⚡ Voltage Drop Calculator

Calculate voltage drop, resistance, and remaining voltage for electrical cables. Checks compliance with NEC recommended 3% (branch circuit) and 5% (feeder + branch) limits.

Supply Voltage (V)

Current (A)

Cable Length — one-way (m)

Conductor Material

Cable Cross-Section (mm²)

Resistivity Values

Material	ρ (Ω·m × 10⁻⁸)	Ω/km for 1mm²
Copper	1.72	17.2
Aluminum	2.82	28.2

What is a Voltage Drop Calculator?

A voltage drop calculator determines how much voltage is lost along an electrical conductor due to the conductor's resistance, based on the wire material (copper or aluminium), cross-sectional area (or AWG gauge), conductor length, and current flowing through it. Every real conductor has resistance, and that resistance consumes some of the supply voltage according to Ohm's law (V = I × R) — the voltage available at the load is the supply voltage minus the voltage lost in the conductors on both the live and return paths. This voltage reduction is called "voltage drop" and, when excessive, causes poor equipment performance, overheating, and safety hazards.

The resistance of a conductor is determined by the formula R = ρ × L / A, where ρ (resistivity) is a material constant (copper ≈ 1.72 × 10⁻⁸ Ω·m at 20°C; aluminium ≈ 2.82 × 10⁻⁸ Ω·m), L is the conductor length in metres, and A is the cross-sectional area in m². Voltage drop equals current × total conductor resistance (both outgoing and return conductors): V_drop = 2 × I × ρ × L / A for single-phase circuits. Percentage voltage drop = (V_drop / V_supply) × 100. The NEC recommends a maximum 3% voltage drop on branch circuits, with a 5% maximum for the total path from source to load including feeder and branch circuit combined.

Voltage drop is a critical design consideration in any electrical installation with long cable runs — particularly in residential and commercial solar PV systems (long DC cable runs from panels to inverter), EV charging installations (high current loads at potentially significant distances from the consumer unit), outdoor lighting circuits, agricultural power installations, data centre power distribution, and marine/RV electrical systems. The voltage drop calculator determines the minimum conductor cross-section that keeps voltage drop within acceptable limits for a given current, cable length, and supply voltage — allowing designers to select appropriately sized cables before installation rather than discovering problems after commissioning.

How the Voltage Drop Calculator Works

Formula, assumptions, and calculation steps for this engineering tool.

Methodology

Engineering calculators apply standard unit conversions and formula relationships after normalizing measurements to compatible units.

Calculation Steps

Enter dimensions, loads, rates, or electrical values.
Convert the inputs into the formula unit system.
Apply the engineering equation or conversion factor.
Return the result with units and supporting context.

Assumptions and Limits

Material behavior is assumed ideal unless fields specify otherwise.
Code checks, safety factors, and site conditions may require professional review.
Use a qualified engineer for design-critical decisions.

Frequently Asked Questions

What is the NEC recommended voltage drop limit?

The NEC (National Electrical Code) recommends (not mandates) a maximum 3% voltage drop on branch circuits and 5% total from source to the load (feeder + branch). Higher voltage drops cause equipment to run hot, motors to draw more current, and lights to dim. Always design to stay within 3% for sensitive loads.

How is voltage drop calculated?

Voltage Drop = 2 × L × I × ρ / A, where L is one-way cable length (m), I is current (A), ρ is resistivity (Ω·m), and A is cross-section (m²). The factor of 2 accounts for both the live and neutral (return) conductors. Percentage drop = (V_drop / V_supply) × 100.

Should I use copper or aluminum cable?

Copper has lower resistivity (better conductivity) and is preferred for most residential and commercial wiring. Aluminum is lighter and cheaper per metre but requires larger cross-sections for equivalent current-carrying capacity. Aluminum is commonly used for large feeders and service entrance cables. Never mix copper and aluminum connections without proper anti-oxidant compound and listed connectors.

What happens if voltage drop is too high?

Excessive voltage drop causes: reduced motor torque (motors may stall), overheating of motors and other loads, reduced lighting output, unreliable operation of electronic equipment, and increased energy waste as heat in the cable. For long runs, upsize the cable to reduce resistance.

Real-World Applications

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Solar PV DC Cable Sizing

Solar photovoltaic installations have long DC cable runs from roof-mounted panels to the inverter — sometimes 15–30 metres of cable carrying 8–15 A DC at 30–50V per string. Voltage drop on these DC runs reduces the voltage delivered to the inverter, directly reducing energy yield. The voltage drop calculator determines the minimum cable cross-section that keeps DC-side losses within 1–3% — larger cable costs more but recovers the additional investment over the system lifetime through improved energy yield.

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EV Charging Circuit Design

Home EV chargers draw 7–22 kW, corresponding to 30–100 A at 230V AC — much higher current than typical domestic circuits. Long cable runs from the consumer unit to the garage or driveway charging point can cause significant voltage drop, reducing charging efficiency and potentially tripping the charger's protective devices. Electricians designing EV charging circuits use the voltage drop calculator to select cable sizes that keep the drop within the IET Wiring Regulations limit of 3% for new circuits.

