The owners of the 48-foot cruiser couldn’t understand why their DC electrical system was unreliable. They’d recently replaced their batteries, and they’d also replaced all the high-amperage cables from the batteries to the battery switches. Now, they had electrical gremlins, the worst of which was that the electronics kept randomly shutting down underway. We set about trying to determine the problem, so we could recommend the correct repairs.
Is the Flow Too Low?
The main purpose of a DC electrical system on a boat is to supply onboard devices with as close to the required battery voltage as possible. Equipment is manufactured to operate within a specific voltage range. If the voltage is too low, then the equipment performance can suffer, or damage can occur.
With any electrical system, there is a compromise between what size wire is practical to use to supply the device, and the distance from the battery to the device. Using too small a wire means the supply voltage at the other end could be too low to power the unit. We call this voltage drop. It is caused by resistance within the wire, and by the wire’s length, including all the connections. (Think of multiple water hoses linked together. There may only be a dribble of water at the end, especially if there are kinks in more than one hose.)
The American Boat and Yacht Council says there can be no more than 3 percent voltage drop for safety-related equipment such as panel board main feeders, bilge blowers, electronics, navigation lights and other critical circuits. The wiring for everything else should not exceed 10 percent voltage drop. This means that if the voltage at the battery is 12.5 volts, the voltage at the noncritical device while it is running should not be lower than 11.25 volts. For critical loads, the 3 percent requirement means no less than 12.1 volts.
In any installation, the diameter of the wire and the length of the conductor—and any connections—add resistance and voltage drop. High temperatures (say, in the engine room) can also reduce the wire’s current-carrying capacity. To mitigate the problem, we can either move the gear closer to the batteries or panel board, or use larger wiring and pay close attention to the quality of our connections.
ABYC’s voltage-drop tables give the wire gauge for each ampacity and length of conductor from the power source to the device and back. The “and back” part is important: As the name implies, a circuit is circular, and must end up back where it started at the battery.
Resistance
In 1847, a German named Georg Ohm published a treatise describing his measurements of voltage and current flowing through simple circuits. His efforts were influential enough that we now call the measurement of resistance an Ohm. And, Ohm’s Law is that volts equal amps times resistance.
Resistance is an important concept for boaters to understand because it affects every electrical circuit. It becomes much more of a factor if wiring connections become damp or wet, especially with seawater, which is somewhat conductive itself. The surface of the connections will form a patina that is less conductive and that increases resistance. If the corrosion grows to bridge a positive and negative connection, then low-level short circuits develop and hasten the corrosion.
These shorts may not draw enough amperage to trip a fuse or breaker, but they can easily heat wiring to the point of fire. (Think about how a stove eye can glow red and not trip its circuit breaker. A 5-amp short circuit can act similarly, creating heat yet still being under its 10-amp breaker-trip rating.)
In the early days of boating, rudimentary wiring might have included solid core wire with solder connections. But the solid wiring or solder could fatigue with vibration. If you bend a solid copper wire enough, it will break in two; this happened frequently enough that something had to be done.
The UL 1426 standard requires at least 19 individual strands for cables 16-gauge and larger. Better marine wire manufacturers use type 3 stranding, which may have up to 26 individual strands for a 16-gauge wire or more than 2,000 strands for a 4/0 cable (pronounced “four ought cable”). Each of these strands are .01 of an inch in diameter and have been tinned by electroplating before being jacketed. Tin is a soft, malleable, reasonably conductive, silvery-white metal that, when electrically applied to the wire, makes the copper much more corrosion-resistant.
A similar advance happened as early fabric- or paper-wrapped wiring transitioned to vinyl. High-quality marine wire insulation is rated to perform in temperatures down to minus 40 degrees Fahrenheit and up to 221 degrees Fahrenheit. The vinyl coat resists salt water, battery acid, oil, gasoline and ultraviolet radiation.
There also was a trend with some builders to use welding wire for high-amperage cabling; while the cable does have a high strand count, its rubber insulation does not react well with oil or gas, which causes it to swell and degrade into a baggy, fragile mess.
Here in the United States, we use the American Wire Gauge (AWG) as our standard, but there is also a British Imperial Standard Wire Gauge (SWG) and a European IEC 60228 based on the metric system. The gauge size in AWG and SWG can be confusing, because the smaller the number, the larger the wire. The American Boat and Yacht Council recommends not using smaller than 16 AWG wiring, with some caveats; if the wire is part of a piece of equipment (such as electronics or inside a panel board), smaller wire can fatigue and break with vibration. The largest wire typically used is 4/0 for heavy loads such as battery cables, thrusters and starters.
Making Connections
Since direct bearing screw terminals (the kind where a machine screw pushes directly on a bare wire to hold it in place) and soldering are frowned upon aboard a boat because of the vessel’s vibration, the industry turns to crimp-on connections. There are quite a few specialized terminals, but the for the most part, ring terminals, butt connections and quick-disconnect terminals are the most commonly used.
Each terminal is sized for the wire with which it will be mated. Smaller connections below 8 AWG will have color coding for their nylon or heat-shrink sleeves. By standard convention, 10 to 12 AWG terminals are yellow, 14 to 16 are blue, and 18 to 22 are red.
