# Using your EV (V2L) as home backup — the changeover and earthing nobody explains > You can run a small essential load — lights and a socket or two, typically up to about 3.6 kW — from an EV's V2L output. You cannot make a cheap voltage-sensing automatic transfer switch select the car on a schedule; that needs a timed relay driving an interlocked two-pole contactor changeover. The hard part is earthing a **floating** V2L output on a PME (TN-C-S) supply — an outbuilding is made TT with a local electrode, while indoors you use an isolation transformer and keep the single PME/MET earth. It must be designed and proven by test by a competent person, and the car maker does not sanction this use. **Safety frame.** This is education, not an instruction to carry out work. Work connecting V2L equipment to fixed wiring is safety-critical and may be notifiable under Part P. It must be designed, installed, inspected and tested by a competent person to the current edition of BS 7671. Vehicle manufacturers generally do not sanction back-feeding fixed wiring from V2L outlets; follow manufacturer instructions. ## In short - An EV's V2L output can run a small essential load — typically up to ~3.6 kW (≈10–15 A) at 230 V. - A cheap voltage-sensing ATS senses *source loss*, not a schedule, and has no control input — you cannot make it switch to the car on command. - A timed smart relay driving an **interlocked 2-pole contactor changeover** is the correct way to switch on a schedule, break-before-make. - Earthing a **floating** V2L output on PME is the crux: outbuilding → make it **TT** (local electrode); indoors → use an **isolation transformer** and keep the one PME/MET earth. - Designed and **proven by test** by a competent person. The vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet. **Where this stops:** This explains the engineering and the standards. It is not a wiring recipe — the design, installation and testing are for a competent person. ## The plan most people start with — and why it's wrong The common idea is to use a Wi-Fi smart relay to 'fool' a cheap automatic transfer switch (ATS) into selecting the car. It fails in three ways. First, the smart relay is a roughly 2 A dry contact, while every supply path here is 63 A — it cannot switch or interrupt them. Second, the auto-ATS decides for itself on *voltage loss*; its status terminals are indicators, not a control input, so there is nothing to command. Third, even if you forced it, you would island a 63 A board onto a roughly 15 A source — a gross overload — and onto a **floating** supply with no earth reference. > **Whole-house onto V2L is a gross overload** — A typical consumer unit is rated around 63 A; an EV's V2L output is around 10–15 A. The job is a small *essential* board — lighting and one radial socket — not the whole house. ## How it actually works An auto-ATS is self-sensing: it transfers in a couple of seconds on source loss and auto-reverts when the source returns. That is the wrong behaviour for moving between two *healthy* sources on a timer. The V2L output itself is around 2.3–3.6 kW (≈10–15 A) at 230 V and cannot be used while the car is charging. On the Hyundai Ioniq 5 the output is reported to be **floating** — no internal neutral-earth bond — so without an external bond an RCD has nothing to operate against. Confirm this on the actual adapter; reports vary. > **Confidence: inference** — Ioniq 5 V2L output is floating (no internal neutral-earth bond). (Reported in field accounts and consistent with the design; behaviour varies by adapter and should be bench-verified before it is relied on. The safe default is to treat the output as floating until proven otherwise.) ## The correct approach — power and control Use a timed smart relay (in maintained mode) to *command* an interlocked **two-pole** contactor changeover: KM1 the grid contactor (the de-energised, fail-safe default) and KM2 the V2L contactor. The relay carries only the small coil current; the contactors carry the load. A **mechanical interlock** is the hard guarantee against paralleling — an EV inverter cannot parallel the grid. On power loss the changeover falls back to grid (fail-safe). Keep actual draw within about 80% of the V2L rating. **Figure: SONOFF control of the interlocked changeover.** The smart relay only commands: its single SPDT contact energises either the grid contactor (KM1, the de-energised fail-safe default) or the V2L contactor (KM2) — never both, because they are mechanically interlocked. _A SONOFF MINI-D relay in maintained mode has one changeover (SPDT) contact: COM, NC and NO. Grid line (L) feeds COM. The NC (normally-closed) output drives the KM1 coil — the grid contactor — so on power loss the load falls back to grid (fail-safe). The NO (normally-open) output drives the KM2 coil — the V2L contactor. Both coil returns go to grid neutral (N). KM1 and KM2 are mechanically interlocked so they can never close together. A schedule (e.g. 05:30 to V2L, 23:30 back to grid) drives the relay. The relay carries only the small coil current; the contactors carry the load._ ## The correct approach — earthing (the crux) Every source needs exactly one neutral-earth reference. The grid's is at the substation; a floating V2L output has none, so you must supply one — at the source side, in circuit only when on V2L, never two bonds in parallel. How you do that depends on **where** the board is. > **§722.411.4.1** (BS 7671, confidence: inference) — On a PME (TN-C-S) supply, you must not simply rely on the distributor's earth for the EV side — an open PEN can raise metalwork to a dangerous voltage. This is why the floating-on-PME approach is contested and must be designed and proven by test. _Reference only; standard text not reproduced._ ### Separate outbuilding → make it TT Drive a local earth electrode and take all the outbuilding's circuit protective conductors to its earth bar — continuous, never switched. Do **not** export the dwelling's PME earth (broken-PEN risk); only line and neutral run over. Make the single V2L neutral-earth bond through a KM2 auxiliary contact, so it exists only on V2L. Protect with a Type A RCD. This only works if no metal pipe, structure or cable armour bridges the two earth systems. **Figure: TT-island earthing — bond and aux-contact detail.