Wire Size Calculator — AWG, mm² & Voltage Drop for DC and AC Circuits

Undersized wire is one of the most common — and most costly — mistakes in DC and AC circuit installations. A 2V drop on a 120V mains circuit barely registers; the same 2V drop on a 12V actuator feed is 16.7%, enough to stall the motor under load. Use this Wire Size Calculator to calculate required conductor gauge, cross-sectional area, and actual voltage drop using circuit voltage, current, cable run length, and maximum allowable voltage drop. It applies directly to low-voltage DC systems, single-phase AC, and three-phase industrial distribution. This page covers the full formula derivation, temperature correction, a worked marine installation example, and answers to the most common wire sizing questions.

Size the wire for the worst moment of the circuit, not the average one — peak current, hot-wire temperature, the longest run in the system. The easy middle never causes failures.

"A 2V drop on a 120V circuit is a rounding error. The same 2V on a 12V actuator feed is 16.7% — enough to stall the motor under load. Low-voltage DC is where wire sizing actually matters, and where most field mistakes happen." — Robbie Dickson, FIRGELLI Automations founder and former Rolls-Royce, BMW, and Ford engineer

What is wire sizing?

Wire sizing is the process of selecting a conductor large enough to carry a circuit's current without losing too much voltage along the way. A wire that is too thin has too much resistance — the load receives less voltage than it needs, and the wire wastes energy as heat.

How does wire sizing work in plain terms?

Think of a wire like a garden hose: a narrow hose limits flow and builds up pressure drop along its length, just as a thin wire limits current and drops voltage. The longer the run and the higher the current, the bigger the wire needs to be. This calculator finds the minimum safe size so your load gets the voltage it requires.

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What does a wire-sizing circuit look like?

Wire Size Calculator

How do you use this wire size calculator?

1. Select your Calculation Mode, electrical system (DC/single-phase or three-phase), conductor material (copper or aluminum), and maximum operating temperature.
2. Enter your source voltage, current draw (use peak or maximum for worst-case sizing), and one-way cable length in metres — the calculator handles the round-trip factor automatically.
3. Set your allowable voltage drop as a percentage (NEC recommends 3% for motors and actuators) or as an absolute voltage value depending on the selected mode.
4. Click Calculate to see your result.

📹 Video — Wire Size Calculator AWG mm Voltage Drop for DC and AC Circuits

What equations does the wire size calculator use?

What does a simple wire-sizing example look like?

A 12V DC actuator draws 20A and is located 5m from the power supply. Maximum allowable voltage drop is 3% (0.36V). Using copper at 25°C:

A = (20 × 1.685×10⁻⁸ × 2 × 5) / 0.36 = 4.68 mm²

Next standard size up: 10 AWG (5.261 mm²). Actual voltage drop with 10 AWG: 0.321V (2.67%). Within limit. ✓

What is the engineering theory behind wire sizing?

Why does voltage drop matter more in low-voltage DC?

Every electrical conductor has inherent resistance governed by Pouillet's Law: R = ρL/A, where resistivity ρ is an intrinsic material property, L is length, and A is cross-sectional area. When current flows through this resistance, Ohm's Law dictates a proportional voltage drop (V = IR) that reduces the voltage available at the load. In 120V or 240V AC mains circuits, a 2V drop represents only 0.8–1.7% — often negligible. In a 12V DC system, the same 2V drop is 16.7%, sufficient to stall an electric motor or cause an actuator to perform well below its rated force. This voltage sensitivity explains why low-voltage DC systems demand wire sizing far more conservative than intuition suggests, and why the National Electrical Code (NEC) recommendations of 3% maximum voltage drop are particularly critical for DC equipment.

Wire resistance depends on four physical factors simultaneously: material resistivity (copper and aluminum are the primary engineering conductors), wire length (resistance increases linearly with length), cross-sectional area (resistance decreases proportionally with area), and temperature (resistance increases approximately linearly with temperature above the reference point). The AWG system encodes cross-sectional area in a logarithmic scale where every three-gauge reduction doubles the area and halves the resistance per unit length. This means that upgrading from 16 AWG to 10 AWG (a six-gauge reduction) quadruples the cross-sectional area, reducing voltage drop to one-quarter of the original value for the same current and distance.

How does temperature change wire resistivity?

