AWG to mm² Conversion Table for Solar and DC Wiring

Solar hardware is a mongrel market. The panels ship with 4 mm² leads, the charge controller manual is written in AWG, the battery lugs are stamped in millimetres, and the cable on the shelf is labelled with whichever unit the seller thought would sell. Getting the conversion wrong by one size is easy and, on a low-voltage system where drop is already tight, expensive.

The conversion table

AWG	Circular mils	mm²	Conductor diameter (mm)
18	1,620	0.823	1.024
16	2,580	1.31	1.291
14	4,110	2.08	1.628
12	6,530	3.31	2.053
10	10,380	5.261	2.588
8	16,510	8.367	3.264
6	26,240	13.3	4.115
4	41,740	21.15	5.189
2	66,360	33.62	6.544
1	83,690	42.41	7.348
1/0	105,600	53.49	8.252
2/0	133,100	67.43	9.266
3/0	167,800	85.01	10.405
4/0	211,600	107.2	11.684

Diameter is the bare conductor, not the outside of the insulation. Cable that must pass through a gland or conduit is considerably fatter than this column suggests.

Why the numbers run backwards

AWG was defined by the drawing process. Wire is pulled through progressively smaller dies, and the gauge number counts how many times it was drawn. More draws means thinner wire, so a larger number means a smaller conductor. When you run out of numbers at 1, the scale continues into 1/0, 2/0, 3/0, 4/0 — pronounced “one aught” through “four aught” — each one larger than the last.

The scale is geometric, not linear. Two facts follow that are worth committing to memory:

• Three gauge steps doubles the cross-sectional area. 10 AWG has roughly twice the copper of 13 AWG; 4 AWG has roughly twice the copper of 7 AWG.
• Six gauge steps doubles the diameter and quadruples the area. 4 AWG has about four times the copper of 10 AWG.

Since resistance is inversely proportional to area, three gauge steps up also roughly halves your voltage drop. That is a useful shortcut when a run fails a drop calculation: dropping three gauge numbers cuts the loss in half.

The nearest metric size is usually smaller

This is the trap. Cable sold as “equivalent to 4 AWG” is frequently 20 mm² or even 16 mm², while 4 AWG is actually 21.15 mm². The substitution looks harmless and is not.

If the design calls for	True mm²	Commonly substituted	Shortfall
8 AWG	8.367	6 mm²	28% less copper
6 AWG	13.3	10 mm²	25% less copper
4 AWG	21.15	16 mm²	24% less copper
2 AWG	33.62	25 mm²	26% less copper
1/0 AWG	53.49	50 mm²	7% less copper
2/0 AWG	67.43	70 mm²	4% more copper

A 25 percent copper shortfall is a 33 percent increase in resistance, and therefore a 33 percent increase in voltage drop. On a run that was designed to 3 percent, that quietly becomes 4 percent. On a run designed to the edge of acceptable, it becomes a problem.

When metric cable is genuinely the right choice, size up to the next metric step rather than down. 25 mm² for 4 AWG, not 16 mm².

The copper-clad aluminium problem

The more damaging trap is not the gauge number at all. It is what the conductor is made of.

A large share of inexpensive “battery cable” and “welding cable” sold through online marketplaces is CCA — copper-clad aluminium. It is aluminium wire with a thin copper plating. It looks identical once terminated. It is sold in AWG sizes. It is dramatically worse.

Aluminium’s conductivity is about 61 percent of copper’s. A CCA cable stamped 4 AWG behaves electrically closer to 6 AWG, so every voltage-drop calculation you did with the copper resistance table is wrong by a wide margin. Worse, aluminium and copper have different thermal expansion rates and aluminium forms an insulating oxide layer, so crimped and bolted terminations tend to loosen and develop resistance over time. A high-resistance connection in a 100 A battery circuit is a heat source in exactly the place you least want one.

Ways to tell before you buy:

• Weight. Aluminium is roughly a third the density of copper. A spool of CCA is startlingly light for its size. If the shipping weight seems too good, it is CCA.
• Price. Pure copper cable tracks the copper commodity price. Cable that undercuts the market by half is not pure copper.
• A cut end. Nick the plating with a blade. Copper is copper all the way through; CCA shows silvery aluminium under a thin copper skin.
• The listing text. Sellers rarely lie outright but will bury “CCA” or “copper clad” in the specification block while the title says “pure copper series.”

For battery interconnects, inverter feeds, and anything carrying serious current, buy cable that is explicitly marked 100% copper, and prefer cable with a recognised marking such as UL 1426 (boat cable), UL 44 (THHN/THWN-2), or a marine tinned-copper specification. The premium over CCA is real but small next to the cost of the batteries it connects.

Stranding matters too

Two cables of identical cross-section can behave very differently in a vibrating or flexing installation. Fine-stranded cable — welding cable, marine battery cable — tolerates vibration and bends tightly. Solid or coarse-stranded building wire work-hardens and eventually fractures, which is why it belongs in walls and not in vehicles.

Terminations have to match the stranding. Lugs and set-screw terminals rated for building wire may not grip fine-stranded cable reliably, and a crimp die sized for one is wrong for the other. If you are wiring an RV, a van, or a boat, use fine-stranded cable with matching lugs and a proper hydraulic or ratcheting crimper, not pliers.

Tinned versus bare copper

Marine-grade cable is tinned: each strand carries a thin tin coating that resists corrosion. In a damp environment — a boat, a van that gets condensation, an outdoor enclosure, a battery box that sees any acid vapour — bare copper wicks moisture up under the insulation and corrodes from the inside out, sometimes well before there is any visible sign at the terminal.

Tinned cable costs roughly 20 to 30 percent more and is the default choice anywhere near salt, moisture, or lead-acid batteries. For a dry indoor shed installation, bare copper is fine.