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100 Amp Wire Size Calculator

Sizing electrical conductors for 100-amp circuits requires balancing safety, code compliance, and efficiency. Wire too small causes dangerous voltage drop and heat buildup; wire too large wastes material and cost. Engineers, electricians, and facility managers use this calculator to determine the minimum cross-sectional area and AWG gauge needed based on system voltage, conductor material, run distance, and ambient temperature.

Last updated:

Creators Steven Wooding, BSc Physics
6,219 people find this calculator helpful

Wire Size Calculation Formula

The cross-sectional area of a conductor carrying 100 amps depends on four primary factors: the resistivity of the material (adjusted for operating temperature), the one-way distance from source to load, the system configuration (single-phase or three-phase), and the allowable voltage drop. Most electrical codes permit a maximum 3–5% drop to prevent equipment malfunction.

A = (2 × ρ × L × φ) ÷ (V × %drop)

  • A — Wire cross-sectional area in square millimetres
  • ρ — Electrical resistivity of conductor material (Ω·mm²/m), adjusted for temperature
  • L — One-way cable run distance in metres
  • φ — Phase factor: 1 for single-phase, √3 (≈1.732) for three-phase systems
  • V — Source voltage in volts
  • %drop — Allowable voltage drop as a decimal (e.g., 0.05 for 5%)

How to Use the 100 Amp Wire Sizing Tool

Start by confirming your system operates at 100 amperes of alternating current. Select the electrical configuration—single-phase or three-phase—which affects the phase factor in the formula. Enter the nominal source voltage (e.g., 120 V, 208 V, 240 V, 277 V, 480 V depending on your installation).

Next, choose your conductor material: copper and aluminium are standard, each with different resistivity values. Specify the maximum operating temperature the wire will experience under load—this affects resistivity due to the temperature coefficient of the material. Define your acceptable voltage drop (typically 3% for branch circuits, 5% for feeder runs combined with branch circuits).

Finally, measure the one-way distance from the power source to the furthest load point. The calculator will return the minimum wire cross-sectional area in mm² and the corresponding AWG (American Wire Gauge) size. Always round up to the next standard AWG if your result falls between gauges.

Why Temperature and Material Matter

Electrical resistivity increases with temperature. A copper conductor at 60 °C has roughly 20% higher resistance than the same wire at 20 °C, due to increased atomic vibration impeding electron flow. This effect is quantified by the temperature coefficient of resistance. Aluminium exhibits even greater temperature sensitivity than copper.

Copper conducts about 60% better than aluminium at the same cross-section, making it the preferred choice for high-current applications despite higher cost. If you use aluminium, you will need a larger wire gauge to carry the same current with equivalent safety margins.

The calculator accounts for temperature derating—wires do not carry their nameplate ampacity at elevated temperatures. A 2 AWG copper conductor might safely carry 95 A at 20 °C, but only 75 A at 60 °C. Always consult local electrical codes (NEC, IEC, etc.) for your region's derating tables.

Critical Considerations for 100 Amp Installations

Several overlooked factors can compromise safety and performance in high-current circuits.

  1. Voltage drop accumulates over distance — A 5% total drop allowed by code may already include branch circuits downstream of your feeder. A 100 A feeder over 150 feet (45 m) at 240 V with a 3% drop limit requires about 2/0 AWG copper. If you then add branch circuits, you risk exceeding code limits. Always calculate the entire circuit path.
  2. Temperature derating is mandatory, not optional — Ambient temperature and conduit temperature rise (due to conductor losses) both derate ampacity. A wire in a hot attic or buried in insulation carries far less current safely than the same gauge in free air. Check NEC Table 310.15(B)(2) or equivalent in your code for derating factors.
  3. Resistance changes at high frequencies — AC impedance (not just DC resistance) governs voltage drop in AC circuits. This is especially critical for three-phase systems and very long runs. The calculator uses the correct phase factor, but confirm your frequency is 50 or 60 Hz, as certain industrial equipment may differ.
  4. Mechanical strength and fault current — Larger conductors are mechanically stronger during installation and less prone to vibration damage. They also better withstand short-circuit heating. A 6 AWG wire might carry 100 A at normal load but fail catastrophically in a fault scenario. Size for both continuous load and available fault current in your system.

