Voltage Selection

• What voltage selection?  Boats use 12 Volts.
• How is voltage selected?
• Calculating
• Breakers

What voltage selection?  Boats use 12 Volts.

Not necessarily.  Most yachts today use 12 Volt direct current because much of the equipment on board is common with wheeled vehicles.  It hasn't always been so -- Before the war it was common for a yacht to use 32 Vdc, and the standard for military vehicles of all types is 28 Vdc maximum.   I am, sadly, old enough to remember cars, tractors, and boats with 6 Volt systems.  Many larger vessels use 24 Vdc, and a lot of equipment will run on it, making it a good choice for yachts as well.

A word on nomenclature:  It is normal in civilian usage to specify the vehicle voltage by the battery voltage:  for a car there are six cells in the battery, each is nominally 2 Vdc, and so the system is referred to as a 12 Volt system.  The military specifies the system by its maximum, or charge, voltage.  Therefore, the same vehicle would be referred to as "14 Volt", since 14 Volts are needed to charge a 12 Volt battery, and is therefore the highest voltage that the system will normally produce.  This means that the civilian 24 Volt system is the same as the military 28 Volt system.

A 32 Volt system, once common on motor yachts, requires a sixteen-cell battery.  Today battery manufacturers make 3 and 6 cell batteries.  I have seen 32 Volt systems with four 4-cell batteries in series, and also with 16 individual cells.

More than 32 Volts is not considered "low voltage" and is not normally used.  An exception of course is electric cars, where 120 Vdc or more is common.  I have worked with dc voltages as high as 450 Volts on shipboard, but such systems have not appeared in yachts.

How is voltage selected?

System voltage is a compromise, where the property to be optimised depends on the application.  If voltage is too low, then wires have to be very large and heavy to carry the current required at an acceptable loss of voltage.  If voltage is too high, then wires large enough to provide an acceptable voltage drop are too small to carry the required current.

For aerospace applications, voltage is often selected, from among several alternatives, to minimize weight. 

For automobiles, the availability of 12 Volt parts makes any other voltage uneconomical.  Many trucks, however, have gone to 24 Vdc to reduce wire sizes.

For civilian aircraft, the choice is between 12 and 24 Volts, with 24 Volts getting the job done with less weight but more expense.

For yachts, the choice is normally between 12 and 24 Volts, with cost often the key factor.  Larger yachts and commercial vessels often drive big loads with 120 Vac or 240 Vac, or even three-phase ac power.

If you are designing a system, and you find that the allowed current for wire of the needed length gives too much voltage drop at the load, then your system voltage should be higher.  If you find that the size of wire that gives acceptable voltage drop at the load cannot carry the current, then your system voltage should be lower.  Generally, the bigger your loads and the bigger the boat, the higher the voltage.

There can be more than one system on board, with battery power for some applications, and ac power for others.

Calculating

Three variables are important in selecting voltage:  Power required by the load, the size of wire, and the distance between the battery and the load.

Engineering practice usually requires that no more than 2% of the available voltage be lost between battery and load. 

Safety requires that the current be no more than the ABYC rating for the wire. 

The layout of the boat determines how far the wire must run.

Example:  An anchor winch requires 1500 Watts, and its location in the bow is 50 feet as the wire runs from the battery.  Assume that 10% voltage drop is acceptable, since the load is a motor.  At different system voltages, we would have:

Notice that 6 Volts is not an option, no surprise that these systems are not seen today. 

A 12 Volt system requires 1/0 wire, large and hard to run.  100 feet of it costs $600. 

24 Volts works with 4 gauge wire, much easier and costing around $250. 

32 Volts only needs 8 gauge, very easy to run, and costing perhaps $100. 

Savings in cost of wire as the system voltage goes up must be balanced against the increased cost of accessories for the higher voltage.  Note that two group 27 gel batteries together, needed for a 24 Volt system, store the same energy as one group 4D gel battery, for a 12 Volt system, and weigh and cost about the same.  But, 24 Volt starters, alternators,  and even light bulbs can be more expensive.

28 Volt military surplus items work with a 24 Volt system, and are often true bargains.  32 Volt components are rare.

Just for fun, I have included calculations for 120 Volts, which could be ac or dc, more likely ac.  Note that the wire required to ensure a voltage drop within 10% is too small to be rated by the ABYC for power, and cannot carry the 12.5 ampere current.  18 AWG would be used instead.  At lower voltages, the wire must be sized to meet the voltage drop requirement, and is larger than needed to meet ABYC current limits.  The extra size of wire is "wasted".  At 120 Volts, the wire is just large enough to meet both needs.

Actually, a 120 volt circuit would be designed to provide a 2% voltage drop, vice 10%, and thus:

There is little to be gained from using wire smaller than 12 AWG, and the increased efficiency offsets some of the power lost in converting from battery voltage to 120 Volts.  Expect to see more winches and thrusters using 120 Vac systems, in addition to the dc system used for primary power.  As the cost of dc to ac inverters drops, the ac option becomes more attractive.

Breakers

Each circuit must be protected against accidental short circuit with a device that will open the circuit when the current exceeds the ABYC rating of any wire connected to it.  If several wires are connected to one breaker, the breaker must protect the smallest.  Each wire from the breaker (red for dc, black for ac) must be paired with a return wire that returns all current to a return bus near the breaker, which must be returned to the battery with yellow wire or to the ac connection with white wire the same size as the red or black wire bringing power to the breakers.

Never "daisy chain" the return wire between loads powered by different breakers.

Note that the primary wire from the battery to the starter solenoid is exempted, although a fusible link is often provided.  If a load, such as a radio, must be fused at some current lower that the wire capacity, it is normally best to provide a fuse or breaker at the load itself. 

It is simpler to use the "engine room" sizes, since many circuits run some distance through the engine room.   If circuits are laid out carefully, and none of the "non-engine" wire runs through the engine room, then the outside engine room ratings can be used.

The "nominal" value of a breaker, such as "30 Amperes",  is normally a guaranteed no-trip value.  Most such breakers will actually trip at a higher current:  usually 10% to 20% more for magnetic breakers, and as much as 100% more for thermal breakers.  It is the "guaranteed trip" value that must be less than the ABYC maximum rating for the wire.  A magnetic breaker rated for 30 Amperes is adequate for 12 AWG wire, although a thermal breaker rated for 30 Amperes might not be.