KEMI Docs · Circuit Practice

Wire Gauge Selection: AWG vs mm², Ampacity and Voltage Drop

Last updated: 2026-10-04

Wire gauge usually gets picked by feel: this much current, about this thick. On short runs that instinct is mostly right. On longer runs, or at lower voltages, the wire may never get hot and the device still resets because the voltage at the far end is too low. This article covers AWG versus metric (mm²) sizes for electronics and harness wiring, the two sizing criteria (ampacity and voltage drop), how wire choice interacts with connectors and crimp terminals, and the wire-spec details people tend to miss. Building and mains wiring is governed by electrical codes (the NEC in the US, BS 7671 in the UK, IEC 60364 elsewhere) and is outside the scope of this guide.

AWG (American Wire Gauge) runs backwards: larger numbers are thinner. Every 3 gauges doubles the cross-section, and every 6 gauges doubles the diameter (AWG 22 to AWG 19 doubles the area). Most of the world outside North America sizes wire by conductor cross-section in square millimeters, using the IEC 60228 series (0.5, 0.75, 1.0, 1.5, 2.5 mm² and so on), with Japanese JIS sizes such as 0.85, 1.25 and 2.0 mm² also common in Asian supply chains. Exact matches between systems are rare, so you convert to the nearest size, rounding up.

Diameter and area are the AWG definitions for solid wire. Stranded wire of the same area has a slightly larger outside diameter because of the gaps between strands. Resistance is calculated from copper resistivity (0.0172 Ω·mm²/m at 20 °C) and rises about 0.39% per °C, so wire running at 60 °C (140 °F) has about 16% more resistance than at room temperature. The 'nearest metric' column gives the size commonly used as the equivalent; where that metric size is slightly smaller than the AWG size (0.5 mm² for 20 AWG, 0.75 mm² for 18 AWG), step up one size if you are near the limit.

Ampacity is the current at which the conductor stays below the insulation's temperature rating. The same gauge therefore varies widely with insulation rating (80 °C or 105 °C PVC, 200 °C silicone), ambient temperature, bundling (several current-carrying wires heating each other inside a harness) and whether the enclosure is sealed. Every standard tabulates it differently under different conditions, so rather than memorizing one number it is safer to keep a conservative working rule. Below is the rule we use for internal wiring in small electronic products (single wire, room temperature, runs under about a meter). It is a conservative in-house guideline, not a code value.

Derate further when several current-carrying wires are bundled in a harness, inside a sealed enclosure, or in high ambient temperatures. Silicone-insulated high-temperature wire can carry more current at the same gauge, but then the connector or terminal at the end of the wire usually hits its limit first.

In R = ρ × L / A, L is the round-trip length. Current goes out on the positive wire and comes back on the negative (ground) wire, so a device 10 ft away means 20 ft of wire in the calculation. Forgetting this is the most common mistake. A few examples build intuition.

The last row is a problem we see constantly. Cheap USB cables sometimes use 28 AWG power conductors, so at 2 A the cable alone drops 0.85 V. The device gets a little over 4.1 V, charging slows down, and motors or radios reset the board when they kick in. The lower the supply voltage, the bigger the share any given drop takes, so low-voltage, high-current wiring is sized by voltage drop, not ampacity. Allow 2 to 5% of the supply voltage as a typical budget, then confirm against the load's minimum operating voltage.

A 24 V, 4 A motor sits 16 ft (about 5 m) from the control panel. With a 3% drop budget (0.72 V), round-trip resistance must be at most 0.72 / 4 = 0.18 Ω. The round trip is 32 ft, so the wire needs no more than 0.18 / 0.032 = 5.6 Ω per 1000 ft. 18 AWG (6.4 Ω/1000 ft) gives 0.82 V, 3.4%, and fails. 16 AWG (4.0 Ω/1000 ft) gives 0.51 V, 2.1%, and passes with margin. In metric terms the minimum area is 0.96 mm², so 1.0 mm² just passes and 1.5 mm² is comfortable. On ampacity alone, 4 A would be fine on 20 AWG; distance pushed it up two gauges. Motors also draw several times their rated current at startup, so check that the startup dip won't reset the control electronics. We would pick 16 AWG here.

• Every crimp terminal part number accepts a specific AWG range, and smaller connectors have lower upper limits. Typical 2.0 mm pitch terminals top out at 24 AWG and 2.5 mm pitch terminals at 22 AWG. Upsizing wire for current can leave you with wire that won't fit the terminal, so choose wire and terminal together.
• Terminals also specify an insulation outside-diameter range. Even at the same AWG, thick-walled 600 V wire may not fit the insulation crimp of a small terminal.
• A connector's per-pin current rating is often stated with the largest wire it accepts. With thinner wire the real limit is lower.
• When current sits near the limit, doubling up two pins and two wires in parallel is usually cheaper and more reliable than moving to a larger connector.
• Our connector selection guide lists current and AWG pairings by connector family.

• Insulation rating. UL 1007 is 300 V / 80 °C, UL 1015 is 600 V / 105 °C, and UL 1061 is a thin-wall 300 V / 80 °C wire. In Europe, harmonized flexible single-core types such as H05V-K (300/500 V) and H07V-K (450/750 V) are common. If mains voltage runs inside the product, check the voltage rating first.
• Stranded vs solid. Use stranded wire wherever there is movement or vibration; solid is for breadboards and fixed runs. Solid wire breaks if crimped or flexed repeatedly.
• Don't clamp solder-tinned wire ends in screw terminals. The solder slowly cold-flows under pressure, the connection loosens, and you get heating and intermittent contact. Crimp a ferrule on stranded wire instead.
• Watch out for CCA. Copper-clad aluminum wire looks like copper but has roughly 1.5 to 1.6 times the resistance of copper at the same gauge. It is common in cheap wire and cables, so for high-current runs check for a pure copper (or oxygen-free copper) specification.
• A fuse protects the wire. Size it for the current the thinnest wire downstream can carry, not for the load. When a thin branch splits off a heavy feed, the branch needs its own fuse.

• Calculating voltage drop with one-way length. It's round trip.
• Sizing by ampacity alone. Long 5 V and 12 V runs are usually limited by voltage drop.
• Getting the AWG direction backwards. Higher numbers are thinner.
• Upsizing the wire without checking the terminal's AWG range.
• Forgetting inrush. Motors, heaters and large capacitive loads draw several times rated current momentarily.
• Trusting cable specs. Power conductor gauge and material on cheap cables may not match the label; for high-current use, measure (run current and measure voltage at both ends).

1. Do you know the rated and inrush (peak) current?
2. Did you calculate voltage drop over the round trip, and is it within 2 to 5% of the supply?
3. Is there margin above the load's minimum operating voltage?
4. Does the gauge meet ampacity after derating for bundling, temperature and enclosure?
5. Does the wire fit the crimp terminal's AWG and insulation-diameter ranges?
6. Is the connector's per-pin rating respected, or are pins paralleled?
7. Are insulation voltage and temperature ratings right for the application?
8. Is the fuse sized to protect the thinnest wire?

This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.