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Circuit Protection

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

16

17.5

14.9

14

24.5

20.8

12

31.5

26.8

10

42.0

.5.7

8

56.0

47.6

6

84.0

71.4

4

112.0

95.2

2

147.0

125.0

1

171.5

145.8

1/0

199.5

169.6

2/0

231.0

196.4

4/0

311.5

264.8

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 (E11.10.1.1.1) 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 detailsfromSystems)

 

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 is capable of interrupting. If the fault current exceeds this amount the fuse or breaker 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