The owner of a 1980s trawler had a problem with his electrical system. He wanted to install a dive compressor, which required 240 volts, but his generator and electrical panel were set up for 120 volts. Even if the genset could be modified to produce 240 volts, the panel had no provision for receiving and distributing that power.

Like many boats from the ’80s and earlier, this boat didn’t come with too many AC loads. It had a single 30-amp inlet to power an air conditioning unit, an inverter/charger, a water heater and a few outlets.

The originally fitted 3-kW generator was just enough to carry those loads as long as the owner kept an eye on the ammeter and practiced a little load management. At some point, the generator was upgraded to an 8-kW unit that allowed more flexibility, although the main 30-amp AC breaker in the electrical panel still limited the total power that could be used dockside.

Searching for a Solution

Planning for new electrical equipment aboard a boat always requires assessing how much power is available, either from shore power or the generator. When thinking your way through electrical power requirements, it helps to convert the power to watts.

A simple formula provides the key: watts = volts multiplied by amps.

You can solve for the unknown as follows: amps = watts divided by volts; or volts = watts divided by amps.

We know from this boat owner’s experience that the 30-amp shore power service was meeting this boat’s power needs. We can compare that to the generator output by applying the formula: 8,000 watts divided by 120 volts = 66.6 amps

The formula tells us that the generator can meet all the power needs handled by the 30-amp shore power service and have 36.6 amps (at 120 volts) available.

Next, we need to assess the dive compressor. The owner is an experienced diver and intends to cruise extensively. He could have avoided the electrical issue by stowing additional air tanks and filling them at a dive shop, but he preferred the convenience of an onboard compressor. He understood the need to adhere to the air filter replacement intervals, as divers using cheap or poorly maintained compressors can breathe oil or contaminants into their lungs and become very ill or die.

The compressor he selected utilizes a 3-hp, 240-volt, 60-Hz, single-phase motor. The full load current is 14 amps (3,360 watts), which, according to our math, his generator should be able to supply.

But we need to dig deeper. 

There are three types of loads: resistive loads, which are lights, stove elements, clothes dryers, water heaters or anything with a resistive element; capacitive loads, which are motors like what you would find driving the compressors in refrigeration or air conditioning units or water pumps; and inductive loads, which are anything with a transformer in it, such as some battery chargers and inverter/chargers.

Resistive loads are nonreactive, while capacitive and inductive are reactive loads. It is important to know what types of loads are aboard because reactive loads take more power to operate than their faceplate would suggest. These load characteristics influence generator sizing.

With electrical usage, there is a term called power factor. If you think of usable power like a quickly poured mug of beer, the power factor would be the ratio of foamy head to liquid. With certain loads such as motors and transformers, a percentage of energy is necessary to make the device work, but sort of gets wasted in the effort (like the foam). The power factor in an AC circuit is the ratio between the apparent power measured in volt-amperes and the real power measured in watts. This difference is expressed as a percentage.

For example, a 0.8 power factor equals 80 percent. An electrical system with a low power factor and with a lot of reactive loads (less than 0.8) will require higher currents to power it than a system with mostly nonreactive loads and a high power factor.

This is all to say that when sizing a generator, the type of loads as well as the size of them have to be taken into account. More reactive loads will require a larger generator.

Two Legs to Stand On

When we talk about the legs of 240-volt hot wires, we are referencing the two hot wires produced from a particular transformer or generator.

A standard U.S. household 240-volt setup utilizes two hot wires (usually black and red) and one grounding wire (usually green). Sixty times a second, each hot wire is either a hot or a neutral, trading roles each cycle. Hertz is the measurement for cycles per second, and the standard for the United States is 60 hZ.

Standard household 120-volt setups consist of one hot wire (black), one neutral wire (white) and one grounding wire (green/copper). This 120-volt setup is created by one of the 240-volt hot wire legs and a neutral wire that is referenced to ground. Because the 120-volt leg only has one hot wire, it needs a separate neutral to complete the cycle.

When a boat has a 50-amp, 240-volt shore power system and no onboard transformer, often the two 240-volt legs will be divided on the electrical panel into A and B legs of 120 volts. Marine electricians try to ascertain how the loads will be used, and then divide them so that each leg will have similar amperage draws. This is called balancing the loads.

In actual use, boat owners should keep an eye on their amperage meters and note both legs, especially when operating the system from a generator. It is a good idea for each leg to be within 10 percent of the other, as we will explain below.