The temperature coefficient of resistance for copper is 0.00404 per °C, meaning copper resistivity increases by 0.404% for every degree Celsius above 20°C. A copper conductor operating at 75°C — typical for a wire running near its ampacity limit in a conduit — exhibits resistivity 22% higher than at 20°C, increasing voltage drop by the same factor. In engine compartments, marine installations, or industrial environments where ambient temperatures routinely reach 40–60°C, neglecting temperature correction can result in undersized conductors that exceed voltage drop limits under actual operating conditions. The correction is straightforward: ρ₂ = ρ₁ × (1 + α × (T₂ − 20)), where T₂ is the maximum expected wire temperature, not just ambient temperature — a loaded wire in a conduit bundle can run 20–30°C above ambient.

Aluminum wire, with a temperature coefficient of 0.00403 per °C, responds almost identically to temperature as copper, but starts from a higher base resistivity of 2.82×10⁻⁸ Ω·m versus copper's 1.68×10⁻⁸ Ω·m. This 68% higher base resistivity means that equivalent electrical performance requires aluminum wire approximately two AWG sizes larger than copper. A 12 AWG copper circuit requires 10 AWG aluminum for identical voltage drop and current capacity. The tradeoff is density: aluminum's 2.70 g/cm³ versus copper's 8.96 g/cm³ makes aluminum wire one-third the weight of copper per unit volume, driving its adoption in overhead transmission lines and aircraft wiring harnesses where weight reduction directly impacts operating cost.

Why does DC wire sizing use a factor of 2?

The factor of 2 in the DC wire sizing formula — A = (I × ρ × 2L) / V_drop — accounts for a physical reality that many installers overlook: current must complete a closed circuit. In a 12V DC installation powering a linear actuator located 8 metres from the battery, current travels 8 metres through the positive supply conductor and 8 metres back through the negative return conductor, encountering resistance in both conductors. Total circuit resistance involves both conductors in series: R_total = ρ × 2L / A. An installer who mistakenly uses total cable length (16 metres) in the one-way formula, or who uses one-way length (8 metres) without the factor of 2, will calculate half the required wire area, resulting in actual voltage drop twice the intended limit. This error is particularly common when pre-made automotive wiring harnesses are cut to a one-way measurement and the return path is routed separately through chassis ground.

Worked Example: Marine 24V DC Winch Installation

A marine electrician is wiring a 24V DC electric anchor windlass on a 38-foot sailing vessel. The windlass motor draws 38A continuous current at full load. The cable run from the house battery bank in the aft cabin to the bow-mounted windlass is 14.7 metres one-way. Copper cable will be used in a conduit with maximum operating temperature of 45°C. Allowable voltage drop is 3%. Calculate the required wire size and demonstrate why temperature correction cannot be skipped.

Step 1: Temperature-corrected resistivity

ρ = 1.68×10⁻⁸ × (1 + 0.00404 × (45 − 20))
ρ = 1.68×10⁻⁸ × (1 + 0.101)
ρ = 1.68×10⁻⁸ × 1.101
ρ = 1.850×10⁻⁸ Ω·m

Step 2: Allowable voltage drop

V_drop = 24V × 0.03 = 0.720 V

Step 3: Required cross-sectional area at 45°C

A = (I × ρ × 2L) / V_drop
A = (38 × 1.850×10⁻⁸ × 2 × 14.7) / 0.720
A = (38 × 1.850×10⁻⁸ × 29.4) / 0.720
A = 2.065×10⁻⁵ / 0.720
A = 2.868×10⁻⁵ m² = 28.68 mm² = 56.6 kcmil

Step 4: Select next standard AWG size up

28.68 mm² exceeds 3 AWG (26.67 mm²), so next size up is 2 AWG at 33.63 mm².

Step 5: Verify actual voltage drop with 2 AWG

V_drop = (38 × 1.850×10⁻⁸ × 29.4) / (33.63×10⁻⁶)
V_drop = 2.065×10⁻⁵ / 33.63×10⁻⁶ = 0.614 V = 2.56% ✓ (below 3% limit)

Step 6: What would happen without temperature correction?

Using ρ at 20°C: A = (38 × 1.68×10⁻⁸ × 29.4) / 0.720 = 26.07 mm² → selects 3 AWG (26.67 mm²)
Actual voltage drop at 45°C with 3 AWG: V_drop = 2.065×10⁻⁵ / (26.67×10⁻⁶) = 0.774 V = 3.23% ✗

Skipping temperature correction selects 3 AWG instead of 2 AWG — a wire that exceeds the 3% voltage drop limit under actual operating conditions. Over a 14.7-metre run, the cost difference between these wire sizes is minor compared to the cost of troubleshooting an underperforming windlass in a remote anchorage.

How is three-phase wire sizing different?

