In earlier chapters we have already figured out how much battery capacity we need, now we need to look at the optimal voltage for our needs: 12, 24, or even 48.
We should do this analysis before we consider adding any of the high-current (amperage)-drawing devices that are becoming common today:
- Electric in-mast or in-boom roller furling
- Bow and stern thrusters
- Electric winches
- Electric cooking
- And on it goes
You will notice that I did not list electric windlasses. The reason is they will almost always be used when the engine is on and the alternator charging, so even a big one is practical on 12-volt boats. That said, 24-volt boat voltage makes windlass installation easier—more in Part 2.
And I left air conditioning off because anyone who thinks they can appreciably cool a boat and keep it that way for long powered by the batteries should move to Canada for two reasons:
- It's cooler
- That stuff you are smoking is legal
Now that we know why we need to look at the base DC voltage of our boats, let's dig into a little theory so we understand why higher boat system voltages are required for high loads, and further, to give us a basis to calculate the crossover between 12 and 24-volt systems.
Yeah, I know, theory is boring. Stop your whining. You know from past bitter experience that I'm going to subject you to this before I get to the fun stuff, so grab a Red Bull so you don't nod off, and let's do it.
The power consumed by a device to do work—spin a motor, heat the tea, light a light, whatever—is measured in watts.
For example, a single induction ring turned full up consumes 1500 watts or 1.5 kW regardless of voltage.
At 120 volts (AC mains current in North America) there will be 12.5 amps passing through the wires feeding the ring. How do I know that?
watts=volts x amps
therefore: amps=watts / volts
Or in this case: 12.5 = 1500 / 120
In the case of our induction ring at AC mains voltages, 12.5 amps can be passed along quite a small wire—as is typical for plug-in portable household appliances—that is inexpensive, flexible, and generally not a problem.
But on most boats we are saddled with 12 volts. So now we plug our induction ring into an inverter and the math changes:
1500 watts / 12 volts = 125 amps!
Actually, it's worse than that because inverters have inefficiencies of at least 5%, and many are more inefficient than that. So let's say 10%, so our 125 amps becomes about 140 amps.
Required Wire Size
We can calculate the wire size required to pass that many amps thusly:
- Decide how much voltage drop over the length of the cable between the batteries and inverter—both conductors (negative and positive) added together—we will accept.
- A good standard for applications like this is 3%.
- Use ohms law to calculate the resistance that will yield that voltage drop from one end to the other of the wire.
- Divide that by the number of meters of wire.
- Look up what cable size has that resistance per meter—known as resistivity—or slightly less, and that's the cable we need.
Was that snoring I heard? Suck up that Red Bull.
The Easy Way
OK, because I'm a really nice guy I will let you in on a secret: