Do-It-Yourself: Shore Power
By Tom Burden, Last updated 3/28/2019
Today’s topic is Shore Power Systems: what they do for boaters, how to ensure compatibility with different electrical sources as you cruise, and how to avoid some common problems.
|Skill Level Description|
|Selecting the correct shore power cord: 1 out of 10|
|Understanding shore power adapters: 2 out of 10|
|Replacing a shore power inlet: 3 out of 10|
|Completely mastering the intricacies of stray current corrosion and dock wiring problems: 10 out of 10|
Separate electrical systems for DC and AC power
Virtually all boats have a DC electrical system onboard, so you can start your engine or engines, operate interior lights, pump your bilge and operate marine electronics. Your boat’s DC system typically operates on either 12V or 24V, from energy stored in the boat’s batteries, and is replenished by the boat’s charging system. A second electrical system, your shore power system, allows you to bring AC electricity onboard from a source on the dock. You literally plug your boat into the local utility and enjoy all of the advantages onboard that you’d enjoy in your home or business. Shown above is a typical single phase 120-volt system with shore-grounded (White) neutral conductor and grounding (green) conductor (see ABYC Standards E-11.17.1. Diagram: ABYC®).
To complicate things a little more, some boats also make their own AC power using a generator or an inverter, which may share some of the same wiring as the shore power system, but the shore power system is an external source of AC electrical power, and an inverter or generator is an internal one. Shore power systems only work when you are near an electrical source on a dock. Generators and inverters produce the same type of electrical power, but are used when you’re underway.
Your shore power system allows you to run all sorts of useful appliances and tools on board when you’re in a marina. One of the most common devices is a battery charger to keep your batteries topped off when you’re not onboard. Others include dehumidifiers, heaters, kitchen appliances, power tools, and entertainment products like TVs and stereos.
Shore power cords
Your shore power system starts at the dock power pedestal. The conventional system used by mid-sized boats in your local marina combines two circuits with circuit breakers rated at 30A, two female receptacles, and a hinged lid to keep water off the connection. These receptacles have a threaded ring that allows the shore power cord to connect securely and make a waterproof connection. The ideal companion to this outlet is a 30A, 50' cord, by far the most common cord in the U.S. The contacts on the plug will only fit one way into the receptacle, which prevents polarity problems and also provides a secure connection when the plug is twisted slightly clockwise.
Larger boats can satisfy their need for more power using a 50A 125V or a 50A 125/250V cordset, assuming that your marina provides 50A service. While these cordsets can handle lots more amperage than the conventional 30A cords, it’s also common to find some boats using two 30A cordsets. The advantage of using the 50A 125/250V cordset is that you can run 250V products, like stoves other large appliances.
Although most boats use 50' cordsets, Marinco makes shorter models that may be just right for the distance from the pedestal to your inlet. You can reduce expense and clutter by considering one of these 12', 25', or 40' cables, or you can buy shore power cable by the foot, purchase the male plug and female connector separately, and custom-build a cordset to the exact length that fits your slip.
It is vital that you use plugs and receptacles that are intended to be used together, or you could create big problems. Never use a cord that has the bent terminal altered, and never force a plug to fit. The plug should fit easily and stay in place after twisting. For extra security and waterproof protection, always use a weatherproof cover and a threaded ring.
Marinco’s EEL system is more secure than your old cordset.
The case for SmartPlug: Larger area of metal-to-metal contact for lower resistance and cooler operation.
Easily Engaged Locking (EEL) System
Marinco’s new system addresses some of the problems with traditional shore power. Marinco uses the traditional old NEMA-configured patterns for the contacts on the entire range of connectors, so their new products are backwards-compatible with your old dock outlets, boat inlets and adapters.
They’ve done away with the threaded sealing ring and created a clever one-handed locking system. It has a better waterproof seal using their new jaw clamp, an integral LED light to help you find your way to the boat inlet in the dark, a secondary lock for a more secure hookup, and an LED power indicator so you’ll know that power is flowing through your cord.
New SmartPlug Alternative
Plugs like the Marinco twist-and-lock systems mentioned above were developed in 1938. Boaters now have an alternative style of shore power connector, a new product of note called the SmartPlug. This 2008 re-invention of the shore power plug has twenty percent more contact area in the electrical pins, so it is less likely to overheat, and if it does, there is an internal thermostat, set for 200° (just under the temperature at which wire insulation starts to soften), that cuts power at the first sign of overheating. Its unique shape only allows you to insert it one way, so there’s no chance of misalignment.
SmartPlug’s electrical pins do not bear any weight or tension in the cord, so they are less likely to work loose, one of the primary causes of electrical arcing and overheating in twist-type designs. The plug pushes straight in and locks automatically, using a three-point system consisting of a locking tab on the cap and two side-locking levers. It also mounts using the same four-hole pattern as the Marinco shore power cord, so it’s directly compatible with what’s on most boats. SmartPlug makes dual-configuration cordsets that fit Marinco dockside hardware, so you can plug your new-style boat into conventional twist-type shore power dock pedestals.
