Wire Gauge Calculator — AWG, Current & Resistance
Look up AWG wire gauge specs: current-carrying capacity (ampacity), resistance per foot, voltage drop, and cross-sectional area. Calculate wire size from current load and find safe circuit wire length.
AWG 12
Kitchen outlets, 20A branch circuits
| Diameter | 2.050 mm / 0.0807 in |
| Cross-section area | 3.310 mm² / 6,532 cmil |
| Resistance (copper) | 1.588 Ω/1000 ft / 5.210 Ω/1000 m |
| Ampacity (copper, in conduit) | 20 A |
| NEC Table | NEC 310.15(B)(16), 60°C column (consult local code) |
AWG Reference Table (Copper)
| AWG | Diam (mm) | Area (mm²) | Res. (Ω/1000ft) | Ampacity (A) |
|---|---|---|---|---|
| 4/0 (0000) | 11.680 | 107.200 | 0.049 | 230 |
| 3/0 (000) | 10.400 | 85.000 | 0.062 | 200 |
| 2/0 (00) | 9.270 | 67.400 | 0.078 | 175 |
| 1/0 (0) | 8.250 | 53.500 | 0.098 | 150 |
| 1 | 7.350 | 42.400 | 0.124 | 130 |
| 2 | 6.540 | 33.600 | 0.156 | 115 |
| 3 | 5.830 | 26.700 | 0.197 | 100 |
| 4 | 5.190 | 21.200 | 0.248 | 85 |
| 6 | 4.110 | 13.300 | 0.395 | 65 |
| 8 | 3.260 | 8.370 | 0.628 | 50 |
| 10 | 2.590 | 5.260 | 0.999 | 30 |
| 12 | 2.050 | 3.310 | 1.588 | 20 |
| 14 | 1.630 | 2.080 | 2.525 | 15 |
| 16 | 1.290 | 1.310 | 4.016 | 13 |
| 18 | 1.020 | 0.823 | 6.385 | 10 |
| 20 | 0.812 | 0.518 | 10.150 | 5 |
| 22 | 0.644 | 0.326 | 16.140 | 3 |
| 24 | 0.511 | 0.205 | 25.670 | 2.1 |
Ampacity: NEC Table 310.15(B)(16), copper in conduit, 60°C, 30°C ambient. Aluminum values adjusted by 0.77×. Consult a licensed electrician — local codes may vary.
What Is the Wire Gauge Calculator — AWG, Current & Resistance?
The Wire Gauge Calculator provides three modes covering the most common electrical wire sizing tasks. Mode 1 (Gauge Lookup) shows all NEC-referenced specifications for any AWG size. Mode 2 (Current Calculator) recommends the minimum safe wire gauge for a given load current and circuit length. Mode 3 (Voltage Drop) calculates the actual voltage drop percentage for a specific wire run, letting you verify compliance with the NEC recommended limit of 3% for branch circuits and 5% combined for feeders plus branch circuits.
- ›AWG gauge lookup — instantly see diameter, cross-sectional area, resistance per 1000 ft, and ampacity (current-carrying capacity) for AWG 4/0 down to AWG 24.
- ›Copper and aluminum toggle — aluminum wire has approximately 1.64× higher resistance than copper and lower ampacity; the calculator adjusts both figures automatically.
- ›Minimum gauge recommendation — enter amps, circuit length, and voltage to find the smallest gauge that stays within your target voltage drop percentage.
- ›NEC 3% guideline check — results clearly flag whether your voltage drop exceeds the recommended 3% threshold for branch circuits.
- ›Common use cases — for each AWG, typical residential and commercial applications are listed so you can sanity-check a gauge selection.
Formula
Voltage Drop
V_drop = (2 × I × L × R_per_ft) / 1000
V_drop% = (V_drop / V_source) × 100
Resistance Method (Circular Mils)
V_drop = (2 × K × I × L) / A_cmil
K = 12.9 Ω·cmil/ft (copper) · K = 21.2 Ω·cmil/ft (aluminum)
| Symbol | Name | Description |
|---|---|---|
| I | Current | Load current in amperes (A) |
| L | One-way length | Distance from panel to load in feet (the factor of 2 accounts for the return conductor) |
| R_per_ft | Resistance per foot | From the AWG table (Ω per 1000 ft ÷ 1000) |
| V_source | Source voltage | Nominal system voltage (e.g. 120V, 240V) |
| A_cmil | Area in circular mils | 1 circular mil = (π/4) × 10⁻⁶ in²; 1 mm² ≈ 1973 cmil |
| K | Resistivity constant | 12.9 Ω·cmil/ft for copper; 21.2 for aluminum |
How to Use
- 1Select a mode: Gauge Lookup to see specs, Current Calculator to find minimum wire size, or Voltage Drop to check an existing installation.
