Wire gauge tables look simple: 22 AWG handles 0.92 A, 18 AWG handles 2.3 A. But those numbers come from specific test conditions that often don’t match your application. Getting this wrong means wires that melt in an enclosure or voltage drop that starves your circuit.
Where wire current ratings come from
The NEC (National Electrical Code) and IPC standards define current-carrying capacity based on allowable temperature rise above ambient. IPC-2221A uses 30 °C rise as the basis for internal PCB traces. UL wire tables often assume open air at 30 °C ambient.
In a sealed enclosure at 50 °C ambient, that same wire running at its “rated” current might hit 80 °C — above the temperature rating of many PVC insulation materials (typically 60–80 °C). You need to derate.
The wire gauge calculator shows voltage drop and current rating for AWG and metric gauges, but the ambient temperature and bundling factors are your inputs — you need to know your environment.
Bundling derating
Wires in a bundle share heat. If you have 10 wires in a harness, each carrying some current, each wire’s current rating must be derated. Standard derating factors from NEC Table 310.15(B)(3)(a):
- 4–6 wires: multiply by 0.80
- 7–9 wires: multiply by 0.70
- 10–20 wires: multiply by 0.50
So that 22 AWG wire rated at 0.92 A in free air drops to 0.46 A effective rating in a 15-wire bundle. If you designed to 0.8 A expecting the free-air rating, you have a thermal problem in the harness.
Voltage drop matters more than current rating
Current rating is a thermal limit. Voltage drop is a functional limit, and it often bites first.
A 1 m run of 24 AWG (0.084 Ω/m per conductor, so 0.168 Ω round trip) carrying 0.5 A drops 0.5 × 0.168 = 84 mV. That’s fine for a 5 V rail. For a 3.3 V rail powering a device with a 3.0 V minimum operating voltage, you have only 300 mV of headroom — and you just spent 84 mV of it on wire resistance before any connector or PCB trace resistance.
For LoRa or cellular modems with peak TX current of 300–500 mA, the voltage drop during transmission can cause brownout resets if wire runs are long. Add capacitance at the load if rerouting wire isn’t practical.
PCB traces vs wire
PCB trace current capacity follows the same physics but different geometry. IPC-2221A gives formulas based on trace width, copper weight, and allowable temperature rise. Rules of thumb:
- 1 oz copper (35 µm), external layer: ~1 A per mm of trace width for 10 °C rise
- 1 oz copper, internal layer: ~0.5 A per mm of trace width (worse cooling)
- 2 oz copper: approximately double the current for the same width
A 1 mm external trace can carry about 1 A with modest temperature rise. A USB power trace at 500 mA on a 0.5 mm trace is borderline — fine if the trace is short, problematic if it runs 50 mm across a board inside an enclosure.
For high-current paths (motor drivers, battery discharge paths, USB VBUS), route them wide and short. On a 4-layer board, via stitching to internal copper layers helps spread the heat.
Connector ratings
Wire can handle the current; connectors are often the weak link. Molex KK 2.54 mm connectors are typically rated 3 A per contact. JST PH is rated 2 A. Micro-USB is specified at 1.8 A in the standard — fine for 500 mA charging, insufficient for fast-charge applications without a better connector.
At high current, check the contact resistance specification, not just the current rating. A connector with 10 mΩ contact resistance carrying 5 A dissipates I² × R = 25 × 0.01 = 0.25 W per contact. In a board-to-board connector with 10 pins all carrying 5 A, that’s potentially 2.5 W of heat inside the connector housing.
Automotive and industrial harnesses
In automotive applications (operating to 85 °C or 105 °C ambient), derate wire ratings significantly. ISO 6722 and SAE J1128 specify wire constructions and temperature ratings for automotive use — standard 60 °C PVC wire is not suitable.
For CAN bus harnesses: termination resistors, impedance matching, and shielding matter more than current capacity (CAN bus signal current is small). For power distribution in vehicles, use 85 °C or 105 °C rated wire and calculate voltage drop for the full harness length at peak load, including cold-start battery voltage (10.5 V minimum in 12 V systems).