Choosing the Correct Wire Size for a DC Circuit

Choosing the right wire size for your DC electrical project is important, since a wire that is too small can overheat and possibly start a fire. The American Boat and Yacht Council (ABYC) publishes charts with valuable detail to help experienced boatbuilders and installers determine what wire size they need. Although these charts are an excellent resource, they are a bit intimidating. This technical brief distills the information on these charts to a more manageable size for installers and boatowners alike.

Quality marine wire, as specified by ABYC standards, will always be stranded rather than solid, and always tin-plated copper. In addition, the DC Wire Selection Chart shown below assumes a wire insulation rating of 105°C. A lower rating will decrease the current-carrying capacity of the wire.

To use the chart included with this technical brief, follow the instructions below.

Choosing the correct wire

A  Locate the CURRENT IN AMPS of your appliance across the top of the chart. Most electrical products include a rating label, or you can find the amperage rating in the documentation that came with the product.

B  Find circuit LENGTH IN FEET along the left side of the chart. Note that the total length of the circuit is the roundtrip distance from power source (usually the battery) to the product and back.

C  Select the CIRCUIT TYPE. Allowable voltage drop is based on whether a circuit is critical or non-critical.

Critical circuits, with 3% allowable voltage drop, include

• Panel main feeders
• Bilge blowers
• Electronics
• Navigation lights

Non-critical circuits, with 10% allowable voltage drop, include

• General lighting
• Windlasses
• Bait pumps
• General appliances

Follow down the column until you find your circuit’s LENGTH IN FEET.

D  Intersect CURRENT IN AMPS with LENGTH IN FEET to identify the wire size.

Example: A windlass rated 80A is 25’ from the battery. Circuit length is 50’, circuit type is ‘non-critical’, and correct wire size is 4 AWG.