Pigtail and Y Adapters
While docks for smaller boats are usually equipped with 30A power pedestals, and docks for larger boats are equipped with 50A pedestals, you may find when you arrive at a new marina that the pedestal at the dock doesn’t match your shore power cord. Let’s say, for example, that you’ve got a powerboat equipped with 50A, 125V service but the harbormaster sends you to an end-tie, and the dock only has 30A service.
Reverse Y-Adapter with one 50A female connector and two 30A male dockside plugs.
To get connected, you’ll need a pigtail adapter. These devices have a male plug at one end, and a female connector at the other, but in different plug configurations. In this case, we’d use a 30A to 50A adapter that makes the conversion from the 30A power pedestal to the 50A shore power cord on your boat. Note that these pigtail adapters have the same waterproof covers and rings as other shore power connections, so it’s a safe and secure connection.
We’ll mention some fine points here. The boat in the above example was wired with a 50A circuit because the builder anticipated that it would consume lots of power while docked. You can’t pull 50A out of a 30A circuit, or at least you can’t for very long! Therefore, you have to restrict your use onboard to less than 30A, or you’ll spend your time resetting the pedestal breaker.
There is a way for this vessel to find the power it needs in this situation, and that’s by using a Reverse Y Adapter. This converter connects to both of the 30A outlets on the shore power pedestal and determines if the hot and neutral are correctly polarized, and if the hots are opposite phase, and if both male plugs are connected properly. If not, a special circuit in the Reverse Y does not allow current to flow. If the wiring is OK, the Reverse Y provides 50A of 125/250V power to your boat.
Another variation on the Y cable allows you to connect into a single pedestal outlet, and split the output between two boats or two inlets on the same boat. Once again, there’s no free lunch: your two circuits will be limited to the 30A capacity of the connection. That may mean some cooperation if two boats are trying to use the same outlet without tripping the breaker.
Identifying shore power connections
Deciding which pigtail adapter you need to get plugged in can be tricky, especially when you’re standing in the pouring rain with your cell phone, trying to place an order. Check the charts below carefully to ensure that you choose the correct cordsets and adapters for your boat.
Note: these connectors aren't all the same size (despite the above illustration). The 15A and 20A straight blades measure 1 3/8"dia. The 20A and 30A measure 1 13/16", and the 50A is 1 15/16". The connectors are usually marked with the rated voltage and amperage, but you may have to look carefully. The diameter of the circle formed by the connectors is the best indicator of 20A, 30A and 50A units. Examine connectors carefully, and never try to force mismatched connectors together.
Now let’s look at how all this power gets inside your boat. Most boats over about 25' in length come from the builder with an AC power system installed. A key component is the power inlet, and there are several designs to choose from. The power inlet provides a waterproof, strain relieved connection for your shore power cord, so you don’t have to leave a hatch or the companionway open and dangle a garden-variety cord down below.
Here’s an example of an inlet with a stainless steel base and gasketed cover. It’s a perfect match for Marinco 30A shore power cords, and for other manufacturer’s cords as well, since they are made to a national standard. This inlet is waterproof with the plug in place or with it removed. Another option is to use either the Standard Inlet or this smaller Contoured Inlet. All three are an exact match for the shore power cord. If you look on the back, you can see that there’s strain relief built-in for the panel feed wire that runs to your boat’s AC distribution panel.
From the shore power inlet, a length of 10 ga. triplex wire runs to the AC distribution panel. The distribution panel has a main breaker (which incidentally will open both the hot and the neutral wire in case of an overload condition) and a reverse polarity indicator. Finally, there are branch circuit breakers and the devices that consume power, also called the “electrical loads”, that are either hardwired or plugged into duplex outlets somewhere onboard.
There’s a right way and numerous wrong ways to install a shore power system on your boat, and the final authority is the ABYC Standard E-11. It’s the authority for boatbuilders and those who repair boats, and the majority of marine companies follow the standards closely. That virtually eliminates the chance of a dangerous situation on the dock or onboard.
Reverse polarity protection
The distribution panel has a reverse polarity indicator, and you should understand why it is important.
There are occasions when a dock pedestal is wired incorrectly, and the consequences can be dangerous for your boat and your crew. This happens when the black and white wires are accidentally interchanged, which renders the green ground wire ineffective. That’s where the reverse polarity light on your distribution panel comes in. The ABYC requires that your panel either have an audible or visual alarm if you have reverse polarity, and that’s your signal to disconnect from shore power immediately. Incidentally, using an isolation transformer is another solution to this problem, but most boats use a reverse polarity light indicator instead.
Another innovative way to identify reverse polarity situations is to use Marinco’s GalvanAlert. This product is installed inline with your shore power cord, and it alerts you to two potentially dangerous situations: reverse polarity, and stray current corrosion. Stray current corrosion may occur if an adjacent boat has a problem with their DC wiring system, and since your boat and the other boat are connected via shore power grounding conductors, you run the risk of underwater metal damage due to stray currents. If either of these two LEDs lights up when you’re connected to shore power, you might want to disconnect and investigate the problem. New boats are required to have an Equipment Leakage Circuit Interrupter (ELCI) within 10' of the shore power inlet. What’s an ELCI? See our West Advisor on ELCI/GFCI Electrical Shock Protection for more on this important safety item.
