When sourcing wire for a project, the terminology can quickly become a barrier. Standard Wire Gauge (SWG), also known as Imperial Wire Gauge, and American Wire Gauge (AWG) are two entirely different logarithmic sizing systems. AWG is the undisputed standard for North American electrical installations, while SWG is a legacy British standard now mostly relegated to specialty applications like guitar strings, craft wire, and vintage electronics. For modern global projects, you will also frequently encounter IEC metric sizing (mm²).
Confusing these systems isn't just a theoretical error; selecting a wire based on the wrong gauge standard can result in undersized conductors, overheating, and catastrophic failure. Below is the definitive reference for translating between these systems and applying the correct ampacity ratings to your installation.
The Master Wire Gauge Conversion & Ampacity Table
How to read this table: This chart cross-references AWG with its closest SWG equivalent, IEC metric cross-sectional area (mm²), and physical diameter. The ampacity columns are sourced directly from NEC Table 310.16 for copper conductors. The temperature columns (60°C, 75°C, 90°C) represent the insulation rating of the wire. Always verify your specific insulation type (e.g., THHN is 90°C, NM-B is 60°C) before selecting a column. Note that SWG steps are irregular, meaning the SWG equivalent is the closest physical match, not an exact 1:1 mathematical translation.
| AWG Size | Closest SWG | IEC Metric (mm²) | Diameter (in / mm) | 60°C Ampacity | 75°C Ampacity | 90°C Ampacity |
|---|---|---|---|---|---|---|
| 14 | 16 | 2.08 | 0.0641 / 1.628 | 15A * | 20A * | 25A * |
| 12 | 14 | 3.31 | 0.0808 / 2.053 | 20A * | 25A * | 30A * |
| 10 | 12 | 5.26 | 0.1019 / 2.588 | 30A * | 35A * | 40A * |
| 8 | 10 | 8.37 | 0.1285 / 3.264 | 40A | 50A | 55A |
| 6 | 8 | 13.30 | 0.1620 / 4.115 | 55A | 65A | 75A |
| 4 | 6 | 21.15 | 0.2043 / 5.189 | 70A | 85A | 95A |
| 2 | 3 | 33.62 | 0.2576 / 6.543 | 95A | 115A | 130A |
Decoding the Columns: Which Rating Applies to Your Install?
Looking at three different ampacity columns for a single wire size is a common point of confusion. The column you must use depends entirely on the weakest link in your circuit's temperature rating chain.
Which Column Applies?
The rule of thumb is that your circuit's allowable ampacity is limited by the lowest temperature rating of any connected device, termination, or conductor.
- 60°C Column: Use this for non-metallic sheathed cable (NM-B / Romex), as well as when terminating to older devices or receptacles not explicitly marked with a temperature rating. Most residential branch circuits under 100A default to this column.
- 75°C Column: Use this when pulling individual THHN/THWN conductors in conduit and terminating to modern breakers, lugs, and receptacles that are explicitly stamped "75°C" or "AL/CU". This is the standard for commercial work and residential feeders.
- 90°C Column: You can almost never use the 90°C ampacity for the final overcurrent protection sizing because terminations are rarely rated for 90°C. This column exists almost exclusively for calculating derating adjustments.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors in a single raceway or conduit, the wires heat each other up. NEC 310.15(C)(1) requires you to apply a derating multiplier. You always start your derating math in the 90°C column.
Worked Example: You are pulling four current-carrying 10 AWG THHN conductors in a conduit to feed a multi-wire branch circuit.
1. Base 90°C ampacity for 10 AWG = 40A.
2. Four conductors require an 80% derating multiplier (40A × 0.80 = 32A).
3. Check termination limits: Your breaker and receptacle are rated 75°C. The 75°C ampacity for 10 AWG is 35A.
4. Compare the derated value (32A) to the termination limit (35A). The lower value is 32A.
5. Apply NEC 240.4(D): Because it is 10 AWG, the maximum breaker size is hard-capped at 30A. Therefore, you must protect this circuit with a 30A breaker, even though the derated wire could technically handle 32A.