This particular boat owner wondered if the generator could be converted to 240 volts so that he could power the compressor and have the rest of the boat run off one hot leg of the 240-volt setup, to make 120-volt power for his panel. Unfortunately, putting all the boat’s loads on one leg would severely imbalance the generator. Pulling dramatically more amperage from one 120-volt leg of a 240-volt setup creates what is called negative sequence magnetic flux within the windings of the generator. In short, that imbalance will create a great deal of unwanted heat in the generator windings that could severely damage them.

Generator Rewiring

The generator owner’s manual indicated that it could be rewired to produce either 120 or 240 volts. But how does that work?

The electrical generation component of a generator comprises a rotor and a stator. The rotor has wire wound around a shaft that spins inside the stator, which encircles the rotor and also is wound with wire. The spinning of the rotor at a precise speed magnetically induces current, which then gets regulated to produce the amperage needed.

If a generator can be rewired to produce different voltages, the generator’s manufacturer will include taps in the wires that shorten or lengthen the windings to boost or cut the voltages. The process is similar for most generators, but the details must come from the manufacturer and a careful study of the electrical diagrams for the generator. Typically, this means that jumpers or wires on a terminal strip in the generator’s electrical box would be changed from one set of terminals to another.

Transformers

With the genset reconfigured to produce 240 volts and providing the power needed for the dive compressor, this boat owner’s problem would appear to be resolved. But as all boat owners know, it’s never that easy.

Reconfiguring the genset solves one problem but creates a new one: no 120-volt power for the electrical distribution panel.

One option would require replacement of the panel with an upgrade to include a 240-volt breaker that could supply the dive compressor, and the service split into two 120-volt legs to supply the rest of the boat’s loads. An argument could be made to do this because it would offer more AC capacity aboard the boat. However, this path would also require upsizing the shore inlet and cord to a 50-amp, 240/120-volt setup, replacing the wiring from the inlet to the panel, replacing the AC panel itself, reorganizing all the loads, and replacing the AC service switching (which in this case is a roll switch). In short, it’s a complicated and expensive approach. A more affordable solution was needed.

To utilize the existing 120-volt service, we need a way to convert 240 volts to 120 volts. Devices that step voltage up or down, called transformers, can meet this need.

Transformers have two coils of wire wrapped around an iron core. The primary coil is connected to the power source (in this case, the generator). The secondary coil is connected to the load (the equipment connected to the AC panel). When energized, the primary coil induces a magnetic current in the secondary coil, forcing the electrons in the wire to move. By varying the relationship between the number of coils, different output voltages can be derived.

If we installed a step-down transformer, we could convert the generator’s 240-volt output down to 120-volt output, keeping the existing electrical system intact. Because the transformer utilizes both legs of 240 to produce the 120 volts, the generator remains balanced.

On the other hand, transformers are heavy, in this case 75 pounds, so they must be through-bolted to ensure that they remain secure. That’s also expensive and impractical.

One more option surfaced: Could the air compressor run on 120 volts instead of 240 volts?

Either voltage could potentially power the motor that runs the compressor. Many AC motors are set up with dual voltage, meaning there are two coils of wire inside: a full-length coil if the motor will run on 120 volts, and a split coil if 240 volts is supplied. Terminals inside the motor’s junction box, when connected, properly determine which voltage the motor will use. Many heavy-equipment motors are set up for 240 volts or more because the wiring and breakers for the motors can be smaller, there are slightly better torque characteristics, and the motors tend to last longer because they can run cooler. They don’t, however, use less wattage.

Our formula of watts = volts multiplied by amps shows that our 240-volt motor would draw 14 amps while a 120-volt motor would draw 28 amps, but the wattage would stay at 3,360. Unfortunately, this motor was not dual voltage.

Air to Spare

While the options described above provide a window into understanding high-voltage principles, they all were too expensive and too complex for this situation. Sometimes it pays to question the original premise: the need for a 240-volt compressor.

A call to the manufacturer of the dive compressor revealed that while the motor was not dual voltage, the compressor could be purchased with a 120-volt motor. This option eliminated the need to reconfigure the genset, and only required a new breaker and an outlet to supply the compressor with power.

This solution had only one shortcoming. When plugged into shore power, the dive compressor could not be run from ship’s AC power because it would overload the 30-amp main breaker. Since this owner intended to spend most of his time on the go and not tied to the dock, he accepted this limitation, making it possible to go diving without getting his checkbook underwater.