Professional note
Hot tub and spa supplies must be designed, installed, and certified to BS 7671, manufacturer instructions, and—for locations analogous to swimming pools—the special-location rules that apply to your build (commonly Section 702 for “swimming pools, fountains, and basins”). In England, work in a special location (including a new circuit to a hot tub) is notifiable to Building Control or must be done by a registered competent person. This page explains the engineering principles; it is not a substitute for your electrician’s assessment, adiabatic calculation, and test results.
Key takeaways
- On many TN-C-S (PME) supplies, a garden hot tub is treated with the same earthing caution as other outdoor “in contact with the general mass of earth” equipment: a dedicated earth electrode (earth rod or array) is often required, with measured earth resistance to prove that protective devices will operate in time.
- Where a TT system is used for the hot-tub final circuit, the PME (combined) earthing is not simply continued to the shell of the unit: the installation is engineered so the protective path is local to the rod at the point of supply change—effectively a separation of the PME earthing for that part of the installation at a defined isolator / origin of the outbuilding or dedicated circuit, documented on the certificate.
- Fixed isolating switchgear, local isolators, and RCD/RCBO enclosures (anything you might operate while standing on wet paving) are positioned at least 2 metres away from the hot tub (or further if Section 702 zones demand it) so the equipment sits outside the immediate wet/hazard area.
- The buried garden run is almost always armoured (SWA) (or an equivalent protected and mechanically robust arrangement), with size chosen from design current, method of installation, and voltage drop—not from a rule-of-thumb in a forum post.
Why earthing matters: PME, earth rods, and “severing” at isolation
Most UK homes—including in Margate, Ramsgate, and Broadstairs—are supplied with TN-C-S, which people often call PME. The distributor’s combined neutral and earth (PEN) is re-separated at the service position so your installation has a protective earth (PE) conductor for normal use. The concern for outdoor fixed metal equipment in garden soil (hot tubs, steel housings, pipework) is the same in principle as for EV points and outbuildings: a PEN fault on the network (open neutral) can, in the worst case, raise touch voltages on bonded metalwork. That is why BS 7671 and industry practice have long used local TT (earth electrode + RCD) for the dedicated outdoor circuit, or other solutions accepted for the product and supply.
When your electrician installs a TT branch for the hot tub, you will see this described on the Electrical Installation Certificate: the earth rod (or array) is tested (RA, loop impedance) so that a fault to earth trips an RCD (typically 30 mA) within required times. The conceptually important point—what is often called separation of the earthed reference for that circuit—happens at the origin of the separated system (e.g. at the isolator, garage sub-board, or dedicated outbuilding / pod origin): the hot tub’s protective conductor is now referenced to local earth, not the PME earth of the main house for that part of the installation, in a way the designer is prepared to evidence and test.
There are alternative engineered approaches in specific product and DNO situations; the headline for homeowners is: an earth rod is not a cosmetic extra — it is often the difference between a compliant outdoor supply and a dangerous assumption that “it’s earthed at the house” is always enough for a tub sat in the lawn.
Where the isolator goes: the 2 metre rule
A fixed means of local isolation and emergency switch-off must be in a safe, accessible place — and not where someone must lean over the water. Good practice, aligned with the intent of Section 702 (separation of electrical equipment from water and zones) is to mount lockable / clearly labelled isolator switches, RCD housings, and any socket-outlets for maintenance at least 2 metres from the water surface / tub shell, in a dry, reachable position (often on a wall, post, or inside a weatherproof enclosure in the line of the supply route). If the tub sits inside a full pool-type enclosure, your designer may apply zone dimensions that require greater separation—your installer will measure against the particular layout.
If someone proposes switching or jointing inside the skirting of the hot tub with no other isolation point outside the splash zone, treat that as a red flag and insist on a design that gives you a local disconnect in the right place, plus emergency switching where the manufacturer and BS 7671 require it for that product type.
Cables, armouring, and why size is not one number
Tub nameplates are quoted in watts and amps (e.g. 20 A, 32 A, 40 A) but the final circuit cable is selected after:
- Design current (Ib) and the rating of the overcurrent device (In)
- Method of installation (buried, clipped, in conduit, thermal insulation)
- Grouping and ambient correction (hot plant rooms, stud walls)
- Voltage drop (especially on long garden runs—Thanet garden lengths vary from compact patios to long coastal plots)
- RCD/RCBO type: many spas use electronic controls; Type A (or B where specified) is usually considered, not Type AC alone
- 30 mA (or less) RCD or RCBO protection for the final circuit, as required for the special / wet context
Steel wire armoured (SWA) cable is the default for a direct-buried or surface outdoor route: the armour is an earthing and mechanical shield; glands are weatherproof and IP-rated where they enter a tub enclosure or sub-panel. Non-armoured T&E laid loose in a garden is not a professional end-to-end solution for a fixed 230 V hot-tub final circuit — and often fails Part L / inspection perspective on mechanical protection and moisture.
Because of correction factors, a “32 A tub” might correctly land on 4 mm² on a short run or 6 mm² (or more) for a long, thermally derated, buried route—that is normal and is why a formal design is required, not a guess from the nameplate.
Further points your electrician will cover
- Supplemental and protective equipotential bonding for extraneous-conductive-parts in the vicinity (perimeter metalwork, deck structures, handrails) where the BS 7671 702/705 criteria are met—especially if a structural steel frame or rebar is within arm’s reach of the water
- External influences: IP rating of enclosures, UV-resistant materials, and condensation inside small plant pockets under the shell
- Back-up power: never a casual generator feed; the earthing and switching arrangements must be designed if off-grid use is a serious requirement
- Future part exchange: a slightly larger sub-main or spare ways in the OCP/RCD layout if you uprate the tub later
- Gas vs electric heat: the electrical control, pumps, and blower circuits are still a fixed installation—do not conflate a gas heater with “no electrical work”
If you are planning a Thanet install
We survey supply capacity (is there headroom in the main fuse and consumer unit for another high-demand circuit?), the earthing arrangement, and the route and burial depth for the armoured run, then specify isolation position, RCD, and cable type in line with BS 7671 and notification rules.
Get the supply and earthing right first
Ask for a written pre-installation view of: supply spare capacity, PME/TT plan, and whether your tub position meets separation to switchgear before the decking is fixed.
Request a quote Domestic servicesLast updated: April 2026 — written for BS 7671:2018+A4:2026 and UK special location / notification practice. Always follow your equipment manual and a qualified designer.