Proper Circuit Protection for your Boat

Proper Circuit Protection for Your Boat

Working on the electrical systems of your vessel can be a daunting and dangerous task.  In fact, approximately 55% of all boating fires result from faulty electrical circuitry.  In order to assist boat builders, owners and mechanics with a safe and reliable standard for onboard electrical systems, the ABYC (American Boat & Yacht Council) has dedicated significant effort in defining the correct methods for protecting a boat’s wiring and devices from overcurrent failures. They even offer certification courses focused on this topic.

Incorrect rating of overcurrent protection devices - such as fuses or circuit breakers - is common. In some installations, circuit protection is completely ignored or overlooked. Correct rating and installation of circuit protection is a requirement to prevent component failure or, worse yet, an electrical fire. Always calculate load to determine proper conductor size and required circuit protection.

The ABYC standards focus on minimizing the amount of electrical cable that would fail on the boat before the fuse or circuit breaker activates and stops current flow.  For that reason, you should locate the fuses and/or breakers as close to the source of power as possible.  ABYC recommends that circuit protection be 7 inches or less, although there are a few exceptions.

Starter motor circuits and conductors are one exception to the 7-inch rule for overcurrent protection.  While starter or cranking motor circuits aren’t required to have overcurrent protection, it is still recommended if space and arrangement of equipment permits. If circuit protection is installed, the rule of thumb for rating should be the actual intermittent cranking current and NOT the short duration inrush current.

Another exception for the 7-inch rule is for wires connected directly to a battery terminal since access can be limited. When this is the case, the overcurrent device can be placed up to 72 inches down the length of the wire run.  However, the cable MUST be sheathed or enclosed.  This cable protection is important to minimize the risk of chafe on the conductor.  A sheath is a material that is used as continuous protective covering on the cable, such as overlapping electrical tape, woven sleeving, molded rubber, molded plastic, loom or flexible tubing.  The sheath is wrapped around one or more of the insulated conductors. Proper sheathing is absolutely necessary and required for safe overcurrent protection.

One final exception to the 7-inch overcurrent protection device rule is when power-feed conductors are connected to a source other than the battery post.  The large lug on a starter motor or the lug on the back of a battery switch are common examples. When this scenario occurs, the overcurrent protection device can be placed up to 40 inches away from the connection point.  Once again, sheathing the cable is required.

The diagrams below illustrate the various scenarios of proper overcurrent protection.