People size smart switches by one number on the listing: 600 W per gang, or 1000 W, or whatever the box says. Then they install one, the load is well under the rating, and months later the relay clicks once and never clicks again. The wattage number was never the whole rule, and in the two most common failure cases it was not even the relevant part.
What the rating actually measures
That per-gang figure is almost always a resistive load rating. It describes a heater, an incandescent bulb, a kettle — a load where current tracks voltage smoothly and the only thing that matters is steady-state heat in the relay contacts.
Three of the most common household loads are not resistive:
- LED drivers and switch-mode supplies draw a large inrush current in the first few milliseconds as their input capacitors charge
- Motors and compressors draw locked-rotor current several times their running current at startup
- Fans and pumps present an inductive load that stresses contacts at switch-off, not switch-on
A 400 W fixture made of a dozen LED panels can hit the relay with a surge of tens of amps for a millisecond at power-on. The steady-state number is nowhere near the limit. The surge is what welds the relay contacts shut, and a welded contact shows up later as a gang that is always on.
The minimum-load problem in the other direction
At the opposite end, there is a floor, not just a ceiling. No-neutral designs steal their operating power through the load circuit, which means the load has to conduct enough current for the electronics to feed on. A single 5 W LED bulb can be below that floor.
The symptoms of a load that is too small are the ones people misdiagnose most: the bulb glows faintly when off, the switch reboots when toggled, or the bulb flickers in a slow pulse. The fix is not a better switch — it is either a bigger load on that circuit or a bypass capacitor across the lamp, and the honest answer is that some no-neutral products simply state a minimum wattage you have to respect.
Designs with a neutral wire do not have this problem, which is one of the quiet reasons an electrician will steer you toward a neutral-wire model whenever the wiring allows it.
Where the rating quietly shrinks
Two installation realities reduce what a gang can carry:
- Ganged switches share a body, and the heat from neighboring relays adds up. Manufacturers derate multi-gang units — a four-gang body might allow full rating on one gang and substantially less if several gangs run heavy loads together.
- Enclosed metal back boxes and shallow plaster-depth boxes trap heat. A relay is fine at room temperature in open air and marginal in a full box behind insulation in summer.
If your real load sits above about 70 percent of the rating, check the derating table in the manual instead of trusting the headline number.
A practical sizing checklist
- Add up the true steady-state watts on the circuit, then find the startup type: heaters are easy, anything with a motor or a driver is not
- For LED or electronic loads, look at inrush current in the load's own specification if it is published; if it is not, be conservative
- For no-neutral designs, check the minimum load figure before buying, not after the flicker starts
- For anything over roughly two-thirds of the rated watts in a shared or enclosed box, move that load to its own gang or its own product
None of this is exotic. It is the same arithmetic any panel builder does, and it is why the switch you can buy for a consumer price and the switch a factory will certify for your market differ more in the relay and the test data than in the glass front.
Where this matters when you are buying
If you are specifying loads per room, start from the product side: smart switches and smart sockets list protocol and load details per model.
The load-rating and derating logic we apply in production is described on the smart switch board manufacturing page. For OEM load profiles, certifications and MOQ, the factory site has the overview, and sourcing questions go to the factory team.
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