Reference

DC Wire Gauge Chart: AWG, Amps & Fuse Sizes

To pick a DC wire gauge, you need two numbers: the current the wire will carry in amps, and the one-way length of the run. Find both in the sizing chart below and read off the smallest gauge that keeps voltage drop at or under 3%. The surprise for most first-time builders is how thick the wire has to be. A 12V system carries a lot of current for not much power, and high current over a long run sheds voltage as heat, so a modest load can still call for heavy cable.

Prefer to skip the tables? The interactive Wire Size Calculator does this math for you. Type in your amps, your run length, and 12V, 24V, or 48V, and it returns the gauge, the exact voltage drop, and the fuse size. The charts below use the same formula and the same copper resistance values, so the two always agree.

Table 1: AWG ampacity and fuse size

Ampacity is the current a wire can carry continuously without overheating. The numbers depend on the insulation rating and on how the wire is run. Two figures matter for off-grid work. The single-conductor column is a wire in open air. The bundled column is the conservative real-world value when wires run together in a loom, sheath, or conduit, which is how most of a van or cabin is actually wired. Use the bundled column unless you know the run is in free air, and remember that heat and tight bundles only pull these lower.

Basis: single-conductor values are ABYC E-11 for 105°C-rated copper, outside engine spaces (30°C ambient). Bundled values reflect a 3-conductor derating and are the same conservative numbers the wire-size calculator uses. The suggested fuse is a starting point that protects the wire and leaves headroom below its rating. Higher temperatures, more conductors in a bundle, and lower-rated insulation all reduce ampacity.

Table 2: Wire gauge by amps and run length (12V, 3% drop)

This is the table you will reach for most. It is sized by voltage drop, which is usually what forces 12V wire to be thick, long before ampacity does. Find your current down the left side, find your one-way run length across the top, and read the minimum gauge where they meet. Every value keeps round-trip voltage drop at or under 3% of 12V.

Basis: copper, round-trip drop = (2 × one-way feet × ohms-per-1000ft × amps) ÷ 1000, kept at or under 0.36V (3% of 12V). "Parallel" means the run wants doubled cable or a higher system voltage. On the highest-current, shortest cells, also check Table 1: if ampacity calls for a thicker wire than voltage drop does, use the thicker one.

Table 3: Fuse and breaker sizing by component

Fuses protect wire from a short or overload. The rule of thumb is to size the fuse at about 125% of the continuous load, round up to a standard fuse value, and confirm that value sits at or below the ampacity of the wire it is protecting. These are conservative starting points. Your charge controller, DC-DC charger, and inverter manuals will often name an exact fuse, and ABYC has the final word.

Get the right parts

Two things make wiring safe and durable: fine-stranded marine-grade copper cable and a proper fuse block close to the battery. These search links are filtered to the right kind of product.

Why does 12V need such thick wire?

It comes down to one equation: current equals power divided by voltage. Run a 600W load at 12V and the wire carries 50A. Run that same 600W at 48V and it carries only 12.5A. The watts are identical, but the lower-voltage circuit pushes four times the current through the wire.

Current is what heats a wire and what drains voltage along a run, and both effects scale with how many amps are flowing. So a 12V system fights two problems at once. It needs thicker wire to carry the high current safely, and it needs thicker wire again to keep voltage drop in check over distance. That is why a simple 12V fridge run across a van can ask for cable that looks oversized to a newcomer. It is not overkill. It is the physics of low voltage.

How to read this chart

Work in three steps. First, find your amps. If you only know watts, divide by the system voltage: a 360W panel array on 12V is about 30A. Second, measure the one-way run, meaning the distance from the source to the load along the real cable path, not straight-line. Third, in Table 2 find where your amps row meets your length column and read the gauge.

Then sanity-check against Table 1. The voltage-drop chart gives the minimum gauge for clean power, but on short, very high-current runs the ampacity limit can be the stricter of the two. Whichever table demands the thicker wire is the one to follow. When a run sits right on the line, size up. Copper is cheap compared with tearing a finished build apart.

Where to put fuses

Every positive wire that leaves the battery gets a fuse or breaker, and it goes as close to the battery as you can manage. The reason is simple: the job of the fuse is to protect the wire from the battery's enormous short-circuit current. If the fuse sits far down the run, the unprotected stretch between the battery and the fuse can still catch fire in a fault.

Size each fuse to protect the wire it feeds, not the device on the end. A main fuse or Class-T fuse guards the big cable to the inverter and main bus. Smaller branch fuses in a fuse block guard each circuit. Negative and ground wires are not fused. If you take one rule from this page, take this one: no positive run without a fuse.

Stranded vs solid, and pure copper vs CCA

For off-grid power, use fine-stranded pure copper with marine-grade insulation. Stranding lets the cable flex and survive constant vibration in a van, RV, or boat. Solid wire work-hardens and eventually cracks where it moves, and its stiff insulation is not built for damp, mobile environments.

Avoid copper-clad aluminum, sold as CCA, for any power run. It is aluminum with a thin copper coating, so it has higher resistance than the same gauge of pure copper, runs hotter, and is weaker at terminals. The ampacity and voltage-drop numbers in these tables assume real copper. CCA does not meet them. The few dollars you save are not worth the fire risk, and CCA is not ABYC-compliant for marine work. Buy pure copper and crimp it with the right lugs.

Keep building

• Wire Size Calculator, the interactive version of these charts.
• How to wire a campervan electrical system, the full wiring walkthrough.
• 12V vs 24V vs 48V, why voltage choice changes everything downstream.
• Inverters, the biggest current draw in most systems.
• Batteries, where every positive cable starts and where the fuse goes.