The short answer. Enter the amps, how far the device is from the battery, and the kind of circuit. The calculator counts the wire both ways, out and back, and returns the smallest AWG size that stays under the voltage drop you picked: 3% for things that must work, 10% for things that just run a bit dimmer.
Enter the current and the run length.
This picks the size for voltage drop only. A cable also has a current limit set by its insulation rating and where it runs (engine spaces are hotter), printed on the cable maker’s spec sheet. Use whichever size is bigger, and fuse the cable to protect it. Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
How the math works
Current runs out on the positive wire and back on the negative, so a device 12 feet from the battery puts 24 feet of copper in the circuit. Blue Sea calls that the roundtrip distance. Victron’s Wiring Unlimited counts it the same way: the positive cable plus the negative cable.
Every wire size has a resistance per meter. The calculator uses Victron’s AWG chart for that number, multiplies it by the counted length to get the circuit’s resistance, then multiplies by your amps to get the volts lost. It starts at 18 AWG and works up until the loss fits under your limit.
Here is Blue Sea’s own example, run through the tool. A windlass draws 80 amps and sits 25 feet from the battery. That’s 50 feet of wire, about 15.24 m. A windlass counts as non-critical, so the limit on a 12V system is 10%, or 1.2 volts. 6 AWG loses 1.58 V on that run, which is too much. 4 AWG loses 0.99 V, so the answer is 4 AWG, the same size Blue Sea’s chart gives.
3%, 10% or 2.5%?
Blue Sea, whose chart draws on ABYC E-11, sorts circuits like this:
- 3%, critical: panel main feeders, bilge blowers, electronics, navigation lights.
- 10%, non-critical: general lighting, windlasses, bait pumps, general appliances.
For the heavy cables between the battery, chargers and the main busbar, Victron suggests aiming for 2.5% or less. On 12V that’s 0.3 volts. Those cables move the most current, and every volt they lose is a volt your charger never puts in the battery.
What this calculator doesn’t check
It sizes for voltage drop. It doesn’t check how much current a cable can carry before it runs too hot, and that’s a separate limit.
How much a cable can carry depends on its insulation temperature rating, where it runs, and how many cables share the bundle. Engine spaces run hot. Blue Sea’s chart assumes 105°C insulation. Victron warns that cables packed in a closed conduit can’t shed heat, and the fix is a thicker cable. On short, heavy runs, the heat limit can call for a bigger cable than voltage drop does. Look up your cable’s rating on its maker’s spec sheet and use whichever size comes out bigger.
Then fuse it. The fuse protects the cable, so pick it for the cable, as close to the battery or busbar as you can. If the tool says “bigger than 4/0”, shorten the run or run two cables side by side. Victron says two cables are fine as long as their combined size matches the one big cable.
Parts for the job
| Product | Where it goes |
|---|---|
| Pacer duplex wire 14/2 red/black | Light loads and electronics runs |
| Pacer duplex wire 18/2 red/black | Small LED lights and sensors |
| Pacer triplex cable 16/3 | Lights and pumps that need three wires |
| Pacer heat-shrink butt connector 12-10 AWG | Joining 12 and 10 AWG runs |
| Pacer heat-shrink ring terminal 8 AWG, 3/8 in stud, 25-pack | 8 AWG feeds onto 3/8 inch studs |
Wire it up with the Boat lithium battery wiring diagram: 2 simple setups. For why boats use tinned wire, see Marine wire: why tinned, and which size. To size the battery first, use the Battery bank size calculator.
Bundles for this job: Wiring and fusing kit (12-circuit panel)
Quick answers
Do I measure the wire one way or both ways? Both ways. Measure from the battery (or busbar) to the device, and the calculator doubles it for the return wire. If your negative takes a different route, add the two real lengths together and enter half.
What voltage drop is OK on a boat? 3% for critical circuits like electronics, nav lights, bilge blowers and panel feeders. 10% for non-critical ones like general lighting and windlasses. Victron aims for 2.5% on battery and charger cables.
Does 24V need smaller wire than 12V? For the same power, yes. At 24V the same power takes half the current, and the drop limit in volts doubles, so the wire can be smaller. Switch the voltage in the tool to see it.
Can I use two smaller cables instead of one big one? Victron says yes, as long as their combined cross-section matches the single cable. Victron also says to fuse each cable.
Sources
- Victron AWG to Metric Conversion Chart: https://www.victronenergy.com/upload/documents/AWG%20to%20Metric%20Conversion%20Chart.pdf
- Blue Sea Systems, Choosing the Correct Wire Size for a DC Circuit: https://www.bluesea.com/resources/1437
- Victron Wiring Unlimited: https://www.victronenergy.com/media/pg/The_Wiring_Unlimited_book/en/theory.html
