3% / 5%
Common UK design guidance
UK voltage drop calculator
Voltage Drop Calculator
Calculate voltage drop, percentage loss and the minimum standard cable size for DC, single-phase and three-phase circuits in seconds.
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Voltage drop calculator
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Complete calculation guide
Plan safer, more efficient cable runs
Whether you are wiring a home, designing a solar system, installing an EV charger or planning an industrial circuit, knowing the expected voltage drop is essential for safety, efficiency and reliable operation.
Our voltage drop calculator estimates the voltage lost across a conductor from cable length, wire size, material, current, supply voltage and circuit type. Use the calculator above for instant results, then use this guide to understand the formulas, inputs and practical design decisions behind them.
The essentials
What is a voltage drop calculator?
A voltage drop calculator is an electrical design tool that estimates how much voltage is lost while current travels through a conductor from the source to the connected load. Every wire has resistance, so some electrical energy becomes heat instead of reaching the equipment. The difference between source voltage and load voltage is called voltage drop.
Instead of performing each resistance and circuit calculation manually, the calculator applies the appropriate relationship and presents the result in volts and as a percentage of the supply voltage.
What can this calculator calculate?
The estimated number of volts lost across the complete cable run.
The loss expressed as a percentage so it can be compared with design guidance.
A standard conductor size that keeps the estimated drop within the selected target.
The approximate voltage available to the connected equipment after conductor losses.
Resistance and load
What causes voltage drop?
Voltage drop is determined by several electrical characteristics working together. Changing any one of them changes the result.
Conductor material
Copper has lower resistance than aluminium. For the same cross-sectional area and length, aluminium normally produces a greater voltage drop and may require a larger conductor.
Wire size
Larger conductors provide more cross-sectional area and lower resistance. Increasing the cable size is usually the most effective way to reduce voltage loss.
Cable length
Resistance increases with conductor length, so detached buildings, pumps, outdoor lighting, solar arrays and other long runs need particular attention.
Electrical current
Voltage drop rises as current rises. Heavy loads therefore need larger conductors than lighter loads over the same distance.
Conductor temperature
Electrical resistance increases as a conductor gets hotter. A realistic operating temperature produces a more conservative estimate than a room-temperature value.
Installation conditions
Bundling, conduit, insulation, ambient temperature and grouping can raise operating temperature and also affect current-carrying capacity.
Input guide
How to use the voltage drop calculator
- 1
Select the circuit type
Choose DC, single-phase AC or three-phase AC so the correct circuit factor is applied.
- 2
Enter the source voltage
Use the nominal voltage available before cable losses, such as 12 V, 24 V, 120 V, 230 V, 240 V, 400 V or 480 V.
- 3
Enter the current
Add the design or load current in amperes. Higher current produces a larger voltage drop.
- 4
Select the conductor material
Choose copper or aluminium to use the appropriate resistivity value.
- 5
Select the wire size
Choose the installed or proposed cross-sectional area in mm².
- 6
Enter the cable length
Use the one-way distance from supply to load; the calculator includes the return path where required.
- 7
Review the results
Compare voltage loss, percentage drop, load voltage and suggested cable size with the requirements of the installation.
Show your working
How is voltage drop calculated?
The calculator derives conductor resistance from material, cross-sectional area and operating temperature, then applies the appropriate circuit relationship.
DC & single-phase ACVd = 2 × I × L × R
The factor of two represents the outgoing and return conductors.
Three-phase ACVd = √3 × I × L × R
The √3 relationship applies to a balanced three-phase circuit.
Percentage drop(Vd ÷ V) × 100
Expresses the calculated loss as a percentage of source voltage.
I Current (A)L One-way lengthR Resistance per unit lengthV Source voltage
Worked percentage example240 V supply − 6 V drop
6 ÷ 240 × 100 = 2.5% voltage drop
Practical calculations
Voltage drop calculation examples
Outdoor workshop
240 V, 25 A, copper, 8 AWG and a 150 ft one-way run.
Example result: 4.3 V / 1.8%12 V solar battery system
12 V DC, 40 A, copper and a 20 ft run.
Example result: 0.56 V / 4.7%Level 2 EV charger
240 V, 48 A and a 110 ft run. Compare conductor sizes until the design target is met.
Larger cable reduces loss and heatingIndustrial motor
480 V three phase, 65 A, copper and a 250 ft run.
Use the √3 three-phase relationship!
Example values are illustrative Actual results depend on conductor data, temperature, power factor, installation method and applicable electrical rules.
Regional guidance
Recommended voltage drop limits
Design targets vary by jurisdiction and equipment. Always confirm the standard and manufacturer requirements applicable to the installation.
| Standard | Common design guidance |
|---|---|
| NEC · United States | Approximately 3% for branch circuits and 5% total for feeder plus branch circuit |
| BS 7671 · United Kingdom | Typically 3% for lighting and 5% for other circuits supplied by a public low-voltage system |
| AS/NZS 3000 & 3008 | Generally up to 5%, subject to installation type and applicable requirements |
Material comparison
Copper vs aluminium conductors
| Feature | Copper | Aluminium |
|---|---|---|
| Conductivity | Higher | Lower |
| Resistance | Lower | Higher |
| Voltage drop at equal size | Less | More |
| Weight | Heavier | Lighter |
| Typical cost | Higher | Lower |
| Common use | Residential and commercial circuits | Utility distribution and larger feeders |
Copper generally provides better electrical performance for a given size. Aluminium can be cost-effective for large installations when it is correctly sized, terminated and installed for its material properties.