** For an outbuilding made TT: a local electrode feeds the earth bar and all CPCs continuously; the V2L neutral-earth bond is made through a KM2 auxiliary contact, so it exists only on V2L. _The TT-island arrangement. A local earth electrode connects to the outbuilding earth bar; all circuit protective conductors (E/CPC) connect to that bar continuously and are never switched. The PME (distributor's) earth is NOT exported to the outbuilding — only line (L) and neutral (N) run over. The V2L neutral-earth (N–E) bond is made through an auxiliary contact on the V2L contactor (KM2), so the single bond exists only while on V2L. An RCD protects the island. The point: one continuous local earth, one switched source bond, no exported PME earth._ ### Inside the dwelling → isolation transformer You cannot TT a single indoor sub-board — it creates a simultaneous-reach hazard with the rest of the PME-earthed installation. Keep one earth. An isolation transformer in the V2L feed galvanically separates the floating output; make one neutral-earth bond on the transformer **secondary** (on V2L), and the earth stays continuous to the main earthing terminal. No spike, no indoor TT island. **Figure: Isolation-transformer method (indoors).** An isolating transformer in the V2L feed galvanically separates the floating output; a single neutral-earth bond is made on the transformer secondary, keeping the one PME/MET earth continuous. _Indoors, an isolating transformer sits in the V2L feed. Its primary takes line (L) and neutral (N) from the floating V2L source; its secondary is galvanically separated. A single neutral-earth (N–E) bond is made on the secondary, giving the home-side circuit a defined earth reference. The installation keeps its one PME/MET earth continuous to the main earthing terminal — there is no second electrode and no TT island indoors (which would create a simultaneous-reach hazard). The transformer breaks the floating-source problem while preserving the single house earth._ > **§551.4.3.2.1** (BS 7671, confidence: inference) — A switched-alternative source needs an independent means of earthing — the distributor's earth may be disconnected during network maintenance, so the island cannot depend on it alone. _Reference only; standard text not reproduced._ ## The non-obvious traps - **Bonding the V2L neutral to earth feels wrong but is correct** — it is the source reference, the equivalent of the substation transformer's earthed star point. One source, one bond, made only on V2L. - **A floating output passes a socket tester but offers no RCD protection** until that bond exists — the dangerous 'looks fine' case. - **Never export a PME earth to an outbuilding** — and a stray metal pipe or cable armour can silently re-import it. - **You can't TT one indoor sub-board** — simultaneous reach with the rest of the PME installation. **Figure: V2L / PME changeover earthing circuit.** On a PME (TN-C-S) supply the V2L island gets its own earth reference — the CPC stays continuous, the neutral-earth bond is switched in only on V2L, and a local electrode provides the earth. _The diagram shows two sources feeding an essential board through a changeover. From the grid, line (L) and neutral (N) arrive at the changeover; the circuit protective conductor (E/CPC) runs continuously to the board and earth bar and is never switched. From the V2L source, L and N arrive at the other side of the changeover. A neutral-earth (N–E) bond is made only when the board is on V2L, providing the floating output its single earth reference. A local earth electrode connects to the earth bar. The point: every source has exactly one neutral-earth reference, and the protective conductor is continuous in both switch states._ ### How this is made and proven compliant **What governs it:** BS 7671 §722.411.4.1 (PME / open-PEN protective measures) and §551 island/switched-alternative source provisions; Switchgear product standards for the changeover (BS EN IEC 60947-6-1 / 60947-4-1 / 60947-3) — see the changeover deep-dive; BS EN IEC 61558-2-4 (isolation transformer, indoor route) or BS 7430 (TT-island electrode, outbuilding route) **Who may do it:** Design, installation, inspection and testing by a competent person. Adding an inlet circuit, changeover switch or consumer-unit alteration is normally notifiable under Part P (England; Wales/Scotland/NI differ). **How compliance is demonstrated:** Initial verification to BS 7671 Part 6 with an Electrical Installation Certificate; RCD operation proven by test in **both** grid and V2L modes; For TT: electrode resistance (Ra) measured low and stable enough for disconnection; Confirm the actual adapter's neutral-earth behaviour on the bench before relying on it ## FAQ ### Can I use a smart relay to make the ATS switch to my EV? No. A cheap ATS senses source loss and has no control input, and a ~2 A smart relay cannot switch a 63 A supply path. Scheduling needs a timed relay commanding an interlocked contactor changeover. ### Why am I bonding the V2L neutral to earth — isn't that wrong? It is correct. A floating source has no earth reference; the bond supplies one, exactly as the substation transformer's star point does for the grid. Make exactly one bond, on the source side, in circuit only on V2L. ### Is TT always better — can't I just run an earth spike outside? Not indoors. A single indoor TT sub-board creates a simultaneous-reach hazard with the rest of the PME-earthed installation. Indoors you keep one earth and use an isolation transformer; the TT route is for a genuinely separate outbuilding. ### How much can an EV's V2L actually power? Typically up to about 3.6 kW (≈10–15 A) at 230 V — a small essential board, not the whole house. Keep the actual draw within roughly 80% of the rating, and remember you cannot use V2L while the car is charging. ### Can I use V2L while the car is charging? No — the V2L output is not available while the vehicle is charging. A scheduled scheme typically charges the car on a cheap overnight rate and runs the board off V2L during expensive hours, returning to grid before the next charge. --- _Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._ _Last reviewed: 14 June 2026. Written against: BS 7671:2018 + A4:2026._ _Status: published. General information, not project-specific design advice._ _[How we source this](/methodology) — evidence hierarchy, confidence flags and source policy._ ## Sources 1. BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (cited by clause only; standard text not reproduced) 2. IET — Amendment 4 updates to the 18th Edition — https://electrical.theiet.org/amendment-4-updates-to-18th-edition 3. V2L Workshop technical reference (internal) — verified design facts and confidence flags