Voltage Drop Calculator

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Calculate voltage drop for electrical circuits and find the optimal wire size to meet NEC, IEC, or BS 7671 code requirements.Learn more ▾Show less ▴
Designed for electricians, engineers, and contractors who need to verify voltage drop compliance and select the right wire gauge for any circuit. Supports residential, commercial, and industrial applications with copper or aluminum conductors.
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Wire Size Ampacity VD% Status
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About This Tool

This calculator helps electricians, engineers, and homeowners determine whether a wire size is adequate for a given circuit, or find the smallest wire that meets both ampacity and voltage drop requirements. It supports three international standards: NEC (US), IEC 60364 (EU), and BS 7671 (UK).

Sources: NEC (NFPA 70) · IEC 60364-5-52 · BS 7671 (IET)

How to Use

  1. Select your standard (NEC, IEC, or BS 7671) and choose between voltage drop calculation or wire size recommendation
  2. Enter your circuit parameters: voltage, current, wire length, conductor type, and insulation
  3. Review the results showing voltage drop percentage, pass/fail status, and recommended wire size

How to Use

  1. Select your standard (NEC, IEC, or BS 7671) and choose between voltage drop calculation or wire size recommendation
  2. Enter your circuit parameters: voltage, current, wire length, conductor type, and insulation
  3. Review the results showing voltage drop percentage, pass/fail status, and recommended wire size

Methodology

For NEC calculations, voltage drop uses the K-factor formula: VD = (K × I × L × 2) / CM for single-phase, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum in ohms·cmil/ft), I is current in amps, L is one-way length in feet, and CM is the wire area in circular mils. For three-phase, the factor of 2 is replaced by 1.732. For IEC and BS 7671 calculations, the mV/A/m method is used with tabulated impedance values from the relevant standard tables. Ampacity is determined from NEC Table 310.16 (60/75/90 degree columns) or equivalent IEC/BS tables, then adjusted for ambient temperature and conduit fill derating factors.

Sources: NEC (NFPA 70) · IEC 60364-5-52 · BS 7671

Understanding Your Results

A green "Pass" result means the voltage drop is within the code limit for the selected circuit type. A red "Exceeds Limit" result means you need a larger wire size, shorter run, or lower current to comply with the standard. In Wire Size mode, the tool shows two recommendations: the minimum wire by ampacity (can safely carry the current) and the minimum wire by voltage drop (stays within the code limit). The larger of the two is the recommended size, and the controlling factor tells you which requirement drove the selection.

Sources

Practical Examples

Example 1: A 20A, 120V single-phase branch circuit using #10 AWG copper THHN, 100 ft one-way. Voltage drop = (12.9 × 20 × 100 × 2) / 10380 = 4.96V, which is 4.13% — exceeds the 3% NEC limit. Solution: upsize to #8 AWG (16510 CM), giving 3.12V drop (2.6%) — passes. Example 2: A 50A, 240V single-phase feeder using #6 AWG copper, 75 ft. Voltage drop = (12.9 × 50 × 75 × 2) / 26240 = 3.69V, or 1.54% — well within the 5% feeder limit.

Practical Tips

For long cable runs, voltage drop often requires a larger wire than ampacity alone. Always check both. A wire that passes ampacity but fails voltage drop will still cause problems at the load. Higher system voltage reduces percentage voltage drop proportionally. A 240V circuit has half the voltage drop percentage of a 120V circuit with the same wire, current, and distance. Parallel conductors can solve voltage drop problems on very long runs where a single large wire would be impractical or too stiff to install. Two sets of smaller wire can be easier to pull and bend. Always account for derating when wires share a conduit or run through hot spaces. The base ampacity from the table assumes a 30 degree Celsius ambient and no more than 3 current-carrying conductors.

All calculations are performed locally in your browser. No data is sent to any server.

