DC Wire Gauge by Run Length and Amperage: A Reference Table

Most wire gauge charts on the internet do not tell you what assumptions they were built on, which makes them impossible to check and unsafe to rely on. Here are the tables, and here is exactly how they were calculated, so you can verify any cell in them.

Two separate constraints determine wire size in a low-voltage DC system, and they bind at different times:

Ampacity — how much current the conductor can carry without its insulation degrading. This is a safety limit. Exceeding it is how fires start.

Voltage drop — how much of your system voltage is lost as heat in the wire over the run. This is a performance limit. Exceeding it means brownouts, chargers that never reach absorption voltage, and inverters that cut out.

On short runs, ampacity usually decides. On long runs, voltage drop decides, and it decides by a wide margin. A 20-foot 12V run at 30A needs 10 AWG on ampacity grounds and 2 AWG on voltage drop grounds. The wire you buy is the larger of the two, always.

How to use these tables

Find your current down the left, your one-way run length across the top. The tables already account for the return conductor — you do not double the length yourself.

If your exact figures fall between rows, use the next row up in current and the next column right in length. Do not interpolate. Wire is sold in fixed sizes and rounding down is the expensive mistake.

12V systems at 3% voltage drop

Use 3% for anything that cares about voltage: charging circuits from solar or alternator, inverter feeds, DC-DC charger inputs and outputs, and any main battery cable.

A dash means no standard single conductor up to 4/0 satisfies 3% at that combination. That is not a printing error — it is the table telling you the layout is wrong. Move the component closer to the battery, run parallel conductors, or raise the system voltage.

12V systems at 10% voltage drop

Use 10% only for non-critical loads where a small voltage reduction is invisible: interior LED lighting, fans, USB outlets, and similar. Never use it for charging circuits.

Notice how many cells in this table are identical across several columns. That is ampacity taking over from voltage drop as the binding constraint. At 100A over 3, 5 and 10 feet the answer is 6 AWG in every case — not because of drop, but because that is the smallest conductor rated to carry 100A.

24V systems at 3% voltage drop

Doubling system voltage halves the current for the same power, and voltage drop scales with the square of that reduction in percentage terms. This is why 24V and 48V systems use dramatically less copper.

48V systems at 3% voltage drop

Ampacity limits used

These are the current ratings the tables enforce. They correspond to marine-practice figures for 105°C insulation, single conductors outside an engine space.

Three derating conditions apply and are not built into the tables:

• Bundled conductors. Three or more current-carrying conductors in a sheath or loom run hotter. Derate by roughly 30 percent for 4–6 conductors, more above that.
• Engine spaces and high ambient temperature. Conductors near an engine or in an unventilated compartment in summer lose a substantial fraction of their rating.
• Lower-temperature insulation. If your wire is 60°C or 75°C rated rather than 105°C, its ampacity is lower. Check the jacket printing.

The assumptions behind these numbers

Every cell above was computed from:

Voltage drop = (2 × L × I × K) / CM

where L is the one-way run length in feet, I is current in amps, CM is the conductor’s circular mil area, and K is 10.75 ohm-cmil/ft — the resistivity of copper at 20°C (68°F). The factor of 2 accounts for the return path.

Worked example: 20A over a 10-foot one-way run in 10 AWG (10,380 cmil).

(2 × 10 × 20 × 10.75) / 10,380 = 0.414 V
0.414 / 12 = 3.45% — over the 3% limit

So the table gives 8 AWG for that cell, which yields 0.26V, or 2.17%.

Two caveats you should apply yourself. First, K rises with temperature — a conductor running hot carries perhaps 5–10 percent more resistance than these figures suggest. Second, the tables count copper only. Terminals, crimps, fuses, switches and busbars all add resistance, and a poor crimp can add more than the entire cable run. If a circuit is marginal, size up.

For arbitrary values rather than table rows, the wire gauge and voltage drop calculator runs the same formula and shows the working.

