An overview of wiring standards for boats in the US

The standards that apply to wiring your boat in the US are the ABYC standards and the Code of Federal Regulations  Title 33 CFR Sections 183.401 - 183.460   The ABYC standards are voluntary and the CFR standards are mandatory.   The ABYC standards incorporate the CFR standards so a boat built to the ABYC Standards should automatically meet the CFR standards.  This article is an introduction to the subject and further articles will delve in a bit deeper to the material.

All wire must be at least 16 awg with some minor exceptions.  It must have written on it the type/style, voltage rating, size and temperature rating.  In engine spaces it must be oil resistant and have a temperature rating of at least 75 Degrees C.  It must be stranded to ensure that it is flexible and vibration resistant.  All wires must be identified and this is normally achieved by means of color coding.

AC wires should be sheathed or bundled separately from DC wires.  If individual conductors are used then this means a separate bundle of AC wires and a separate bundle of DC wires.  If the wires are Duplex and Triplex cables where an outer jacket is wrapped around a set of conductors then that is considered separate sheathing.

DC wire must be rated for at least 50 Volts, it must be stranded, it must be sized to take account of both Ampacity Tables and Voltage Drop Tables. 

In practice most DC wire sold for marine use normally has an insulation rating of 105 Degrees C and a Voltage rating of 600 Volts.  Although it is not required to be tinned it would be better if it were. In practice the Voltage Drop Table usually take precedence over the Ampacity Tables for DC wires because in most cases the Voltage Drop table will come up with a larger wire size. The Voltage Drop Tables say that the allowable voltage drop for general purpose wires can be up to 10% but the Voltage Drop in essential wires should be no more than 3%.  Essential wires include wires to bilge pumps and blowers, navigation lights and navigation electronics as well as the wires that feed the circuit breaker panel. Sizing wires for 3% voltage drop results in some surprisingly big wires at times.

DC wires should be identified by color coding.  Negative wires can be either Black or Yellow.  To avoid confusion between the DC and AC wires it is preferable if the DC Negative is Yellow and this practice is becoming increasingly prevalent in new boat construction.  The Positive wire can be Red or for certain specific purposes another color from the Table of Wire Colors

For most purposes wire for AC use must have a 600 Volt rating, flexible cords are the exception and are allowed to have a 300 Volt rating. Wire must be stranded and although it is not a requirement is would be best if it were tinned. The minimum insulation temperature rating is 60 Deg C generally or 75 Deg C in Engine Spaces but in practice most wire sold for boats has an insulation rating of 105 Degrees C.  Voltage drop is not an issue with AC wires because of the higher voltage of the system and so the sizing of the conductors is determined only by the insulation rating of the wire, the number of wires in a bundle, and whether the wires are in an engine room or not.  As the number of wires in a bundle increases the temperature of the bundle the more wires there are the more the current carrying capacity is de-rated.  Only current carrying conductors are considered in the count, the ground wire is excluded.  When wires pass through an engine room it is assumed by the standards that the engine room temperature is 20 Deg C higher than the ambient temperature and so the current carrying capacity of the wires is de-rated accordingly.

AC wires should be identified by color coding.  The standards go into some detail on unusual color coding but for normal purposes the Ground wire is Green, the Neutral is White, and the Hot is Black.  Additional Hot wires can be Red, Orange or Blue but are normally Red

Crimped connectors are allowed, the standards go into some detail about the pull force that the crimp must withstand and without getting bogged down in the details the intention is that the connections cannot under normal circumstances come undone.  Twist-on or Wire nut connections are not permitted.  When fork type connectors are used the forks must be the captive type which will not come undone if the screw comes loose.  Metals used for connections cannot be Aluminum or unplated Steel. 

Solder is permitted but cannot be the sole means of making the connection.  This is because if the connection becomes very hot the solder might melt.  Solder also tends to make the end of the wire into a solid wire as it wicks into the stranded conductors.  The end of the wire then loses it flexibility and so is more prone to cracking induced by vibration.

The shanks of connectors must be insulated except those used in grounding systems.

When connections are made at terminals that use set screws there must be a pressure plate under the screw to prevent damage to the wire strands.  Many set screw terminals are designed for use with solid wire and are missing this pressure plate.  If these type of connections are used with stranded wire the screw tends to push the strands out of the way as it is tightened and so doesn't do an adequate job of tightening the connection.

Wires should be supported throughout their length, either by being in conduit or by being clamped every 18 inches.  Wires should be routed away from hot areas of the engine and should be protected from chafing when coming in contact with sharp edges. They should also be routed to avoid them coming in contact with moving parts of the engine. Junction boxes should be made of weatherproof materials or installed in a protected area.  Openings in junction boxes should be sealed up.  Cable entry to junction boxes should be clamped to avoid stressing the connections.  Wires should be routed above the bilge area as much as possible and any connections in the bilge area should be waterproof. 

USCG standards for Electrical Systems on boats