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Voltage Drop Calculator

Everyday

Calculate voltage drop, percentage drop, and wire resistance for any wire gauge, length, and current. Covers copper and aluminium wire for electrical wiring.

Reviewed by the thecalcu.com team · Last updated June 30, 2026

240 V
20 A
100 ft
Wire Gauge (AWG)
AWG 12 (20A)
Wire Material
Voltage Drop
— V
—%
0%3%5%10%
NEC Compliance
Excellent (≤ 3%)
Wire Resistance
—
Voltage at Load
—
NEC recommends max 3% voltage drop for branch circuits and 5% total (including feeder). Aluminium resistance is ~1.64× copper at the same gauge.

What is a Voltage Drop?

A Voltage Drop Calculator determines how much voltage is lost along an electrical cable run due to wire resistance, and expresses this as both an absolute voltage (volts) and a percentage of the supply voltage. It uses the wire gauge (AWG), wire length, current load, and wire material (copper or aluminium) to compute the drop, and then evaluates the result against NEC (National Electrical Code) compliance thresholds of 3% (excellent) and 5% (maximum acceptable).

When electrical current flows through any conductor, the conductor's resistance dissipates some energy as heat and causes a voltage reduction along the wire's length. The longer the wire run or the higher the current draw, the greater the drop. This is not merely a theoretical concern, equipment at the end of a long, undersized cable run may receive 10–15% less voltage than the supply, causing motors to overheat, electronics to malfunction, and lights to dim.

The Voltage Drop Calculator is used in three primary contexts: residential electrical work (sizing the wire for a sub-panel, a pump, or a long branch circuit run), commercial and industrial installations (verifying compliance before inspection), and educational settings (learning Ohm's Law applications in AC circuit design). The Ohm's Law Calculator addresses the load side; the Voltage Drop Calculator addresses the supply cable side.

Why Use a Voltage Drop Calculator?

Ensure NEC compliance. Electrical inspectors check voltage drop as part of the wiring inspection process. Being able to demonstrate the calculation and show drop is under 3% prevents failed inspections and costly rewiring.

Protect equipment. Running sensitive electronics, variable frequency drives, CNC machines, medical equipment, networking gear, on a circuit with excessive voltage drop shortens equipment life and causes operational errors. A voltage drop calculation at the design stage prevents this.

Size wire correctly, not conservatively. Many electricians use the next larger wire gauge than needed "to be safe," adding unnecessary material cost. The calculator shows exactly how much margin exists, allowing right-sizing rather than over-sizing.

Diagnose existing problems. If a motor is running hot or a light circuit is flickering, calculating the theoretical voltage drop on the existing wiring can confirm whether under-voltage is the cause.

Who Should Use This Calculator?

Electricians and electrical contractors designing residential, commercial, or industrial wiring layouts need to verify voltage drop for each circuit before running cable.

Homeowners planning extensions or subpanels who are adding a new workshop, garage, pump, or outdoor lighting circuit far from the main panel can calculate whether their planned wire gauge is adequate for the run length.

Solar PV and battery system installers designing DC cable runs from solar panels to charge controllers, or from batteries to inverters, where even 1–2% voltage drop can meaningfully reduce system efficiency.

Industrial engineers and maintenance technicians troubleshooting why a motor or drive is tripping on low-voltage faults can use the calculator to determine whether the cable is undersized for the load.

Electrical engineering students learning circuit theory and NEC code compliance can use the calculator to verify textbook wire sizing problems.

What Insights Does the Voltage Drop Calculator Give You?

Voltage Drop (V) is the absolute voltage lost along the round-trip wire run. If your supply is 240 V and this value is 7.2 V, the equipment receives 232.8 V, a concrete figure useful for equipment specification comparison.

Drop Percentage (%) expresses the loss relative to the supply voltage. This is the NEC compliance figure: ≤ 3% (excellent, shown in green), 3–5% (acceptable, shown in amber), > 5% (exceeds limit, shown in red).

Wire Resistance (Ω) is the total round-trip resistance of the cable run, used in verifying manual calculations and understanding how the gauge and length interact.

Voltage at Load is the supply voltage minus the drop, the actual voltage the connected equipment receives. Compare this against the equipment's rated operating voltage range (typically ±10% of nominal).

How to use this Voltage Drop calculator

  1. Set System Voltage, 120 V for North America standard, 230 V for India and UK, 240 V for heavy circuits, 12 V or 24 V for DC systems.
  2. Set Current (Load), the amperage drawn by the connected load. Check the equipment nameplate; for circuits serving multiple loads, add the load currents.
  3. Set Wire Length, the one-way length of the wire run in feet (the calculator automatically doubles this for the round-trip calculation).
  4. Select Wire Gauge (AWG), click one of the eight AWG options from AWG 14 (lightest, 15A rated) to AWG 1/0 (heaviest, 125A rated). The ampacity label helps you verify the gauge is rated for your current.
  5. Select Wire Material, copper (lower resistance, standard for residential) or aluminium (1.64× resistance, used in feeder cable).
  6. Read the compliance colour, green = excellent, amber = acceptable, red = oversize the wire.

