The American Wire Gauge (AWG) system dictates that for standard residential copper wiring, a 15-amp circuit requires 14 AWG, a 20-amp circuit requires 12 AWG, and a 30-amp circuit requires 10 AWG. However, pulling the right wire isn't just about matching amps to gauge. The physical diameter, cross-sectional area, and insulation temperature rating all dictate whether your wire will safely carry the load or melt inside the conduit.

This reference provides the exact physical conversions and the National Electrical Code (NEC) ampacity limits you need to size your next branch circuit, feeder, or subpanel run.

How to Read the American Wire Gauge Conversion Chart

Before pulling wire, you need to understand the four distinct data sets in a proper AWG conversion chart: physical dimensions, cross-sectional area, and the three temperature-rated ampacity columns.

  • Physical Dimensions (Diameter & Area): AWG is a logarithmic scale. Every 3-gauge decrease (e.g., 12 AWG to 9 AWG) doubles the cross-sectional area. Every 6-gauge decrease doubles the diameter. This matters when calculating conduit fill capacity.
  • 60°C Column (140°F): Use this column for older equipment, NM-B (Romex) cable assemblies, and UF-B underground feeder cables. Even if the wire inside NM-B is rated for 90°C, NEC 334.80 mandates you must use the 60°C ampacity column for final overcurrent protection sizing.
  • 75°C Column (167°F): Use this for most modern branch circuit terminations. Standard residential breakers and receptacles manufactured after the 1990s are typically rated for 75°C.
  • 90°C Column (194°F): Use this only as your starting baseline for derating calculations (bundling and ambient temperature adjustments) when using THHN/THWN-2 wire in conduit. You cannot use the 90°C ampacity for final breaker sizing unless both the wire and the termination lugs are explicitly rated for 90°C, which is rare in residential work.
NEC 240.4(D) Small Conductor Rule: Regardless of the 75°C or 90°C columns showing higher numbers, the NEC strictly limits standard overcurrent protection for 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A for general branch circuits. Do not put a 25A breaker on 12 AWG wire just because the 75°C column says 25A.

Complete AWG Conversion and Ampacity Reference Table

The following table merges physical metric/imperial conversions with the copper ampacity limits defined in NEC Table 310.16 (formerly 310.15(B)(16)). This data assumes copper conductors, an ambient temperature of 30°C (86°F), and not more than three current-carrying conductors in a raceway.

AWG Size Diameter (in) Diameter (mm) Area (kcmil) Ampacity 60°C (TW/UF) Ampacity 75°C (THW/THWN) Ampacity 90°C (THHN)
14 AWG0.06411.6284.1115A*20A*25A*
12 AWG0.08082.0536.5320A*25A*30A*
10 AWG0.10192.58810.3830A35A40A
8 AWG0.12853.26416.5140A50A55A
6 AWG0.16204.11526.2455A65A75A
4 AWG0.20435.18941.7470A85A95A
3 AWG0.22945.82752.6285A100A110A
2 AWG0.25766.54366.3695A115A130A
1 AWG0.28937.34883.69110A130A145A
1/0 AWG0.32498.252105.60125A150A170A
2/0 AWG0.36489.266133.10145A175A195A

*Ampacities shown for 14, 12, and 10 AWG are the physical wire capabilities, but per NEC 240.4(D), standard branch circuit overcurrent protection is capped at 15A, 20A, and 30A respectively.

Derating Factors: When the Base Chart Fails You

The ampacity numbers above assume ideal conditions: 30°C (86°F) ambient temperature and no more than three current-carrying conductors bundled together. When you stuff conduit or run wire through a hot attic, the wire cannot shed heat efficiently. You must apply derating multipliers to the 90°C column to find your new true ampacity.

Example: Bundling in a Hot Attic
You are pulling four current-carrying conductors (two 240V circuits sharing a neutral, or just two 120V circuits) through an attic where the ambient temperature hits 110°F (43°C). You want to use 12 AWG THHN for a 20A circuit.

  1. Base Ampacity: 12 AWG at 90°C = 30A.
  2. Bundling Derating: 4 to 6 conductors requires an 80% multiplier (30A × 0.80 = 24A).
  3. Temperature Derating: 110°F ambient requires an 87% multiplier for 90°C wire (24A × 0.87 = 20.88A).
  4. Final Termination Check: 20.88A is greater than your 20A load, and the final termination at the breaker is rated for 75°C (which allows 25A for 12 AWG). Result: 12 AWG THHN passes. If you had tried 14 AWG (base 25A × 0.80 × 0.87 = 17.4A), it would fail and create a fire hazard.

For comprehensive bundling and temperature tables, reference the Cerrowire official ampacity and derating charts, which map directly to NEC Article 310.

Wire Sizing Decision Tree

Stop guessing. Use this decision matrix to terminate your wire sizing process with a concrete material pick.

Application Scenario Load / Breaker Size Installation Method Concrete Wire Pick
Standard bedroom/living room 120V receptacles 15A / 15A Breaker Stapled to studs inside drywall 14/2 NM-B (Romex)
Kitchen small appliance or bathroom GFCI circuits 20A / 20A Breaker Stapled to studs inside drywall 12/2 NM-B (Romex)
Electric water heater or heavy window AC unit 30A / 30A Breaker Fished through walls or surface conduit 10/2 NM-B or Two 10 AWG THHN in 1/2' EMT
Electric range, oven, or 50A hot tub disconnect 40A-50A / 50A Breaker Conduit run to appliance/disconnect 6 AWG THHN/THWN-2 (Copper) in 3/4' EMT
100-Amp detached garage subpanel feeder 100A / 100A Breaker Underground PVC conduit (Schedule 80) 1/0 AWG THWN-2 (Copper) or 2/0 AWG XHHW-2 (Aluminum)
Pro-Tip for Subpanels: When pulling large gauge wire like 1/0 or 2/0 through conduit sweeps, use a high-quality wire pulling lubricant (like Polywater J). Dry pulling large AWG wire will strip the insulation and create micro-tears that lead to ground faults years down the line.

What This Chart Cannot Tell You

An AWG conversion chart gives you the thermal limits of the wire, but it ignores physics over distance. Keep these three blind spots in mind before buying your spools:

  1. Voltage Drop: The NEC recommends keeping voltage drop under 3% for branch circuits and 5% overall. If you are running a 120V circuit to a shed 150 feet away, 12 AWG wire will suffer a ~4.5% voltage drop under a full 15A load. The chart says 12 AWG is fine for 15A thermally, but your power tools will bog down. You must upsize to 10 AWG or 8 AWG purely for voltage drop mitigation on long runs.
  2. Aluminum vs. Copper: The table above is strictly for copper. If you are buying SER (Service Entrance Round) or XHHW aluminum wire for a subpanel feeder, aluminum has higher resistance and lower ampacity. As a rule of thumb, aluminum must be sized two AWG sizes larger than copper for the same ampacity (e.g., use 2 AWG aluminum where you would use 4 AWG copper).
  3. Local AHJ Overrides: Some local municipalities ban 14 AWG entirely, requiring 12 AWG as the minimum for all 15A and 20A residential branch circuits to reduce voltage drop and provide a margin of safety. Always verify with your local Authority Having Jurisdiction (AHJ) before rough-in.

De-energize the panel, verify the bus bars are dead with a non-contact voltage tester and a multimeter, and torque all terminal lugs to the manufacturer's inch-pound specifications. Sizing the wire correctly is only half the job; terminating it properly prevents the arcing and heating that cause electrical fires.