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Domestic Ring Main & Spur Circuit Verification

UK electricians use voltage drop calculations to verify compliance with BS 7671 (IET Wiring Regulations), which limits voltage drop to 3% of nominal supply voltage (6.9V on a 230V system) for lighting and socket circuits. Long spur cables from a ring main junction to remote sockets, or extended ring main circuits in large houses, can exceed this limit with standard 2.5mm² T&E cable — the calculator identifies where up-sizing to 4mm² or 6mm² is required.

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Agricultural & Outdoor Power Distribution

Farms, equestrian centres, and large outdoor events distribute power over long cable runs to remote buildings, pumps, lighting, and equipment — often using underground armoured cable over 50–200 metres. Voltage drop becomes the dominant cable sizing criterion at these distances, often requiring significantly larger conductors than the current-carrying capacity alone would suggest. The voltage drop calculator determines the minimum cross-section for the specific load, run distance, and required voltage quality at the end of the run.

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Marine & RV 12V/24V Low-Voltage Systems

Boats, motorhomes, and campervans use 12V or 24V DC electrical systems where even small absolute voltage drops represent a significant percentage of the supply voltage. A 0.5V drop is 4.2% of 12V — enough to affect lighting brightness, motor efficiency, and battery charger performance. Marine electricians and RV builders use voltage drop calculators extensively to size cable for bilge pumps (high current, often long run), navigation lights (critical safety equipment), and bow thruster motors (very high current, short burst).

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Industrial Motor Circuit Design

Electric motors are particularly sensitive to voltage drop — a 10% voltage reduction causes approximately 19% reduction in starting torque (torque varies as the square of voltage), which can prevent large motors from starting under load. Industrial electrical engineers use voltage drop calculations as part of motor circuit design, ensuring that the combined voltage drop of the supply cable and motor control centre busbars stays within the maximum permissible value for the motor's starting torque requirements.

Common Mistakes

Using single conductor length instead of total circuit length (round trip)

Voltage drop occurs in both the outgoing live conductor and the returning neutral conductor — the current flows through both. The voltage drop formula uses the total circuit conductor length (2 × one-way cable run for single-phase circuits): V_drop = 2 × I × ρ × L / A. A common error is using only the one-way run length, which underestimates voltage drop by 50%. For three-phase circuits, the factor is √3 (≈1.732) times the one-way run length rather than 2.

Not accounting for conductor temperature on resistance

Copper's resistivity increases approximately 0.39% per °C above 20°C. A copper conductor operating at 70°C (typical for PVC-insulated cable at full load) has approximately 20% higher resistance than at 20°C — and therefore 20% higher voltage drop than a room-temperature calculation suggests. For circuits loaded to near their thermal capacity, the elevated operating temperature significantly increases actual voltage drop compared to the nominal resistivity calculation. Always apply a temperature correction factor for loaded cables in conduit, duct, or buried installations.

Ignoring voltage drop on the supply side (before the consumer unit)

The NEC and IET both specify voltage drop limits as a percentage of the nominal supply voltage. But the nominal voltage is measured at the supply terminals — not at the load. If the utility supply voltage is already at the lower end of its permitted range (207V for a nominal 230V supply in the UK — −10% tolerance), a further 3% drop in the installation wiring leaves only 200.8V at the load socket. Sensitive equipment (some motors, electronic devices) may not function correctly at this level; oversizing conductors to limit total drop to 2% rather than 3% provides margin.

Selecting cable size based only on current-carrying capacity without checking voltage drop

For short cable runs, current-carrying capacity (thermal rating) is the dominant cable sizing criterion; for long cable runs, voltage drop becomes the governing criterion. A 2.5mm² cable has a current-carrying capacity of 18.5A in free air — adequate for most 20A circuits on a short run — but over a 40-metre single-phase run at 16A, the voltage drop exceeds 3.5% (above the recommended limit). Always check both current-carrying capacity AND voltage drop when sizing cables; the larger of the two requirements determines the minimum cable cross-section.

Applying DC voltage drop formulas to AC circuits without considering power factor

For AC circuits with purely resistive loads (heaters, incandescent bulbs), DC voltage drop formulas (V = I × R) are accurate. For AC circuits with reactive loads (motors, fluorescent lighting, electronic power supplies), the power factor of the load and the reactance of the cable (which becomes significant at larger conductor sizes and higher frequencies) affect voltage drop. Professional electrical design uses impedance (Z = √(R² + X²)) rather than resistance alone, and applies the load power factor. For rough calculations, DC formulas are acceptable; for precise HV or high-current AC calculations, use impedance-based methods.

Voltage Drop Limits by Standard & Application

Standard / Application	Max Drop	Scope
IET BS 7671 (UK)	3% branch / 5% total	Residential & commercial
NEC (US)	3% branch recommended	Branch circuits
NEC (US) total	5% total recommended	Feeder + branch
Solar PV (IEC)	1–3% DC side	String to inverter
12V marine (ABYC)	3% lighting / 10% motor	Boat wiring

References

IET. BS 7671:2018 Requirements for Electrical Installations (18th Edition). theiet.org, 2018.
NFPA. NFPA 70: National Electrical Code. nfpa.org, 2023.
IEC. IEC 60364: Electrical Installations of Buildings. iec.ch, 2024.
ABYC. E-11: AC and DC Electrical Systems on Boats. abycinc.org, 2022.
Hughes, E. Electrical Technology. Pearson Education, 2016.