Wire Gauge Diagram: AWG to mm² Ampacity, Voltage Drop, and Selection Guide

Wire Gauge Diagram: AWG to mm² Ampacity, Voltage Drop, and Selection Guide — interactive diagram. Open it in the editor to customise components and wiring.

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A wire gauge diagram maps conductor sizes — in AWG or mm² — to safe ampacity and acceptable voltage drop, helping you select the correct wire for any circuit.

Wire gauge is the first question in any wiring job and arguably the most consequential one. Select too small a gauge and the conductor overheats: insulation melts, short circuits develop, and fires start. Select too large a gauge and you waste money on copper and create cables that are hard to terminate in compact enclosures. The two dominant sizing systems are the American Wire Gauge (AWG) scale used in North America and the metric cross-sectional area in square millimetres (mm²) used in IEC-aligned systems across most of the world.

AWG is a logarithmic scale that runs backwards: larger AWG numbers mean smaller conductors. 14 AWG is smaller than 12 AWG. 4/0 AWG (written as 0000) is a heavy cable used in battery banks and main service feeds. The relationship is fixed: every 3 AWG steps roughly halves the cross-sectional area; every 6 AWG steps roughly doubles the resistance.

In metric systems, conductors are specified by cross-section in mm²: 1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm², 16 mm², 25 mm², and so on. These are the standard sizes in IEC 60364-compliant wiring (Europe, UK, South Africa, Australia).

Ampacity — the maximum current a conductor can safely carry — depends on the conductor size, the insulation temperature rating, the installation method (clipped direct, enclosed in conduit, buried), the ambient temperature, and grouping correction factors when multiple cables are bundled together. A 2.5 mm² conductor clipped to a wall might be rated at 23 A; the same cable drawn through thermal insulation might be de-rated to 15 A.

Voltage drop is the second critical constraint. Long cable runs lose voltage across the conductor's resistance. On a 12 V DC system, a 3 % drop limit (0.36 V) is much more restrictive than on a 230 V AC system (6.9 V at 3 %). Formula: V_drop = (2 × L × I × ρ) / A, where L is one-way run length in metres, I is current in amps, ρ is resistivity (0.0175 Ω·mm²/m for copper), and A is cross-section in mm².

How to wire wire gauge diagram

Determine the circuit's maximum continuous current Identify the maximum continuous load current the circuit will carry. For motor loads, use the full-load current from the motor nameplate (not the running current alone — starting current can be 6–8× the full-load current, but ampacity is sized for continuous rated current). For heating loads, calculate from wattage: I = P / V.
Identify the installation method Determine how the cable will be installed: clipped direct to surface, enclosed in conduit, buried in wall, in free air, or bundled with other cables. Each installation method has a reference method code in your applicable wiring standard (e.g. BS 7671 Appendix 4, NEC Table 310.15, IEC 60364-5-52) that determines the base ampacity and the applicable correction factors.
Apply correction factors Apply ambient temperature correction (cables derate in high-temperature environments), grouping correction (multiple cables bundled together share heat and derate each other), and any burial depth correction for underground cables. Multiply the base ampacity by all applicable correction factors to get the derated permissible current for the conductor.
Select the minimum conductor size from the ampacity table From your applicable standard's ampacity table, select the smallest standard conductor size whose derated permissible current equals or exceeds the circuit's maximum continuous current. This gives the minimum size based on thermal capacity.
Check voltage drop for the run length Using V_drop = (2 × L × I × ρ) / A for DC or the equivalent formula from your standard for AC, calculate the voltage drop for the cable run at the design current. If the calculated drop exceeds the permitted limit (typically 3–5 % depending on standard and circuit type), select the next larger conductor size and repeat.
Verify protective device coordination The upstream overcurrent protective device (fuse or circuit breaker) must be rated at or below the conductor's ampacity after all correction factors are applied. A 25 A MCB protecting a 2.5 mm² cable (derated to 20 A by its installation method) is a code violation and a fire risk. Adjust either the conductor size or the protective device rating.
Document and label Record the conductor size, the calculated voltage drop, the correction factors applied, and the protective device rating in the installation's circuit schedule. Label cable runs at both ends and at accessible intermediate points. This documentation is required for BS 7671 Electrical Installation Certificates and supports safe future maintenance.

Specifications

AWG to mm² key equivalents	14 AWG ≈ 2.08 mm²; 12 AWG ≈ 3.31 mm²; 10 AWG ≈ 5.26 mm²; 8 AWG ≈ 8.37 mm²; 6 AWG ≈ 13.3 mm²
Copper resistivity at 20 °C	0.0175 Ω·mm²/m (17.5 mΩ·mm²/m)
1.5 mm² copper, ampacity clipped to surface (IEC, 30 °C ambient)	Approximately 15 A
2.5 mm² copper, ampacity clipped to surface (IEC, 30 °C ambient)	Approximately 23 A
4 mm² copper, ampacity clipped to surface (IEC, 30 °C ambient)	Approximately 31 A
14 AWG copper, ampacity in conduit (NEC 310.15, 30 °C)	15 A (THWN/THHN insulation)
12 AWG copper, ampacity in conduit (NEC 310.15, 30 °C)	20 A (THWN/THHN insulation)
Voltage drop limit (BS 7671 recommendation)	3 % for lighting; 5 % for power circuits from installation origin

