Voltage Drop Calculator
About Voltage Drop
Voltage drop is the decrease in electrical potential along the path of a current flowing in an electrical circuit. In cables, this occurs due to the resistance of the conductor material, resulting in energy loss and reduced voltage at the load end compared to the source.
Why Voltage Drop Matters
Excessive voltage drop can cause:
- Lights to dim or flicker
- Motors to run inefficiently and overheat
- Appliances to perform poorly or fail prematurely
- Increased energy consumption and higher electricity bills
- Non-compliance with BS 7671 wiring regulations
Voltage Drop Formula:
Voltage Drop (V) = (mV/A/m × Current × Length) ÷ 1000
Where mV/A/m is the millivolt drop per ampere per metre from BS 7671 tables
Voltage Drop (V) = (mV/A/m × Current × Length) ÷ 1000
Where mV/A/m is the millivolt drop per ampere per metre from BS 7671 tables
UK Voltage Drop Limits (BS 7671)
| Installation Type | Lighting Circuits | Power Circuits |
|---|---|---|
| Public LV Supply | 3% | 5% |
| Private LV Supply | 6% | 8% |
Cable Selection Guide
Factors Affecting Voltage Drop
- Cable Length: Longer cables have higher resistance and greater voltage drop
- Current Load: Higher current increases voltage drop proportionally
- Cable Size: Larger cross-sectional area reduces resistance and voltage drop
- Material: Copper has lower resistance than aluminium
- Temperature: Higher temperatures increase resistance
Common Cable Sizes and Applications
- 1.0-1.5 mm²: Lighting circuits, low-power applications
- 2.5 mm²: Ring circuits, socket outlets
- 4.0-6.0 mm²: Cooker circuits, large appliances
- 10.0+ mm²: Distribution circuits, high-power equipment
Frequently Asked Questions
What is acceptable voltage drop in the UK?
According to BS 7671, voltage drop should not exceed 3% for lighting circuits and 5% for power circuits in installations supplied from public low-voltage systems. For private supplies, limits are 6% and 8% respectively.
How do I reduce voltage drop in my installation?
You can reduce voltage drop by: using larger cable cross-sectional area, shortening cable runs, reducing load current, choosing copper over aluminium conductors, or installing multiple smaller circuits instead of one large circuit.
Does cable type affect voltage drop calculations?
Yes, different cable types have different resistance values per metre. Single-core, multicore, armoured, and various insulation types all have specific voltage drop values listed in BS 7671 Appendix 4 tables.
What happens if voltage drop exceeds the limits?
Excessive voltage drop can cause equipment malfunction, reduced efficiency, overheating of motors, dim lighting, and non-compliance with wiring regulations. The installation may fail inspection and require remedial work.
Should I include both live and neutral conductors in calculations?
Yes, voltage drop occurs in both live and neutral conductors. The calculator accounts for the total circuit length, which includes both conductors. For three-phase calculations, different factors apply as specified in BS 7671.
Installation Best Practices
Planning Your Installation
- Calculate voltage drop at the design stage
- Consider future load increases
- Plan cable routes to minimise length
- Account for ambient temperature conditions
- Consider grouping factors if multiple cables are bundled
Testing and Verification
After installation, verify voltage drop by measuring voltage at the source and load under full load conditions. The difference should not exceed the calculated values and regulatory limits.