Category Archives: 50A Power from 30A Sources

50A Power from 30A Sources

Facility managers for marinas, yacht clubs, boatyards, condos and municipal walls must make investment choices about the electrical infrastructure that they will install to support their customer’s needs. Systems that provide maximum flexibility in electrical connectivity for boaters are expensive in capital cost and maintenance. In many facilities boaters will encounter more modest wiring alternatives. Wiring configurations will also vary between docks in larger facilities. Different docks at facilities that support a wide size range of both resident boats and transient visitors may be wired differently. Very large boats would normally slip on a dock with other large boats. These docks will likely be powered with only 208V/240V, 50A service outlets. Docks intended for mid-sized boats may have a mix of 208V/240V, 50A outlets and 120V, 30A outlets, or may have only 120V, 30A outlets. Facilities that cater to only transient visitors may have a mix of 30A and 50A outlets, or may have only 20A and 30A twistlock outlets. Electrically, there are many possible code-compliant wiring variations.

Cruisers must assess their personal desire for, and dependence on, shore power. Before departure, cruiser’s should obtain a set of adapters to provide the desired personal flexibility. The specific adapter(s) needed aboard the boat depends on the shore power inlet configuration of the boat. Along the Great Loop route, several variations of shore power may be encountered. The goal would be to have the flexibility to be able to connect the boat’€™s shore power inlet connection to each of the following commonly found power sources:

  1. residential 120VAC, 15A and 20A duplex outlets,
  2. marine 125V, 20A twist outlet (sometimes alone and sometimes in pairs),
  3. marine 125V, 30A twist outlet (sometimes alone and sometimes in pairs),
  4. 208V/240V, 50A marine twist outlet.

We never encountered a 120V, 2-Pole, 3-Wire NEMA type SS1 50A shore power source; they exist, but are very uncommon. We have encountered NEMA 14-50 (residential 240V, 50A outlets used on electric range/ovens and kilns) in various places along the Erie Canal system for use by canal system work boats. Because we had an adapter to access those outlets, we enjoyed shore power when others did not.

Boats fit with 50A Shore Power inlets will regularly encounter situations where 50A outlets are not available. For these situations, 30A-to-50A adapters can provide access to a AC power sufficient to meet short-term needs. Simply put, adapters create options, flexibility and alternatives to boaters. Among the options, adapters can provide enough power to avoid the need to run gensets at a dock.

Note: this article applies to boats which are NOT fit with polarization/isolation transformers.

In North America, the national standard for power delivered to residential and light commercial customers is a “single phase, three-pole, four-wire, center-neutral” wiring configuration. This system is sometimes referred to as a “240V grounded-neutral” system. In these systems, the service’s Neutral (white) conductor is bonded (electrically connected) to the system’s Ground conductor. The bonding point is located at the “derived source,” ashore. Boats connected to shore power systems should never have the neutral and ground bonded aboard the boat. Connections to outlets fed from single phase sources in the utility distribution system will receive service voltages of 120V/240V. Connections to outlets fed from the phase legs of three phase sources in the utility distribution system will receive service voltages of 120V/208V.

Figure 1 shows a typical single-phase residence or dock source fed from a street transformer.  The “secondary” of the transformer is the feed’s source:


Figure 1: Single Phase Transformer with 3-Pole, Center-Neutral Secondary to Customer

Figure 2 shows the common dock power distribution components found on docks at marinas, yacht clubs, boatyards, condos and municipal walls throughout North America. The “derived source” is defined by code to be the point where the Neutral-to-Ground Bond and the dock’s main disconnect circuit breakers are located.

Dock electrical system feeders must be designed to support a number of boats at the same time, which means the current-carrying conductors of the dock feeder need to be quite large. In the US, the National Electric Code, Article 555.12, specifies the ampacity calculations of dock feeders.


Figure 2: North American Dock Shore Power Layout – Typical

Figure 3 shows a simplified example of the most common configuration of 50A shore power outlets found on docks.  This example shows the 3-pole, 4-wire dock feeder with drops to six 208V/240V, 50A shore power receptacles.  The source for the dock feeder can be either 120V/208V or 120V/240V.  This wiring configuration is mandatory in order to support boats fit with 50A shore power services.  Note that the dock feeder is a direct electrical extension of the power transformer shown in Figure 1, and consists of the two energized conductors (L1 and L2), the Neutral conductor (N), and a safety ground conductor, (G).


Figure 3: Typical 240V, 50A, 3-Pole, 4-Wire Dock Feeder

Figure 4 shows a portion of a 120V, 30A shore power configuration.  In this example, each outlet provides 120V at up to 30A to the boat.  Note that adjacent outlets in this wiring configuration are alternately connected to the two energized legs, L1 and L2.  Since both energized legs are necessary for 208V/240V service, this would be the most common way to connect 30A outlets in the case of a dock with a large number of 50A outlets.


