Circuit Training: Improving Overcurrent Protection

class t fuseSteve D'Antonio | Professional Boatbuilder Magazine

Not all fuses are created equal. Class t fuses, like the one shown here, have a significantly higher interrupt capacity than most other common fuses.

I vividly recall my first encounter with overcurrent protection, or more accurately the lack thereof. As a teenage would-be marine electrician, with more ambition than knowledge, I’d wired a light under the foredeck of the family runabout. The wire passed adjacent to the steel fuel tank and the shelf on which it rested. After a summer’s worth of use, the improperly secured wire worked its way under the tank, where a sharp metal edge chafed through the insulation. In an instant, a cloud of dense white smoke erupted from under the forepeak, enveloping the cockpit and its occupants.

Mercifully, it was over in just a few seconds. The wire was a small gauge, and it melted, breaking the circuit before it could plasma cut its way through the light-gauge sheet metal fuel tank. While I quickly figured out what had happened, at that young age I assumed my error was in not properly or chafe-protecting the wire. I didn’t know about the importance of overcurrent protection. That episode, however, and the acrid smell of that short circuit has stayed with me ever since.

General Requirements

Overcurrent protection (OCP) is simply a catchall phrase for fuses and circuit breakers. With just a few exceptions, which are detailed below, OCP protects wiring, and not the power consumer or appliance, from overheating and a potential fire.

no overcurrent protectionSteve D'Antonio | Professional Boatbuilder Magazine

A disaster waiting to happen: These cables passing through a generator’s sound enclosure have no overcurrent protection, and their chafe guards don’t comply with ABYC standards.

In vessel inspections, all too often I see incorrectly installed, or absent, OCP, which is an occurrence all savvy marine technicians, not just electricians, must understand and be able to identify. Additionally, there is a common misunderstanding about what OCP actually protects. A low-resistance fault, essentially a short circuit, can be dealt with by OCP. High-resistance faults cannot. Therefore, a poor, high resistance connection—think of a corroded, salt-encrusted shore-power plug—can generate significant heat, leading to a fire without ever causing the OCP to react, trip, or blow.

inline circuit breakerSteve D'Antonio | Professional Boatbuilder Magazine

An inline circuit breaker (foreground) and a fuse for alternator output (background). Regardless of the guidelines, overcurrent protection should always be installed as close to the power source as possible.

American Boat & Yacht Council guidelines are clear. For DC systems, OCP must be installed within 7” of the source of power, measured by wire length. The exceptions to this mandate, which are critically important, are as follows (quoted from ABYC E-11.10.1.1):

EXCEPTIONS:

  • Cranking motor conductors [Because no OCP is required, special care must be taken in routing of the positive cable. Among other things, it must not make contact with the engine other than at the point where it connects to the starter positive post.]
  • If the conductor is connected directly to the battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the battery, but not to exceed 72” (1.83m).
  • If the conductor is connected to a source of power other than a battery terminal [this could be a battery switch, starter post, or bus bar] and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the point of connection to the source of power, but not to exceed 40” (1.02m).
  • Overcurrent protection is not required in conductors from self-limiting alternators with integral regulators if the conductor is less than 40” (1.02m), is connected to a source of power other than the battery, and is contained throughout its entire distance in a sheath or enclosure.
  • Overcurrent protection is not required at an alternator if the ampacity of the conductor is equal to or greater than the rated output of the alternator.
  • Pigtails less than 7” (178mm) in length are exempt from overcurrent protection requirements.

The italics are mine, emphasizing the need to place OCP as close as possible to the source, rather than at the maximum allowable distance. Remember, each inch of wire between the power source and OCP is unprotected and, in the event of a short circuit, could lead to a fire.

inline circuit breakerSteve D'Antonio | Professional Boatbuilder Magazine

All nonstock, high-output alternator positive supply cables that leave the engine should be equipped with overcurrent protection at both the alternator and positive bus connection ends.

Alternators

If the alternator’s positive output cable leaves the engine, it must have overcurrent protection where it connects to the vessel’s batteries or DC bus, following the standards detailed above. Furthermore, I don’t fully agree with exception number five. If a high output alternator’s output cable shorts to a grounded structure (such as an engine block, shaft or fuel tank), regardless of whether the cable is capable of conveying the current, the ground source may not be able to convey it. Thus, the potential for arcing and substantial heat generation remains a significant concern for me. (For more on this see the article, “Risks of Undersized Bonding Wire.” I recommend that OCP be installed at both ends of alternator positive output cables that leave the engine.

different fusesSteve D'Antonio | Professional Boatbuilder Magazine

Different fuses for different applications: The two fuses on the left are class T, protecting wiring for main battery banks, while the ANL-style fuse on the far right has a lower interrupt capacity that’s appropriate for an alternator output application.

