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Overcurrent Protection

Posted by Peter Kennedy on 11/5/2013 to ABYC and USCG standards

ABYC standard E11.10 Overcurrent Protection


Overcurrent Protection Device Location
 

Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches of the point at which the conductor is connected to the source of power.. The standard then goes on to list exceptions: 
  • Starter Motor conductors are exempt from this rule. 
  • The distance to the circuit protection can be 40 inches instead of 7 inches if the wire is sheathed in conduit, in a junction box, or otherwise protected. 
  • Internally regulated self limiting alternators are exempt.

Main Battery Protection

This section specifies that if batteries are in an area with potential gasoline fumes the circuit protection be ignition protected.  It says that the voltage rating of the circuit protection must match the voltage of the system, and it specifies the interrupt rating of the circuit protection device which depends on the size and voltage of the battery. If circuit breakers are used they must be of the trip free type.

Motors

There is a special provision for motors that they have to be able to stay in a locked rotor position for seven hours without causing a fire.  That is why many winches and windlasses have an internal thermal self resetting circuit breaker (the only place such a breaker is permitted) For things like bilge pumps that don't have internal circuit protection this is a difficult standard to meet and the standard suggests testing as installed, the aim being to have the fuse or circuit breaker blow if the motor locks but otherwise be functional for normal startup loads.  Not easy to achieve in practice.

Non-motor Loads 

The fuse or circuit breaker shall not exceed the maximum current carrying capacity of the conductor being protected.  If an exact match is not available the next size up circuit breaker is permitted as long as it doesn't exceed the current carrying capacity of the wire by more than 150%

Branch Circuits 

Each branch circuit should have its own fuse or circuit breaker unless the main breaker covers it appropriately.  Fuses and circuit breakers should be of the appropriate type, voltage etc.

Panelboards and Switchboards

The wire feeding a panelboard or switchboard must have circuit protection to match the wire and the rating of the panel. If the panel has a main breaker then the circuit protection for the feed wire can be 150% of the panel main breaker 

Circuit Breakers

Circuit breakers exposed to gasoline vapors need to be ignition protected. They need to be of the trip free type, be rated appropriately for the voltage of the circuit they are protecting and have an interrupt capacity as stated in the table that is included in the standard. A fuse ahead of the circuit breaker can be used to meet the interrupt capacity requirement. Some exemptions apply to circuit breakers built into a device

AC Systems

AC circuit breakers must also be ignition protected if exposed to gasoline vapors, must also meet the interrupt capacity specified in the standard, must be manual reset trip-free and must meet  international standards.  They must have the appropriate voltage rating for the circuit and must be sized to not exceed the current carrying capacity of the wire they are protecting. They must also have an appropriate temperature rating for the location they are installed.

The standard has some special provisions for transformers.

Branch circuits: More to follow

Motors: More to follow

Location of overcurrent devices: More to follow

The standard specifies circuit protection for three phase and other power supplies with multiple ungrounded conductors

Disclaimer

There is more to this section of the standard than I have indicated above, it is written as if it was a legal document and so only a reading of the original will give you the full intent.  What is here is just a brief summary of some of the main points.

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