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Wire Size Calculator

Wire Size Calculator

Calculate minimum wire gauge (AWG) based on voltage drop requirements

100% FreeNEC CompliantVoltage Drop Analysis
Electrical Wire Sizing
Enter circuit parameters to find minimum wire size

NEC recommends 3% for branch circuits

Voltage Drop Theory

Ohm's Law Application:

V=I×RV = I \times R

Voltage drop occurs due to wire resistance

Wire Resistance Formula:

R=K×LCMR = \frac{K \times L}{CM}
  • • K = resistivity (12.9 for copper at 75°C)
  • • L = one-way distance in feet
  • • CM = circular mil area

Phase Multipliers:

  • • Single-phase: 2 (accounts for return path)
  • • Three-phase: 1.732 (√3 factor)
NEC Code Requirements

Voltage Drop Limits

  • • Branch circuits: 3% maximum
  • • Feeders: 3% maximum
  • • Total system: 5% maximum
  • • Critical loads: 2% recommended

Wire Selection Criteria

  • • Ampacity must exceed circuit current
  • • Voltage drop within acceptable limits
  • • Appropriate insulation rating
  • • Environmental considerations

Installation Factors

  • • Ambient temperature derating
  • • Conduit fill considerations
  • • Proper termination methods
  • • Code compliance verification
AWG Wire Size Reference
AWGCircular MilsDiameter (in)Ampacity (A)Weight (lbs/1000ft)
144,1100.0642012.43
126,5300.0812519.77
1010,3800.1023531.43
816,5100.1285050.97
626,2400.1626579.46
441,7400.20485125.1
266,3600.258115197.4
183,6900.289130248.1
1/0105,6000.325150312.1
2/0133,1000.365175392.8
3/0167,8000.410200494.1
4/0211,6000.460230621.9

* Ampacities based on 75°C conductor rating in raceway

Authoritative Wire Sizing Resources

NEC Wire Sizing Tables

National Electrical Code official ampacity and wire sizing standards

Cerrowire AWG Guide

American Wire Gauge reference tables and conductor specifications

EE Portal

Electrical Engineering Portal wire sizing calculations and guides

Frequently Asked Questions

How do I calculate wire size?
Wire size depends on current, distance, and acceptable voltage drop. CM = (2 × K × I × D) / VD, with K = 12.9 for copper and 21.2 for aluminum. Pick the next larger AWG with that area.
What is acceptable voltage drop?
NEC recommends ≤ 3% for branch circuits and ≤ 5% total (feeder + branch). Sensitive equipment may require 2-3%. Higher drops waste energy and can damage motors.
Copper or aluminum?
Copper has lower resistance and is preferred for short runs and standard branch circuits. Aluminum is cheaper for long feeder and service conductors but needs anti-oxidant paste and proper terminations.
How does wire size affect ampacity?
Larger wires carry more current without overheating. NEC Table 310.16 gives ampacities per AWG at standard insulation temperatures. Derating applies for conduit fill and ambient temperature.
What if calculated AWG is huge?
For long runs with large loads, split into parallel feeds, upsize the breaker, or increase voltage (e.g. 240 V instead of 120 V). Consult an electrician for any critical feeder design.
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