provide the necessary overcurrent protection, is to run a single heavy conductor, normally 2/0 gauge (70mm²), from the positive post on the house batteries to a battery-isolation switch. This conductor must be properly fused as close to the battery as possible. If the conductor is bolted (as opposed to clamped) to the battery post, there are purpose-built fuses that can be added at the battery post itself (MRBF fuses).

More comfort on board means more high-current circuits to be protected.

Another conductor (the same size as the first) is run from the isolation switch to a high-current bus bar, which is nothing more than a heavy copper plate (preferably tin-plated for corrosion resistance) drilled and tapped to take a series of bolts. The conductor from the isolation switch is bolted to the bus bar. The bus bar is mounted on a phenolic or plastic base plate, with another series of bolts set up parallel to the bolts in the bus bar. Fuses are bolted in place between the bus bar and the second set of electrically isolated bolts. With this installation you can attach the boat’s various high-current circuits, such as a windlass, bow thruster, high-output alternator, the DC panel, etc., to the isolated bolts.

The various fuses are sized according to the current-carrying capability of the conductors bolted to them. For example, a windlass may need an AWG 2/0 (70mm²) cable with a 300-amp or lesser fuse, while a distribution panel will normally have cables on the order of AWG 4-gauge (25mm²), protected with a fuse of around 150 amps or less.

Circuits Bypassing the Isolation Switch

Some circuits will still need to bypass the isolation switch so that they may be left on when the rest of the boat is shut down. These circuits typically include a bilge pump and any charging devices (including solar panels, maybe a wind generator, and the inverter if it also doubles as a battery charger). Other examples are a carbon monoxide or propane alarm, DC-systems-monitoring devices, power supplies to electronics with a memory function, impressed current cathodic protection systems, and sometimes a refrigerator and freezer, although these last two are not currently permitted under ABYC standards.

This is part of the circuit diagram we developed for the complex example boat in our Boat Electrics course. The devices connected to the fuse block in the upper right are all bypassing the isolation switch S1. If you sign up for Boat Electrics 101, you will learn how to read such a diagram and also how to make one for your own boat.

All devices are wired to a separate auxiliary bus bar or fuse holder fed by a fused conductor that is tied into the battery side of the isolation switch. Each circuit is, in turn, fused at the bus bar. 

The net result of such an approach is that every single circuit on the boat will be fully overcurrent protected at its source. If the boat is wired as suggested, the circuits that bypass the battery switch will be fused but not switched; in other words, they can never be turned off. I prefer this method, because it prevents the bilge pump and other key circuits from being accidentally turned off, but others prefer a switch or circuit breaker in this circuit.

Location of Fuses and Circuit Breakers

The goal is to provide protection at the source of power for each circuit. With DC circuits, the OCP is always placed in the positive side of DC circuits. (Apart from anything else, an unbroken connection to DC negative must be maintained at all times to protect against stray-current corrosion.)

Note that some European boatbuilders install fuses and battery switches in the DC negative side and on the positive side, but this is not recommended except in some isolated (floating) ground DC systems.

The ultimate source of power for any DC circuit is the battery, but the practical source of power is the connection point of the circuit to the DC system. This point may be at the battery, the battery switch, the distribution panel, a subsidiary panel, some distribution bus bar, or other connecting point.

If the conductors in the new circuit are no smaller than the conductor that feeds the new circuit’s point of connection, then the OCP for the feeder conductor will sufficiently protect the new circuit. The same will apply even if the feeder conductor is larger than the cables in the new circuit, as long as the feeder conductor's OCP has been downsized to the point at which it effectively protects the new circuit. But if the new circuit is not adequately served by overcurrent devices already in place, additional protection is required at its point of connection, i.e., at its source of power.

Placement of fuses in the positive conductor. Note how a smaller fuse is used every time a smaller conductor is connected

Sometimes space limits how close an OCP device can be placed to the source of power. (In any case, fuses, circuit breakers, and switches are best not installed in battery compartments, because of the risk of corrosion coupled with the potential presence of explosive gases. If they must be installed in battery compartments, they should be ignition protected.) The question then arises, how close is close enough? The generic answer, from the ABYC, is within 7" (17.5cm).

