Wire Size Calculator: How to Choose the Right AWG for Any Circuit
Learn how to select the correct wire gauge (AWG) based on ampacity, run length, and NEC requirements. Includes the standard wire-to-breaker pairing table and when to upsize.
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How Wire Gauge Is Selected
Electrical wire size (AWG — American Wire Gauge) is determined by two separate criteria, and the wire must satisfy both:
- Ampacity — the wire must carry the full circuit current without overheating
- Voltage drop — the voltage at the end of the run must stay within acceptable limits (NEC recommends ≤3% for branch circuits)
A wire that satisfies ampacity but produces excessive voltage drop needs to be upsized. A wire that satisfies voltage drop but is too small for the breaker is a code violation.
Standard AWG Ampacity Table (Copper, 60°C/75°C Rating)
Most residential conductors use the 75°C column for sizing unless the device terminals specify otherwise.
The 80% Continuous Load Rule
NEC 210.20 requires circuit breakers to operate continuously at no more than 80% of their rated ampacity when the load runs for 3 or more hours.
Practical effect: A 20A circuit should carry no more than 16A of continuous load (20 × 0.8). For a circuit that runs HVAC, office equipment, or lighting for extended periods, add 25% to your load when selecting wire size.
Example: A commercial office lighting circuit draws 18A. Since lighting runs continuously, the design load is 18 × 1.25 = 22.5A → use 12 AWG with a 30A breaker, or 10 AWG with a 30A breaker to also satisfy voltage drop on a long run.
NEC Wire-to-Breaker Pairing Requirements
NEC Article 240.4(D) sets maximum overcurrent protection for small conductors:
Installing a 20A breaker on 14 AWG wire is a code violation — the wire can overheat before the 20A breaker trips. The breaker must be sized to protect the wire, not the load.
When to Upsize for Voltage Drop
For runs over 100 feet, check voltage drop separately. The rule of thumb: upsize one AWG for runs of 75–150 feet, two AWG for runs over 150 feet, compared to the ampacity minimum.
Example: A 20A circuit (normally 12 AWG) running 120 feet produces: - Voltage drop: 2 × 20A × 1.98 Ω/1000ft × 120ft ÷ 1000 = 9.5V = 7.9% on a 120V circuit — well over the 3% NEC recommendation - Fix: use 10 AWG → drop = 2 × 20 × 1.24 × 120 ÷ 1000 = 5.95V = 5.0% — borderline, consider 8 AWG for a clean result
Three-Phase Circuits
For three-phase circuits, the voltage drop formula changes:
Vd = √3 × K × I × L ÷ CM (or approximately 1.732 × single-phase formula)
The √3 factor (1.732) appears because three-phase current is distributed across three conductors in a balanced configuration. Three-phase circuits experience lower voltage drop per conductor than equivalent single-phase circuits.
How the Calculator Works
The calculator takes your current draw (amps), one-way run length (feet), voltage (120V or 240V), phase (single or three), and optionally wire material (copper or aluminum), then outputs:
- Minimum AWG for ampacity
- Minimum AWG for voltage drop compliance (≤3% NEC recommendation)
- The larger of the two recommendations as the required wire size
Always confirm your local AHJ (authority having jurisdiction) requirements, as some localities adopt NEC amendments.
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Frequently Asked Questions
- What wire size do I need for a 20-amp circuit?
- 12 AWG copper wire is required for a 20-amp branch circuit per NEC 240.4(D). For runs over 75–100 feet, check voltage drop and upsize to 10 AWG if the drop exceeds 3%. Never use 14 AWG on a 20A breaker — it's a code violation.
- What wire size is needed for a 30-amp circuit?
- 10 AWG copper is the minimum for a 30-amp circuit. Common uses: electric dryers (240V, 30A), window air conditioners, water heaters. For runs over 50 feet, check voltage drop and consider 8 AWG.
- Can I use a larger breaker with the same wire size?
- No — NEC 240.4(D) requires the breaker to protect the wire, not the load. Using a 30A breaker with 12 AWG wire (rated for 20A) allows the wire to overheat to unsafe levels before the breaker trips. Upsize the wire to match the breaker, not the other way around.
- What's the difference between 60°C and 75°C wire ratings?
- The temperature rating determines how much current the wire can carry without the insulation degrading. Most modern residential wiring is rated 60°C or 75°C. Use the 75°C column when connecting to devices with 75°C-rated terminals (marked on the device). Breakers and panels rated 'AL75/CU' can use the 75°C column.
- When do I need to upsize wire for voltage drop?
- For any run over 75 feet at 120V or 150 feet at 240V, run the voltage drop calculation. Upsize by one AWG from the ampacity minimum for runs of 75–150 feet, and by two AWG for runs over 150 feet. This applies especially to subpanels, outdoor workshops, and detached garages.
Last updated 7/28/2026