Safety warnings

Undersized conductors overheat and are the leading cause of electrical fires in fixed wiring. Always select conductor sizes from the ampacity tables in your applicable wiring standard (NEC, BS 7671, IEC 60364, AS/NZS 3000, SANS 10142) and apply all required correction factors. Never guess wire gauge — calculate it.
Fixed electrical wiring must be installed by or under the direct supervision of a licensed electrician in most jurisdictions. An incorrect wire gauge that passes initial inspection can become dangerous as loads increase over the life of the installation. Obtain the required permit and inspection.
Aluminium conductors have approximately 60 % of the conductivity of copper by cross-section — a 6 mm² aluminium conductor cannot be treated as equivalent to a 6 mm² copper conductor for ampacity. Aluminium also requires compatible terminals and anti-oxidant compound. Consult the applicable standard for aluminium conductor requirements.
Conductor ampacity tables assume specific ambient temperatures (typically 30 °C for IEC, 30 °C for NEC 310.15). In high-temperature environments — roof spaces, engine bays, near heating equipment — apply the appropriate derating factor from the standard. Failure to derate in hot environments is a documented cause of insulation failure.
Verify that all ferrules, terminal blocks, and connectors are rated for the conductor size and current being used. An undersized terminal creates a hot-spot at the connection that can cause failure even if the cable itself is correctly sized.

Tools needed

Wire strippers and crimping tool matched to conductor size
Digital multimeter (resistance measurement for voltage drop verification)
Cable sizing chart or wiring standard reference (NEC, BS 7671, IEC 60364)
Clamp meter (for measuring actual load current during commissioning)
Cable labels and permanent marker
Vernier calliper (for measuring unknown cable cross-section)

Common mistakes

Using the AWG table without considering installation method correction factors — a 14 AWG conductor in a bundle of ten cables has a significantly lower safe ampacity than a single 14 AWG cable run in free air.
Sizing wire only for current and ignoring voltage drop — a wire that is safely rated for the current may have an unacceptably high voltage drop on a long run, causing equipment to malfunction or fail to start.
Confusing AWG with metric mm² and using the wrong column in an ampacity table — a 14 AWG is not the same as 1.4 mm² (14 AWG is approximately 2.08 mm²), and using the wrong table leads to systematic under-sizing.
Terminating stranded wire in a terminal designed for solid wire without a ferrule — the strands can fan out, and over time a single strand contacting the adjacent terminal creates a short circuit.
Assuming the circuit breaker rating indicates the maximum safe conductor size — the breaker protects the conductor from overload, not from chronic overheating below the trip threshold. The conductor must be rated above the breaker, not the other way around.

Troubleshooting

Cable feels warm or hot under normal load conditions: Cause: Conductor is undersized for the actual load current, or the conductor has been derated by its installation method but an adequate size was not selected Fix: Measure actual load current with a clamp meter. Compare to the conductor's derated ampacity for its installation method. If the current exceeds the derated ampacity, the conductor must be replaced with a correctly sized one or the load must be reduced. Do not simply replace the overcurrent device with a higher-rated one.
Equipment at the end of a long cable run fails to start or runs sluggishly: Cause: Excessive voltage drop along the conductor, resulting in a supply voltage at the load that is below the equipment's minimum operating voltage Fix: Measure voltage at the supply end and at the load end under operating current. Calculate the actual voltage drop and compare to the permitted limit. Select a larger conductor cross-section for the run, or reduce run length by relocating the distribution board closer to the load.
Overcurrent device trips intermittently under normal load: Cause: Conductor resistance has increased due to a loose or corroded terminal connection, creating a local hot-spot that adds resistance and dissipates heat into the thermal-magnetic trip element Fix: Isolate the circuit and inspect all termination points with an infrared thermometer or thermal camera under load (before isolation). Tighten or reterminate corroded connections. If conductors are discoloured from heat, the damaged section of cable must be replaced.

Frequently asked questions

What AWG wire is equivalent to 2.5 mm²?

2.5 mm² is approximately 13.3 AWG, which means the nearest standard AWG size is 14 AWG (2.08 mm²) or 12 AWG (3.31 mm²). In practice, 2.5 mm² metric cable is used where 12 AWG is used in North America for 20 A circuits — check local code, as ampacity tables differ between IEC and NEC standards.

How does installation method affect wire ampacity?

Significantly. A 2.5 mm² cable clipped direct to a surface has a higher ampacity than the same cable drawn through conduit with other cables, buried in thermal insulation, or bundled with additional circuits. Installation method correction factors (as defined in IEC 60364-5-52 or BS 7671 Appendix 4) can reduce the rated ampacity by 30–60 % in worst-case grouping and thermal conditions.

What is the maximum voltage drop allowed on a circuit?

BS 7671 (UK) recommends a maximum voltage drop of 3 % for lighting circuits and 5 % for power circuits from the origin of the installation. NEC does not mandate a specific drop limit but recommends less than 3 % on branch circuits and 5 % total from service to the final outlet (as a recommended practice, not a hard code requirement). IEC 60364 similarly recommends 4 % for lighting and 6–8 % in some applications depending on the standard edition.

Does the same AWG or mm² have different ampacity for AC versus DC circuits?

The conductor's resistance is similar at the frequencies involved in residential/commercial wiring (50 Hz or 60 Hz AC has minimal skin effect impact below about 10 AWG / 6 mm²). However, AC circuits use RMS voltage and current whereas DC circuits use absolute values. Practical wiring codes provide AC-specific ampacity tables; for DC systems (solar, automotive, marine) use the same conductor resistance but apply the appropriate DC voltage drop formula and correct for the absence of neutral conductor.

How do I calculate the correct wire size for a long DC cable run?

Use the voltage drop formula: V_drop = (2 × L × I × ρ) / A. Rearranged for area: A = (2 × L × I × ρ) / V_drop_max. For a 10 m run at 20 A on a 24 V DC system with 3 % drop limit (0.72 V): A = (2 × 10 × 20 × 0.0175) / 0.72 = 9.72 mm². Select the next standard size up: 10 mm². Then verify the chosen size also meets the ampacity requirement for its installation method.