What the Table Cannot Tell You
Ampacity tables assume ideal conditions. They do not account for:
- Voltage Drop: A 12 AWG wire can safely carry 20A indefinitely without melting, but if that run is 150 feet long, the voltage at the receptacle will drop below the acceptable 114V threshold under load. You must calculate voltage drop using NEC Chapter 9, Table 8 resistance values for runs exceeding 100 feet.
- Conduit Fill Limits: The table tells you how much current a wire can carry, but NEC Chapter 9, Table 1 dictates how many wires physically fit inside a specific trade size of PVC or EMT conduit without jamming or damaging the insulation during the pull.
- Ambient Temperature: If your conduit runs across a 110°F rooftop or through a hot attic, the base ampacities in this table must be multiplied by ambient temperature correction factors found in NEC Table 310.15(B)(1).
SWG vs AWG vs Metric: Regional Standards and When to Use Which
While AWG dominates in North America, international sourcing requires fluency in other standards. Understanding the origin of these standards prevents costly procurement errors, particularly when ordering from overseas suppliers on platforms like AliExpress or Digi-Key.
| Feature | AWG (American Wire Gauge) | SWG (Standard / Imperial) | IEC Metric (mm²) |
|---|---|---|---|
| Primary Region | USA, Canada, Mexico | UK (Legacy), Specialty Crafts | EU, UK (Modern), AU, NZ, Asia |
| Governing Body | ASTM B258 / NEC | BS 3737 (Withdrawn/Legacy) | IEC 60228 |
| Sizing Logic | Smaller number = Larger wire | Smaller number = Larger wire | Larger number = Larger wire |
| Common Mains Use | Yes (Residential & Commercial) | No (Obsolete for mains wiring) | Yes (Global standard for mains) |
| Typical Receptacle Wire | 12 AWG (20A circuits) | N/A | 2.5 mm² (20A circuits) |
Quick-Jump Reference: The 5 Most Queried Wire Sizes
Bookmark this section for fast lookups on the most common residential and light-commercial branch circuit and feeder sizes. These guidelines assume standard copper conductors in a typical 30°C ambient environment.
14 AWG (2.08 mm²)
Max Breaker: 15A.
Primary Use: General lighting circuits, low-draw switch loops, and doorbell wiring. Never use 14 AWG for standard 20A kitchen or bathroom receptacles, even if the lighting and receptacles are technically on the same general-purpose branch circuit in older, non-compliant homes.
12 AWG (3.31 mm²)
Max Breaker: 20A.
Primary Use: The modern standard for all 120V general-purpose receptacles, kitchen countertop small-appliance circuits, and bathroom GFCI circuits. It is highly recommended to use 12 AWG exclusively for all 120V receptacle runs to eliminate voltage drop concerns and allow for future load upgrades.
10 AWG (5.26 mm²)
Max Breaker: 30A.
Primary Use: 240V electric water heaters, standard electric clothes dryers (though many modern dryers now require 30A or 40A depending on the nameplate), and heavy-duty 120V window AC units or RV hookups.
8 AWG (8.37 mm²)
Max Breaker: 40A (Copper).
Primary Use: Level 2 EV chargers (requiring a 40A continuous load on a 50A breaker), larger electric ranges, and baseboard heater arrays. Note that 8 AWG solid wire is incredibly stiff; always use stranded THHN in conduit for this size and larger to make pulling and terminating manageable.
6 AWG (13.30 mm²)
Max Breaker: 60A (Copper).
Primary Use: 60A subpanel feeders (using 4 wires: two hots, neutral, ground), large workshop welders, and heavy-duty hot tubs. When running 6 AWG as a subpanel feeder in NM-B cable, remember that the ampacity drops to the 60°C column (55A), meaning you must protect it with a 50A or 55A breaker, not a 60A breaker, unless using THHN in conduit.