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Frequently Asked Questions
What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage as electrical current flows through a conductor. Every wire has some resistance, and this resistance causes a portion of the supply voltage to be "lost" as heat along the wire. Excessive voltage drop can cause equipment to malfunction, motors to overheat, lights to dim, and energy to be wasted. Electrical codes limit voltage drop to protect equipment and ensure safe operation. The NEC recommends no more than 3% for branch circuits and 5% total for the entire system from the service entrance to the farthest outlet.
What are the NEC, IEC, and BS 7671 voltage drop limits?
The NEC (NFPA 70) recommends 3% maximum voltage drop for branch circuits and 5% total for feeder plus branch circuits combined. These are recommendations in the informational notes to NEC 210.19(A) and 215.2(A), not mandatory requirements, though most jurisdictions enforce them. IEC 60364-5-52 limits voltage drop to 3% for lighting circuits and 5% for other circuits from the origin of the installation. BS 7671 (UK Wiring Regulations) allows 3% for lighting and 5% for other uses, measured from the meter to the furthest point.
How do I choose between copper and aluminum conductors?
Copper has about 61% higher conductivity than aluminum, meaning smaller wire sizes carry the same current. Copper is preferred for branch circuits, tight spaces, and connections to equipment terminals rated only for copper. It is more durable and easier to terminate. Aluminum costs significantly less per ampere of capacity and is lighter, making it the standard choice for large feeders, service entrance cables, and utility connections. When using aluminum, you typically need to go up two wire sizes compared to copper. Always use connectors and lugs rated for aluminum (marked AL or CU-AL).
When should I use single-phase vs three-phase calculations?
Use single-phase calculations for most residential circuits, including 120V and 240V circuits for lighting, outlets, appliances, water heaters, and residential HVAC equipment. Single-phase power uses two conductors (hot and neutral for 120V, or two hots for 240V). Use three-phase calculations for commercial and industrial installations with 208V, 240V three-phase, 480V, or 600V systems. Three-phase power is more efficient for large motors, commercial HVAC, and heavy industrial loads. The voltage drop formula includes a factor of 1.732 (square root of 3) instead of 2 for the return path.
What is ampacity derating and when does it apply?
Ampacity derating reduces the maximum current a wire can safely carry based on installation conditions. Two main derating factors apply. First, ambient temperature correction: when the surrounding temperature exceeds 30 degrees Celsius (86 degrees Fahrenheit), the wire's ampacity is reduced because insulation degrades faster at higher temperatures (NEC Table 310.15(B)(1)). Second, conduit fill adjustment: when multiple current-carrying conductors share the same raceway, heat builds up and each conductor's ampacity is reduced. For example, 4 to 6 conductors in a conduit reduces ampacity to 80% of the base value (NEC Table 310.15(C)(1)). Both factors are multiplied together when both conditions exist.
How does wire length affect voltage drop?
Voltage drop is directly proportional to wire length. Doubling the distance from the panel to the load doubles the voltage drop. This is because the current must travel through more conductor material, encountering more total resistance. The formula uses one-way distance but accounts for the round-trip path automatically (the factor of 2 in single-phase or 1.732 in three-phase). For long runs, you may need to upsize the wire beyond the minimum ampacity requirement. For example, a 20A circuit on #12 AWG copper works fine at 50 feet (1.9% drop at 120V), but at 150 feet the drop reaches 5.8%, requiring #8 AWG instead.
What is the difference between branch and feeder circuits?
A branch circuit runs from the final overcurrent protection device (breaker or fuse) to the outlets or equipment it serves. Examples include a 20A circuit to kitchen outlets or a 15A circuit to bedroom receptacles. The NEC recommends a maximum 3% voltage drop on branch circuits. A feeder runs from the main service panel to a sub-panel or distribution board. It carries the combined load of all branch circuits downstream. The NEC recommends that the total voltage drop from the service entrance through the feeder and branch circuit to the farthest outlet should not exceed 5%. So if your feeder has 2% drop, each branch circuit should stay under 3%.
Can this tool be used for motor circuits?
Yes. Enable the "Motor Circuit (125%)" option in Advanced Options to automatically size conductors at 125% of the motor's full-load current, as required by NEC 430.22. Enter the motor nameplate full-load amperes (FLA) as the current value, and the tool applies the 125% factor. For motor circuits, also consider setting the power factor to 0.85 for typical induction motors (rather than 1.0 for resistive loads). Voltage drop is especially important for motors because low voltage causes higher current draw, increased heating, and reduced torque. Keep voltage drop under 3% for motor branch circuits to ensure proper starting and running performance.
What voltage does Australia use?
Australia uses 230V single-phase and 400V three-phase power at 50 Hz, as specified by AS/NZS 3000 (the Wiring Rules). When using this calculator for Australian installations, select 230V for single-phase circuits. Note that Australian voltage was historically 240V but was harmonized to 230V (+10%/−6% tolerance) in the 2000s, so most equipment operates across the 216–253V range.
What voltage standard does the UK use?
The UK uses 230V single-phase and 400V three-phase at 50 Hz, governed by BS 7671 (the IET Wiring Regulations). When using this calculator for UK installations, select 230V for single-phase circuits and the BS 7671 standard. UK voltage was historically 240V but harmonized to 230V (+10%/−6% tolerance) in 1995, meaning most UK supplies operate around 230–240V.