Your customers have come to expect shore power and other AC systems as part of life afloat. The safety and reliability standards for AC shore-power systems—which include the shore cord supply, generators, and inverters, referred to simply as “AC” from here on—are necessarily strict. It’s up to the professional to fully understand how they work, as well as recognizing the subtleties that differentiate them from systems ashore. Incorrect wiring of, these systems could have catastrophic consequences ranging from fire to electrocution.
Incorrect wiring is insidious in that it often exhibits no obvious signs or clues until a fault occurs. So it’s necessary that all AC systems fully comply with the American Boat & Yacht Council guidelines. Regrettably, violations of the guidelines are as likely to be found aboard new boats as used ones. As industry professionals know, even the guidelines are not always easily deciphered, understood, or applied.
Common Single-Phase Shore Power
Although there are exceptions, typical AC wiring installations aboard boats up to about 75′ (23m) are single phase. This means, for 120V service, that the wiring configuration has three colored conductors:
- hot (black)—analogous to DC positive, it has more voltage and is alternating current and potentially lethal;
- neutral (white), sometimes referred to as grounded—analogous to DC negative;
- safety grounding (green). Note the “ground” terminology ending in “ing” versus “ed.” Normally this conductor is not considered a current carrier unless and until there is a fault.
Connections between the hot and the neutral provide 120V. For 240V service, another hot conductor—typically red—is added. Connections between the black and red provide 240V. Other colors are used within the parameters of the standards. Some Asian and European builders, for instance, utilize light blue in place of the white, or neutral; and brown, orange, or blue for hot (black) conductors. Various striped patterns are also used, the most common of which is for the green grounding conductor; this is often green with a yellow stripe. Other colors within this system might violate ABYC guidelines and should be avoided to prevent confusion.
The Green Grounding Conductor
The function of the green grounding conductor, as mentioned above, is to conduct fault current safely to ground. This may seem intuitive, but it’s what happens during or after the fault current is conducted safely to ground that is not often discussed or well understood. When fault current is conducted to ground, the immediate result should trip a breaker or blow a fuse, effectively shutting off the power. The green grounding conductor must be sized so it’s capable of safely carrying fault current without overheating. This conductor could be thought of as insurance. You hope you never need it, but on the rare occasions when you do, you want to be certain it will work.
The green AC safety-grounding conductor is often part of a larger onboard ground system that consists of the bonding system and the DC negative system. If ABYC guidelines are followed, these other “ground” systems are interconnected.
The following real-life example illustrates why a working ground system is so important. During a plumbing repair in the galley, I brushed my sleeved arm against the stainless-steel case of the microwave oven. I felt a slight tingle. My first reaction was to touch the oven casing with my hand to confirm what I’d felt. Having been shocked a few times in my career, I resisted. Instead, I used a multimeter to measure voltage between the microwave and the sink, which I was touching with my hand when I felt the tingle. To my amazement the meter read 113V. I shut down the power and removed the microwave. A screw had recently been driven through the cord into the microwave case, exposing the hot conductor, and the receptacle to which it was wired lacked a ground. This enabled the case to become energized, and, because the AC safety-grounding conductor was not intact, it remained that way. Had I been wearing short sleeves I likely would have received a shock or worse. If the receptacle had been properly wired, fault current would have been safely conducted to ground.
Why then, you may ask, is it important for the aforementioned ground systems to be interconnected? Take the above scenario and substitute a raw water strainer for the microwave. A shore-power hot conductor chafes against the strainer; if it’s bonded and the bonding system is connected to the AC safety grounding system as it should be, the fault is conducted safely to ground, and it trips an AC breaker.
In a residential or commercial electrical service, it’s common practice for the AC safety grounding conductor bus to be connected to the neutral bus. If you’ve ever looked into a main electrical panel ashore (not a subpanel as might be found in a garage or outbuilding), you will typically see white wires sharing a bus with uninsulated wires. The latter are AC safety grounding conductors; or you may see two separate buses that are connected. While this practice is fine on land, it most certainly does not work aboard a boat, and it’s where many do-it-yourselfers or otherwise experienced residential/commercial and some marine electricians make a potentially fatal flaw when installing or servicing marine electrical systems.
In a properly wired system, current traveling aboard on the hot wire returns solely through the white wire. Because a boat floats in a conductive medium, interconnecting the green and white wires aboard affords current returning to its source—the transformer ashore in this case—through three paths: the white wire, the now-connected green wire, and the water via bonded underwater metal hardware such as the engine/propeller, strainers, and seacocks. Contrary to popular belief, electricity takes all paths back to its source, not just the path of least resistance. This presents a clear hazard to swimmers, and those aboard may also be at risk, depending upon the nature of the fault.
Therefore, it is vitally important that the connection between the AC safety grounding wire and the neutral wire occur only at the power source. That means, at the onshore transformer supplying the dock shore cord, or at a generator or inverter—when and only when one of those is supplying power to the vessel. There are exceptions to this rule that involve the use of transformers; however, I’ll save those for another ATD column.
If you have doubts about the neutral-to-ground connection(s) aboard any boat’s AC safety grounding system, check it with a multimeter.
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 Inc.” He is a contributing editor of Professional BoatBuilder, and awaits publication (by McGraw-Hill/International Marine) of his book on marine systems.