Troubleshooting shore power
Your shore power system is engineered for years of reliable service, but the marine environment can cause even the best-designed products to fail. As mentioned earlier, the majority of dock power troubles can be linked to two simple problems: overheating and saltwater immersion.
If you suspect a bad connection, turn off the dock power at the outlet and unplug the dock cord couplings at the outlet and boat. Examine the ends of the shore power cord. Look for discoloration or melting around the blades of the plug (male end) and around the slots on the connector (female end). Look at the face of the inlet on the boat, and check for discoloration or melting around the blades and the inlet. Check the receptacle on the dock and look for deterioration or discoloration around the slots.
Causes of overheating
Boaters often suspect that they’ve caused overheating by overloading the shore power circuit, but the folks at Marinco say that is rarely the case. Overheating generally results from one or two conditions: corrosion on the metal blades or contacts, or bad connections between the wiring device and the wires connected to it. Badly corroded contacts or blades are typically caused by exposure to a corrosive environment (usually salt water). If the end of your cord gets dropped into salt water, and isn’t thoroughly cleaned and dried, the contacts corrode, causing a poor electrical connection and overheating.
Bad connections between electrical terminals and a wiring device can usually be traced to loose termination, corrosion on the wires or terminals, or wires that are not stripped properly so that the wire insulation is under the terminals. A bad connection results in overheating of the terminal, and this is often visible on the face of the device.
Replace the connector immediately if you find signs of overheating. Be sure to inspect wire ends when replacing connectors. Make sure they’re clean, with no signs of corrosion or overheating. Marinco suggests that many dock cord connectors are replaced unnecessarily because boat inlet and dock receptacle connectors are at fault. Be sure to inspect all components with equal care.
If your dock cord connector gets dunked in saltwater, disconnect quickly and rinse well in freshwater. Dry thoroughly before use (Marinco recommends several days of drying). Afterward, spray the contacts with an electrical contact cleaner to displace any remaining water.
It’s tempting to use a shore power pedestal as a handy way to power drills, sanders, and other power tools. This is a particularly dangerous activity for two reasons.
You’re connecting a light gauge extension cord into a 30A circuit, so the cord is not sufficiently protected against short-circuits and fires. The only circuit protection is the 30-amp breaker on the shore power center, which will undoubtedly exceed the power rating of the cord that’s plugged into it.
There’s no operator protection from a GFCI, since that would normally be found onboard the boat, and there’s no boat involved. Therefore, the operator of the power tool, while using it in a wet environment, is in danger of electrocution. In fact, shore power centers are only intended to supply power to a boat, using an approved shore power cord, and not as general-purpose AC outlets. More importantly, if you’re working around water, you must have a Ground Fault Circuit Interrupter (GFCI) in place to prevent potentially fatal shocks. That’s exactly why Marinco developed this dockside 30A to 15A adapter with built-in GFCI protection. It gives you three protected outlets for power tools and will trip if the current leakage exceeds 4–6mA.
It’s not clear exactly how this differs from a circuit breaker, or what makes a GFCI or Ground Fault Circuit Interrupter better, so we’ll explain this critical bit of safety protection in detail. Circuit breakers and Ground Fault Circuit Interrupters are both safety devices, but they protect against different types of problems. Circuit breakers open a circuit (which means they cut the power) if you encounter either of the following two dangerous conditions. One is when you consume more power from an outlet than it’s designed to provide, which might cause the shore power cable and other wiring to get hot and possibly cause a fire or other dangerous situation. The other condition is a short-circuit from a damaged wire or appliance that causes excessive current to flow in the circuit. If the hot and neutral or hot and ground wires touch each other, it creates an overload condition, which is enough to trip the circuit breaker. This might happen if a shore power cord has been damaged, or if a power tool has been dropped overboard.
A Ground Fault Circuit Interrupter is intended to prevent a shock hazard when working with AC power. It measures the current flowing in the hot and neutral wires, and if it determines that the current is not identical, it opens the circuit. This is critical, since the likely path for the missing current is through the body of someone using a power tool, or someone using some sort of appliance, and this can be fatal, especially on a boat.
Shore Power Organizers
Now that we’ve identified the cords, inlets, adapters and Y cables that you’re likely to need, we’ll also mention a variety of ways to keep this gear neat and tidy when you’re at the dock or underway. Consider adding a shore power storage bag, which holds up to six adapters in addition to your cord, and can be used to cover the extra cord when you’re connected. Shore power clips support your cable using handrails on board, and these shore power deck clips keep the cord snug underfoot. Finally, this zip sleeve can contain two shore power cords and a cable TV connection, and it protects them from the elements and keeps them off the ground using these hook and loop attachments. You’ve invested in your shore power system, so you might as well protect your investment!