[Editor’s note: In a series of articles, we’ll be taking a look at the intricacies of AC shore power. This first installment covers AC power basics, terms, practices and procedures.]
Thirty or more years ago, it was a rarity to find vessels less than 40 feet that were equipped with shore power. Of those that did have it, it was often used for a limited number of devices, a battery charger, the occasional bulkhead-mounted receptacle and perhaps a light fixture. Generators were even less common, found only on large luxury yachts. The inverter as we know it today had yet to be invented. As frightening as it is to consider, most of these vessels were wired with common, solid Romex wiring and household circuit breakers. Today, however, the ubiquitous shore power cord is so common, even on the smallest of vessels, it is simply de rigueur. Generators and inverters have also become quite common. The latter is nearly as common as shore power itself.
Today’s voyaging vessel, even one that spends little time dockside, is certain to have at least a basic AC shore power system. In all likelihood, this includes a weatherproof, deck-mounted shore power receptacle, main AC electrical panel, some type of meyer system (volts, amps and perhaps cycles as well), main and branch circuit breakers, a galvanic isolator, battery charger/inverter, receptacles, hot water heater, and perhaps even reverse cycle heat and air conditioning. Larger vessels with greater demand, and more sophisticated electrical systems, may add to this list a generator and isolation transformer. All of this equipment will, or should, be interconnected with flexible type II or III (preferably the latter) stranded wire (type II no. 12 AWG wire has, for instance, 19 strands, type III no. 12 has 65), in all non-bonding applications, electrical cables and connections designed for use afloat. Additionally, the required safety equipment will include ground-fault circuit interrupters, reverse-polarity indicators and an extremely rugged grounding (that’s grounding, as opposed to grounded, more on this later) system. It’s clear to see, even from this brief comparison, that as the years have passed, the list of accessories and requirements has evolved considerably.
There is good reason for this leap in the popularity of shore power systems (120 and 240 VAC, or volts-alternating current, more on this later as well). Tools, appliances and equipment that operate on this higher voltage, and use AC, are more efficient and smaller. Most important of all, this gear is comparatively inexpensive because it is mass-produced. Virtually every home in America has a blender and coffee maker that operates on 120 VAC. When was the last time, however, you saw these devices on the shelves at your local department store in 12 kDC (volts direct current)? Not recently, if ever, I suspect. Of course, many household-type appliances are available in 12-volt versions (blenders, coffee makers and even microwave ovens); however, they are not readily available. Because of low production numbers, they are not often long-lived (unlike household gear, they’re not designed for everyday use), and they are more expensive than their 120 VAC cousins.
Additionally, some equipment simply must operate from 120 VAC. These items include battery chargers (whether integral with an inverter or not), air-conditioning/heating units and electric hot water heaters. Finally, one of the most useful features of AC power, compared to DC, is its ability to be easily stepped up or down by using a transformer. This is why both a 12 VDC cigarette-lighter receptacle and a 120 VAC wall receptacle can charge your cell phone. The AC is tailored to match the requirements of the device.
Having made a compelling argument for shore power, I must confess, I am certain boats would be much safer without it. However, so would our homes, shops and businesses. We can keep the yoke on AC power, to ensure that we remain the master and it remains the servant, by following a few clearly defined safety standards. Make no mistake about it; developing an understanding of these regulations through study and implementing them with hands-on practices are two entirely different tasks. I’m not suggesting that by reading the following information you can become as proficient as a seasoned American Boat and Yacht Council (ABYC) certified marine electrician (not to be confused with even the best, licensed, land-based electricians) where onboard AC electrical systems are concerned. What you may be able to do, however, is determine if the system on your own boat is safe and reliable. If it is not and you do not possess the skills or confidence to repair it yourself, you may at least gain a basic understanding of the principles by which it works. Furthermore, you will also be able to assess the abilities of those whom you choose to carry out upgrades or repairs.