- 2Choose wire material: Select Copper (default) or Aluminum. Aluminum is common for large service entrance conductors and feeders but is not permitted for branch circuits under 8 AWG in most jurisdictions.
- 3Enter your values: For Gauge Lookup enter an AWG number. For Current Calculator enter load amps, one-way circuit length, and target max voltage drop %. For Voltage Drop enter AWG, current, length, and source voltage.
- 4Press Calculate or Enter: Results appear immediately. For Current Calculator, the recommended gauge and next-size-up are both shown.
- 5Check the NEC flag: A clear indicator shows whether the calculated voltage drop is within the NEC 3% recommendation for branch circuits.
- 6Verify with a licensed electrician: This calculator is a planning and educational tool. All electrical work must comply with local codes and be performed or inspected by a licensed electrician.
Example Calculation
Scenario: 20-amp 120V circuit, 75 ft one-way run, copper wire.
The 3% Rule Explained
NEC Article 210.19(A) informational note recommends branch circuit conductors be sized to limit voltage drop to 3% or less. For longer runs the NEC recommends 5% total (feeder + branch combined). A 4% voltage drop on a 120V circuit means the device at the end of the run sees only 115.2V — acceptable for many loads but can cause heating, reduced motor efficiency, and tripped GFCI breakers.
Understanding Wire Gauge — AWG, Current & Resistance
Understanding American Wire Gauge (AWG)
The American Wire Gauge system is the standard for specifying conductor diameter in North America. The scale is logarithmic and inverse — every 6-gauge increase halves the cross-sectional area, and every 3-gauge increase halves the diameter. This means AWG 10 has half the cross-sectional area of AWG 7, and about twice the resistance per unit length.
The largest commonly used sizes are designated 4/0 (four-ought, also written 0000), 3/0, 2/0, and 1/0 for very large service entrance and feeder conductors. Above 4/0, conductors are sized in kcmil (thousands of circular mils) rather than AWG.
Why Voltage Drop Matters
Voltage drop is not just an efficiency concern — it affects equipment performance and safety. Electric motors running at reduced voltage draw more current to maintain output power, which increases heating and shortens motor life. LED drivers and sensitive electronics may behave erratically below their rated minimum voltage. For long circuit runs in large homes, workshops, outbuildings, and commercial spaces, upsizing the wire gauge is the most reliable way to maintain voltage within acceptable limits.
The NEC 3% recommendation provides a practical design target. For critical loads (medical equipment, audio/video systems, CNC machinery), aim for 1–2% or less. For non-critical general-purpose circuits, up to 5% is usually acceptable, especially if load is intermittent.
Copper vs. Aluminum Conductors
- ›Resistivity: Copper = 1.72 × 10⁻⁸ Ω·m; Aluminum = 2.82 × 10⁻⁸ Ω·m. Aluminum is about 1.64× more resistive than copper for the same cross-section.
- ›Weight: Aluminum is about 70% lighter than copper by weight for equivalent current capacity, making it preferred for overhead transmission lines and large feeders.
- ›Cost: Aluminum conductors are significantly less expensive than copper, an important factor for large feeder runs and service entrance cables.
- ›Connection requirements: Aluminum requires anti-oxidant compound and aluminum-rated connectors/terminals to prevent oxidation-caused resistance increase at connections.
- ›Code restrictions: Most jurisdictions restrict aluminum to 8 AWG and larger. Check local codes before specifying aluminum for any circuit.
Common AWG Applications
- ›AWG 14: 15-amp branch circuits; lighting, general-purpose outlets.
- ›AWG 12: 20-amp branch circuits; kitchen outlets, bathroom GFCI circuits, workshop outlets.
- ›AWG 10: 30-amp circuits; clothes dryers, water heaters, window air conditioners.
- ›AWG 8: 40–50-amp circuits; electric ranges, EV chargers (Level 2).
- ›AWG 6: 60-amp subpanel feeders; large HVAC units.
- ›AWG 4 and larger: Main service entrance conductors, large subpanels, commercial HVAC.
Frequently Asked Questions
What is AWG and how does the gauge number work?
AWG stands for American Wire Gauge — the North American standard for conductor diameter. The scale is counterintuitive:
- ›A smaller AWG number = a thicker, larger wire.
