Wire Gauge Chart: How to Choose the Right Wire and Cable Size

By Patrick Martiro · 11 September 2026

Picking a wire gauge looks like a quick lookup, yet the chart on the wall answers only one of the questions that decide whether a cable runs cool or overheats.

A conductor that is too small runs hot and can damage its own insulation. One that is far too large wastes copper and money, so the real goal is the smallest safe size for the actual installation. Both mistakes are avoidable once you know which checks to run.

What Wire Gauge Actually Measures

Two sizing systems dominate the world's wiring. The United States uses American Wire Gauge, while most other markets describe a conductor by its cross-sectional area in square millimetres.

How the AWG Numbers Work

American Wire Gauge is the US standard that defines the area and diameter of solid, round wire. The numbers trace back to the number of dies used to draw copper down to size, so a higher wire gauge number means a thinner wire. Beyond 1 AWG, larger conductors run through 1/0, 2/0, 3/0 and 4/0 before the scale switches to kcmil.

• Dropping three gauge numbers doubles the cross-sectional area.
• Dropping six gauge numbers doubles the diameter.
• Stranded conductors bundle many thinner wires: - Fine strands suit cables that bend or vibrate in service. - The overall size is still quoted as a single gauge.

Metric Cable Sizes in mm²

Metric cables are named by conductor area, such as 4, 6, 10 or 16 mm². Unlike a wire gauge number, a bigger metric number always means a bigger conductor, which makes single-core house wiring cables easy to compare at a glance.

Why the Two Systems Never Line Up

There is no exact conversion between a wire gauge and a metric size, only close equivalents. Lapp Tannehill advises checking the product data sheet whenever an exact metric figure matters for the design.

The gap is usually small, but it tends to run in one direction. Most wire gauge sizes are slightly smaller than the metric size they are usually matched with.

Wire Gauge Chart: AWG to mm²

The wire gauge chart below lists the exact area of common AWG sizes beside the metric size they are usually matched with and the diameter of a solid conductor.

Wire gauge (AWG)	Exact area (mm²)	Common metric match (mm²)	Solid diameter (mm)
14	2.08	2.5	1.63
12	3.31	4	2.05
10	5.26	6	2.59
8	8.37	10	3.26
6	13.3	16	4.11
4	21.2	25	5.19
2	33.6	35	6.54
1/0	53.5	50	8.25
2/0	67.4	70	9.27
4/0	107	120	11.68

Treat the metric column as a starting point rather than a direct swap. An exact design should always work from the true area shown on the data sheet.

How Much Current Each Wire Gauge Can Carry

Area alone does not tell you the safe current for a wire gauge. In the United States, the National Electrical Code lists allowable ampacities in its main conductor table. That table covers up to three current-carrying conductors in a raceway, a cable or the earth.

Reading Table 310.16

Read across the row for your wire gauge, then down to the column that matches the insulation rating. The terminal rating can lower that figure, as the rule further down explains.

Wire gauge (AWG), copper	60 °C rating	75 °C rating	90 °C rating
14	15 A	20 A	25 A
12	20 A	25 A	30 A
10	30 A	35 A	40 A
8	40 A	50 A	55 A
6	55 A	65 A	75 A
4	70 A	85 A	95 A
2	95 A	115 A	130 A
1/0	125 A	150 A	170 A
2/0	145 A	175 A	195 A
3/0	165 A	200 A	225 A

Each column matches an insulation temperature rating, and every value assumes an ambient temperature of 30 °C. Hotter surroundings reduce the usable figure through correction factors published alongside the table.

The Small-Conductor Limits

The code also caps the overcurrent protection on the three smallest common sizes. That cap applies whatever the table column says, even after correction factors.

• 14 AWG copper is limited to 15 A of protection.
• 12 AWG copper is limited to 20 A of protection.
• 10 AWG copper is limited to 30 A of protection.

The Termination Temperature Rule

A cable with 90 °C insulation does not automatically earn the 90 °C ampacity for its wire gauge. The terminals on the equipment usually set the limit.

• Equipment rated 100 A or less usually restricts you to the 60 °C column.
• Equipment rated above 100 A usually allows the 75 °C column.
• The 90 °C column mainly serves as the starting point for derating.

The Checks Behind Every Cable Size

A cable size is only correct when it passes every one of these checks at the same time.

Current-Carrying Capacity

This first check keeps the cable from overheating in normal use. It is also the check that most sizing charts are built around.