Real-World Example: Three-Phase 480 V Feeder

A factory receives three-phase power at 480 V and must deliver 100 A to a motor control centre 200 feet (61 m) away. Using copper (resistivity 0.0171 Ω·mm²/m), a maximum wire temperature of 60 °C, and a 5% voltage drop allowance:

  • Phase factor φ = √3 ≈ 1.732 (three-phase)
  • Allowable drop = 0.05 × 480 = 24 volts
  • Required area A = (2 × 0.0171 × 61 × 1.732) ÷ (480 × 0.05) ≈ 7.6 mm²

This corresponds to roughly 10 AWG copper. However, always verify that the wire's ampacity at 60 °C (derating from the 90 °C nameplate rating) meets or exceeds 100 A. In this case, 10 AWG copper is rated 55 A at 60 °C—too small. You would need to use 6 AWG or larger, even though voltage drop alone permits smaller gauge. This illustrates why the calculator's systematic approach prevents dangerous undersizing.

Frequently Asked Questions

What size wire should I use for a 100-amp, 240-volt single-phase circuit running 150 feet?

For a single-phase 240 V circuit over 150 feet with a 3% voltage drop limit, you would require approximately 1 AWG copper or 0 AWG aluminium. If the ambient temperature is 40 °C or higher, apply derating factors from your electrical code—you may need to go larger. Always verify the wire's ampacity at the actual operating temperature exceeds 100 A.

Why does three-phase wiring allow smaller conductors than single-phase for the same amperage?

Three-phase systems distribute current across three conductors instead of two, and the phase factor √3 (1.732) mathematically reduces the voltage drop for the same cross-sectional area. A 480 V three-phase feeder carrying 100 A over 200 feet will have less absolute voltage drop than a 240 V single-phase feeder at the same distance and wire gauge, because both the voltage is higher and the phase configuration is more efficient.

How does operating temperature affect the wire size I need to select?

Higher wire temperature increases electrical resistivity, which raises voltage drop and heat generation. A wire calculated at 20 °C will have about 20% more resistance if operating at 60 °C. This means the same amperage requires a larger cross-sectional area at higher temperatures. Additionally, the wire's safe ampacity (maximum current it can carry) is derated by local code tables—for example, a 2 AWG copper at 90 °C might be derated to 75% of its nameplate rating at 60 °C operation.

Can I use aluminium instead of copper for a 100-amp feeder?

Yes, but you must use a larger gauge. Aluminium's conductivity is about 60% that of copper, so the same feeder would require roughly one or two sizes larger (e.g., 1 AWG copper becomes 0 AWG or 2/0 AWG aluminium). Aluminium also oxidises at connection points, requiring special terminals and periodic maintenance. Local code may restrict aluminium for certain applications, so verify before purchasing.

What does a 5% voltage drop limit mean, and why is 3% sometimes recommended instead?

Voltage drop is the reduction in voltage from source to load, expressed as a percentage. A 5% drop on a 240 V circuit equals 12 volts at the load. Electrical equipment typically operates within ±10% of rated voltage. A 5% drop on the feeder leaves only 5% for branch circuits before equipment may malfunction. Most codes allow 5% total; hence 3% on feeder and 2% on branch circuits is the common design practice for critical loads.

Do I need different calculations for DC and AC circuits?

Yes. This calculator addresses AC circuits, where the phase factor and frequency affect impedance. DC circuits use pure resistance (no reactance), so the formula simplifies to A = (2 × ρ × L) ÷ (V × %drop), with phase factor φ = 1. For DC applications, use a dedicated DC wire sizing tool or adjust the phase factor manually.

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