Hot Tub & Spa Voltage Drop Calculator
Calculate voltage drop for hot tub and spa dedicated circuits
Hot Tub Quick Facts
- Hot tubs need dedicated circuits - never share with other loads
- Most UK hot tubs require 13A, 16A, 20A or 32A supply depending on heater size
- 32A hot tubs (7.5kW) are most common for quality models with fast heat-up
- Outdoor installation requires RCD protection and weatherproof isolator within sight
- SWA cable or protected conduit required for buried/outdoor cable runs
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
Need a Qualified Electrician?
Get quotes for hot tub electrical installation
Common Hot Tub Scenarios
13A plug-in hot tub (3kW) - extension not recommended
Plug-in tubs should use dedicated socket. If long run needed: 13A with 2.5mm² at 15m: (18 × 13 × 15) ÷ 1000 = 3.51V (1.53%) ✓
16A hot tub (3.7kW) - 15m garden run
16A with 4mm² (11 mV/A/m). At 15m: (11 × 16 × 15) ÷ 1000 = 2.64V (1.15%) ✓
20A hot tub (4.5kW) - 20m to patio
20A with 4mm² (11 mV/A/m). At 20m: (11 × 20 × 20) ÷ 1000 = 4.40V (1.91%) ✓
32A hot tub (7.5kW) - 25m garden run
32A with 6mm² (7.3 mV/A/m). At 25m: (7.3 × 32 × 25) ÷ 1000 = 5.84V (2.54%) ✓
32A hot tub - long run (40m)
32A with 10mm² (4.4 mV/A/m). At 40m: (4.4 × 32 × 40) ÷ 1000 = 5.63V (2.45%) ✓
Related Outdoor Installations
Adding a garden lighting circuit near your hot tub? For larger pool installations, see swimming pool electrical. Use our cable sizing calculator to check current capacity.
Hot Tub Electrical Requirements
Hot tubs require dedicated circuits because their heaters draw significant current continuously. A 32A (7.5kW) hot tub running its heater and pumps draws close to full rated current for extended periods. Sharing with other loads risks overloading the circuit and tripping. The dedicated circuit ensures reliable operation and meets manufacturer warranty requirements.
Hot Tub Electrical Installation Costs (2024)
Typical UK costs for hot tub electrical installation.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| 13A dedicated socket circuit | £100-200 | £200-350 | £300-550 |
| 16/20A hot tub circuit (<15m) | £150-300 | £250-450 | £400-750 |
| 32A hot tub circuit (<25m) | £250-450 | £350-550 | £600-1000 |
| 32A hot tub - long run (25-40m) | £400-700 | £400-650 | £800-1350 |
| Rotary isolator + installation | £50-120 | £80-150 | £130-270 |
Prices as of 2024. Costs vary by cable run length and installation complexity.
Local Isolator Requirements
BS 7671 requires a means of isolation within sight of the hot tub or within 3m. A weatherproof rotary isolator (IP65 minimum) mounted near the tub allows safe disconnection for maintenance without going back to the consumer unit. The isolator should be lockable for safety during servicing and positioned where users exiting the tub can't accidentally touch it.
RCD Protection for Hot Tubs
As outdoor equipment that combines electricity and water, hot tubs require 30mA RCD protection (BS 7671 Section 701 principles apply near water). The RCD can be at the consumer unit or as an RCBO. Some installers add a local RCD at the isolator for additional protection, but this isn't required if main board protection exists.
Cable Installation Methods
Underground cables to garden hot tubs must be SWA (Steel Wire Armoured) or protected in suitable conduit. Burial depth: minimum 450mm (600mm under driveways). Warning tape should be placed above the cable. Above-ground runs can use SWA on cable clips or standard cable in conduit. All outdoor sections need UV-resistant materials or protection from sunlight.
Frequently Asked Questions
Related Electrical Calculators
Get Quotes from Local Electricians
Looking for a qualified electrician for your hot tub installation? Tell us about your project and we'll connect you with vetted contractors in your area. Free, no obligation quotes.
Trade professional or electrical business? Use the form above and let us know - we offer lead referrals in your area, bulk calculation tools, and white-label partnerships for merchants and suppliers.
More Electrical Calculators
Voltage drop, cable sizing, conduit fill and more. All BS 7671 compliant, all free.
View All Calculators