Electrical Wire Calculator

About the Electrical Wire Calculator

The Electrical Wire Calculator helps you determine wire resistance, voltage drop, current-carrying capacity, and power dissipation for any conductor. Whether you're wiring a residential circuit, designing an industrial panel, or planning a low-voltage DC system, proper wire selection is critical for safety and efficiency.

This calculator uses the American Wire Gauge (AWG) standard and handles both copper and aluminum conductors. It factors in wire length, temperature, and insulation type to estimate wire sizing in a way that stays aligned with NEC and IEC assumptions. Understanding the relationship between wire size, resistance, and voltage drop prevents overheated conductors, tripped breakers, and wasted energy.

Enter your wire parameters to see resistance per foot, total resistance, voltage drop percentage, maximum safe current, and estimated power loss. The calculator also shows a comparison table of common wire gauges so you can evaluate trade-offs between cost and performance. It gives you a practical check before you pick a conductor size. That makes it easier to size the run before you pull cable.

When This Page Helps

Use this calculator when you want to see what a chosen wire gauge actually does in terms of resistance, voltage drop, and power loss before you commit to it. It is useful for checking whether an existing size is acceptable, not just for picking a new one. That makes it easier to compare ampacity with real-world drop on the run length you have.

How to Use the Inputs

1. Select the wire gauge (AWG) from the dropdown or choose a preset scenario.
2. Enter the total wire run length in feet (one-way distance, calculator doubles for round trip).
3. Choose the conductor material — copper or aluminum.
4. Enter the current flowing through the wire in amps.
5. Set the ambient temperature if different from the default 25°C.
6. Review resistance, voltage drop, power loss, and ampacity outputs.
7. Check the reference table for alternative wire gauges.

Example Calculation

Tips & Best Practices

• Always use the round-trip distance (2× one-way length) when calculating voltage drop.
• Stranded wire has slightly higher resistance than solid wire of the same gauge due to air gaps.
• For underground burial, use wire rated for wet locations (UF-B, THWN, XHHW).
• Voltage drop is more critical at 120V than at 240V — same current and distance produces double the percentage drop.
• When running multiple circuits in the same conduit, derate ampacity per NEC 310.15(C).
• Label wire with gauge and circuit number at both ends for future maintenance.

AWG Wire Gauge Reference Table

The American Wire Gauge system assigns numbers inversely to wire diameter — smaller numbers mean larger wires. Each 3-gauge step roughly doubles the cross-sectional area. Common residential gauges: 14 AWG for 15A circuits, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40-50A, 6 AWG for 60A, and 4 AWG for larger loads. Service entrance wires typically range from 2 AWG to 4/0 depending on the panel size.

Copper vs. Aluminum Conductors

Copper dominates residential branch circuits due to its superior conductivity, ease of termination, and corrosion resistance. Aluminum is standard for service entrance cables and large feeders (100A+) because of significant cost and weight savings. When using aluminum, always use AL-rated connectors and apply anti-oxidant paste. Modern NEC editions allow AA-8000 series aluminum alloy, which is more flexible and resistant to creep than older alloys.

Temperature Derating and Installation

Wire ampacity ratings assume specific ambient temperatures and installation conditions. In attics, engine compartments, or sun-exposed conduit, temperatures can easily exceed 40°C, requiring derating. NEC Table 310.15(B)(1) provides correction factors: at 40°C, multiply rated ampacity by 0.88 for 75°C-rated wire. Multiple current-carrying conductors in a single conduit also require derating per NEC 310.15(C)(1) — 4-6 conductors derate to 80%, 7-9 to 70%.

Frequently Asked Questions

• What gauge wire do I need for 20 amps?

  For 20A circuits, NEC requires minimum 12 AWG copper or 10 AWG aluminum. For runs over 50 feet, consider 10 AWG copper to keep voltage drop under 3%.

• How does temperature affect wire resistance?

  Wire resistance increases roughly 0.4% per °C above 20°C for copper. At 75°C, copper wire has about 22% more resistance than at 25°C. This matters for current-carrying capacity and voltage drop in hot environments.

• What is the difference between copper and aluminum wire?

  Aluminum has about 61% of copper's conductivity, so it needs to be roughly 2 AWG sizes larger for the same ampacity. Aluminum is cheaper and lighter but requires special connectors and anti-oxidant compound.

• What wire size for 50 amps at 100 feet?

  For a 50A copper circuit at 240V over 100ft, you need at minimum 6 AWG for ampacity. Checking voltage drop: 6 AWG gives about 3.4% drop — borderline. Use 4 AWG copper for comfortable margin.

• How do I convert AWG to metric (mm²)?

  12 AWG ≈ 3.31mm², 10 AWG ≈ 5.26mm², 8 AWG ≈ 8.37mm², 6 AWG ≈ 13.3mm², 4 AWG ≈ 21.1mm². Metric sizes round to nearest standard: 2.5mm², 4mm², 6mm², 10mm², 16mm², 25mm².

• Why is voltage drop important?

  Excessive voltage drop causes dim lights, slow motors, overheated wire insulation, and equipment malfunction. NEC recommends no more than 3% drop on branch circuits and 5% total including feeders.