Tutorial
Smart switch with no neutral wire
Many UK switch boxes only contain the live feed and the switched live. A smart module must stay powered while the light is off: here is what happens and how to avoid it.
Why LEDs flicker
A smart module has to listen to Wi-Fi or Zigbee even with the light off. Without a neutral, the only way to complete its circuit is to leak a small current through the bulb to the neutral at the ceiling rose. A filament passes it without lighting; an LED stores it in its driver capacitor, then flashes or glows faintly.
Order of magnitude: a module drawing 0.3 W on standby needs about 1.3 mA at 230V (0.3 ÷ 230), or 2.5 mA at 120V. A bypass is a small component fitted in parallel with the lamp at the ceiling: it gives that current a path to neutral that no longer goes through the LED.
US note: since the 2011 edition, NEC 404.2(C) requires a neutral at most switch locations, so newer US homes usually have one; older ones may not.
Check the minimum load
| Connected lighting | Power | Module rated from 3 W | Module that needs a bypass with LEDs |
|---|---|---|---|
| 1 × 2.5 W E14 LED bulb | 2.5 W | below minimum: bypass | bypass |
| 1 × 8 W E27/B22 LED bulb | 8 W | OK | bypass |
| 4 × 5 W GU10 spots | 20 W | OK | bypass |
| Chandelier, 5 × 40 W filament | 200 W | OK | OK without bypass (incandescent) |
Based on the SONOFF ZBMINIL2 (from 3 W) and Shelly 1L Gen3 (bypass required with any LED load) data sheets, both 230V products. In the US, choose a 120V, UL-listed no-neutral module. Even above the minimum, some cheap LEDs flicker: the bypass remains the fix.
Which solution to choose
| Your situation | Solution | Watch out |
|---|---|---|
| I don’t want to touch any wires | Smart bulb + wireless wall remote | The switch must stay on; one bulb per lamp holder |
| Keep my switch, I have a Zigbee hub | No-neutral Zigbee module (from 3 W depending on model) | Does not act as a Zigbee repeater |
| Wi-Fi without a hub | No-neutral Wi-Fi module + bypass at the light | Bypass mandatory with LEDs; needs access to the ceiling |
| Two-way switching | Module that supports two-way, wired at the switch that gets the live | Check the module instructions |
| Fitting with 20 W or more of LEDs | Zigbee or Wi-Fi module; bypass often unnecessary | Stay under the module’s maximum load |
Safety
This is mains work: switch off the circuit at the consumer unit (breaker panel in the US) and test that it is dead. In the UK it falls under BS 7671 and, in England, replacing a switch is not notifiable but must still be done competently; in the US, follow the NEC and local permit rules. For 12V MR16 spots behind a magnetic transformer, read replace halogen spots with LED first.
Where to buy in the UK
If you go the Wi-Fi route without a hub, the Shelly 1L Gen3 is sold directly on the Shelly UK store (affiliate link: we may earn a small commission at no extra cost to you). Remember the bypass if you only run LED bulbs.