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Hot Tub Cable Size Calculator

Calculate cable size, MCB rating, and RCD requirements for hot tub installations

Reference Info & Formulas
Quick Reference

2.5 kW (13A plug): No hardwired circuit needed

4.5 kW (20A): 4mm² SWA usually ok

7.5 kW (32A): 6mm² SWA typical

SWA required for outdoor/underground runs

Requirements

Dedicated radial circuit

30mA Type A RCD

DP isolator within sight

IP65+ outdoor enclosure

Part P notifiable

Hot Tub Details
Select the hot tub power rating
Cable Route
Enter cable run and installation details

Data: BS 7671:2018+A2:2022 — Tables 4D4A, 4D5, Section 701

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How Does This Hot Tub Cable Size Calculator Work?

Hot tub circuits are dedicated radial circuits supplying a continuous resistive and motor load — cable must handle 100% of the rated power with no diversity.

The design current is calculated as Ib = P / V. For example, a 7.5kW hot tub at 230V draws 7500 / 230 = 32.6A. No diversity is applied because the heater and pump operate simultaneously at full rated power during the heating cycle.

Ib = P / V
P
= Hot tub power rating in watts
V
= Nominal supply voltage (230V)

BS 7671 Appendix 4

Correction factors are then applied to derate the cable's tabulated current capacity: ambient temperature (Ca from Table 4B1), grouping (Cg from Table 4C1), and thermal insulation (Ci from Table 52.2). The cable is selected so that its derated capacity exceeds the MCB rating, which must in turn exceed the design current: Ib ≤ In ≤ Iz.

What Cable Sizes Do Hot Tubs Require?

SWA cable sizing for common hot tub ratings (buried direct, Method D)

Table 4D4A
Hot Tub (kW)Design Current (A)Cable SizeMCB Rating
2.510.92.5mm²16A
4.519.62.5mm²20A
7.532.66mm²32A
939.16mm²40A
1147.810mm²50A

Based on Method D (buried direct), 30\u00B0C ambient, no grouping. Always verify voltage drop for your cable length.

Why Is SWA Cable Required for Outdoor Hot Tubs?

SWA cable provides essential mechanical protection for outdoor and underground installations. The steel wire armour protects the conductors from damage during installation and from external impacts such as garden tools, and it also serves as the circuit protective conductor (CPC). T&E cable lacks this protection and must not be used for buried or exposed outdoor runs.

PME Earthing Restriction

If your supply uses PME earthing (TN-C-S) — the most common UK arrangement — you cannot use the supply earth for the exposed-conductive-parts of equipment installed outdoors. Regulation 411.4.2 requires a separate earth electrode (earth rod) for outdoor circuits. Failure to comply creates a serious shock risk if the PEN conductor is lost.

SWA Gland Selection

When terminating SWA cable, the correct gland size is essential for maintaining IP rating and earth continuity. Use the SparkyHub SWA Cable Gland Calculator to select the correct CW or BW gland for your cable size.

Practical Installation Notes for Hot Tub Circuits

Local Isolation Required

BS 7671 Regulation 701.512.3 requires a double-pole isolator to be installed within sight of the hot tub. This must be IP65 rated or better for outdoor use, mounted at a minimum height of 1.2m, and positioned where the user can reach it in an emergency without crossing the hot tub area.

Voltage Drop on Long Runs

Hot tubs are often positioned at the far end of a garden, resulting in cable runs of 20-30m or more. At 32A, a 25m run of 6mm² SWA drops about 5.1V (2.2%) — well within limits. But at 40m, the drop reaches 8.2V (3.6%), which is still compliant but leaves less margin. Always check voltage drop for long runs, and upsize to 10mm² if necessary.

Part P Notification

Installing a new circuit for a hot tub is notifiable work under Part P of the Building Regulations. You must either be registered with a Competent Person Scheme (CPS) or notify your Local Authority Building Control (LABC) before starting work.

Frequently Asked Questions

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