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| AWG | mm² | Cu 60°C | Cu 75°C | Al 75°C |
|---|---|---|---|---|
| 14 | 2.08 | 15A | 20A | -A |
| 12 | 3.31 | 20A | 25A | 20A |
| 10 | 5.26 | 30A | 35A | 30A |
| 8 | 8.37 | 40A | 50A | 40A |
| 6 | 13.3 | 55A | 65A | 50A |
| 4 | 21.2 | 70A | 85A | 65A |
| 3 | 26.7 | 85A | 100A | 75A |
| 2 | 33.6 | 95A | 115A | 90A |
Based on NEC Table 310.16 for conductors in raceway or cable
| AWG | mm² | Cu 60°C | Cu 75°C | Al 75°C |
|---|---|---|---|---|
| 14 | 2.08 | 15A | 20A | -A |
| 12 | 3.31 | 20A | 25A | 20A |
| 10 | 5.26 | 30A | 35A | 30A |
| 8 | 8.37 | 40A | 50A | 40A |
| 6 | 13.3 | 55A | 65A | 50A |
| 4 | 21.2 | 70A | 85A | 65A |
| 3 | 26.7 | 85A | 100A | 75A |
| 2 | 33.6 | 95A | 115A | 90A |
| 1 | 42.4 | 110A | 130A | 100A |
| 1/0 | 53.5 | 125A | 150A | 120A |
| 2/0 | 67.4 | 145A | 175A | 135A |
| 3/0 | 85 | 165A | 200A | 155A |
| 4/0 | 107.2 | 195A | 230A | 180A |
Based on NEC Table 310.16 for conductors in raceway or cable
Wire ampacity is the maximum amount of electrical current a conductor can carry continuously without exceeding its temperature rating and degrading the insulation. Ampacity depends on the wire gauge (AWG or mm²), conductor material (copper or aluminum), insulation temperature rating (60°C, 75°C, or 90°C), ambient temperature, and the number of current-carrying conductors bundled together. The National Electrical Code (NEC) Table 310.16 provides standardized ampacity values that electricians and engineers use to select wire sizes for safe installation. Undersized wiring is one of the leading causes of electrical fires, making accurate ampacity determination critical for every electrical installation.
Determine the total current the circuit will carry by dividing total power (watts) by voltage: I = P / V. For motor loads, use the full-load current from the nameplate. For continuous loads (running 3+ hours), multiply the current by 1.25 per NEC requirements to ensure the wire is not loaded beyond 80% of its ampacity.
Copper wire has higher ampacity than aluminum for the same gauge but costs more. Common insulation types include THHN/THWN (rated 90°C dry / 75°C wet), NM-B Romex (rated 90°C but used at 60°C ampacity per NEC), and XHHW (rated 90°C). Higher temperature ratings allow more current.
Using NEC Table 310.16, find the ampacity for your wire gauge, material, and insulation temperature rating. For example, 12 AWG copper at 60°C is rated for 20 A, while the same wire at 75°C is rated for 25 A. Always match the insulation type to the correct temperature column.
Reduce the base ampacity for ambient temperatures above 30°C (86°F) using NEC Table 310.15(B)(1). Apply conduit fill derating from Table 310.15(C)(1) when more than 3 current-carrying conductors share a raceway. These factors can significantly reduce the allowable current, often requiring a larger wire gauge.
Undersized wiring is a leading cause of electrical fires. When current exceeds a wire's ampacity, the conductor heats up, degrading insulation over time. Eventually, the insulation melts or chars, potentially igniting nearby combustible materials. Proper wire sizing per NEC standards is the first line of defense against electrical fires.
All electrical installations in the US must comply with the National Electrical Code (NEC), and local jurisdictions may have additional requirements. Using wire that meets ampacity requirements is mandatory for passing electrical inspections. Non-compliant wiring can result in failed inspections, fines, and insurance claim denials.
While a wire may have sufficient ampacity, long runs can experience excessive voltage drop. NEC recommends no more than 3% drop for branch circuits and 5% total for feeder plus branch. Larger wire gauges reduce both voltage drop and energy waste from I²R losses, which is especially important for long-distance runs and high-current circuits.
| AWG | mm² | Cu 60°C (TW) | Cu 75°C (THWN) | Cu 90°C (THHN) | Al 75°C | Typical Use |
|---|---|---|---|---|---|---|
| 14 | 2.08 | 15 A | 20 A | 25 A | - | 15 A lighting circuits |
| 12 | 3.31 | 20 A | 25 A | 30 A | 20 A | 20 A general receptacles |
| 10 | 5.26 | 30 A | 35 A | 40 A | 30 A | 30 A dryer, water heater |
| 8 | 8.37 | 40 A | 50 A | 55 A | 40 A | 40-50 A range, large AC |
| 6 | 13.3 | 55 A | 65 A | 75 A | 50 A | 60 A subpanel feeder |
| 4 | 21.2 | 70 A | 85 A | 95 A | 65 A | 70-80 A subpanel |
| 3 | 26.7 | 85 A | 100 A | 115 A | 75 A | 100 A subpanel feeder |
| 2 | 33.6 | 95 A | 115 A | 130 A | 90 A | 100 A service entrance |
| 1 | 42.4 | 110 A | 130 A | 145 A | 100 A | 110 A large loads |
| 1/0 | 53.5 | 125 A | 150 A | 170 A | 120 A | 150 A service |
| 2/0 | 67.4 | 145 A | 175 A | 195 A | 135 A | 200 A (Al 4/0) |
| 4/0 | 107 | 195 A | 230 A | 260 A | 180 A | 200 A service (Al) |
Based on NEC Table 310.16 for not more than 3 current-carrying conductors in raceway/cable at 30°C ambient
For a 200 amp residential service using copper conductors, you need 2/0 AWG (67.4 mm²) at 75°C rated for 175 A (acceptable because service entrance conductors have different rules). For aluminum, which is more common for service entrance due to cost, use 4/0 AWG rated for 180 A. Always verify with local code requirements.
Aluminum is commonly used for service entrance conductors, large feeders, and utility connections where cost savings are significant (aluminum costs roughly 50% less than copper). However, aluminum requires larger gauges for the same ampacity, special connectors rated for aluminum (marked AL-CU), and anti-oxidant compound on connections. For branch circuits, copper is preferred for its higher conductivity and easier termination.
When more than 3 current-carrying conductors are installed in the same raceway or cable, heat dissipation is reduced and ampacity must be derated per NEC 310.15(C)(1): 4-6 conductors derate to 80%, 7-9 to 70%, 10-20 to 50%, and 21-30 to 45%. Neutral conductors carrying only unbalanced current and equipment grounding conductors are not counted.
NEC ampacity tables assume a 30°C (86°F) ambient temperature. Higher ambient temperatures require derating because there is less temperature differential for heat dissipation. At 40°C, the correction factor for 75°C insulation is 0.88, reducing a 20 A wire to 17.6 A. In attic spaces during summer, temperatures can exceed 50°C, requiring significant derating.
Yes, standard 15 A duplex receptacles are permitted on 20 A circuits with 12 AWG wire per NEC 210.21(B)(3). This is the most common residential wiring configuration. However, a single receptacle on a 20 A circuit must be rated for 20 A. The 12 AWG wire and 20 A breaker protect the circuit; the 15 A receptacle simply limits any single device to 15 A.
Select the optimal wire gauge based on current, distance, voltage drop requirements, and NEC compliance for your installation.
Calculate voltage drop across wire runs to ensure equipment receives adequate voltage, especially for long-distance circuits.
Determine the current draw of electrical loads from power and voltage ratings for proper wire and breaker sizing.