Does this calculator replace the National Electrical Code?

No. This tool gives a general estimate based on simplified, published ampacity and resistance values. Always verify sizing against the current NEC (or local equivalent) and a qualified electrician before installation.

Wire Size & Ampacity Calculator

Calculate the recommended American Wire Gauge (AWG), continuous ampacity, and expected voltage drop based on circuit load, voltage, and conductor distance.

Unit System:

1. Circuit Parameters NEC Table 310.16 Reference

Current / Load

System Voltage

Cable Length (One-Way)

Conductor Material

Current Type

Engineering estimate only: Sizing follows published NEC reference values (Table 310.16 & Ch. 9 Table 8). Always verify conduit fill, termination temperature ratings, and local electrical codes with a licensed professional before installation.

Advanced Options & Derating (Ambient Temp, Parallel Runs, Drop Limit)

Length Entered Above Is

Ambient Temperature

°C

Conductor Insulation

Installation Method

Max Voltage Drop Target

Parallel Conductors Per Leg

Voltage Drop Curve vs. Wire Gauge

Conductor Comparison Table

How Wire Sizing Works

Wire sizing balances two core criteria: safety (thermal ampacity) and performance (voltage drop):

Ampacity check: The conductor must carry the full design current without overheating the insulation. Ambient temperatures above 30°C reduce rated capacity via NEC correction factors.
Voltage drop limit: Resistive losses over distance cause voltage to sag. Excessive drop causes lights to flicker, motors to stall, and electronics to malfunction.

The standard single-phase two-way drop formula is:

V_drop = (2 × Length × Current × Resistance) / 1000

Where Length is the one-way run in feet, Current is in amperes, and Resistance is conductor DC resistance in ohms per 1,000 feet from NEC Chapter 9, Table 8.

Worked Example

For a 20 A, 120 V circuit running 50 ft with copper conductor: 10 AWG copper has a resistance of 1.24 Ω/kft. The voltage drop is 2 × 50 × 20 × 1.24 / 1000 = 2.48 V, which is 2.07% of 120 V (safely below the 3% branch circuit threshold). The 10 AWG 90°C ampacity of 40 A provides ample thermal headroom.

Technical Assumptions

Standard ampacities are referenced from the 90°C column of NEC Table 310.16.
Conductor resistance uses uncoated DC copper and aluminum values from NEC Chapter 9 Table 8.
Temperature derating above 30°C applies standard correction multipliers. Conduit fill derating for more than 3 current-carrying conductors is not automatically bundled.

Frequently Asked Questions

What wire gauge do I need for a given current and distance?

The required gauge depends on both conductor ampacity and circuit length. Even if 12 AWG handles 20 A safely for heat, runs over 60 feet will typically require 10 AWG or 8 AWG to avoid excessive voltage drop.

What voltage drop percentage is acceptable?

NEC Informational Note 210.19(A) recommends keeping voltage drop under 3% for branch circuits and under 5% for the combined feeder and branch circuit.

Should I choose Copper or Aluminum?

Copper has lower electrical resistance per gauge size and is standard for small branch circuits (14 AWG to 10 AWG). Aluminum is widely used for larger subpanel feeders and service entrances (4 AWG and larger) due to cost and weight advantages.