Figure 4: Desirable, Best-Case 125V, 30A Dock Wiring Usually Found on Docks with 50A Pedestal Outlets

Figures 5 and 6 show two alternative configurations for providing 30A shore power at a slip.  As in Figure 4, each outlet in Figures 5 and 6 provides 120V at up to 30A to the boat.  The difference in these examples is that all of the 30A outlets are connected to the same energized leg, rather than to alternate legs, of the dock feeder.  For 30A boats, this configuration is functionally equivalent to the example in Figure 4.  Boats requiring two 30A shore power services will never notice or be affected by the difference between the configurations shown in Figure 4, Figure 5 or Figure 6.


Figure 5: Alternate “One” for a 125V, 30A, 2-Pole, 3-Wire Configuration, with Only One Energized Dock Feeder Leg Used for Power


Figure 6: Alternate “Two” for a 125V, 30A, 2-Pole, 3-Wire Configuration, with Only One Energized Dock Feeder Leg Used for Power

For boats that require 208V/240V shore power services via a 50A, 4-Wire shore power cord, a “Smart Wye” splitter can provide shore power from the example shown in Figure 4, albeit at reduced total amperage capacity of 30A, total.  However, the “Smart Wye” will not deliver any power at all if connected to the configurations shown in Figures 5 and 6.

Figure 7 shows the electrical diagram of a “Smart” Reverse Wye Splitter.  To the left are two 30A male plugs which are fit to  30A pedestal outlets.  On the right is a 50A female, which  receives the boat’s regular shore power cord.  In the box at the center, a relay is used to forward power from the pedestal outlets (dock feeder) to the shore power cord.  A 208V/240V relay, K1, is connected between the two energized conductors of the two incoming 120V, 30A lines.


Figure 7: “Smart Wye” Reverse Splitter

Figure 8 shows a “Smart”€ Reverse Wye connected to the dual-leg dock feeder wiring configuration (as previously shown in Figure 4).


Figure 8: Smart Wye Reverse Splitter Connected to Both Energized Legs of a 208V/240V Shore Power Service

This wiring alternative places 208V/240V on the relay coil, K1. The relay “picks,”€ meaning the contacts of the relay close, and this allows 208V/240V shore power to feed through the splitter to the boat. With this adapter, 208V/240V appliances will work. Because the input source is limited to 30A, boat loads may need to be manually limited and controlled to avoid drawing more than 30A. However, with attention, most boat equipment can be used successfully, even if not at the same time.

Figures 9 and 10 show the Smart Wye Reverse Splitter connected to the two alternative single-leg dock feeder configurations. In these cases, since both 30A inputs are connected to the same energized dock feeder leg, there is no voltage between them; that is, zero volts. Since the relay requires 208V/240V to operate, the relay in this case does not “pick,” and no power at all is allowed to pass to the boat.


Figure 9: Both Smart Wye 30A Connections Made to L1


Figure 10: Both Smart Wye 30A Connections Made to L2

In cases where only one energized dock feeder leg is available, the only way to get any shore power at all to this 50A boat – however limited – is with another type of power adapter. Options are available. To understand the options, it is necessary to first understand how the branch circuits aboard the boat are wired.

Figure 11 shows an incomplete but representative view of a 208V/240V boat electrical system. Although I have modified the diagram, credit for the base is to the American Boat and Yacht Council (ABYC), Annapolis, MD. This diagram shows a “typical”€ AC shore power configuration for a boat built with a 208V/240V, 50A AC power system, and found without a polarization/isolation transformer.


Figure 11: Partial View, Representative of a “Typical” 208V/240V Boat Electrical System

The left side of Figure 11 shows the dock feeder discussed above. At the center-right of the drawing, the AC power buss shown in colors is the AC power buss of the wiring of the boat. All boat equipment gets power from the boat’s AC buss via branch circuit breakers. The 120V utility outlets on a 208V/240V boat can be attached to either one of the energized conductors; to L1 alone, or to L2 alone, or some to L1 and some to L2. The drawing shows two utility outlets. The top outlet is fed by L1 and the bottom outlet is fed by L2. Any adapter that’s used to supply some limited power onto a 208V/240V boat must provide that power to both L1 and L2.

Figure 12 shows a 30A-to-50A adapter that will accomplish the goal. Power from one of the energized dock feeder legs is brought through the pigtail to feed both the L1 and L2 blades of the 50A receptacle. Each 50A receptacle blade will have 120V, but because they are fed from the same point, there will be no 208V/240V power.


Figure 12: 30A-to-50A Straight Adapter

Commercial straight pigtail adapters like this are available.  Power is limited to 30A, total.  With this adapter, 208V/240V appliances will not work, but important 120V refrigeration, lighting, entertainment systems and computers connected to 120V utility outlets will be OK as long as the total load is managed to be less than 30A.