Calculate wire gauge, resistance, voltage drop, current capacity, and power loss for electrical wiring. Supports AWG and metric wire sizes.
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 tool uses the American Wire Gauge (AWG) standard and handles both copper and aluminum conductors. It factors in wire length, temperature, and insulation type to provide accurate results that align with NEC and IEC standards. Understanding the relationship between wire size, resistance, and voltage drop prevents overheated conductors, tripped breakers, and wasted energy.
Enter your wire parameters to instantly 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.
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.
Resistance (Ω) = ρ × L / A. Voltage Drop = I × R_total (round trip). Power Loss = I² × R_total. Where ρ = resistivity (Ω·cmil/ft), L = one-way length (ft), A = cross-sectional area (cmil), I = current (A).
Result: About 6.35V drop on a 120V circuit, or 5.3%
A 100 ft one-way run means about 200 ft of conductor path for the round trip. Using a typical 12 AWG copper resistance of about 1.588 ohms per 1,000 ft gives roughly 0.318 ohms total, so the drop at 20 A is about 6.35 V. That is about 5.3% on 120 V, or about 2.6% if the same run were part of a 240 V circuit.
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 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. The 2010+ NEC allows AA-8000 series aluminum alloy, which is more flexible and resistant to creep than older alloys.
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%.
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%.
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.
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.
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.
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.
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.