Design, installation and troubleshooting of marine electrical systems            PETER KENNEDY YACHT SERVICES
                   Marine Electrical Systems


Shop for wire and cable at shop.pkys.com



These large format DC circuit breakers come in sizes up to 200 Amps and are used for winches, windlasses and all kinds of other DC loads

Shop for circuit breakers at shop.pkys.com




The ATO fuse shown here is an automotive style blade fuse for DC circuits and is available in sizes up to 40 Amps



Shop for fuses at shop.pkys.com


The Class-T fuse shown above is used to provide super fast protection for sensitive electronics such as inverters and comes in sizes from 200 to 400 Amps



Shop for Fuseblocks at shop.pkys.com

This fuseblock takes 12 ATO or ATC fuses and is used in cars, trucks, rv's, boats and for all kinds of other mobile or DC applications







This terminal fuseblock is good for retrofits because of its compact size and the fact that it can bolt right on to a battery terminal of a battery switch.  Its can take an ignition protected fuse of up to 300 Amps










                 What is Circuit Protection all about

The purpose of circuit protection is to protect the wire from carrying more current than it is able.  If the current becomes excessive the circuit protection should interrupt the circuit.  If the current is not interrupted the wire can become hot enough to melt the insulation, and the result is often a short circuit with adjacent wires causing a meltdown and possibly a fire. The principals of circuit protection are the same for AC and DC systems.

The amount of current a wire can safely carry is determined by the size of the conductor, the temperature rating of the insulation, the ambient temperature and the number of wires in the bundle.  On boats subject to ABYC standards the ambient temperature is taken to mean that wires located in the engine room are assumed to be 20 degrees warmer than elsewhere and their current carrying capacity is derated accordingly.  Wires in bundles have their current carrying capacity derated because of the assumed ambient heat of the other wires.  Boat cable normally has an insulation rating of 105 degrees C.  See the table below for ampacity ratings.

The table below is a simplified version of the ABYC  E11 Table IV B and shows the ampacity for wire with 105 degrees C insulation in bundles of no more than three. 


Ampacity for wire with 105 degree C insulation rating in bundles of up to three conductors  AMPS

Wire size in AWG

Outside engine rooms

Inside engine rooms





































Since the purpose of the circuit protection is to protect the wire the circuit protection needs to be as close to the source of power as possible.  Any conductor between the source of power and the circuit protection is not covered.  This is an important concept that is often misunderstood.  The ABYC E.11 states that on DC circuits the circuit protection must be within 7” of the power source.  It then goes on to list some exceptions for sheathed wires up to 72” and for pigtails less than 7” as well as for self limiting devices and starting motor conductors.

Starter motor exemption

Starter motor circuits are exempt from the requirement to provide circuit protection in the ABYC standards (E11.  and the Code of Federal Regulations. (Sec. 183.460)  The explanation for this is simple:  On occasion your starter motor might require all that the battery can give. Circuit protection on the wire would have to be higher than the battery can provide, which is meaningless, so the circuit is exempted from the requirement.  It’s not in the rules but common sense indicates that the unprotected wires be treated with special care in regard to chafe protection and routing.

Fuses and Circuit Breakers

There are two methods of achieving circuit protection, fuses and breakers.  Breakers can be reset, fuses contain a fusible link which melts if the current exceeds the rating and they have to be replaced when they blow.  In general breakers are more expensive than fuses, especially for higher ratings.  The choice between using a breaker or a fuse will depend on both cost and convenience;  for example having a main breaker for your DC panel  is more expensive than having a main fuse, but if you are left in the dark you would appreciate having spent the extra money on the breaker. It is always prudent of course to investigate why the breaker has tripped before resetting it.  High current loads, for example those serving inverters or alternators, usually have fuses because breakers for these kind of loads are either unavailable or too expensive.  If one of these high current fuses blows then it would be particularly prudent to investigate the cause before replacing it.