Where calculations matter
Voltage drop in real-world applications
Residential circuits
Check long runs supplying lighting, outlets, garages, workshops, HVAC equipment and large appliances.
Solar power systems
Evaluate runs between panels, combiner boxes, batteries, inverters and distribution equipment where small losses reduce delivered energy.
EV charging
Assess high-current, continuous-load cable runs to support reliable and efficient charging.
Automotive and marine DC
Low-voltage battery systems are especially sensitive because even a small loss represents a large percentage of the supply.
Commercial and industrial
Check feeders, three-phase motors, pumps and machinery installed a significant distance from the switchboard.
Improve the design
How to reduce voltage drop
- Increase wire size: a larger cross-sectional area lowers conductor resistance.
- Shorten the route: every additional unit of cable length adds resistance.
- Reduce circuit current: distribute loads where practical and permitted.
- Use copper: copper has lower resistance than aluminium at the same size.
- Increase system voltage: transmitting the same power at a higher voltage reduces current.
- Manage heat and bundling: appropriate spacing and installation conditions help control conductor temperature.
Check before relying on a result
Common calculation mistakes
Using total distance
Enter the one-way cable length unless a calculator specifically asks for the complete circuit length.
Selecting the wrong material
Copper and aluminium have different resistance values, so the material selection must match the installed conductor.
Ignoring temperature
Conductor resistance increases with temperature; use a realistic operating temperature for a conservative estimate.
Mixing AWG and metric sizes
AWG and mm² are different sizing systems and should not be treated as equivalent labels.
Using the wrong circuit formula
DC and single-phase calculations include the return path; balanced three-phase circuits use the √3 relationship.
Checking voltage drop only
Ampacity, protection, grouping, installation method, fault conditions and local code requirements must also be verified.
Final design check
Choose cable size with the complete installation in mind
A voltage drop calculator is a valuable design aid for residential circuits, commercial installations, solar arrays, EV chargers and industrial motors. It helps compare conductor sizes, estimate load voltage and identify cable runs that deserve closer attention before installation.
Voltage drop is only one part of cable selection. Final designs must also meet ampacity, protective-device, fault-loop, grouping, insulation, ambient-temperature, installation-method and local code requirements. Where safety or compliance is involved, consult a qualified electrical professional.
Calculate your circuit ↑Common questions
Voltage drop FAQs
Clear answers about cable length, conductor size, circuit types and acceptable design targets.
What is voltage drop?
Voltage drop is the reduction in electrical voltage between the power source and the load as current flows through conductor resistance.
What is an acceptable voltage drop in the UK?
BS 7671 guidance commonly uses 3% for lighting circuits and 5% for other circuits supplied from a public low-voltage system. The connected equipment and the complete installation design may require a lower value.
How is voltage drop calculated?
For a resistive estimate, DC and single-phase circuits use 2 × current × one-way length × resistance per unit length. Balanced three-phase circuits use √3 × current × one-way length × resistance per unit length.
Does cable length mean one-way or return length?
Enter the one-way distance from the supply to the load. The calculator automatically includes the return path for DC and single-phase circuits.
Does longer cable always increase voltage drop?
Yes. With the same conductor material, size and current, voltage drop increases as cable length increases because the circuit has more conductor resistance.
Which has less voltage drop, copper or aluminium?
Copper has lower electrical resistance and therefore less voltage drop than aluminium of the same cross-sectional area and length. Aluminium can still be suitable when it is correctly sized and installed.
Can I use the calculator for 12 V and other DC systems?
Yes. The calculator supports battery, automotive, marine, RV and solar DC circuits. Low-voltage systems are especially sensitive because a small voltage loss represents a larger percentage of the supply.
Can I calculate voltage drop using metric cable sizes?
Yes. Select a conductor cross-sectional area in mm². Do not treat an AWG label as the same value as a metric cable size.
Can I use this result to choose a cable?
Use it as a voltage-drop estimate only. Final cable selection must also satisfy current-carrying capacity, protective-device, fault-loop, grouping, insulation, ambient temperature and installation-method requirements.
Why does cable temperature matter?
A conductor's resistance rises as it gets hotter. Including an estimated operating temperature produces a more conservative and realistic voltage-drop estimate.
How can I reduce voltage drop?
Increase the cable cross-sectional area, shorten the route, reduce current where practical, and ensure every connection is correctly terminated and maintained.
Is voltage drop the same as power loss?
Not exactly. Voltage drop describes the reduction in voltage at the load, while power loss describes electrical energy converted into heat by conductor resistance. They are closely related but measure different effects.
Knowledge base
Browse all articles →Practical voltage drop guides
Clear explanations for cable sizing, circuit calculations and common design decisions.
How to Calculate Voltage Drop: A Practical UK Guide
Understand the voltage drop formula, the values you need, and how to check a cable run before installation.
8 min readRead article →
3% vs 5% Voltage Drop: Which Limit Should You Use?
A clear explanation of common UK voltage-drop design limits for lighting and other final circuits.
6 min readRead article →
How to Choose Cable Size for a Long Cable Run
See how cable length, current and conductor size interact—and why voltage drop is only one part of cable selection.
9 min readRead article →
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