Electrical system
Single or dual voltage?
DC system
Given the expense of high gauge wire and its weight, combined 12v/24v system should probably be deployed providing that the cost of additional high-end batteries and 24v special order equipment do not overweigh the wire cost. Wire cost can also be minimized by utilizing multiple distribution panels.
To achieve 24v, two 12v batteries are wired sequencially. The whole boat can then be wired for 24v and 24-to-12v converters can be tapped in where needed for 12v devices. This works great for light loads where converter also acts as a "filter" supplying cleaner power. Its output voltage is constant and does not depend on input voltage as oppose to equalizer (see below).
Since converters are in series with the load, heavy loads require expensive converters. Alterntively, an equalizer can be used in expense of extra wiring. In the later case the load is applied directly to the half of the 24v battery bank bypassing the equalizer. To avoid this problem, heavy loads should be 24v.
Diesel or genset 12v starter is normally connected to a separate 12v battery. 12v alternator is used to charge the 12v battery and 24v albertnator is used to charge 24v bank.
In case of emergency, when cranking battery is dead, the engine that has 12v starter can be started from 24v bank with an equalizer arrangement or 12v battery can be wired via emergency cranking circuit to half of the 24v bank providing this circuit is rarely used (only in emergency).
It seems that optimal solution may be to have dual voltage electical system with couple of converters installed near distribution subpanels and one equalizer used for cranking the engine in emergency cases when the cranking 12v battery is dead.
AC system
To sail around the world one need universal dual voltage system: 120/240 V 50/60 Hz. The easy way to achieve it is by installing the isolation transformer that will accept both voltages and frequencies and allow four way conversions: straight pass-through for both voltages and step-down/step-up transformer. The good examples of such devices will be Vicron Isolation Transformers.
To deal with frequency and voltage variations, one should choose only appliances that operate in both worlds. Alternatively, invertors may be installed that provide both frequencies or two different invertors: one supplies 120 V 60Hz, another - 240V 50Hz.
Battery
The question is what type of battery to choose: wet-cell, gel-cell or AGM?
I'm leaning towards AGM because of the following features:
- Maintenance free. No cost and worries.
- Sealed units. No explosive gases, corrosive fumes or acid spills.
- Lowest internal resistance means least time to re-charge, saving on genset hours and fuel.
- Lower self-discharge rate minimizes sulfation. This is unlikely to apply for live-aboard.
- Long manufacture warranty - up to 10 years.
The recommended depth of discharge is about 40% but the battery life will be exponensially longer with less depth. If genset and enough fuel are available, then the battery can be re-charged daily or even more often if needed to extend its life.
If DC-to-AC inverter is used, then it shouldn't draw more than 25% of the rated capacity of the battery bank to prevent the batteries from overheating.
Battery bank capacity should in most cases be 2 to 4 times higher than electricity consumption between re-charges.
The rule of thumb is to parallel small number of high capacity batteries, rather than large number of small capacity batteries.
Also, it's much better to have a single large capacity bank than two small capacity banks because of increased battery life time expectancy due to shallower discharge. Plus the greater the capacity of a bank (read voltage) in relation to the load (read drawing current), the smaller the initial voltage drop will be, extending the life of voltage sensitive electrical applicances. Less capacity (lower voltage) will cause higher current to be drawn by an applicance, which will in turn cause higher voltage drop. Said the above, a separate emergency cranking battery should be available and only connected to the starter.
Batteries should be kept at no more than 20 gedrees Celcius. Every 6 degree raise will shorten their life time by half.
The battery charging device should have temperature sensor that will taper the charge rate in addition to voltage regulators. Ignition protected fan may also need to be installed to cool the batteries off.
Wires
Use only tinned copper wires, ABYC Type 2 (19 strands) or 3 (many strands depending on gauge) UL 1426 high quality self-extinguishing insulation boat cable (BC) 5W2 oil resistant, where 5 means heat rating in dry environment 105° C and 2 - for wet environment 75° C (W - wet, moister resistant).
ABYC/ISO colour coding for DC: red - positive, yellow - negative, green - grounding, bonding. For AC: black, red - hot, white - neutral, green - grounding, bonding.
