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USCG standards for Electrical Systems on boats

Posted by Peter Kennedy on 10/28/2013 to ABYC and USCG standards
The USCG electrical regulations in 33 C.F.R. Part 183 apply only to boats with inboard or stern drive I/O gasoline engines. Here are some of the main provisions:

Ignition Protection

Electrical equipment must be Ignition protected unless it is isolated from gasoline fuel sources such as engines, tanks, vent lines, connections, fill lines etc. The standard goes on to describe in considerable detail and with numerous drawings, what exactly is meant by "isolated".  This is all common sense but it describes in detail the letter of the law on the subject.


If a boat has more than one gasoline engine, grounded cranking motor circuits must be connected to each other by a common conductor circuit that can carry the starting current of each of the grounded cranking motor circuits.

The intention of this rule is to ensure that current is never carried along fuel lines.  The common conductor described above is in addition to the normal negative battery cables that would go to each engine. Again the details or the rule are explained with drawings and additional explanation.


Batteries must be restrained so they cant move around.  Exactly what is meant by restrained is spelled out in some detail.

Each battery must be installed so that metallic objects cannot come in contact with the ungrounded battery terminals.  To comply the battery must be in a box with a lid or have a rubber boot over the terminals.

Each metallic fuel line and fuel system component within 12 inches and above the horizontal plane of the battery top surface as installed must be shielded with dielectric material..  Since metallic fuel lines are normally grounded having an unshielded one above a battery might cause a short when the battery is being installed or removed.

Each battery must not be directly above or below a fuel tank, fuel filter, or fitting in a fuel line.

A vent system or other means must be provided to permit the discharge from the boat of hydrogen gas released by the battery.  This applies even to sealed batteries such as AGM and Gel

Each battery terminal connector must not depend on spring tension for its mechanical connection to the terminal.


Each conductor must be insulated, stranded copper.

The standard specifies mandatory minimum wire sizes, these are shown in a table and various values are given depending on the insulation temperature rating of the wire being used.  Most boat cable on the market has an insulation temperature rating of 105 deg C.

The table goes on to show how the allowable current for any particular wire is derated when there are multiple wires in a bundle. This is because wires get hotter when bundled together and as the insulation rating is fixed the allowable current needs to be reduced when more wires are together.

The regulations say the minimum size for any wire is 16 awg except for certain exemptions. They give a specification for wire that it is permissible to use and they give detailed guidelines and example calculations on how to select an appropriate wire for any particular application.

Special rules apply to conductors in ignition systems.

Conductors must be protected from abrasion when passing through a bulkhead

Exposed positive terminals must be protected by a boot or other means to prevent accidental short circuits

Circuit protection

Each ungrounded current-carrying conductor must be protected by a manually reset, tripfree circuit breaker or fuse placed at the source of power for each circuit or conductor except:
(1) If it is physically impractical to place the circuit breaker or fuse at the source of
power, it may be placed within seven inches of the source of power for each circuit
or conductor, measured along the conductor.
(2) If it is physically impractical to place the circuit breaker or fuse at or within
seven inches of the source of power, it may be placed within 40 inches of the source
of power for each circuit or conductor, measured along the conductor, if the
conductor is contained throughout its entire distance between the source of power
and the required circuit breaker or fuse in a sheath or enclosure such as a junction
box, control box, or enclosed panel.
There are a few notable exceptions, see the regulations for details.  There are numerous drawings and explanations of what is meant by the above.

There is a section describing the allowable value of the circuit protection device in relation to the current carrying capacity of the wire. I am  not going to summarize that part, see the actual regulations for details.

Circuit protection devices must meet the voltage rating of the circuit they are protecting. Pretty obvious but they had to say it anyway!

Each ungrounded output conductor from a storage battery must have a manually reset, trip-free circuit breaker or fuse, unless the supply conductor is in the main power feed circuit from the battery to an engine cranking motor. The circuit breaker or fuse must bewithin 72 inches of the battery measured along the conductor, unless, for boats built prior to August 1, 1985, the circuit has a switch that disconnects the battery.

Each ungrounded output conductor from an alternator or generator, except for self-limiting alternators or generators, must have a circuit breaker or fuse that has a current rating that does not exceed 120 percent of the maximum rated current of thealternator or generator at 60° C.

Drawings, tables, reference and resources

The regulations end up with numerous drawings and finish with references to additional resources, standards etc


There is more to these regulations than I have indicated above, they are 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 covered.


Date 9/21/2014

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