DC wire size and voltage drop calculator

Enter the system voltage, the current, and the distance from the battery to the load. You get the smallest copper wire that keeps the voltage drop in limit and carries the current, with the wire drawn and shaded by the voltage it loses.

Inputs

Amps the wire carries. If the circuit has a fuse that is bigger than the load, use the fuse rating. Watts ÷ volts gives amps.

Measure along the route the wire takes. The calculator doubles it for the return wire.

ABYC: 3% for important circuits, 10% for the rest. Many installers use 3% everywhere.

Continuous loads are sized at 125% of the current (NEC).

Pick the table that matches the wire you will buy. Building wire (THHN) is rated lower than tinned marine or automotive cable.

Minimum wire size

4AWG

Copper, 15 ft each way at 30 A and 12 V, under 3% drop.

4 AWG drops 0.28 V (2.3%), inside your 3% limit. The voltage drop sets the size.

Voltage drop
0.28 V (2.3%)
Voltage at the load
11.72 V
Power lost in the wire
8 W
Wire cross-section
21.1 mm²
Wire rating
160 A
Fuse size
38 A or just above
Voltage along the wire

A 30 A load 15 ft from a 12 V battery needs 4 AWG copper wire to keep the voltage drop under 3%. The drop is 0.28 V, so the load sees 11.72 V. The voltage drop decides the size. The wire is rated 160 A; fuse the circuit just above your load and never above that rating. The picture colours the wire from green at the battery toward amber and red as the voltage falls.

Safety first

Undersized wire and missing fuses start fires. A wire that carries more than its rating gets hot enough to melt its insulation, and a battery can push hundreds of amps into a short circuit.

  • Fuse every circuit at the battery end of the positive wire. The fuse rating must be above the load and no more than the wire’s rating.
  • Follow the manufacturer’s instructions for the battery, inverter, charge controller and every appliance, including their cable and fuse recommendations.
  • Follow your local electrical code and any marine, RV or vehicle standard that applies. This calculator is not a substitute for them.
  • Have a qualified person check the installation before you power it up. Bundled wires, hot spaces and long cable runs all need a larger wire than the tables here show.

How it works

voltage drop (V)  = current × 2 × one-way length × (ohms per 1,000 ft ÷ 1,000)
drop (%)          = voltage drop ÷ system voltage
current needed    = load × 1.25   (continuous loads, 3 hours or more)

The calculator goes down a list of copper wire sizes from 18 to 4/0 AWG and picks the first that passes both tests.

  • Resistance is from NEC Chapter 9 Table 8, uncoated stranded copper at 75 °C. That is hot wire, so the answers lean slightly cautious. The ABYC formula, which uses a constant of 10.75, gives about 17% less resistance at 6 AWG, and the ABYC published table sits between the two.
  • Ampacity comes from one of two tables. The marine and automotive choice uses the US Coast Guard table for 105 °C insulation outside the engine space. The building-wire choice uses NEC Table 310.16 at 75 °C, held to the small-conductor limits of NEC 240.4(D): 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG.
  • Bundling and heat are not included. More than three current-carrying wires in a loom, or a hot engine space, call for derating or a bigger wire.

Worked example

A 30 A load, 15 ft from a 12 V battery, running all day, at 3% drop:

  • The allowed drop is 12 × 0.03 = 0.36 V.
  • 6 AWG: 30 × (2 × 15 × 0.491 ÷ 1,000) = 0.44 V, which is too much. 4 AWG: 30 × (2 × 15 × 0.308 ÷ 1,000) = 0.28 V (2.3%), which passes.
  • Ampacity: 30 × 1.25 = 37.5 A, which 12 AWG carries on the Coast Guard 105 °C table, so the drop decides. The answer is 4 AWG.
  • The same load at 48 V has 1.44 V to spare, and 10 AWG is enough.

Tips from the road

  • Move the big loads to the battery. An inverter next to the battery with a short fat cable, then thin AC wire to the outlets, beats long runs of DC.
  • Include the return wire. If the negative goes to the chassis, measure the route it really takes, not the straight line.
  • Watch the connections. A loose lug or a bad crimp costs more voltage than an extra foot of wire and gets hot; crimp with the right tool and check them after a few weeks.

FAQ

What size wire do I need for 12 volts?

It depends on the current and the distance. A 30 A load 15 ft from the battery needs 4 AWG copper to keep the voltage drop under 3%, because the 12 V circuit only has 0.36 V to spare. The same load on 48 V can use 10 AWG. Enter your own numbers above.

How much voltage drop is acceptable?

ABYC, the US marine standard, allows 3% for critical circuits such as lights, electronics, pumps and inverters, and 10% for less important ones. Many installers use 3% for everything on a 12 V system. The calculator offers both.

What is the voltage drop formula?

Drop = current × resistance of the whole wire. The wire goes out and comes back, so the length used is twice the one-way distance. Resistance is the wire's ohms per 1,000 ft × length ÷ 1,000. NEC Table 8 gives 0.491 ohm per 1,000 ft for 6 AWG, so 15 ft each way at 30 A drops 30 × 30 × 0.491 ÷ 1,000 = 0.44 V. ABYC uses circular mils = 10.75 × amps × round-trip feet ÷ volts dropped, which gives slightly smaller wire.

Why is the wire bigger than I expected?

Two tests decide the size, and the larger wire wins. They are the voltage drop, and the wire's current rating (ampacity). The calculator also takes 125% of the current for loads that run 3 hours or more, as the NEC does. Long runs are controlled by drop, short heavy ones by the rating.

Does wire size change the fuse I use?

Yes. The fuse protects the wire, so it must not be larger than the wire's rating, and it should be just above your load. The calculator shows the size to start from and the wire's limit. Put the fuse at the battery end of the wire.

Sources

  1. National Electrical Code, Chapter 9, Table 8 (conductor properties), NFPA 70 (2017 edition, as reproduced for a course). DC resistance at 75 °C of uncoated stranded copper in ohms per 1,000 ft, with circular mils, from 18 AWG to 4/0. Table 8 in NEC 2020 has the same values.
  2. NEC Table 310.16 and Chapter 9 Table 8, Wire Size Guide. Copper ampacity at 60, 75 and 90 °C and resistance per 1,000 ft (NEC 2020); the 125% rule for continuous loads; voltage drop = 2 × length × resistance × current.
  3. Allowable amperage of conductors for systems under 50 volts (33 CFR 183.40), Everything Boat Building (US Coast Guard table). Ampacity by insulation temperature (60–200 °C), outside and inside the engine space.
  4. ABYC E-11 marine DC wiring guide, ToolGrit. Circular mils = 10.75 × amps × round-trip feet ÷ volts dropped; 3% drop for critical circuits, 10% for others; ampacity and voltage drop are separate tests and the larger wire wins.
  5. How to Calculate Wire Size for Voltage Drop (12 V, 10% table), Everything Boat Building. The ABYC 12 V table at 10% drop, by amps and total circuit length.
  6. Overcurrent protection, NEC 240.4(D) small conductors, NFPA 70 (2017 edition, as reproduced for a course). Overcurrent protection of 14 AWG copper limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A.

Formula and sources last checked September 30, 2026. How we test formulas.

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Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.