Battery Banks

Overcurrent protection is required for positive cables connected to battery banks used for purposes other than starting loads. Common ANL-style fuses are typical for comparatively small banks, i.e., 2200 cold cranking amps (CCA) or 500 amp-hours or less (see ABYC Table IV-A for specific guidance). However, for banks that exceed this capacity, the ABYC Standard mandates a higher ampere interrupt capacity (AIC): it must be at least equal to the battery manufacturer’s short-circuit rating (this could be up to five times the CCA rating). This level of OCP often calls for a class T or equivalent fuse, as the interrupt capacity of the ANL fuse is woefully inadequate. In some cases of exceptionally large banks, even a class T fuse may not meet the requirements.

Motors

Motor loads present yet another set of challenges for OCP. ABYC E-11.10.1.3.1 provides that motors “shall be protected internally” or “by branch circuit overcurrent protection devices suitable for motor current.” The OCP should be able to prevent a fire hazard “for seven hours under any conditions of overload, including locked rotor.”

positive cableSteve D'Antonio | Professional Boatbuilder Magazine

These ring terminals are incorrectly installed; they should always progress from largest to the smallest. Since the positive cable supplying an engine’s starter is exempt from overcurrent protection requirements, its routing and chafe protection must eliminate the possibility of a short circuit.

To meet this requirement, the standard’s guidance notes state that you may need to install thermal protection “if the motor is not capable of operating continuously at maximum possible loading.” The notes also state that testing may be necessary to assure compliance with the locked rotor requirement: “Voltage drop, due to wire size, and delay characteristics of the overcurrent protection device may have to be adjusted to protect the motor.”

Ultimately, unless a motor possesses the requisite internal thermal protection, and some do not, it may be necessary to install OCP at the motor to meet this requirement, in addition to the OCP for the branch circuit serving the motor. Alternatively, there is no requirement to size OCP for a conductor’s maximum current carrying capability. The main OCP for a motor’s branch circuit may, therefore, be sized to provide adequate protection for the motor itself, even if it’s undersized per se for the wire size, as the wire size may be larger in order to reduce voltage drop.

About the Author: For many years a full-service yard manager, Steve now works with boat builders and owners and others in the industry as Steve D’Antonio Marine Consulting. He is an ABYC-certified Master Technician, and sits on that organization’s Hull and Piping Project Technical Committee. He’s also the technical editor of Professional BoatBuilder.

  • Thomas Rogers

    Awesome article! …but no mentioned of digitally controlled circuit breakers. Digitally controlled circuit breakers can be programmed to perform in a more consistent and controlled manor. Existing model available mimic ANL type breakers. Some models are even user adjustable to break 12v circuits in a range from 15-60amps and a models for circuits up to 75-175 amps are coming soon. Why is digital worth noting? Consider the user or installer can choose the current break point. For example, a given wire gauge may be capable of carrying 60amps, but the device on that circuit requires protection at 40amps. A Digital circuit breaker could be sized for the current carrying capacity of the wire, but can be adjusted down to 40amp for the specific device. As the boater changes electrical devices, this adjustable feature would allow the breaker to meet most any device requirement. What do you think?

    • Melissa F. Wood

      Hi Thomas, thanks for your question! I emailed it to Steve. His response is below:

      Tom, thanks for sharing your thoughts. I didn’t include digital circuit breakers, along with a host of other breaker and fuse types, simply because doing so was outside the purview of the column. If it had been an article I would have had more room to comment on such products. You say existing digital breaker models’ mimic ANL type breakers’, I presume you mean fuses, and I assume you mean for trip time and interrupt capacity? I’m curious, are they UL Listed, CE or ABYC compliant?

      While I can’t immediately think of applications where it may be useful to adjust circuit breaker trip current, the thought of placing this responsibility in the hands of the boat owner, or even some industry professionals, concerns me. Every marine electrician has encountered cases where boat owners resorted to installing a larger fuse to deal with frequent blown fuses, rather than identify and resolve the underlying cause. If they could simply crank up the circuit breaker the result could be an overheated wire, and a fire.

      The real time voltage and current display are a nice feature, and I can see the advantage in the example you use, wherein OCP could be adjusted to account for a change in gear, however, those cases are rare, and the risk of the user miscalculating the wire’s maximum current carrying ability remains a very real concern. Fixed breaker ratings have no such risk provided they are selected based on the ampacity of the wire they serve, or the maximum allowable current for inductive load, motor or device they supply, when the system is initially designed and installed.