• Find the design current of the load.
• Choose a protective device rated at or above that current.
• Select a cable whose corrected rating still covers the device rating.

Installation Method

The same wire gauge carries very different currents depending on how it is installed. A 16 mm² four-core cable rated at 73 A on an open perforated tray might manage only 57 A inside enclosed conduit. Always identify the method before reading any rating table.

Temperature, Grouping and Insulation

Tabulated ratings assume ideal conditions that real sites rarely match. Each factor below adjusts the rating, usually downward.

• Ambient temperature is referenced to 30 °C under the NEC, BS 7671 and IEC 60364: - Australia and New Zealand use a 40 °C reference instead. - Hotter rooms reduce the usable rating.
• Grouping reduces the rating when circuits share a tray or conduit: - Two circuits bunched together keep about 80 percent of their rating. - Six bunched circuits keep only about 57 percent.
• Thermal insulation wrapped around a cable can cut its rating by up to half.

Voltage Drop

A cable can pass the current check and still deliver too little voltage at the far end of the run. Every major standard therefore sets a limit.

• BS 7671 allows 3 percent for lighting and 5 percent for other uses.
• The NEC recommends 3 percent on a branch circuit and 5 percent in total.
• AS/NZS 3008 allows 5 percent from the supply to the point of use.

On runs longer than about 50 metres, voltage drop often decides the final size. The fix is either a larger conductor or a shorter route.

Short-Circuit Withstand

The cable must also survive fault current until the breaker or fuse clears it. This check rarely governs on distribution cables, but it can matter on long sub-mains where fault levels stay high. A larger size is the fix when it fails.

Wire Gauge for Common Jobs

A few sizing questions come up again and again, and the code answers some of them directly.

15 A and 20 A Branch Circuits

Under the small-conductor limits, a 15 A circuit needs at least 14 AWG copper and a 20 A circuit needs at least 12 AWG copper. A long run or a hot space can push either one up a wire gauge.

200 A Home Services

The NEC has a separate table for 120/240 V single-phase dwelling services and feeders. For a 200 A rating it permits conductors smaller than the general ampacity table would require at 75 °C. The allowance applies only to that dwelling case, not to commercial feeders.

• NEC Table 310.12 permits 2/0 AWG copper for this service.
• The same table permits 4/0 AWG aluminium as the alternative.

Copper Versus Aluminium

Aluminium needs a larger wire gauge than copper for the same job, as the 200 A example shows. Its ampacities sit in their own columns of the NEC table, so never read an aluminium cable size from the copper figures.

Common Wire Gauge Mistakes

Most wire gauge errors come from skipping a step rather than misreading a chart.

• Choosing the wrong installation method, such as rating a cable for open tray when it actually runs in enclosed trunking.
• Forgetting grouping, which is the derating factor engineers miss most often.
• Mixing reference temperatures between one standard's tables and another's correction factors.
• Skipping the voltage drop check on long runs.
• Using the 90 °C column directly when selecting a conductor.

Frequently Asked Questions

What wire gauge do I need for a 20 amp circuit?

Copper wire of at least 12 AWG is the minimum under the NEC small-conductor rule. Choose a larger wire gauge if the run is long or the space is hot.

How does 14 AWG compare with metric cable?

A 14 AWG conductor has an area of about 2 mm², so it is slightly smaller than its usual metric match. Check the data sheet whenever the exact area matters.

What size wire is used for a 200 amp service?

The NEC dwelling service table lets a 200 A single-phase home service use 2/0 AWG copper or 4/0 AWG aluminium. Other buildings follow the general ampacity tables instead.

Does a higher AWG number mean a thicker wire?

No, a higher wire gauge number means a thinner wire. Dropping three gauge numbers doubles the cross-sectional area.

Why does voltage drop change the cable size?

Every metre of conductor adds resistance, so long runs lose voltage before power reaches the load. On runs beyond about 50 metres, meeting the limit often needs a larger wire gauge than the current rating alone. Check both before you order.

Conclusion

The right wire gauge comes from a sequence of checks, not from a single chart lookup.

• Convert carefully between AWG and mm², using exact areas.
• Read the right column for the insulation and terminal ratings.
• Correct for conditions such as heat, grouping and installation method: - Apply every factor before comparing the result with the protective device. - Size for the final installation, not just today's circuits.
• Confirm voltage drop and fault withstand before you order.

Get those steps right, and the cable you buy will be both safe and economical. Browse the full range of copper cables and compare each wire gauge option against your calculated size.