Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel
Posted: Sat May 02, 2020 9:32 am
Typically the 12-Volt power feed to the secondary power distribution panel at the helm console from the battery on a properly designed OEM installation (such as found on Boston Whaler boats) will be done with 8-AWG wiring. The wire size is usually clearly marked on the conductor insulation. The wire size is chosen to be sufficient to handle the current load from the secondary panel, but, more significantly, the wire size is chosen to limit the voltage drop to not more than three-percent at maximum load current. This often results in use of a much larger wire size than if only the current rating for the wire were considered. For more advice on selecting wire size for power distribution, see
Conductor Size for Power Distribution
http://continuouswave.com/whaler/reference/powerConductorSize.html
The purpose of a circuit breaker located close to the battery on the circuit that feeds power to the secondary panel from the primary power source is to shut off electrical current in the event of a wiring fault. The circuit breaker should be chosen so that it has a rating high enough that it would NOT trip off on the normal electrical loads due to use of electrical accessories like lamps, pumps, radio transmitters, SONARs, and so on, but low enough that it will very quickly trip off if there is a dead short at the secondary panel where a very high current could flow.
If 8-AWG wire is used, the wire can handle about 75-Amperes. As long as the circuit breaker rating is below 75-Amperes the wire will be protected. However, the lower the circuit breaker rating, the faster it will operate in case of a dead short high-current fault. If 10-AWG were used to feed power to the secondary panel the maximum current the wire can handle is 55-Amperes, so any circuit breaker less than 50-Amperes would be okay. For these reasons, the rating of the circuit breaker should be in proportion to the total load current expected to be consumed at the Secondary Power Distribution Panel.
A typical total load current expected at the Secondary Power DIstribution Panel on a smaller boat, say an open center console boat like a Boston Whaler MONTAUK 17, might be 25 to 30-Amperes. Note that the total load current does not mean that every circuit will be drawing the maximum current it is fused for at the same time. Typically a load panel can have a total fused current in all its branches that is higher than the expected normal maximum load current because not every device will be operating at full current simultaneously. The total of all circuit protection device ratings on a secondary panel may exceed the panel's rating for maximum current. I don't know the precise scheme or rule used in making this judgement.
Conductor Size for Power Distribution
http://continuouswave.com/whaler/reference/powerConductorSize.html
The purpose of a circuit breaker located close to the battery on the circuit that feeds power to the secondary panel from the primary power source is to shut off electrical current in the event of a wiring fault. The circuit breaker should be chosen so that it has a rating high enough that it would NOT trip off on the normal electrical loads due to use of electrical accessories like lamps, pumps, radio transmitters, SONARs, and so on, but low enough that it will very quickly trip off if there is a dead short at the secondary panel where a very high current could flow.
If 8-AWG wire is used, the wire can handle about 75-Amperes. As long as the circuit breaker rating is below 75-Amperes the wire will be protected. However, the lower the circuit breaker rating, the faster it will operate in case of a dead short high-current fault. If 10-AWG were used to feed power to the secondary panel the maximum current the wire can handle is 55-Amperes, so any circuit breaker less than 50-Amperes would be okay. For these reasons, the rating of the circuit breaker should be in proportion to the total load current expected to be consumed at the Secondary Power Distribution Panel.
A typical total load current expected at the Secondary Power DIstribution Panel on a smaller boat, say an open center console boat like a Boston Whaler MONTAUK 17, might be 25 to 30-Amperes. Note that the total load current does not mean that every circuit will be drawing the maximum current it is fused for at the same time. Typically a load panel can have a total fused current in all its branches that is higher than the expected normal maximum load current because not every device will be operating at full current simultaneously. The total of all circuit protection device ratings on a secondary panel may exceed the panel's rating for maximum current. I don't know the precise scheme or rule used in making this judgement.