Voltage Drop Calculator UK
Verify BS 7671 Regulation 525 compliance for electrical circuits in seconds
Voltage Drop Calculator
BS 7671 Table 4D1B mV/A/m method
mV/A/m values from BS 7671 Table 4D1B (PVC copper at 70°C)
⚡ Important Compliance Notes
- • BS 7671 limits: 3% for lighting, 5% for other circuits (Regulation 525.1)
- • Calculation method: Uses official Table 4D1B mV/A/m values at 70°C operating temperature
- • Formula: VD = (mV/A/m × I × L) ÷ 1000 — do NOT multiply by 2
- • mV/A/m values: Already account for both live and neutral conductors
- • Temperature: Values at 70°C (conductor temperature under load), not 20°C reference
- • Always consult: A qualified electrician for professional installations and complex scenarios
Voltage Drop Calculators by Application
Jump to a specific use-case calculator with pre-filled values for your scenario:
Submain to Outbuilding
Garden offices, garages, workshops, annexes
10mm²-25mm² SWA • 32A-63A
EV Charger Installation
7kW and 22kW home charging points
6mm²-10mm² • 32A continuous
Garden Lighting
Path lights, security lights, festoons
1.5mm²-2.5mm² • 3% lighting limit
Shower Circuit
8.5kW, 9.5kW, 10.5kW electric showers
6mm²-10mm² • 37A-46A
Cooker Circuit
Electric ovens, hobs, range cookers
6mm²-10mm² • 32A-45A with diversity
Three-Phase Motor
Industrial motors with 0.866 factor & starting current
Solar PV
Inverter to consumer unit AC cable sizing
Heat Pump
ASHP & GSHP circuits - MCS compliant
Marina Shore Power
Section 709 - stricter 3% limit applies
Caravan & Camping Site
Section 708 pitch supplies & PME considerations
Commercial Lighting
3% limit - offices, retail, emergency
Warehouse & Industrial
Long runs, three-phase distribution
Server Room / Data Centre
UPS feeds, critical power, 2-3% target
Agricultural & Farm
Section 705 - grain dryers, livestock
Swimming Pool
Section 702 - zones, SELV, bonding
Hot Tub & Spa
13A-32A dedicated outdoor circuits
Home Battery Storage
Tesla, GivEnergy, hybrid inverters
Workshop & Garage
Welders, compressors, machinery
Annex & Granny Flat
Separate dwelling supplies
Construction Site
Section 704 - temporary supplies
Ring Circuit
UK ring main - divide by 4 method
Radial Circuit
20A and 32A socket circuits
Domestic Lighting
Indoor lighting - 3% limit
Immersion Heater
3kW hot water circuits
12V DC Systems
Caravans, boats, solar, garden
SWA Armoured Cable
Buried and external cable runs
Underfloor Heating
Electric UFH mats and cables
Understanding Voltage Drop in Electrical Circuits
Voltage drop is the reduction in voltage that occurs as electrical current flows through cables due to conductor resistance. As current travels along a conductor, it encounters resistance which dissipates energy as heat and causes a voltage reduction. BS 7671:2018+A2:2022 Regulation 525 sets strict limits on acceptable voltage drop to ensure electrical equipment operates safely and efficiently.
Key principle: Long cable runs with high current draw experience significant voltage drop. A 32A circuit running 30 meters in 2.5mm² cable: VD = (18 × 32 × 30) ÷ 1000 = 17.28V (7.5%) — this exceeds the 5% limit and requires larger cable.
BS 7671 Regulation 525 Voltage Drop Limits
According to Regulation 525.1, the voltage drop between the origin of the installation and any load point must not exceed specific percentages. These limits ensure equipment operates properly, safely, and reliably.
Lighting Circuits - 3% Maximum
Lighting is more sensitive to voltage reduction. At 3% drop (6.9V at 230V), LEDs and fluorescents dim noticeably. Best practice targets <2% for LED circuits.
Power & Other Circuits - 5% Maximum
Motors and appliances tolerate greater voltage drop but still require adequate voltage for proper operation and to prevent overheating during high-load periods.
Why Voltage Drop Matters
Excessive voltage drop is one of the most common causes of electrical installation failures. The consequences are serious and costly:
❌ Effects of Excessive Voltage Drop
- • LED and fluorescent lights dim significantly
- • Motors overheat and operate inefficiently
- • Equipment malfunction or failure
- • Reduced appliance lifespan due to stress
- • BS 7671 non-compliance
- • EICR certification failures
✓ Benefits of Proper Design
- • Equipment operates at rated efficiency
- • Longer equipment and cable lifespan
- • Full compliance with BS 7671
- • Passes EICR inspections first time
- • Reduced operating costs
- • Customer satisfaction
How Voltage Drop is Calculated (BS 7671 Method)
This calculator uses the official BS 7671 method with mV/A/m values from Table 4D1B. These values are for thermoplastic (PVC) insulated copper conductors at 70°C operating temperature.
Formula:
VD = (mV/A/m × I × L) ÷ 1000
- • VD = Voltage drop in volts
- • mV/A/m = Millivolts per amp per metre (from Table 4D1B)
- • I = Current in amps
- • L = Cable length in metres
- • 1000 = Conversion factor (mV to V)
Important: Do NOT multiply by 2 — the mV/A/m values already account for both conductors.
