A specification line says one gateway supports up to two hundred devices. A project with two hundred devices per floor buys one gateway per floor. Six months later the network is unreliable and the factory is blamed for a number that was never a project figure.
What the datasheet number measures
Protocol address space, or a laboratory count with favourable radio conditions, or the largest number achieved in a single test. It is a ceiling, not a planning figure. The distance between the ceiling and the reliable figure depends on the building.
Three variables move it. Physical distance and wall construction, because a masonry wall between floors attenuates more than an open plan office. Device type mix, because battery sensors sleep and route nothing while mains devices can extend the mesh. And traffic pattern, because two hundred devices that report once an hour behave nothing like twenty cameras that stream.
The practical planning figure
For battery sensors on a mesh network in a residential building, plan on a fraction of the ceiling, commonly a third to a half, and place gateways per zone rather than per count. A floor, a wing, a cluster of apartments around one riser. The zone boundary should follow the building, not the device tally.
For devices that route, mains powered switches and plugs, the mesh is extended and the figure improves. For devices that stream or that switch frequently, the figure degrades and the gateway's processor, not its radio, becomes the limit.
Why the failure looks like a software bug
When a gateway is over-subscribed, the symptom is not a clean refusal. It is intermittent. One device drops and rejoins. Scenes fire late or partially. A group command reaches most devices and misses three. From the user's side this reads as flaky software, and the support conversation goes to firmware while the real cause is radio congestion and queue depth.
Diagnosing it requires per-device last-seen timestamps, which is why the audit view matters more than the dashboard.
The cost of adding a gateway later
Adding capacity after handover is not a hardware cost. The gateway is cheap relative to the labour of pulling a new network drop, re-pairing devices, rebuilding scenes, and re-testing every room. In an occupied building the disruption is the expense.
That is the argument for over-provisioning gateways at design stage: not because the hardware is fragile, but because retrofitting the network is the expensive part.
Zoning also buys resilience
A single gateway for a whole building is a single point of failure for the whole building. Zoned gateways mean a failure takes out one zone. The commercial case is straightforward once the operator has experienced one outage.
Zoning also makes expansion cheap: a new wing gets a new zone and does not disturb the existing configuration.
What to ask a supplier before fixing the count
Ask for a planning figure rather than a maximum, and ask what conditions produced it. Ask whether the figure assumes mains routing devices present or only battery sensors. Ask how many devices per gateway the supplier has running in a delivered project, and in what building type.
A supplier who answers with a delivered reference and a device count per zone is giving a usable number. A supplier who repeats the datasheet maximum is not.
Ask also about the gateway's own limits: how many scenes, how many schedules, how many concurrent app connections. These saturate before the radio does in some designs.
Writing the number into the specification
The specification line should read as a planning figure with a stated condition, for example: one gateway per floor for up to sixty devices where at least a quarter are mains powered. That sentence is defensible in a design review. "Up to two hundred" is not.
The gateway options we build, together with the zone planning guidance we give project buyers, are set out on the home automation system manufacturing page. It is worth reading before the device schedule is finalised, because the gateway count is one of the few decisions that is genuinely cheap now and expensive later. Device schedules can be sent through the contact page for a zone count review before the tender goes out.
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