Measuring the run length correctly

Almost every undersized cable in a vehicle was sized from a straight-line estimate.

Measure the route the cable will actually take: down the wall, along the frame rail, around the tank, up through the floor. Then add 10–15 percent for service loops, bends and the slack you need at each termination. A battery-to-inverter run that looks like six feet across the compartment is regularly eleven feet once it is routed properly, and eleven feet is two gauge sizes different from six at inverter currents.

Measure the positive conductor’s path, one way. The tables handle the return. If your negative return is significantly longer than your positive — common when a chassis ground is used at one end — measure the longer of the two and use that.

One more thing to check before buying: real AWG. A significant volume of cheap cable is sold at a nominal gauge with a conductor cross-section well below specification, padded out with thicker insulation. If the jacket does not carry a printed AWG marking and a standard reference, weigh a known length against the published copper mass or buy from a supplier who publishes strand count and conductor diameter.

Typical circuits and where they land

For orientation rather than substitute for calculation. Every one of these should still be checked against the tables using your own run length.

That last row is worth dwelling on. The factory charge wire in a 7-pin connector is typically 10 or 12 AWG over 25–30 feet with a long chassis return, several connectors and often a diode in the path. The tables put a 30-foot 12 AWG run at roughly 10 amps before drop becomes unacceptable, and in practice the delivered current at the battery is frequently under 5A. It will maintain a lead-acid bank against parasitic loads. It will not meaningfully charge a lithium house bank while driving. That is why DC-DC charging on a towable needs its own dedicated conductor run rather than reuse of the existing pin.

Parallel conductors, and when to stop adding copper

Above 4/0, single conductors become genuinely difficult to work with: minimum bend radius grows, lugs get expensive, and routing through a vehicle becomes a fabrication exercise. Two 2/0 conductors in parallel carry roughly the same current as one 4/0 and are far easier to route.

If you run parallel conductors, three rules apply without exception. They must be the same gauge, the same length, and the same routing and termination type. Unequal paths do not share current equally — the lower-resistance conductor takes a disproportionate share and can exceed its own ampacity while the other loafs. Each parallel conductor also needs to be protected as part of a properly fused assembly.

Before adding copper, though, ask whether the layout is the problem. Moving an inverter three feet closer to the battery is free and eliminates more resistance than a gauge increase. Where a run cannot be shortened and 4/0 is not enough, the correct answer is usually a higher system voltage rather than more parallel cable — which is the whole argument for 24V and 48V house systems in larger builds, visible in the difference between the tables above.

Four things the tables cannot tell you

Size for surge, not for average. An inverter’s cable has to survive the DC current at motor start, which can be three to five times the running figure. Size the run at the surge current or accept that the inverter will cut out on every compressor start — this is the single most common cause of “my inverter is too small”, explained in full in inverter sizing for surge loads.

Aluminium is not copper. Aluminium conductors need roughly two gauge sizes larger for equivalent resistance, plus anti-oxidant compound and correctly rated terminals. Copper-clad aluminium sold as “copper” cable is common in the cheap end of the market and is a genuine hazard when sized from a copper table.

Stranded, tinned, fine-strand. Use fine-stranded conductor in any vehicle. Solid or coarse-stranded building wire work-hardens and fractures under vibration. Tinned copper costs 15–25 percent more and is worth it anywhere near moisture.

The fuse protects the wire, not the device. Fuse to the conductor’s ampacity, not to the load’s draw, and place the fuse within a few inches of the power source. On a lithium bank this needs a fuse with an interrupt rating high enough for the available fault current — a detail covered in the lithium retrofit guide, and one that ANL fuses frequently fail to meet.

That last point has a consequence beyond the technical. Owner-installed high-current DC wiring is one of the things insurers look at closely after a vehicle fire, and undersized or unfused conductors are the sort of finding that converts a claim into a dispute. If you are doing substantial electrical work on a converted or self-built vehicle, the declaration side of it is worth understanding before you need it — see insuring a custom or converted vehicle.