For 8 AWG and above, open- and closed-barrel connectors are available. The choice is like the difference between a sandal and a boot. The sandal-style open barrel allows the installer to confirm visually that the wire is extending just past the barrel, but not so far as to interfere with the ring terminal. This can be more reliable than the mystery of what is going on inside the boot of a closed-barrel connector, since the act of crimping a large-gauge cable can take some muscle. It is easy to cause the cable to slide partway out of the terminal during the process.
A proper crimp actually cold-welds the metal of the connector to the wire. The crimping process is designed to reduce the barrel of the connector with tapers to the middle, so there is a smooth transition on both sides of the barrel. This decreases the chance of a sharp point that might break off or nick the copper strands.
Only use the correct crimper for the terminations you are using. Ideally, the crimper would be the same brand as the terminals, since there can be variations between the diameters and amount of metal in different brand terminals. In a well-executed crimp, the metal of the terminal and the wire form a solid bar that excludes air and moisture. This plug helps to prevent corrosion that could cause resistance, heat or separation.
Terminals smaller than 8 AWG are available with plain nylon sleeves or heat-shrink sleeves. Heat-shrink terminals are recommended for areas that could be damp, such as outside lockers or bilges. The heat-shrink gives another layer of protection against water migration up the wiring. Just be careful when you crimp: Avoid cutting the heat-shrink with your crimp. That small hole will be an avenue for moisture and corrosion.
Connections in damp areas should also be coated with a corrosion inhibitor and installed inside junction boxes whenever possible.
The American Boat and Yacht Council has a table that indicates how much pull a proper crimp should resist. This ranges from a 10-pound pull for an 18 AWG wire up to a 225-pound pull for a 4/0 terminal. Techs who do this work regularly develop a sense of how much to pull to check each crimp they make, but typically, a little tug after a crimp will tell you if you have a problem. The terminal will come right off in your hand. Conversely, too much crimping pressure will mangle the terminal.
A Proper Crimp
There’s a correct tool for every job. For wiring smaller than 8 AWG, a ratcheting double crimping tool won’t release until it applies the correct amount of pressure to the terminal. The dies are color-coded to match the terminal size, and the double crimp ensures that the terminal won’t come off.
For very tight areas, a high-quality, plier-type crimper can be used. Skip the cheap, thin, automotive plier-strippers. For stripping wires these sizes, a compound action stripper clamps the wire and peels the insulation in one smooth action. The guillotine of the cutter is also coded for wire size so that only insulation is removed, not wire strands.
For cable 8 AWG and larger, a ratcheting box crimper is the way to go. These crimpers have rotating dies to match the wire size, and smoothly and repeatably create the welded copper plug we want inside the terminal. To make the termination, use a sharp box knife. Carefully trim off the insulation so that the bare wire just peeks through the crimp. Remember to slide a 1-inch piece of heat-shrink on the cable before you crimp. It is also a good idea to index your ring terminal crimps before crimping, so that the ring ends up flat against the terminal.
Larger wire may require a second crimp. When performing the second crimp, be careful to keep the crimps parallel. Don’t allow the terminal to become misaligned in the crimper, as it will unnaturally distort the smooth box we want.
Inspect your crimp and give it a pull test before shrinking the heat-shrink. You can use a heat gun or flame to heat the shrink evenly all the way around the terminal. Be careful either way, but it can be difficult to control the heat with an open flame, and you don’t want to singe the wire insulation or heat-shrink.
Ring terminals must match the stud or screw diameter used. You can place up to four ring terminals on a single stud. Place the biggest wires on the bottom. Never place a stainless steel washer between a ring terminal and another connection. Stainless steel is not as good a conductor as copper, and the added resistance can cause enough heat to melt plastics. Plus, we’re trying to avoid adding resistance in our circuit.
This is also a good time to label your connection. You know what it is right now, but it will be just another wire as soon as you turn away.
Since vibration occurs constantly aboard a boat, carefully securing wiring against movement is critical. Never stuff a wad of wire behind a panel or bulkhead—doing so can strain the wiring and terminations. While a short service loop is handy if a wire needs to be reterminated, big coils of wire add unnecessary voltage drop. The American Boat and Yacht Council recommends that all wiring be secured every 18 inches, as a minimum. Obviously, smaller-gauge wiring in tight locations, such as behind panel boards, may require additional security.
Don’t cut the tails of nylon wire ties with standard diagonal cutters. Use flush cutting pliers that eliminate the sharp dagger that standard cutters create. Reaching into an electrical system with these sharp points is like reaching into a shark’s mouth. There will be blood.
Connected, but Not Together
We determined that the voltage on the 48-footer’s batteries was fine, but when the terminals were installed on the new cables, the wrong crimping die was used. A light pull on the cables was all it took to remove them from the terminals.
The poor crimps made good enough contact to show voltage on a voltmeter, but not good enough contact to carry a heavier load. Thus, when the boat’s DC-powered refrigerator cycled on, the system voltage dropped, causing the electronics to shut down.
Reterminating the cables allowed the amps to flow freely and solved the problem of intermittent electronics dropout.
