EV Charger Voltage Drop Calculator
Check voltage drop compliance for 7kW and 22kW EV charger installations
EV Charger Quick Facts
- 7kW charger draws 32A continuously - requires dedicated radial circuit
- 22kW three-phase draws 32A per phase - uses 0.866 factor for voltage drop
- Voltage drop affects charging speed - smart chargers may throttle if voltage low
- Standard cable: 6mm² (7.3 mV/A/m) for short runs, 10mm² (4.4 mV/A/m) for longer
- Part P notifiable - must be installed by registered electrician
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common EV Charger Scenarios
7kW charger - short driveway (10-15m)
32A with 6mm² (7.3 mV/A/m). At 15m: (7.3 × 32 × 15) ÷ 1000 = 3.50V (1.52%) ✓
7kW charger - long driveway (25-30m)
32A with 10mm² (4.4 mV/A/m). At 30m: (4.4 × 32 × 30) ÷ 1000 = 4.22V (1.84%) ✓
22kW three-phase charger (20m)
32A per phase with 6mm². (7.3 × 32 × 20 × 0.866) ÷ 1000 = 4.04V (1.01% on 400V) ✓
Charger in detached garage (40m)
32A with 10mm² SWA. At 40m: (4.4 × 32 × 40) ÷ 1000 = 5.63V (2.45%) ✓
Workplace charger - long run (50m+)
32A with 16mm² (2.8 mV/A/m). At 60m: (2.8 × 32 × 60) ÷ 1000 = 5.38V (2.34%) ✓
Complete Your EV Installation
Installing in a detached garage or outbuilding? Check submain voltage drop too. Need to size the cable? Use our cable sizing calculator. For the full voltage drop calculator with all options, see our main voltage drop tool.
EV Charger Voltage Drop
EV chargers draw high current continuously for extended periods - often 4-8 hours overnight. Unlike intermittent loads, this sustained draw means voltage drop is a critical design factor. Smart chargers monitor incoming voltage and may reduce charging power if it drops too low.
EV Charger Installation Costs (2024)
Typical UK installation costs. OZEV grant may cover up to £350 for eligible installations.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Simple install - charger near CU | £400-600 | £200-300 | £600-900 |
| Standard install (15-25m run) | £500-800 | £300-450 | £800-1250 |
| Long run to garage (30m+) | £700-1100 | £400-600 | £1100-1700 |
| Three-phase 22kW installation | £1200-2000 | £500-800 | £1700-2800 |
| CU upgrade required | +£300-600 | +£200-400 | +£500-1000 |
Prices as of 2024. Excludes charger unit cost (£300-1000 depending on brand).
Why EV Charger Voltage Drop Matters
Unlike typical domestic loads, EV chargers draw their rated current continuously for hours. A 7kW charger pulling 32A for 6 hours generates significant heat in the cable. Smart chargers actively monitor voltage and reduce power output if it drops below acceptable levels - meaning you might only get 5-6kW from a 7kW charger if voltage drop is excessive.
PME Earthing Considerations
Most UK supplies use PME (Protective Multiple Earthing). Some EV charger manufacturers require additional earth electrodes or specific protective devices for PME supplies. Check manufacturer requirements and consult a qualified electrician.
Frequently Asked Questions
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