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Wire Size Calculator (AWG & mm²)

Calculate recommended minimum conductor gauge and cross-sectional area to keep voltage drop within allowable limits.

V
A
°C

For NEC 310.15(B)(1) correction. 30°C is the table baseline.

Ampacity follows the NEC procedure: Table 310.16 ampacity corrected for ambient temperature and conductor count, capped by the termination rating and the 240.4(D) small-conductor limits, then checked against your voltage-drop target. For continuous loads, enter the load current multiplied by 1.25.

Recommended Conductor Gauge

Recommended AWG Size
10 AWG

Nominal Area: 5.26 mm² (drop alone would need 3.99 mm²)

Allowable Ampacity (Derated)
30A

Table 310.16 ampacity × 1 ambient × 1 conductor-count, capped by termination rating

Estimated Voltage Drop
2.7338 V (2.2781%)
Voltage at Load
117.266V
Target Max Allowed Drop
3.6V

Step-by-Step Engineering Calculation

  1. 1Load: I=15 AI = 15\text{ A} at 120 V120\text{ V}, one-way length L=75.0 ftL = 75.0\text{ ft}, COPPER conductors.
  2. 2Ampacity derating (NEC 310.15): ambient 30°C correction Famb=1F_{\text{amb}} = 1 (90°C column) × conductor-count adjustment Fccc=1F_{\text{ccc}} = 1 (3 current-carrying) = combined factor 1.000\mathbf{1.000}.
  3. 3Termination limit (NEC 110.14(C)): allowable ampacity is capped at the 75°C column of Table 310.16, and at the NEC 240.4(D) small-conductor limits where they apply.
  4. 4Voltage drop limit: 3% of 120 V=3.600 V3\%\text{ of }120\text{ V} = \mathbf{3.600\text{ V}}, checked with NEC Chapter 9 Table 8 resistance at 75°C. (Drop alone would need at least 3.99 mm².)
  5. 5Selected conductor: 10 AWG (5.26 mm²). Allowable ampacity after derating and caps: 30.0 A≥15 A\mathbf{30.0\text{ A}} \ge 15\text{ A} ✓. Actual voltage drop: 2.73 V\mathbf{2.73\text{ V}} (2.28%) ✓.
  6. 6Voltage at load: Vload=120 V−2.73 V=117.27 VV_{\text{load}} = 120\text{ V} - 2.73\text{ V} = \mathbf{117.27\text{ V}}

Formula & Theory

A_min = (Multiplier × ρ × L × I) / ΔV_max

Calculates the minimum required cross-sectional conductor area (mm² and AWG) to ensure circuit voltage drop stays below the target threshold.

Variable Definitions
A_min:
Minimum Area (mm² / kcmil) – Minimum required cross-sectional conductor area
AWG:
Gauge (AWG) – Recommended American Wire Gauge
ΔV_max:
Max Drop (Volts (V)) – Maximum allowable voltage drop

Worked Engineering Example

24V Off-Grid Solar Battery Cable Run

A 24V DC inverter circuit pulling 40A over 20 feet with a 2% allowed voltage drop.

Inputs:
  • Voltage: 24 V
  • Current: 40 A
  • Length: 20 ft
  • Max Drop: 2%
Result: Recommended: 4 AWG Copper (Actual Drop: 1.66%, 0.40V)

Key Assumptions

  • •Pure conductor resistance sizing based on allowed voltage drop.

Limitations & Safety Scope

  • •Thermal ampacity derating (insulation class 60°C/75°C/90°C, ambient temperature, conduit fill) must be verified against NEC Article 310 or IEC 60364 tables.

Frequently Asked Questions

How do I choose between 3% and 5% voltage drop?

3% is standard for branch circuits powering sensitive electronics, appliances, and lighting. 5% is the standard total maximum for the entire run from utility entrance to final load.

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Educational & Reference Notice: This calculator provides technical estimates based on standard physical formulas. Actual electrical installation, protective device sizing, and conductor selection must be verified against the applicable electrical code (NEC, IEC, CEC) by a licensed electrician or qualified professional engineer.