Annex & Granny Flat Voltage Drop Calculator
Calculate voltage drop for annex and granny flat electrical supplies
Annex Quick Facts
- Annexes may require separate metering depending on occupancy and planning status
- Full dwelling loads include heating, hot water, cooking - size for 60-100A typical
- Building Regulations Part P applies - installation must be certified
- Consider future EV charging and heat pump requirements in sizing
- Fire separation between main house and annex may affect cable routing
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common Annex Scenarios
Small annex (studio flat) - 20m run
40A with 10mm² (4.4 mV/A/m). At 20m: (4.4 × 40 × 20) ÷ 1000 = 3.52V (1.53%) ✓
Standard granny flat - 30m run
63A with 16mm² (2.8 mV/A/m). At 30m: (2.8 × 63 × 30) ÷ 1000 = 5.29V (2.30%) ✓
Full annex (2-bed) - 40m run
80A with 25mm² (1.75 mV/A/m). At 40m: (1.75 × 80 × 40) ÷ 1000 = 5.60V (2.43%) ✓
Garden annex - long run (50m)
63A with 25mm² (1.75 mV/A/m). At 50m: (1.75 × 63 × 50) ÷ 1000 = 5.51V (2.40%) ✓
Annex with heat pump + EV
100A with 35mm² (1.25 mV/A/m). At 30m: (1.25 × 100 × 30) ÷ 1000 = 3.75V (1.63%) ✓
Related Calculations
For simpler outbuilding submains. Adding EV charging to the annex? Heat pump calculations for electric heating. Use our cable sizing calculator for current capacity.
Annex Electrical Requirements
An annex is effectively a small dwelling requiring full electrical facilities: lighting, power, heating, hot water, cooking. Unlike outbuildings, annexes have sustained loads similar to houses. Size the supply cable for maximum demand: all heating + cooking + general loads with appropriate diversity. Typical annexes need 40-80A supplies; larger ones with modern loads may need 100A.
Annex Electrical Installation Costs (2024)
Typical UK costs for annex electrical installations.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Small annex supply (<20m) | £500-900 | £600-1000 | £1100-1900 |
| Standard annex (20-35m) | £800-1400 | £800-1300 | £1600-2700 |
| Large/distant annex (35-50m) | £1200-2000 | £1000-1500 | £2200-3500 |
| Annex consumer unit (10-way) | £300-500 | £300-500 | £600-1000 |
| Complete annex wiring (per room) | £200-400 | £250-450 | £450-850 |
Prices as of 2024. Full annex fit-out costs vary significantly by size and specification.
Metering Options for Annexes
Annexes can be supplied from the main house (shared meter) or have separate metering. Separate meters are required for independent letting and may be required by the DNO for larger annexes. Sub-metering (meter after main house meter) is an option for family use. Consult your DNO and local planning authority for specific requirements.
Part P Building Regulations
Annex electrical installations are notifiable work under Part P of Building Regulations. The work must be done by a registered competent person (registered electrician) or inspected by Building Control. You'll receive an Electrical Installation Certificate and Building Regulations compliance certificate. Non-compliant work can cause problems when selling.
Future-Proofing Annex Supplies
Modern annexes may eventually need EV charging (7kW = 32A), heat pumps (16-32A), and high-power appliances. Size the main supply cable with growth in mind - upgrading buried cable later is expensive and disruptive. A 100A supply provides excellent headroom and costs little more than 63A for the cable run.
Frequently Asked Questions
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