This article discusses the electrical behavior of the two 120V AC circuits on a boat that is natively wired for 125V/250V, 50A shore power service. Topics include current flow (Amps) in the different appliance loads, power limitations when connected through a “Smart Splitter,” and the constraints and limitations encountered with the use of certain shore power transformers when powered from 208V dock utility voltages.
Use Case 1: a boat wired with a 125V/250V, 50A shore power cord, but not fit with 240V appliance loads.
Figure 1 is a generic wiring diagram illustrating this use case. The system includes a genset and a Galvanic Isolator. In Figure 1, the dock power source is on the far left and the boat’s appliance loads are on the far right. Dockside 50A circuit breakers are omitted for simplicity. The 50A shore power cord is highlighted in the red oval. One 120V load (the heat pump) is highlighted in red. Other 120V loads (house loads) are shown in black. This boat DOES NOT have 240V loads. This use case is a very common “50A” boat configuration.
Use Case 2: a boat wired with a 125V/250V, 50A shore power cord adapted to two 120V, 30A pedestal outlets to obtain limited 208V/240V power.
Figure 2 is a generic wiring diagram illustrating this use case. Most commonly, a “Smart Wye” splitter adapter is used (ref: Appendix 1). A “Smart Wye” splitter has two 30A twistlock plugs (NEMA L5-30P) and one 50A receptacle (NEMA SS2). The two 30A receptacles (NEMA L5-30R) are on the dock pedestal. The splitter and the 3-pole, 4-wire, 50A power cord are shown in the red ovals. The rest of this system is identical to Figure 1.
Figure 3 applies to both Use Case 1 and Use Case 2 configurations. Figure 3 shows logical blocks instead of actual circuit detail in order to make it easier to visualize the electrical behavior in this AC system. In Figure 3, incoming power is shown as being derived from “any suitable 240V source.” Electrically, we really don’t care how we get shore power as long as it’s “3-pole, 4-wire” of the right voltages. In Figures 1 and 2, the loads were shown as they are wired, but Figure 3 shows them as they are logically arranged in the overall electrical circuit. As the drawing shows, the red-highlighted 125V, L2 heat pump load is connected in series with the black-highlighted 125V, L1 appliance loads. These two load groups share a common “Neutral” conductor. The Neutral conductor anchors and maintains the midpoint voltage of the series connection under varying demand conditions.
Visualizing this electrical configuration in the mind’s eye as two 120V loads connected in series across a 240V source is the first key concept in this article.
Having identified the electrical arrangement of the two 120V appliance load groups of this 240V system, further analysis is on a) the voltages present, b) current flows, and c) power available to do work.
Figure 4 shows the two series load components of this boat’s 240V boat system, each with 120V across them. The L2 load group is comprised of the boat’s heat pump(s) and raw water circulator. The L1 load group is comprised of the hot water heater, fridge, battery charger(s) and multiple utility outlets. Measuring across the L2 load between points A and B, there are 120V. Measuring across the L1 load between points B and C, there are 120V. The series pair receive the 240V mains supply voltage measured between points A and C.
Next, consider the electrical currents (measured in Amps) flowing through the two series load groups in a variety of specific but different load circumstances. Understand that in the following analyses, different specific devices are “on” and others are “off” at any specific point in time. Assume the following scenario: the boat’s owners have been away from their boat for a mid-summer week. Upon late day arrival at the boat, outside air temperatures are in the mid-to-high 80s with 85% relative humidity. Our boat owners will turn on some space lighting, and will immediately turn on the heat pump for air conditioning. They will turn on the hot water heater and battery charger, stow fresh veggies, ice cream and adult beverages into the fridge, and perhaps turn on the DVR/TV.
Electrically, assume the heat pump draws 20A. Also assume that house loads (hot water heater, battery charger, fridge, space lighting, computers and DVR/TV) add up to drawing 20A.
In Figure 5, the heavy red line represents this 20A flow of current (Amps). This example is a special case called a “balanced-load” condition; that is, both of the 120V loads just happen to draw the same amount of current (20A). The Amps flow from the dock pedestal into the loads on one of the energized line legs (L1), and flow back to the pedestal on the other energized line leg (L2). In this balanced-load condition, no current flows in the neutral conductor (N).