- ›AWG 4/0 (the largest common size) has a diameter of 11.68 mm.
- ›AWG 24 is only 0.511 mm — thin enough for small signal wiring.
- ›The scale originated from the number of drawing dies used to reach a given diameter. More draws = thinner wire = higher gauge number.
Every 6-gauge increase halves the cross-sectional area; every 3-gauge increase halves the diameter.
What is ampacity and why does it depend on installation method?
Ampacity is the maximum continuous current a conductor can carry without exceeding its insulation temperature rating. It varies based on:
- ›Wire size: Larger conductors dissipate heat more effectively and carry more current.
- ›Insulation type: 60°C, 75°C, and 90°C rated insulations have different temperature limits.
- ›Ambient temperature: Higher surrounding temperatures reduce how much heat the wire can shed.
- ›Bundling: Multiple conductors in one conduit or bundled together trap heat and reduce ampacity.
This calculator uses NEC Table 310.15(B)(16) values for copper in conduit (60°C column) — a conservative, widely-applicable reference.
When should I use aluminum wire?
Aluminum conductors have legitimate uses, but not everywhere:
- ›Good uses: Service entrance cables (SEU), feeder conductors, and large branch circuits 8 AWG and larger. Aluminum is lighter and cheaper than copper at these sizes.
- ›Avoid for: Small branch circuits (10 AWG and smaller). Aluminum oxidises at connections, has higher resistance, and requires anti-oxidant compound and aluminum-rated terminals.
- ›Size up: When substituting aluminum for copper, go up 2 AWG sizes (e.g., replace 10 AWG copper with 8 AWG aluminum) to match ampacity.
What voltage drop percentage is acceptable?
The NEC provides two voltage drop guidelines:
- ›3% maximum on any single branch circuit or feeder — the common design target.
- ›5% maximum total from the service panel to the final outlet — the sum of feeder and branch drops.
These are recommendations, not hard inspection requirements. That said, exceeding 5% causes real problems:
- ›Electric motors draw more current at low voltage, running hotter and wearing faster.
- ›LED drivers and switching power supplies may malfunction below rated minimum voltage.
- ›For critical loads (medical, audio/video, CNC), target 1–2% or less.
Why does wire resistance increase with length?
Resistance is directly proportional to length: R = ρL/A, where ρ is resistivity, L is length, and A is cross-sectional area.
- ›Every additional foot of wire adds a small but real resistance.
- ›In a circuit, current travels to the load AND back — so the effective conductor length is 2× the one-way distance. This is why voltage drop formulas use 2L.
- ›A 100-ft run actually involves 200 ft of conductor.
- ›Long, thin wire runs (e.g., AWG 14 at 150 ft) can easily exceed the 3% voltage drop guideline at full load.
What is the difference between wire gauge and cable size?
These terms are related but distinct:
- ›Wire gauge (AWG) refers to the diameter of a single conductor.
- ›Cable is a bundle of multiple conductors with a common outer jacket. A 12/2 cable contains two 12 AWG conductors plus a bare ground wire.
- ›Notation: "12/2 NM-B" means 12 AWG, 2 current-carrying conductors, non-metallic sheathed (NM), type B jacket. Used for standard 20A 120V household circuits.
When ordering materials, always specify both the AWG and the conductor count (e.g., "14/3" for a 15A 240V circuit).
How do I calculate the circular mils area?
Circular mils (cmil) is the area unit used in AWG voltage drop formulas:
- ›1 circular mil = the area of a circle with a diameter of 1 mil (0.001 inch).
- ›Formula: A_cmil = d² where d is the diameter in mils.
- ›Conversion: 1 mm² ≈ 1,973.5 cmil.
- ›AWG 12 (2.05 mm diameter) has an area of ≈ 6,530 cmil.
The calculator handles all circular mil conversions internally — you never need to compute this manually when using Mode 3 (voltage drop).
Is this calculator suitable for 3-phase circuits?
This calculator covers single-phase 2-wire circuits only. For 3-phase circuits, the voltage drop formula differs:
- ›Single-phase: V_drop = (2 × I × L × R_per_ft) / 1000
- ›3-phase balanced: V_drop = (√3 × I × L × R_per_ft) / 1000
- ›The same AWG ampacity and resistance data applies to both — only the multiplier changes.
- ›Always consult the NEC (Article 215, 310) and a licensed electrical engineer for 3-phase feeder and distribution designs.