The ABYC allows the following exceptions, which have been tightened up in recent years:

  • A conductor connected directly to a battery that is also “contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel” must have its overcurrent protection “as close as practicable to the battery, but not to exceed 72 inches (1.83m).” Note the “as close as practicable” phrase. Gone is the blanket 72" allowance that used to be there.
  • A conductor connected to a source of power other than the battery (e.g., the battery switch, the distribution panel, or some other point in the DC circuits) that is similarly contained in a sheath, etc., must have its overcurrent protection “as close as practicable to the point of connection to the source of power, but not to exceed 40 inches (1.02m).” Once again, the “as close as practicable” phrase has replaced the former blanket acceptance, 40 inches here.
  • “Overcurrent protection is not required in conductors from self- limiting alternators with integral regulators if the conductor is less than 40 inches (1.02m) in length, is connected to a source of power other than the battery, and is contained throughout its entire distance in a sheath or enclosure.” This exception covers the standard alternator that comes with a marine engine or generator and is wired to the starter motor solenoid, and the batteries, and which is almost never overcurrent protected.
  • If the regular alternator is replaced by a high-output model with an external regulator, or an alternator with an internal regulator but which is wired back to the batteries, or if a second alternator is fitted with an external regulator or wired back to the batteries, the new alternator requires OCP at its point of connection to the source of power—i.e., wherever it ties into the DC system. Given that alternators themselves are a source of power, it has been debatable whether these added alternators required OCP at the alternator itself. This has been resolved in the latest version of the ABYC E-11 standard as follows: “Overcurrent protection is not required at an alternator if the ampacity of the conductor is equal to or greater than the rated output of the alternator.”
  • Cranking-motor circuits are not required to have overcurrent protection. This exception originates in the automotive field, where starter-motor cables are short and well secured. In the marine field, where cranking circuits may be long, this practice might create a hazard. If a car catches fire, the occupants can pull over and jump out. If a boat catches fire, it is not so simple. It makes no sense to have any unprotected circuits on a boat. Protecting cranking circuits, however, is problematic. In cold weather, the inrush current on a 12V starter motor may be as high as 1,500 amps; the cranking current may be as much as 200 amps. Often, cranking conductors are undersized even for the cranking current, let alone the inrush current. This situation does not pose a safety problem per se, because these currents are sustained for only a few seconds, so the conductors do not have time to get hot enough to create a fire hazard. But if OCP is based on the ampacity of the conductors (as required by ABYC standards for all other circuits), any fuse will blow whenever the engine is cranked. My solution is to oversize the fuse to avoid nuisance blowing, knowing that in the event of a dead short, the very high current flows will cause the fuse to blow. (For the typical auxiliary diesel with a 12V starter motor, I use a 300-amp slow-blow fuse. Given that the standards require no fusing in the cranking circuit, I am not concerned by the fact that this fuse does not comply with the general prescription for sizing overcurrent devices).
This article is quite technical and if you are new to boat electrics, you might be overwhelmed by the amount of information. If you want to learn how to wire a boat, step-by-step in 56 video lessons, check out our Boat Electrics 101 course. We made the course with absolute beginners in mind.

Check All Boats

As the electrical load on boats increases, so too does the complexity of electrical circuits and the potential for short circuits and electrical fires. It is more important than ever to wire all circuits with proper OCP, and yet so often this protection is omitted— sometimes even by the original boatbuilder.

It is important when performing electrical work on any boat to take a close look at all the conductors attached to the battery positive posts. If any are unfused (other than wiring to temperature sensors), take immediate steps to correct the situation. Ideally, all the circuits on a boat will then be traced to their final circuits, ensuring that at each reduction in conductor size there is appropriate OCP in place. The time and money will be well spent.

A fire at sea can be a terrifying experience. Smoke billowing out of the companionway hatch is not the way to find out that a boat has inadequate overcurrent protection.

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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