Very importantly, notice that no more than 20A is flowing anywhere in this system. A double-pole 30A circuit breaker that serves the boat via a Smart Splitter at the dock pedestal sees 20A on both legs, L1 and L2. Since there is no place in the system carrying more than 20A, the 30A pedestal circuit breaker is perfectly happy. The second extremely key concept to take from this article is that the 20A flowing to power the heat pump circuit is the same 20A that flows through the House circuits to power the water heater, battery charger, fridge and utility outlets.
The word “power” is highlighted above to make the point that the same 20A flowing in the two 120V loads does useful work in both 120V load groups. The basic formula for “Power” is P = Volts x Amps. So in the heat pump load group, we have 120V * 20A = 2400 Watts. In the house appliance load group, we also have 120V * 20A = 2400 Watts of power doing useful work. In total, we have 4800 Watts of work being done at this time, in this system.
Up to 30A is available from a 30A shore power pedestal without exceeding the capacity of the circuit breakers. The maximum power possible for each load is 120 * 30 = 3600 Watts. Because the two load groups are in series, the maximum work that can be done by 30A, in total, is 7200 Watts. If the boat had access to its design maximum of 125V/240V, 50A shore power, there would be the potential for 240 * 50 = 12000 Watts, total. It quickly becomes clear why careful load management is necessary when running with two 30A cords feeding a 50A boat through a 30A Smart Splitter.
Following from our earlier scenario, after an hour or so, the hot water heater has done its water heating work, the fridge has done its cooling/freezing work, and the batteries are fully charged. But, the heat pumps are still running to cool the boat. Now, although we have 20A flowing in the heat pump load, current on the house side has dropped to 4A for the DVR/TV and space lighting. Figure 6 shows what happens electrically.
The heavy red line represents the 20A needed by the heat pump. But this time, there are only 4A needed by the house, represented by the thin red line continuing through the House circuit. There is no longer a balanced-load. The arithmetic difference between the heat pump demand and the house demand is 16A. That 16A returns to the pedestal in the system’s neutral (N) conductor. In this example, as before, there are 120 * 20 = 2400 Watts of work being done in the heat pump load group, and 120 * 4 = 480 Watts of work being done in the House load group. There are never more than 20A flowing in any part of this system. Neither the shore power pedestal breakers nor the Neutral conductor are overloaded. All is safe and well within specifications.
At the end of the evening, when our sample boaters retire to bed, assume they turn off all of the house loads. The hot water heater is satisfied, the battery charger is satisfied, the fridge is satisfied, the TV is “off,” the laptop and iGadget batteries are charged (and the screens have gone “dark”), and the space lighting is “off.” Now, there is no current at all flowing in the House loads. Ah, yes, but the air conditioning is still needed.
Figure 7 represents the electrical status in this case. Since the heat pumps are still running, there are 20A flowing in the heat pump circuit. Since there is nothing “on” in the House load group, the arithmetic difference is 20 amps, which returns on the neutral (N) conductor. Again, no part of the circuit carries more than a total of 20A.
Use Case 3: a boat wired with a 125V/250V, 50A shore power cord, but fit with 240V appliance loads aboard.
Figure 8 shows the addition of pure 240V loads at the far right of the drawing. Boats with 125V/250V, 50A shore power service which have both 120V and 240V appliance loads (hot water heater, cooktop, electric dryer, heat pump compressor) are electrically very similar to those without 240V appliances. Very few “240V appliances” are “pure” 240V devices. The only ones that come to mind are 2-pole, 240V deep well pumps and 2-pole, 240V hot water heaters. Appliances like heat pumps, cook tops, ovens, clothes dryers and watermakers, are usually “hybrid devices;” ie, they need both 120V and 240V to operate. The control circuits in hybrid appliances are generally 120V circuits. In a dryer, for example, the heating elements are 240V but the motor that turns the drum and the clock timer circuit both require 120V. Hot water heaters can be pure 240V-only loads which do not need or have a neutral conductor.
