What size cable do I need?

Choosing the right cable size for your DC electrical project is important from both a safety and performance perspective. Using cable that is too small will cause it to overheat and potentially cause a fire. Cables that are too small also cause resistance in the circuit and prevent connected devices from functioning properly.

In addition to choosing wire that is an appropriate size, you should also make sure that you purchase high quality stranded cable from a reputable supplier. More information on choosing the correct type of cable can be found here.

You will also need to choose an appropriate sized fuse for most circuits in your electrical system. The current carrying capacity of your cables should be higher than your circuit protection. This helps ensure that your circuit protection will activate before your cable melts in most situations. Information on how to choose a fuse can be found here.

Choosing a Wire

The table below shows recommended cable sizes based on recommendations from the American Boat and Yacht Council (ABYC), an organisation which provides technical information for the marine industry. The following guide will show you how to choose cable sizing based on the chart. Please note that this table assumes that you are using high quality stranded wire with an insulation rating of 105°C. Lower insulation ratings will decrease the current carrying capacity of the wire.

STEP 1: Determine the current draw of your appliance in Amps.

Most of your appliances should list their maximum current draw on a specifications label or in the owner's manual.

STEP 2: Determine the the length of the circuit

The total length of the circuit will be the round-trip distance from the power source to the appliance and back.

STEP 3: Determine Circuit Type

Allowable voltage drop is based on whether a circuit is critical or non-critical.

Critical DC circuits, with 3% allowable voltage drop, include

• Battery Chargers
• Inverters
• Inverter/Chargers (Combi's)
• Fridges/Freezers
• Electronics
• Navigation lights

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

• General lighting
• Windlasses/Winches
• Bait/Water pumps
• General DC appliances

STEP 4: Determine Cable Size

Once you have determined whether your circuit is critical or non-critical, follow the column down until you find the length of your circuit. Once you have found the length, move across the chart until you reach the maximum current flow of your appliance. The required cable size will be located where the current flow and circuit length intersect.

Click the image the enlarge

Example

You plan to install a 40A AC charger 2 meters away from your battery. Battery chargers are considered to be critical load, so you need to use the critical circuit type column. You would then find the intersection of where the current flow (40A) meets the critical circuit length (up to 4.6m) and see that this circuit requires 16mm2 cable. 

More Information

Owner's Manuals

Many of the Enerdrive Owner's manuals also include suggested circuit protection and cable sizing for installations at different circuit lengths.

Blue Sea Circuit Wizard

The Circuit Wizard, at circuitwizard.bluesea.com, is a resource for a more detailed treatment of cable size selection for DC circuits. It allows you to input detailed information including cable insulation temperature rating and other de-rating factors. The Circuit Wizard is also available for download via the iOS App Store or Android apps for mobile devices.

The American Boat & Yacht Council Wire Sizer App

The American Boat & Yacht Council also offer a cable size calculator than can be downloaded on the iOS App Store or Android.