Practical Example:
A 20A circuit running 30 metres in 2.5mm² cable (18 mV/A/m):
VD = (18 × 20 × 30) ÷ 1000 = 10.8V (4.7%)
This exceeds the 3% lighting limit but passes for power circuits. For lighting, use 4mm² (11 mV/A/m) = 6.6V (2.87%) ✓
Common Voltage Drop Scenarios
🏠 Long Cable Runs to Outbuildings
Garages, workshops, and garden buildings often require sub-mains of 20–50 metres. Example: 32A at 40m with 10mm² (4.4 mV/A/m) = (4.4 × 32 × 40) ÷ 1000 = 5.63V (2.4%) ✓
⚡ Electric Vehicle Charging Points
32A EV chargers draw continuous high current. 32A at 25m with 6mm² (7.3 mV/A/m) = (7.3 × 32 × 25) ÷ 1000 = 5.84V (2.5%) ✓
❄️ Large Motors and Air Conditioning
High inductive loads have poor power factor and starting inrush currents 5–7× higher than running current. Calculate voltage drop for starting current to prevent equipment failure.
💡 LED Lighting in Long Runs
LED drivers are sensitive to low voltage. 6A at 20m with 1.5mm² (29 mV/A/m) = (29 × 6 × 20) ÷ 1000 = 3.48V (1.5%) ✓ — within 3% lighting limit.
🏢 63A Submain Circuits
63A circuits are common for large residential installations. 63A at 30m with 16mm² (2.8 mV/A/m) = (2.8 × 63 × 30) ÷ 1000 = 5.29V (2.3%) ✓
✓ Pass Inspections & EICR Every Time
Verifying voltage drop compliance during the design phase saves costly remedial work. Use this calculator before installation to ensure your design passes certification first time. Document your calculations using Table 4D1B mV/A/m values to prove BS 7671 compliance to inspectors.
BS 7671 Table 4D1B - mV/A/m Reference
Voltage drop values for thermoplastic (PVC) insulated copper conductors at 70°C operating temperature. These values already include both conductors — do NOT multiply by 2.
| Cable Size (mm²) | mV/A/m | Typical Uses |
|---|---|---|
| 1.0 | 44 | Low-current single circuits |
| 1.5 | 29 | Lighting circuits |
| 2.5 | 18 | Ring finals, radial sockets |
| 4.0 | 11 | Immersion heaters, high-load radials |
| 6.0 | 7.3 | Cookers, showers up to 9kW |
| 10.0 | 4.4 | Large showers, small submains |
| 16.0 | 2.8 | Submains, long runs, 63A circuits |
| 25.0 | 1.75 | Main supplies, large submains |
Source: BS 7671:2018+A2:2022 Table 4D1B — Voltage drop (mV/A/m) for single-phase circuits. For three-phase, multiply result by 0.866.
Frequently Asked Questions
Q: What if my voltage drop calculation exceeds the limit?
Increase the cable size to get a lower mV/A/m value. For example, going from 2.5mm² (18 mV/A/m) to 4mm² (11 mV/A/m) reduces voltage drop by 39%. Alternatively, reposition the distribution board closer to the load, or split the circuit into two smaller circuits.
Q: Does the 3% or 5% limit apply to the whole installation or just final circuits?
The limit applies to the TOTAL voltage drop from the origin of the installation to the load point. This includes distribution circuits AND final circuits combined. You cannot exceed 3% (lighting) or 5% (power) total.
Q: Why don't you multiply by 2 when using mV/A/m values?
The mV/A/m values in Table 4D1B already account for both conductors (live and neutral). The old "multiply resistance by 2" method uses conductor resistance at 20°C, which is a different approach. Don't mix methods — use mV/A/m without the factor of 2.
Q: Can I temporarily exceed voltage drop limits during motor starting?
BS 7671 recognizes that transient voltage drops during motor starting (inrush current 5–7× running current) are acceptable if they don't cause equipment damage. However, design should minimize starting inrush voltage drop. Continuous operation must meet the stated limits.
Q: How do I account for future load growth in my design?
Design for anticipated future maximum demand rather than current load. This avoids costly cable upgrades later. For example, if a distribution might eventually need 40A, design for 40A now rather than upgrading from 20A later.
Q: Do I need to apply temperature correction to mV/A/m values?
No — the Table 4D1B values are already at 70°C operating temperature. Unlike the conductor resistance method which uses 20°C and needs correction, mV/A/m values represent real operating conditions under load.
Q: Is this calculator compliant with 18th Edition?
Yes, all calculations follow BS 7671:2018+A2:2022 Regulation 525 requirements and use mV/A/m values from Table 4D1B for thermoplastic (PVC) insulated copper conductors at 70°C operating temperature.
Q: What is a 63A circuit used for?
A 63A MCB is typically used for submain circuits to outbuildings, large commercial feeds, or significant domestic circuits. It's common for main distribution feeds and substantial loads requiring high current capacity. 63A circuits must be carefully designed to keep voltage drop within BS 7671 limits, often requiring 10mm² or larger cable.
Q: Do 63A circuits need larger cables?
Yes, 63A circuits almost always require larger cable than smaller circuits. For example, a 63A circuit over 20 metres needs 16mm² minimum to stay compliant. Always check the quick-select 63A button and test different cable sizes to find the right balance between voltage drop compliance and cost.
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