In Figure 8, the pure 240V appliance loads are electrically in parallel with the two 120V series loads, and the 240V loads add to the amps drawn in the 120V supply mains, L1 and L2. So, if we had the 20A L2 load running a 120V heat pump, as has been the example throughout this article, and in addition, a 240V hot water heater simultaneously calling for 12A, the result would be a 32A total Amps in L2. Attached to a 50A pedestal, all would be OK, but attached to a 30A splitter, the result would be a tripped 30A pedestal circuit breaker. So again for emphasis, it is up to the boat owner/operator to understand load management and ensure that pedestal breaker capacity is not exceeded.
Potential Power Issues with Certain Shore Power Transformers
The utility power on docks can originate from two kinds of public utility sources. “Single phase” sources will appear as conventional 120V/240V. “Three phase” sources will appear as 120V/208V. Because this electrical fact is a well-understood, and very common in boating, UL Marine certified electrical appliances are designed to accommodate the difference between 240V and 208V. Residential appliances MAY NOT have have that same flexibility.
Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for 125V/250V, 50A applications are manufactured in three “flavors:”
- Basic, single input, single output, 240V transformer; least expensive flavor.
- Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input to achieve a constant 240V output.
- High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage. While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.
Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction. Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.” The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor. The output of this transformer (the secondary) provides single phase, 3-pole, 4-wire power to the boat. In English, that means there is a conventional black, red, white and green output. If the input voltage to a basic style transformer is 240V, the output will be 120V/240V. But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances. With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.
One hundred four volts is a low utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances. Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally. Microwaves will run but will take slightly longer to cook. Coffee pots will perk, but will take slightly longer to perk. Electric blankets will keep sleepers warm and cozy. Water Heaters will heat water, but take slightly longer to reach target temperature. Stovetop burners will heat, but not get as hot at the same setting. Heat pump compressors and fans should all run, but some motors may overheat and cut out to protect themselves from damage. Marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible. One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V should be designed to detect the condition, put up a power warning fault light, and self-disconnect. Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer marine heat pump designs do that.
WARNING: if there is a 240V shore power supply voltage applied to a manual transformer set to a 208V input voltage, then the AC voltages aboard can get high enough to “damage” appliances.
- When operating a 125V/250V, 50A boat which does not have 240V loads, total loads of up to 2 * 3600 Watts can be supported with two conventional 30A pedestal outlets. In this case, neither the energized (hot) conductors nor the Neutral conductor are ever overloaded. No individual circuit conductor ever conducts more than 30A.
- When operating a boat with pure 240V loads, the Amps required by the 240V loads add to the Amps needed in the 120V loads. The owner/operator must monitor total amps drawn/power used to keep total power consumed below 3600 Watts per side.
- Some shore power circuit breakers are housed in inaccessible, locked locations ashore. If a boater accidentally trips a shore power circuit breaker, particularly after hours, it may not be possible to gain access to it in order to reset it
- It is necessary for boat owner’s to closely monitor power usage and limit the amount of current used to prevent tripping shore power circuit breakers. Care must be exercised to not run high amp draw appliances (coffee pots, microwave ovens, inductive cookware, hair dryers, clothes washer/dryers and similar devices) at the same time. Boats with multiple heat pumps will probably be unable to run all of them at the same time on 30A services.
- The examples in this article assume that the heat pump circuit is on one 30A load leg and house loads are on the other leg. Obviously, some boats are wired differently. Systems with heat pumps and house loads distributed across both incoming energized 120V legs will have to monitor loads and current draws in the same manner, but the electrical principles discussed above remain the same.
- The specific balance of currents in the load one group and the load two group changes constantly. L1, L2 and Neutral current (Amps) never exceeds 30A.
To the right is the electrical diagram of a typical “Smart Wye” splitter. This Figure represents the electrical circuit detail of the splitter shown in Figure 2 in the earlier text. Note that the splitter contains a relay – labeled “K” in the drawing. The relay requires 208V or 240V to close. Without at least 208V, the relay will not close and the splitter will not pass any power through to the boat.
Following is a link to my article describing Smart Splitters, and the receptacles required for their successful operation.