Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1

Ever wondered why the wiring on our boats is so much heavier than in our houses, even though we draw far less power?

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

The Theory

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 Theory

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

But here's the key thing: The size of wire required to power a device safely is determined only by the current (amperage) that it's required to carry.

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:

  1. 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.
  2. A good standard for applications like this is 3%.
  3. Use ohms law to calculate the resistance that will yield that voltage drop from one end to the other of the wire.
  4. Divide that by the number of meters of wire.
  5. 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:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. The Danger of Voltage Drops From High Current (Amp) Loads
  10. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  12. Battery Bank Separation and Cross-Charging Best Practices
  13. Choosing & Installing Battery Switches
  14. Cross-Bank Battery Charging—Splitters and Relays
  15. Cross-Bank Battery Charging—DC/DC Chargers
  16. 10 Tips To Install An Alternator
  17. Stupid Alternator Regulators Get Smarter…Finally
  18. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  19. Smart Chargers Are Not That Smart
  20. Do You Need A Generator?
  21. Efficient Generator-Based Electrical Systems For Yachts
  22. Battery Bank Size and Generator Run Time, A Case Study
  23. Battery Options, Part 1—Lithium
  24. Battery Options, Part 2—Lead Acid
  25. Why Lithium Battery Load Dumps Matter
  26. 8 Tips To Prevent Lithium Battery Load Dumps
  27. Building a Seamanlike Lithium Battery System
  28. Lithium Ion Batteries Explained
  29. 11 Steps To Better Lead Acid Battery Life
  30. How Hard Can We Charge Our Lead-Acid Batteries?
  31. How Lead Acid Batteries Get Wrecked and What To Do About It
  32. Equalizing Batteries, The Reality
  33. Renewable Power
  34. Wind Generators
  35. Solar Power
  36. Hydro Power
  37. Watt & Sea Hydro Generator Review
  38. Battery Monitors, Part 1—Which Type Is Right For You?
  39. Battery Monitors, Part 2—Recommended Unit
  40. Battery Monitors, Part 3—Calibration and Use
  41. Battery Containment—Part 1
  42. Q&A—Are Battery Desulphators a Good Idea?
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