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Higher current has disadvantages. The higher the current draw, the thicker the wiring required (and the bigger the spark when you accidentally ground it). Thick wiring is more expensive and hard to install and maintain – think jumper cables for boosting your car.
Higher voltages are also feasible. Large ships have long used a higher voltage DC bus. Some saller smaller yachts have used 150 VDC. Some of the advantages this yields are smaller wiring, more efficient thrusters and windlasses, and compatibility with most shore power around the world and existing resistive devices like stoves.
Wires also have resistance and, when a current passes through them, this causes a voltage drop along the length of the wire. The higher the current, the higher the drop in voltage. This voltage drop limits the practical length of a wire. The maximum run for 12-VDC wiring is around 30-35 ft, which translates into a boat length of around 50 ft, given a midships battery, and the need to run wires around corners.<ref>www.computrols.com/file_download/.../Calculating-Wire-Resistance.pdf</ref> <ref>http://www.advanced-energy.com/upload/File/White_Papers/ENG-24VDCInstallation-260-01.pdf</ref>
The trend in the market is to 24 VDC, so that should be your first choice, anyway. However, if your boat is less than 50 ft, 12 VDC is probably still your best price-performance option.
== Bonding ==
Second only to discussions about one house bank or two, are discussions about bonding or not bonding the electrical circuits. The simple fact is that all electrical circuits have to have a common ‘ground’. On shore, this is often the earth. Bonding means connecting all the ground points together with an extra run of wires.
In all cases in a steel hull, the DC system must be a "floating ground" (DC negative bus) type of system, with an insulated return, fully isolated from the hull and all the hull fittings. This means that no electrical items (including common appliances) have a local ground to the hull and lamps should be double pin. Instead, all ground returns are tied to a Common Grounding Point (CCG).
For example, all engine fittings are double insulated. The engine is electrically isolated from the hull via flexible mounts and flexible coupling. A grounding wire runs from the alternators to the DC negative bus. This might seem confusing, because the CCG itself is grounded to the hull. However, a CCG avoids stray electrical currents running through the hull and causing electrolysis. It also provides a grounding point for the lightening-protection system.
Having selected the voltage, the next phase in the design of the electrical system is to determine the requirements for the DC battery primary system – the [[HouseBank|house bank]].
== Charging Systems ==
=== Alternator ===
Each engine (if there is more than one) will have a high-capacity dual-output alternator and multistage regulator, with separate charging circuits for the starter and house batteries. A backup manual switch and regulator are provided. The regulator must be suited to the type of battery: Flooded cells require an equalization charge after the main charge; whereas gel and AGM cells usually do not. Typical vendors are: [http://www.amplepower.com/ Ample Power], [http://www.balmar.net/ Balmar], [http://www.hehrpowersystems.com/ Hehr Power Systems], [http://www.jackrabbitmarine.com/ JackRabbit Marine] and [http://www.salt-systems.com/ Sea Air Land Technologies, Inc.]
If the boat will be unattended for periods at least one engine must autostart on a schedule to keep the batteries charged.
=== Trickle Charge System ===
In case the main charging system fails while the boat is unattended, a DC trickle-charge system can be provided. Trickle charging is also a good idea because there are usually parasitic loads on a battery system that will slowly discharge it. Deep discharge batteries do not want to be trickle charged at a high rate: 3% is recommended. Thus a boat with a house bank of 1000 AH requires a trickle charge of 30 AH.
Wind turbines and solar panels are ideal for a trickle-charge system; although they are not suited as a main power source. Unfortunately, as a main power source, each of them has a significant performance drawback in the context of a small- to medium-size boat. They simply need too much real estate.
=== Shore Power Charger ===
See [[ACSecondarySystems#Inverter/Charger]].
== Distribution ==
=== Controller ===
The controller for engine charging and inverter/charger is a single point of failure, so a back-up controller or a bypass system should be provided.
=== Distribution Panel ===
For easy access the AC/DC distribution panel is located in the pilothouse. Remote latching relays disconnect the batteries in the event of an electrical fire.
[[Category:ElectricalPrimarySupply]]
== References ==
[[Category:BatteriesPrimary]]