Wire gauge is practically determined based on 3% voltage drop. In most cases the size of the cable calculated using the voltage drop approach will be larger than the one calculated using ampacity tables except maybe in very short cable cases.
When bundling 3 wires, derate the ampacity by 0.7; 4 to 6 wires - by 0.6; 7 to 24 - by 0.5; 25 and more - by 0.4.
| AWG | 75° C outside engine space | 75° C inside engine space at 50° C |
|---|---|---|
| 18 | 10 | 7.5 |
| 16 | 15 | 11.3 |
| 14 | 20 | 15 |
| 12 | 25 | 18.8 |
| 10 | 40 | 30 |
| 8 | 65 | 48.8 |
| 6 | 95 | 71.3 |
| 4 | 125 | 93.8 |
| 2 | 170 | 127 |
| 1 | 195 | 146 |
| 1/0 | 230 | 172 |
| 2/0 | 265 | 198 |
| 3/0 | 310 | 232 |
| 4/0 | 360 | 270 |
| Amps | 10' | 15' | 20' | 25' | 30' | 40' | 50' |
|---|---|---|---|---|---|---|---|
| 5 | 18 | 16 | 14 | 12 | 12 | 10 | 10 |
| 10 | 14 | 12 | 10 | 10 | 10 | 8 | 6 |
| 15 | 12 | 10 | 10 | 8 | 8 | 6 | 6 |
| 20 | 10 | 10 | 8 | 6 | 6 | 6 | 4 |
| 25 | 10 | 8 | 6 | 6 | 6 | 4 | 4 |
| 30 | 10 | 8 | 6 | 4 | 4 | 4 | 2 |
| 40 | 8 | 6 | 6 | 4 | 4 | 2 | 2 |
| 50 | 6 | 6 | 4 | 2 | 2 | 2 | 1 |
| 60 | 6 | 4 | 4 | 2 | 2 | 1 | 1/0 |
| 70 | 6 | 4 | 2 | 2 | 1 | 1/0 | 2/0 |
| 80 | 6 | 4 | 2 | 2 | 1 | 1/0 | 3/0 |
| 90 | 4 | 2 | 2 | 1 | 1/0 | 2/0 | 3/0 |
| 100 | 4 | 2 | 2 | 1 | 1/0 | 2/0 | 3/0 |
| Amps | 10' | 15' | 20' | 25' | 30' | 40' | 50' |
|---|---|---|---|---|---|---|---|
| 5 | 18 | 18 | 18 | 16 | 16 | 14 | 12 |
| 10 | 18 | 16 | 14 | 12 | 12 | 10 | 10 |
| 15 | 16 | 14 | 12 | 12 | 10 | 10 | 8 |
| 20 | 14 | 12 | 10 | 10 | 10 | 8 | 6 |
| 25 | 12 | 12 | 10 | 10 | 8 | 6 | 6 |
| 30 | 12 | 10 | 10 | 8 | 8 | 6 | 6 |
| 40 | 10 | 10 | 8 | 6 | 6 | 6 | 4 |
| 50 | 10 | 8 | 6 | 6 | 6 | 4 | 4 |
| 60 | 10 | 8 | 6 | 6 | 4 | 4 | 2 |
| 70 | 8 | 6 | 6 | 4 | 4 | 2 | 2 |
| 80 | 8 | 6 | 6 | 4 | 4 | 2 | 2 |
| 90 | 8 | 6 | 4 | 4 | 2 | 2 | 1 |
| 100 | 6 | 6 | 4 | 4 | 2 | 2 | 1 |
Connectors and terminals
All connectors and crimp-on terminals should be tinned annealed copper. Use dual-walled or heavy-walled adhesive heat shrinks or electrical putty with heat-shrink tape for moister protection.
Estimation of electric energy consumption
I will fill the table below in as I go.
| Device | Amps | Hours | Ah |
|---|---|---|---|
| Running LED lights (0.1A each or 0.13A tri-colour) | 0.3 | 10 | 3 |
| Fridge 2.4 cu. ft | 2.2 | 12 | 27 |
| Water maker 12l/h | 8 | 1 | 8 |
| Radar 2.0kW 24nm | 3 | 10 | 30 |
| Total |