Formula & Methodology

Wire Resistance Table (copper, Ω per 1,000 ft at 75°C):
AWG 14: 3.14 | AWG 12: 1.98 | AWG 10: 1.24 | AWG 8: 0.778
AWG 6: 0.491 | AWG 4: 0.308 | AWG 2: 0.194 | AWG 1/0: 0.122

Aluminium factor: Resistance × 1.64

Wire Resistance (Ω) = 2 × Wire Length (ft) × (R per 1,000 ft) ÷ 1,000

Voltage Drop (V) = Wire Resistance × Current (A)

Drop Percentage (%) = (Voltage Drop ÷ System Voltage) × 100

Voltage at Load = System Voltage − Voltage Drop

Worked example: An electrician in Delhi is wiring a 20 A circuit to a sub-panel 150 ft from the main panel, at 240 V, using AWG 12 copper wire.

- Wire Resistance = 2 × 150 × 1.98 ÷ 1,000 = 0.594 Ω
- Voltage Drop = 0.594 × 20 = 11.88 V
- Drop % = 11.88 ÷ 240 = 4.95%
- Voltage at Load = 240 − 11.88 = 228.12 V

Result: 4.95%, Acceptable (just under the 5% NEC maximum), but borderline. Upgrading to AWG 10 copper would reduce the drop to 2 × 150 × 1.24 ÷ 1,000 × 20 = 7.44 V = 3.1%, comfortably in the Excellent range and leaving headroom if the load is later increased.

Frequently Asked Questions

Voltage drop is the reduction in voltage that occurs along a length of electrical wire due to the wire's resistance. When current flows through a conductor, Ohm's Law dictates a voltage loss (V = I × R). The longer the wire or the smaller its cross-section (higher gauge number in AWG), the more voltage is lost before reaching the load, reducing performance and potentially causing equipment malfunction.
The NEC (National Electrical Code) recommends a maximum 3% voltage drop for branch circuits serving individual outlets, with a combined feeder-plus-branch-circuit maximum of 5%. In practice: ≤ 3% is excellent; 3–5% is acceptable but at the limit; > 5% means the wire is undersized for the run length and load current. The Voltage Drop Calculator colour-codes results to instantly show where you stand.
Voltage Drop (V) = 2 × Wire Length (ft) × Wire Resistance per 1,000 ft (Ω) × Current (A) ÷ 1,000. The factor of 2 accounts for the two-way wire run (current flows out and returns). Drop Percentage (%) = (Voltage Drop ÷ System Voltage) × 100.
AWG (American Wire Gauge) numbers are inversely related to wire size, a lower AWG number means a thicker wire with lower resistance. AWG 12 has a resistance of 1.98 Ω per 1,000 ft while AWG 14 has 3.14 Ω/1,000 ft. Thicker wire (lower AWG) reduces resistance and thus reduces voltage drop for the same current and run length. AWG is the standard system used in India for imported wiring products and in international standards.
Copper has lower electrical resistivity than aluminium, aluminium wire has approximately 1.64× the resistance of copper at the same AWG gauge. This means aluminium wire produces 64% more voltage drop than copper wire of the same gauge, current, and length. Copper is preferred for residential wiring; aluminium is used for high-voltage power transmission lines where weight saving is more important than resistivity.
The three main strategies are: (1) Use a larger wire (lower AWG number), going from AWG 12 to AWG 10 halves the voltage drop for the same run; (2) Increase the system voltage, a 240V circuit drops a smaller percentage than a 120V circuit carrying the same wattage (half the current for double the voltage); (3) Shorten the wire run by placing the distribution panel closer to the load.
At 120V, 20A, and 100 ft (one-way), AWG 12 copper gives a voltage drop of 2 × 100 × 1.98 × 20 ÷ 1,000 = 7.92V, or 7.92 ÷ 120 = 6.6%, exceeding the 5% NEC limit. AWG 10 copper (1.24 Ω/1,000 ft) gives 2 × 100 × 1.24 × 20 ÷ 1,000 = 4.96V, or 4.1%, within the acceptable range. Use AWG 10 for this run.
Yes, voltage drop and voltage loss refer to the same phenomenon: the reduction in voltage measured across the wire (rather than across the load). The voltage at the load end is the supply voltage minus the voltage drop. If your supply is 240V and the wire drop is 7.2V, the appliance receives 232.8V. For sensitive electronics, this reduced supply voltage can cause malfunction or data errors.
The [Ohm's Law Calculator](/ohms-law-calculator/) computes V, I, R, and P for a single circuit element, it answers the question 'what is the resistance of this load?' The Voltage Drop Calculator is specifically designed for wire sizing, it uses a resistance table for standard wire gauges (AWG) to compute the drop across the supply cable, not the load. They complement each other: use Ohm's Law for the load, voltage drop for the cable.
India uses both the AWG (American Wire Gauge) system (primarily for imported products and international specifications) and the metric mm² cross-sectional area system (as specified by BIS standards). Common Indian residential wiring uses 1.5 mm² for lighting circuits and 2.5 mm² for power points, equivalent approximately to AWG 15 and AWG 13 respectively. For industrial and large-load runs, both systems are in use depending on the equipment origin.
At voltages significantly below rated supply, AC induction motors (fans, pumps, compressors) draw higher current to maintain torque, increasing heat build-up and reducing motor life. Fluorescent and LED lights may flicker or fail to start. Computers and sensitive electronics may lock up, produce errors, or activate low-voltage protection. Over time, persistent under-voltage shortens appliance lifespan and increases electricity consumption as equipment runs less efficiently.
Also known as
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