Tacklebox ’n Tools

Wire Size & Voltage Drop Calculator

Find the right wire gauge for any 12, 24, 36 or 48 volt DC circuit on a boat, RV, truck or cabin, plus the voltage drop, fuse size and what to buy.

How to use it

  1. Pick what you're powering, or choose "Other" and enter its amps or watts from the label or manual.
  2. Choose how much voltage drop is allowed. Use 3% for electronics, navigation lights, trolling motors and inverters, and 10% for general lighting and pumps.
  3. Measure the wire run one way, from the battery to the device, along the path the wire will take.
  4. Read the results. The wire size, its voltage drop, the fuse to use, a table of every size and what to buy.

Your setup

An example is loaded to start. Change any field and the results update. Simple shows the essentials. Detailed adds insulation rating, engine space and bundling, a table of every wire size and the math.

View

Your circuit

System voltage
Enter the load in
A
Allowed voltage drop
3% for electronics, navigation lights, bilge blowers, trolling motors and inverters. 10% for general lighting, bait pumps and other gear that runs fine on a little less voltage.

Your wire run

feet, one way
Measure along the path the wire takes. The calculator doubles it, because current flows out on the positive wire and back on the negative.

Where the wire runs

Printed on the wire's jacket. Hotter-rated insulation can carry more current.
Location
Wires packed together run hotter, so each one can carry less.

How the numbers are worked out

Voltage drop

The wire must be at least circular mils = 10.75 × amps × round-trip feet ÷ allowed volts dropped, the formula ABYC and wire makers like Ancor use for copper. Allowed volts dropped is your system voltage times 3% or 10%. The calculator picks the smallest standard size with enough circular mils.

Blue Sea Systems lists panel main feeders, bilge blowers, electronics and navigation lights as critical 3% circuits, and general lighting, windlasses, bait pumps and general appliances as 10% circuits.

Can it carry the current?

Each size's allowable amps come from Table 5 of the U.S. Coast Guard rule 33 CFR 183.425, for your insulation rating. In an engine space they're multiplied by 0.85 (105°C), 0.82 (90°C) or 0.75 (75°C). Bundled wires are multiplied by 0.70 for 3 wires, 0.60 for 4 to 6, 0.50 for 7 to 24 and 0.40 for 25 or more.

The wire has to carry the fuse rating, not just the load, so a fault blows the fuse before the wire overheats.

Fuse size

The suggested fuse is the next standard size at or above 125% of the load, which leaves headroom so it doesn't blow in normal use. If your equipment's manual names a fuse or breaker, use that size and make sure the wire can carry it.

ABYC asks for the fuse within 7 inches of the battery, or up to 72 inches if the wire runs its whole length inside a sheath or enclosure.

What it leaves out

  • Aluminum wire, which needs larger sizes. These figures are for copper.
  • Starting motors and windlasses, whose short, heavy current follows the maker's own wiring chart.
  • AC house wiring, which follows the National Electrical Code instead.
  • Each crimp and switch adds a little resistance. Stay well inside the limit on long runs.

Planning estimates. Follow your equipment's manual for wire and fuse size, use marine-grade tinned copper wire on boats, and protect every positive wire with a fuse or breaker at the battery.

Sources