Wire Gauge (AWG) Calculator

Size copper or aluminum conductors from amperage, circuit length, and voltage — with voltage drop checked against your allowable limit to recommend the minimum AWG gauge that meets both ampacity and drop requirements.

Circuit Details

Amperage

Circuit Length

Voltage

Conductor Material

Max Voltage Drop %3%

1%5%

Live Results

Recommended AWG Gauge

12 AWG

Voltage Drop

3.19 V

Voltage Drop

2.65%

Amperage

20A

Circuit Length

50ft

Conductor Material

Copper

Load too high for small-gauge conductors. Larger wire required.

This is an estimation tool based on standard NEC resistivity and ampacity values. Always consult local electrical codes and a licensed electrician for final design.

How to Use This Wire Gauge Calculator

Enter the circuit amperage. Input the continuous load current in amperes — use the breaker rating or the nameplate FLA for motors and appliances. For general branch circuits, common values are 15 A for lighting, 20 A for kitchen and bathroom receptacles, and 30 A for dryers or water heaters on dedicated circuits.
Measure one-way circuit length. Enter the one-way distance in feet from the panel to the load, not the round-trip wire path. The calculator applies the standard factor of 2 for outbound and return conductors in the voltage drop formula.
Select system voltage. Choose 120 V for standard single-pole branch circuits or 240 V for large appliances, HVAC equipment, and double-pole circuits. Higher voltage reduces the percentage voltage drop for the same wire size and current.
Choose conductor material. Select copper for most branch circuits and short runs, or aluminum for larger feeders where cost savings matter. Aluminum requires proper terminations and is typically limited to 8 AWG and larger for branch-circuit work in many jurisdictions.
Set allowable voltage drop. Use 3% as the default for branch circuits per common NEC guidance (5% total from service to load). Increase the slider only when local code or engineering practice allows a higher drop — the recommended AWG updates instantly as you adjust inputs.

Formulas & Example

Voltage drop uses the standard single-phase formula with a factor of 2 for the outbound and return conductors. Resistivity is expressed in ohm-cmil per foot. The calculator scans AWG sizes from 14 through 4/0 and selects the smallest conductor that meets both ampacity and voltage drop limits.

Voltage Drop (V) = (2 × Circuit Length (ft) × Resistivity × Amperage) ÷ Circular Mil Area
Voltage Drop (%) = (Voltage Drop ÷ System Voltage) × 100

Resistivity (ohm-cmil/ft):
  Copper:    10.4
  Aluminum:  17.0

Required AWG = smallest gauge where:
  Ampacity (material) ≥ Amperage
  Voltage Drop % ≤ Max Allowable Drop %

Worked Example

A 20 A load on a 50 ft one-way run at 120 V with copper conductors and a 3% maximum voltage drop:

14 AWG: ampacity 15 A < 20 A → fails ampacity
12 AWG: ampacity 20 A ≥ 20 A ✓
  CMIL = 6,530
  Voltage Drop = (2 × 50 × 10.4 × 20) ÷ 6,530 = 3.19 V
  Voltage Drop % = (3.19 ÷ 120) × 100 = 2.66% ≤ 3% ✓

Recommended: 12 AWG copper

If the same circuit were 100 ft, 12 AWG would exceed 3% drop and the calculator would recommend 10 AWG. Always confirm ampacity tables, derating factors, and termination requirements against the current NEC and local amendments before pulling wire.

Frequently Asked Questions

What is AWG?▾

AWG stands for American Wire Gauge — a standardized wire sizing system where a smaller gauge number means a larger conductor diameter. For example, 10 AWG is thicker than 14 AWG and carries more current with less voltage drop. AWG sizes above 1/0 (one aught) continue as 2/0, 3/0, and 4/0 before transitioning to kcmil or MCM designations for very large conductors. Residential branch circuits most commonly use 14, 12, and 10 AWG copper.

What are the differences between copper and aluminum wire?▾

Copper has lower electrical resistance per unit volume, so it can be one or two AWG sizes smaller than aluminum for the same ampacity and voltage drop. Aluminum is lighter and less expensive per foot but requires larger conductors, anti-oxidant compound at terminations, and devices listed for aluminum (CO/ALR or AL-CU). Modern AA-8000 series aluminum alloy is used for feeders and service entrance conductors; small-gauge aluminum branch wire is generally discouraged in residential work.

What is an acceptable voltage drop?▾

The NEC recommends limiting voltage drop to 3% on branch circuits and 5% total from the service entrance to the farthest outlet for efficient equipment operation. Excessive drop causes dimming lights, motor overheating, and reduced appliance performance. Sensitive electronics and long outdoor runs often benefit from sizing to 2% or less even when 3% is technically acceptable.

How does circuit length affect wire size?▾

Voltage drop is directly proportional to circuit length because resistance accumulates over the run. Doubling the one-way distance doubles the voltage drop, which may require upsizing one or two AWG sizes. This is why a 20 A garage subpanel 150 ft from the main panel needs much larger wire than a 20 A kitchen receptacle 25 ft away — length is often the limiting factor rather than ampacity alone.

Can I use aluminum wire for branch circuits?▾

Aluminum branch-circuit wiring (12 and 10 AWG) was common in the 1960s–70s and is associated with connection failures when not properly terminated. Current practice favors copper for 14, 12, and 10 AWG branch circuits. Aluminum is widely accepted for feeders, service entrance conductors, and large appliances at 8 AWG and above when installed with listed connectors, torque specifications, and anti-oxidant paste. Always follow your local AHJ and the current NEC edition.

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