Three-phase AC distribution uses three conductors carrying currents 120° out of phase with each other. Because the three currents sum to zero at any instant (in a balanced system), no return conductor is needed — eliminating the factor of 2 in the wire area formula. The formula becomes A = (√3 × I × ρ × L) / V_drop, where I is the line current, L is the one-way cable length, and V_drop is the acceptable line-to-line voltage drop. The √3 factor (approximately 1.732) arises from the vector relationship between phase and line quantities in a balanced three-phase system. Three-phase distribution is significantly more efficient than single-phase for the same transmitted power: three-phase wire uses only 75% of the copper required for an equivalent single-phase system at the same current and voltage drop, which drives its universal adoption in industrial power distribution above approximately 5 kW.

What is special about wire sizing for linear actuators?

Electric linear actuators present specific wire sizing challenges that differ from fixed loads. Actuator current draw varies dramatically across the stroke: running current during low-load operation may be 2–4A, while stall current (when the actuator reaches its end-of-stroke or encounters an obstruction) can reach 10–20A on a 12V model, even higher on heavy-duty units. Wire sizing must account for the maximum expected current — typically taken as the actuator's rated load current rather than no-load current.

For systems using the FCB-2 control board with position feedback via Hall Effect or optical encoder, the feedback signal wire must also be sized — not for voltage drop (the feedback signal is low current) but for noise immunity. Running feedback wires in separate conduit from power conductors, or using shielded cable, prevents PWM switching noise from corrupting the position signal over long runs. A 2V drop on a 12V supply wire is severe; a 0.1V drop on a 5V feedback line can completely corrupt position reading.

AWG ↔ mm² ↔ kcmil ↔ Diameter reference table

The values below are the standard wire sizes used by the calculator. kcmil is computed as mm² / 0.5067 per the article's conversion formula.

What are common mistakes when using this calculator?

1. Forgetting the round-trip factor on DC and single-phase circuits. The "one-way cable length" input expects the source-to-load distance only — the calculator doubles it internally. Entering total round-trip length doubles the conductor area unnecessarily; entering one-way length on a calculator that doesn't double it underestimates voltage drop by 50%.
2. Skipping temperature correction. The 20°C reference resistivity does not represent real operating conditions. Engine compartments, marine engine rooms, and conduit bundles routinely run 40–60°C; a loaded wire can self-heat another 20–30°C above ambient. Enter the maximum expected wire temperature, not ambient.
3. Using running current instead of peak current for actuators and motors. Stall current on a 12V actuator can reach 10–20A while running current is only 2–4A. Size for the worst-case load, not the average.
4. Using one-way length with the three-phase formula. Three-phase already removes the factor of 2 because there is no return conductor. Switching to three-phase mode after entering DC data, without rechecking, gives an undersized wire.
5. Treating the recommended AWG as a hard answer. The calculator returns the next standard size up that satisfies voltage drop — it does not check ampacity, conduit fill, insulation temperature rating, or fuse coordination. Local code (NEC Article 310, Conductors for General Wiring — ampacity tables) governs the final selection.

How can you verify the calculator output is reasonable?

1. Apply the three-gauge rule of thumb. Every three-gauge reduction doubles the cross-sectional area and halves the resistance. If doubling your current pushes the result from 14 AWG to 11 AWG (i.e. ~3 gauges thicker), the math is internally consistent.
2. Check the copper-to-aluminum offset. Switching the material from copper to aluminum at the same current, length, voltage, and drop should bump the recommendation by roughly two AWG sizes. If it jumps by five or stays the same, an input is wrong.
3. Cross-check against the metric ↔ AWG equivalents. 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG, 6 mm² ≈ 10 AWG, 10 mm² ≈ 8 AWG. If the calculator returns 6 mm² and 8 AWG together, something is off.
4. Spot-check the percentage. A 12V system with 3% allowable drop has only 0.36V of margin. If the calculator returns a voltage drop near 1V on a 12V system, you have entered 24V drop tolerance against 12V supply, or used a 10× current.
5. Run a back-calculation in Voltage Drop mode using the recommended AWG. The result should fall just under your target drop percentage — usually between 60% and 100% of the limit. If it returns 5% on a "3% target" run, recheck the inputs.

Industries: Marine (anchor windlass and house battery distribution), Automotive (wiring harnesses, 12V/24V accessory feeds), Industrial (three-phase power distribution above ~5 kW), Linear Actuator Systems (12V/24V DC motion control).
Mechanisms: DC power distribution, single-phase AC, three-phase AC, temperature-derated conductors, voltage-drop-limited cable runs.

Frequently Asked Questions