Time delay

Breakers and fuses have different time delays.  Faster time delays are used to protect sensitive equipment.  Slower time delays are often used in circuits with motors because of their high startup surge.  Even for standard fuses or breakers an overcurrent situation of say 20% will require quite a long trip time. 

The amperage at which fuses actually blow, and circuit breakers actually trip, is considerably higher than their nominal ratings. SEA, Maxi, ATO and AGC fuses, and most circuit breakers, blow or trip at about 130% of their rating. ANL fuses blow from 140% to as high as 266% of their rating. (More details from Blue Sea Systems)


Self resetting breakers

Self resetting breakers are usually used within a piece of equipment to prevent overheating.  Many windlasses have these built in and the purpose is to temporarily interrupt operation.  Once the temperature drops they reset themselves automatically.  These types of breakers should never be used for other than for the purpose they were intended.  If a self resetting breaker is used as primary circuit protection it could trip and reset automatically for as long as it is allowed, eventually failing from arcing at the contacts. If it fails in the closed position the circuit is no longer protected and damage can result.

Trip Free

Circuit breakers on boats subject to ABYC standards are required to be “trip free”.  This means that is should not be possible to manually hold the breaker in a position that prevents it tripping.  This requirement means that most domestic type breakers will not meet the standards for use on boats.

Interrupt rating

The interrupt rating for a fuse or breaker is the fault current it is capable of interrupting. Every circuit breaker has a time delay before it trips and during this period, which may be extremely brief, the current may rise to the maximum the system can sustain.  If this fault current exceeds the interrupt capacity of the fuse or breaker it may weld itself in the closed position or otherwise not function correctly.

The requirement for AC systems are as follows:

Shore power service Main circuit breaker interrupting capacity Branch circuit breaker interrupting capacity
120 volt 30 Amp 3000 Amps 3000 Amps
120 volt 50 Amp 3000 Amps 3000 Amps
120/240 volt 50 Amp 5000 Amps 3000 Amps
Other shore power services See ABYC E.11 Table IV - B  


The requirements for DC systems up to 24 volts are as follows:

Total connected battery capacity in CCA
(cold cranking amps) 
Main circuit breaker interrupting capacity Branch circuit breaker interrupting capacity
Up  to 650 CCA 1500 Amps 750 Amps
650 – 1100 CCA 3000 Amps 1500 Amps
Over 1100 CCA 5000 Amps 2500 Amps
For 32 volt systems See ABYC E.11 Table IV - A  


Further reading:

A quick guide to Blue Sea Systems fuses and circuit breakers

·         Choosing Circuit Protection

·         Circuit Protection

·         DC Circuit Protection

·         DC Main Overcurrent Protection Requirements

·         Fuse/Circuit Breaker Speed Explained

·         Fusing the Negative Circuit of ACRs and Other Electric Relays

Shopping for circuit protection:



When should I use a fuse and when should I use a circuit breaker?

Fuses and circuit breakers both do the same thing, protect the wire from carrying excessive current.  Circuit breakers are more convenient because they can be reset easily but are more expensive.  Fuses can be a bit inconvenient when they blow as you have to hunt around for a replacement, but they are cheaper.  So the simplest answer to the question is that if you don't mind spending the money then circuit breakers will definitely be more convenient.

There are a few additional ideas to consider: 

  • Some circuit breakers can also be used as switches to turn a device on or off. This gives an additional advantage for circuit breakers when switching is needed.
  • Fuses are more readily available in a greater variety of increments. So if your application calls for an exact value of circuit protection then it is usually going to be easier to find the correct fuse than the correct circuit breaker.
  • As the Amperage value of the Circuit Protection rises it become increasingly difficult and expensive to find circuit breakers to do the job.  This is why for high current applications like Inverters where the level of circuit protection might perhaps be 300 Amps it is usual to find a circuit breaker being used.
  • For critical applications it might be more appropriate to use a circuit breaker even if it is a bit more expensive.  For example if you put a fuse on the main supply to your circuit breaker panel and the fuse blows then you might have to find a replacement in the dark which could endanger the vessel if it occurred at a critical time.