Low-voltage DC punishes thin wire: at 12 V even a short run can waste 10% of your power as heat. Enter amps, one-way length and system voltage (12/24/48 V) and this tool finds the smallest copper AWG that passes both ampacity and the critical ≤ 3% voltage-drop rule — then shows exactly how many volts and watts you lose.
🔒 100% client-side — your data never leaves this browser⚡ DC copper · 12/24/48 V▼ ≤ 3% drop rule
Electrical safety: this is a planning tool, not a substitute for your local electrical code or a licensed electrician. Always fuse wire close to the power source, and never exceed a conductor's ampacity — melted insulation causes fires.
▸ The calculator
Fill in your load and run length. The tool checks every copper AWG from 4/0 down to 20 and returns the thinnest wire that is both safe (ampacity) and efficient (≤ 3% drop) — for automotive, marine, solar, and campervan 12 V wiring alike. Metric users (Australia, EU): every result also shows mm², and length accepts metres or feet.
Don't know amps? Use the watts helper below: amps = watts ÷ volts.
Measure positive terminal → load only. The calculator doubles it for the return wire.
Recommended copper wire
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voltage drop
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volts lost
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watts lost as heat
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ampacity (max safe)
Try:
▸ Voltage Drop Lab
Every AWG, side by side, for your load — drop in volts and percent over a 10 m one-way reference run. Wires failing ampacity or the 3% rule are marked.
AWG
mm²
Drop (V)
Drop (%)
Ampacity
Verdict
Copper ampacity reference (conservative DC ratings)
AWG
mm²
Ampacity (A)
Max watts @ 12 V
Max watts @ 24 V
Ampacity values are conservative bundled-conductor DC ratings (ABYC-style); chassis/single-conductor ratings can be higher. The calculator always requires your current to be at or below these.
▸ Worked examples (hand-verified)
Three real scenarios with the full math shown, each independently verified against the tool's engine.
1 · 100 W solar panel, 12 V system, 6 m one-way
Current = 100 ÷ 12 = 8.33 A. Try 10 AWG (3.28 Ω/km @ 75 °C → 0.00399 Ω/m): Vd = 2 × 8.33 × 0.00399 × 6 = 0.399 V → 3.32% — fails the 3% rule. Step up to 8 AWG (2.06 Ω/km → 0.002505 Ω/m): Vd = 0.251 V → 2.09% ✓, and 8.33 A ≤ 55 A ampacity ✓.
→ Use 8 AWG (8.37 mm²). You lose 0.25 V / 2.09 W as heat.
2 · 2000 W inverter, 12 V system, 2 m one-way
Current = 2000 ÷ 12 = 166.7 A. Ampacity rules out 1/0 AWG (165 A < 166.7 A) even before checking drop. 2/0 AWG (0.256 Ω/km → 0.000311 Ω/m @ 75 °C): Vd = 2 × 166.7 × 0.000311 × 2 = 0.208 V → 1.73% ✓, 166.7 A ≤ 190 A ✓.
→ Use 2/0 AWG (67.4 mm²). You lose 0.21 V / 34.6 W as heat. Keep inverter cables as short as possible.
3 · Same 100 W load, but a 24 V system, 6 m
Current = 100 ÷ 24 = 4.17 A — halved. 14 AWG (8.29 Ω/km → 0.01008 Ω/m @ 75 °C): Vd = 2 × 4.17 × 0.01008 × 6 = 0.504 V → 2.10% ✓ of 24 V, 4.17 A ≤ 25 A ✓. (16 AWG would give 3.34% — fails.)
→ Use 14 AWG (2.08 mm²). Doubling system voltage roughly halves the wire you need — a key reason 24 V/48 V exist.
▸ Methodology — the exact math
Voltage drop formula
Vd = 2 × I × RT × L
where I = current (A), L = one-way length (m), and the factor 2 accounts for the round trip through both the positive and negative conductors. Drop percent = Vd ÷ Vsys × 100; watts lost = Vd × I.
Resistance at operating temperature
RT = R20 × (1 + α × (T − 20)) with copper α = 0.00393/°C and conductor temperature T = 75 °C (typical for PVC-insulated wire under load). That gives a ×1.216 multiplier over the 20 °C figures — most thin calculators skip this and understate drop by ~20%.
