Workshop & Home Garage Voltage Drop Calculator
Calculate voltage drop for workshop and garage electrical supplies
Workshop Quick Facts
- Workshop equipment often has high starting currents - welders and compressors can draw 2-3× running current
- Detached garages need SWA cable or protected conduit for the supply run
- Consider future equipment - size the main supply with headroom for additions
- Lighting circuits use 3% voltage drop limit; power circuits use 5%
- Three-phase supply opens up options for larger machinery (lathes, mills, welders)
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common Workshop Scenarios
Basic garage - lights and sockets (15m)
32A with 6mm² (7.3 mV/A/m). At 15m: (7.3 × 32 × 15) ÷ 1000 = 3.50V (1.52%) ✓
Workshop with welder (140A input, 25m)
63A with 16mm² (2.8 mV/A/m). At 25m: (2.8 × 63 × 25) ÷ 1000 = 4.41V (1.92%) ✓
Compressor (3kW) - long run (30m)
16A with 4mm² (11 mV/A/m). At 30m: (11 × 16 × 30) ÷ 1000 = 5.28V (2.30%) ✓
Lathe/mill (2.2kW 3Ø) - 20m run
5A per phase with 2.5mm² (18 mV/A/m). At 20m 3Ø: (18 × 5 × 20 × 0.866) ÷ 1000 = 1.56V (0.39% on 400V) ✓
Full workshop supply (40m detached)
63A with 25mm² (1.75 mV/A/m). At 40m: (1.75 × 63 × 40) ÷ 1000 = 4.41V (1.92%) ✓
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Workshop Electrical Demands
Workshops have diverse electrical loads: lighting, general sockets, and dedicated circuits for specific machinery. Unlike domestic installations, workshop equipment often runs at high power for extended periods and has significant starting currents. A 2kW compressor might draw 15A running but 30-40A starting. Plan the main supply with capacity for your largest equipment plus general loads.
Workshop Electrical Installation Costs (2024)
Typical UK costs for workshop and garage electrical installations.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Basic garage supply (<15m) | £300-500 | £350-550 | £650-1050 |
| Workshop supply (15-25m) | £450-750 | £400-650 | £850-1400 |
| Heavy workshop (25-40m) | £600-1000 | £500-800 | £1100-1800 |
| Three-phase workshop supply | £800-1500 | £600-1000 | £1400-2500 |
| Workshop sub-board (6-way) | £200-400 | £200-350 | £400-750 |
Prices as of 2024. Costs vary by cable run length and equipment requirements.
Welder Electrical Requirements
MIG and stick welders are among the most demanding workshop loads. A 140A welder typically draws 30-50A from the supply (check nameplate). Larger 200A+ welders may need 63A or dedicated circuits. Welders cause voltage fluctuations - undersized cables result in poor arc stability and can affect other equipment. Size generously.
Detached Garage Supply Options
Detached garages need armoured cable (SWA) or protected conduit from the house. Options: extend from house consumer unit (common for short runs), or install a sub-board in the garage fed from the main board. Sub-boards allow local RCD protection and easier circuit additions. For long runs, a garage sub-board often makes more sense.
Three-Phase for Workshops
Three-phase 400V supplies enable larger motors and industrial machinery. Many lathes, mills, and large compressors are three-phase. Three-phase welders provide smoother arc. If you're serious about workshop machinery, consider requesting three-phase from your DNO - the application process takes weeks but the capability is worth it for heavy users.
Frequently Asked Questions
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