It is bad enough that the fundamentals of electricity are filled with a host of esoteric and unfamiliar terms. Marine electrical systems complicate the matter by injecting still more unusual terms and practices. I will, however, attempt to begin the demystification process with a few straightforward definitions. The most commonly used term is, of course, volt, named after the Italian physicist Allesandro Volta, who, incidentally, made some of the first batteries around 1800. Volts simply refer to the quantity of electricity, but are more specifically defined as a difference in potential. Most DC systems are 12, 24 or 32 volts, whereas AC systems are usually 120, 208, 240 volts and above. Incidentally, most experts agree that it requires at least 50 volts, coupled with the requisite amount of amperage (in some cases less than one amp) for electrocution to occur. That is why the average DC system is not capable of electrocuting a crewmember.
The wires used in AC systems are given specific names and color codes. Hot refers to the ungrounded conductor, similar to DC’s positive. However, unlike DC positive, this wire is color-coded black. This has led to some confusion; the DC negative wire, as we all know, is also black. An attempt has been made in the marine industry and ABYC to alleviate this potential problem by switching the color code for DC negative from black to yellow. If you are grounded (if you are at the earth’s potential, you are grounded, whether you are standing on soil or touching a bonded/grounded engine block) and you touch this wire, you will receive a shock. Hot wires are always connected to the black- or brass-colored screw on the backside of 120 VAC receptacles.
Neutral refers to the grounded conductor in the AC system, and its color code is white. The neutral should always be at the same potential as ground, although, for reasons to be explained later, the neutral and grounding conductors must never be connected aboard. The neutral wire should always be connected to the silver-colored screw on the back of receptacles. Grounding (not to be confused with the neutral, which is grounded) is the description given to the third wire in the usual AC arrangement. The importance of this conductor’s mission cannot be overstated. It exists solely to carry fault current safely to ground and its integrity must never be compromised. Phase is one of the less often heard terms in shore power discussions. It refers to the alternating nature of AC power. Most shore power applications use what is referred to as single phase, as do most homes. However, there is a bit of a misnomer in this description. While a 120 VAC, 30-amp shore power system may be single phase, homes and some larger boats are actually two-phase 120/240 VAC. Although, within the electrical industry, both are referred to as single phase, even though the latter actually possesses two phases, each 180 degrees out of sync. The explanation of this phenomenon can get more complicated. Suffice it to say, both 120 VAC shore power and 120/240 VAC shore power (and most homes) are described as single phase.
As a final clarification of this issue, the 120 VAC, 30-amp system utilizes three wires, a hot (black) wire, a neutral (white) wire and a grounding (green) wire. Whereas, the 120/240 VAC system, which provides 50-amp service (at 240 VAC), utilizes the same wiring configuration, with the addition of a “hot” wire, which is usually red. This is the “second” phase.
The final term you may encounter in the realm of shore power is hertz, named after the German physicist Heinrich Hertz and sometimes referred to as cycles. AC current, unlike DC, flows in one direction for a short period, then reverses, flowing in the opposite direction for the same period. Hence, the term alternating, and the current flow is the same for both directions. Hertz, cycles per second, is a measure of the number of reversals that occur each second. In the United States, the standard utility power supply is at 60 hertz, regardless of voltage. Many European countries, on the other hand, supply power at 50 hertz. Resistive loads, such as lights and toaster ovens, are unaffected by this frequency difference. In fact, most will work on DC. However, most motors (including compressors, pumps, fans, etc.) are quite sensitive to fluctuations in hertz, and if designed to operate on 60 hertz AC, they will definitely not operate on 50 hertz, AC or DC.
AC practices and procedures
The debate over whether to bond or not to bond a vessel has raged within the marine industry for years and continues to this day, albeit somewhat less virulently than in the past. Before delving into the fray on this issue, some clarification may be in order. Bonding, or grounding, is the interconnection of all non-current-carrying metal objects with the negative side of the DC and the AC safety ground system. This includes the engine, running gear, metallic through-hull fittings, metallic tanks, metallic fuel system components, spars/rigging, ballast, rails, as well as all AC appliance cases, etc. The purposes for this sought-after (by some) commonality is threefold.