Selection rule
The tool tests AWG 4/0 → 20 (smallest first) and returns the thinnest wire satisfying both: current ≤ ampacity and drop ≤ 3%. If no AWG in range passes, it says so honestly instead of guessing.
Honest limitations
DC only. For AC mains or three-phase voltage drop, see our voltage converter / drop tools — different physics (reactance, power factor).
Copper only. Aluminum conductors need roughly 1.6× the cross-section for the same resistance and are not covered.
Round-trip assumption. The ×2 factor assumes the negative return runs the same distance. Chassis-ground returns in vehicles add unpredictable resistance — measure, don't assume.
75 °C conductor. Hot engine bays or bundled conduits run hotter; derate (use one size up) if in doubt.
Connections not modeled. Every crimp, lug and fuse holder adds a little resistance. Keep connections clean, tight, and few.
Ampacity is conservative. Ratings used are bundled-conductor style; check your local code (NEC/ABYC) for the final word.
Voltage converter — deep voltage-drop analysis including AC and three-phase.
Antenna lab — a different kind of wire math: RF antenna design.
▸ Frequently asked questions
What size wire do I need for a 12V battery?
It depends on two things, not the battery itself: how many amps your load draws and how far the wire runs (one-way). A 12V system is very sensitive to voltage drop, so the wire is usually sized by the 3% drop rule rather than by ampacity alone. As a starting point: a 100 W load at 12 V draws 8.33 A, and over a 6 m one-way run it needs 8 AWG copper to stay under 3% drop. Enter your own amps and distance in the calculator above for an exact answer.
What gauge wire should I use for 12V?
There is no single answer — wire gauge is chosen per circuit from current and length. Light circuits (a few amps, short runs) can use 16–14 AWG, medium loads (10–30 A) typically need 12–8 AWG, and heavy loads like inverters need 4 AWG up to 4/0. The calculator above tests every AWG from 4/0 down to 20 and returns the thinnest copper wire that passes both ampacity and the ≤3% voltage-drop rule, for 12 V, 24 V, or 48 V systems.
Can 18 gauge wire handle 12V?
Voltage is not the constraint — current and distance are. 18 AWG copper is rated about 14 A here, so it will not melt at 12 V, but voltage drop usually disqualifies it: at 14 A over just 3 m one-way, 18 AWG drops about 2.14 V (17.8%), far past the 3% limit. 18 AWG is fine only for tiny loads (1–2 A, e.g. signal wire or a short LED tap) over short runs. Run the numbers in the calculator before committing.
What size is 12V wire? Is there a standard 12V wire size?
No — '12V wire' is not a wire size. 12 V describes the system voltage; the wire's physical size (AWG or mm²) is chosen from the current it must carry and the distance it must run. Any copper wire with insulation rated for at least 12 V (virtually all of it) can be used on a 12V system — the question is only whether it is thick enough for your amps and length without excessive voltage drop.
How do I calculate 12V wire size and voltage drop by hand?
Use Vd = 2 × I × R × L, where I is current in amps, L is one-way length in metres, R is the wire's resistance per metre at operating temperature, and the factor 2 covers the round trip through both conductors. Then drop % = Vd ÷ system volts × 100, and require ≤3%. Example: 8.33 A over 6 m on 8 AWG copper (2.06 Ω/km at 20 °C, ×1.216 at 75 °C = 0.002505 Ω/m): Vd = 2 × 8.33 × 0.002505 × 6 = 0.251 V, which is 2.09% of 12 V — a pass.
How many watts can 12 gauge wire handle at 12 volts?
By ampacity, 12 AWG copper is rated 30 A here, which is 360 W at 12 V — but voltage drop usually limits you far below that on longer runs. At 6 m one-way, 12 AWG carrying 30 A drops about 2.28 V (19%), so the 3% rule caps it at roughly 4.7 A (≈57 W) for that distance. Short runs can use the full ampacity; long runs cannot. The calculator shows both limits so you can see which one binds.
What wire size do I need for 12V LED lights?
