A smart switch failure that starts as "the switch feels warm" almost always ends as a load planning problem. The switch did its job. The circuit behind it was asked to carry more than it was sized for, and the weakest link happened to be the terminal block of the newest device on the wall.
The circuit is the budget, not the switch
In most of Europe a lighting and socket circuit is protected at 16 A. At 230 V that is roughly 3,680 watts for the whole circuit, shared by every outlet and every switch on it. In North America a 15 A branch at 120 V gives about 1,800 watts. The relay inside a smart switch is rated against this: a common spec is 10 A per gang, sometimes 16 A on heavier models.
The mistake happens at the next step. People read "16 A" on the relay and treat it as a personal allowance for one wall position. It is not. The breaker upstream does not know or care which device used the current. When a heater runs on one gang and a kettle boils at the socket two meters away, the shared breaker sees the sum.
Why the relay rating is not the whole number
A relay rated 10 A resistive has a second, lower rating for motor and inductive loads, and almost no product prints both numbers on the box. Fans, pumps and compressor-driven appliances draw several times their running current for a few hundred milliseconds at start. That inrush is what welds relay contacts shut, and it repeats on every start, not once.
There is also the question of what the switch itself consumes. A no-neutral model harvests its operating power through the load, which is why some LED bulbs glow faintly when off. A model with a neutral wire draws standby power across the mains, usually well under half a watt. Neither number threatens a breaker. Both numbers matter when thirty of them share one circuit in a hotel corridor.
Heating loads change the math
Resistive heating is where budgets die. A portable heater, an electric towel rail, a bathroom floor heating mat and a kettle can each be a legitimate load for a 10 A relay, individually. Put two of them behind gangs on the same back box and the same wall now dissipates heat from four directions: the loads themselves, the relay contacts, the driver electronics and the sealed glass panel that traps all of it.
Glass-front smart switches have a specific thermal weakness here. The glass is the visible surface and the insulating surface at the same time. A board that is fine in a hallway becomes marginal in a south-facing kitchen at 35 degrees ambient, which is exactly when every heating appliance in the building is running.
What this means for multi-gang boards
A four-gang board is not four independent switches. It is one heat-dissipating object with one mounting box behind it. Sensible practice when planning a wall position:
- Give any permanently high load its own gang, and its own circuit where possible.
- Keep heating appliances off multi-gang boards entirely.
- Ask for the total board rating, not only the per-gang figure.
- In hot climates, ask for the derating curve above 40 degrees ambient.
A smart switch collection sorted by protocol and gang count makes it easier to match board types to wall positions instead of defaulting every room to the same model.
The acceptance test that takes ten minutes
After installation, run the planned worst case for twenty minutes: every planned load on that wall position switched on together. Then touch the plate, not the screws. Warm is normal at high load. Hot enough that you pull your hand away, or an audible click as a relay cycles, is a finding, not a quirk. Repeat the check in summer, because a board that passed a January test says nothing about August.
An infrared thermometer against the glass center of each gang turns a guess into a number. Boards of the same model should sit within a few degrees of each other under the same load. One outlier gang usually means a loose terminal, and loose terminals only get looser over time.
What to ask before you order
Three questions separate a supplier who has thought about this from one who has not. First: is the printed rating per gang or per board, and what is the other one. Second: what load type is the endurance rating based on, resistive or motor. Third: what happens to the ratings in a 40 degree room.
A smart switch board manufacturer that answers all three without checking has probably shipped this exact question before. If you are planning a project with mixed loads and want the figures in writing, XingLiao publishes per-gang and per-board ratings, and the team can be reached here for project-specific load tables.
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