First of all, and most relevant to the discussion of shore power, it provides a safe path to ground for any stray, hot AC current. If, for instance, a hot AC conductor were to chafe through its insulating coat and make contact with a fuel tank that was not bonded, the tank would become energized. If you were to touch this tank and the engine block, which is grounded, you would complete the path to ground and could be electrocuted. However, if the tank were bonded, as soon as the hot conductor made contact with its surface, the current would be immediately and safely conducted to ground, most likely tripping the circuit breaker in the process. The second purpose for bonding is to enhance lightning prevention and dissipation characteristics. The final value that a bonding system will provide involves DC stray-current corrosion, which emanates within the vessel being bonded. A well-bonded vessel (bonding wires should be no. 6 AWG, type II, stranded, tinned cable) will, under these stray-current circumstances, usually suffer less damage by enabling the destructive, leaking voltage to travel directly back to DC negative bus rather than through immersed hardware.
An entire article could be penned on the subject of bonding, detailing its merits and faults. However, for the purposes of shore power safety alone, bonding is essential and required.
I hear a common refrain in my line of work. It goes something like this, “I have a friend who’s an electrician [residential or commercial, not marine], and he’s going to rewire my boat.” Each time I hear this I cringe because, as good as most licensed electricians are, there are a few subtle, yet critical, differences between wiring a home and a boat. If your friend the electrician is familiar with these differences, you’re okay. If not, however, you could be in for some trouble. The primary difference is the separation of the neutral (white) and grounding (green) conductors at every location aboard any vessel wired for shore power. This is a concept that is anathema to shore-based electricians and those familiar with residential wiring practices; it flies in the face of everything they are taught. Land-based electricians look at a boat as if it were an outbuilding, a garage or a barn. It has a sub panel with a main breaker and branch circuits. In those cases, ashore, the neutral and grounding circuits may share a common bus bar or be interconnected within the panel. This is not the case afloat. The neutral and grounding conductors must never be connected within the electrical panel or anywhere else aboard.
The reason for this clear directive is simple, and it has to do with parallel paths for leakage or fault current when it travels back to ground. If the neutral and grounding conductors are paralleled aboard a vessel, current attempting to return to ground, after coming aboard on the hot conductor, can take three paths: the neutral wire, the grounding wire, below-the-waterline hardware and thence, the water. If this were to occur, a swimmer passing through that current path could be electrocuted, or he or she could be paralyzed by involuntary muscle spasms and drown. The amperage required for the latter case is minute, on the order of just 50 milliamps.
Another common AC grounding problem involves severing the connection between the AC safety grounding system and the bonding system. Unfortunately, this is not an uncommon approach that some skippers take in an attempt to preserve zinc anodes. Herein lies the problem with this “fix.” A fault develops in a piece of AC-powered gear, a motor or charger for instance, and the case becomes energized. Normally, this fault current would be conducted safely to ground; however, it cannot because of a poor connection in the shore power receptacle. The potential in this case or enclosure is now at something above ground potential. A crewmember then touches the energized case and a seacock simultaneously. Normally, both of these pieces of gear would be at the same potential, because they are bonded. However, because of the skipper’s modification, they are not. The current now takes the only available path, traveling through the person, the seacock, seawater and finally back to ground.
As a reiteration to the above AC-grounding stories, the following principles must be adhered to. The neutral and grounding wires must be separate throughout the vessel’s electrical system (this may involve removing the bonding strap within some appliances), and the AC grounding system, the DC negative system and the bonding system (seacocks, tanks, etc.) must all be at the same electrical potential.
In the next issue: Galvanic isolators, isolation transformers, gensets, inverters, GFIs and European shore power systems.
Steve C. D’Antonio is a marine writer and photographer and is the boatyard manager at Zimmerman Marine Inc. in Mathews, Va.