LED strips are light loads, so drop — not ampacity — decides. A 5 A LED run over 3 m one-way: 16 AWG drops 0.48 V (4.0%, fail), while 14 AWG drops 0.30 V (2.5%, pass) with 25 A ampacity to spare. So 14 AWG is the honest answer for that case. For very long LED runs, consider 24 V strips (same watts at half the current) or feed the strip from the middle to halve the effective length.
What size wire do I need for a 12V solar panel?
Size from the panel's Imp (max-power current), not its wattage alone: a '100 W' panel typically delivers about 5.5 A at ~18 Vmp, which is ~8.3 A at 12 V nominal. At 8.33 A over 6 m one-way, 10 AWG drops 3.32% (fail) and 8 AWG drops 2.09% (pass) — so 8 AWG copper. Keep panel-to-controller runs as short as practical, and remember the controller-to-battery run needs sizing too, often at higher current.
What size wire for a 2000W (or 3000W) inverter on 12V?
Inverters are the heaviest common 12V load: 2000 W ÷ 12 V = 166.7 A. At 2 m one-way, 1/0 AWG is already disqualified on ampacity (165 A < 166.7 A); 2/0 AWG passes both tests with 1.73% drop. A 3000 W inverter draws 250 A and needs 4/0 AWG at 2 m (1.63% drop). Two rules: keep inverter cables as short as physically possible, and size for surge current (often 2× rated) if your inverter surges on motor startup.
What wire size for a 12V trolling motor?
Trolling motors typically draw 30–50 A; check your motor's manual for its max draw first. As an illustration, 40 A over 5 m one-way: 6 AWG drops 5.23% (fail), 4 AWG drops 3.30% (just fails), and 2 AWG drops 2.08% (pass) with 130 A ampacity — so 2 AWG. Long bow-to-battery runs on boats are exactly where undersized wire causes the classic 'motor lacks thrust' complaint.
What size fuse or breaker do I need for 12V wire?
The fuse protects the wire, not the device: size the fuse at or below the wire's ampacity, and at or above the load current. Example: a 50 A trolling-motor circuit on 6 AWG (75 A ampacity) gets a 50 A breaker. Place the fuse as close to the power source (battery) as possible, and fuse the positive side. Undersized wire with an oversized fuse is a fire hazard — the fuse must always blow before the wire overheats.
How do I measure wire gauge size?
The reliable way: strip a short section, measure the bare copper diameter with calipers, and compare against an AWG diameter table (e.g. 10 AWG ≈ 2.59 mm, 12 AWG ≈ 2.05 mm, 14 AWG ≈ 1.63 mm). Most wire also has its gauge printed on the insulation — look for 'AWG' followed by a number. For converting between AWG, SWG, and mm², use our dedicated AWG to mm² converter rather than guessing.
What size wire connects two 12V batteries together?
Battery interconnects and parallel links carry the full system current, so size them like any other run: total load amps plus the one-way distance between battery terminals. Keep interconnects as short as possible and make parallel links equal length so batteries share current evenly. For inverter battery banks (100+ A), this usually means 2/0 or 4/0 AWG even for runs under a metre.
What wire size for a 12V fridge, water pump, or campervan circuit?
These are small, steady loads: a 12V compressor fridge draws ~4–6 A, water and bilge pumps ~3–8 A. At those currents, 14 AWG handles most runs up to ~5 m within the 3% rule. In a campervan, the best practice is a fused distribution block near the battery with a home-run pair to each device — never daisy-chain high-draw devices on one thin run, and fuse every branch at the block.
Do electrical codes (NEC/ABYC) agree with this calculator?
This calculator uses conservative bundled-conductor DC ampacity ratings in the ABYC (marine) tradition, which are stricter than some chassis-wiring tables. The NEC's ampacity tables assume specific insulation temperature ratings (60/75/90 °C) and installation conditions, so exact code values can differ. Treat this tool as a planning aid: for inspected or marine work, confirm against the code edition and insulation rating that applies to you, and when in doubt go one size larger.
Does this calculator send my data anywhere?
No. It runs entirely in your browser — every calculation happens in JavaScript on this page, with zero network requests. There is no account, no analytics payload carrying your inputs, and nothing to intercept. You can verify this yourself: disconnect from the internet after the page loads and the calculator keeps working.
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