UniFi Relay - Switching high powered loads - How to guide

The Ubiquiti UniFi Protect Smart Relay (USL-Relay) is a fine piece of kit for pulling physical infrastructure into your UniFi ecosystem. It has one hard limit: 10 amps per port. Here is how to use it to switch a 63 A EV charger, a 100 A commercial heater, or a whole building from grid to generator — without destroying it. Parts and wiring are covered for both UK and US installations.

Bridging smart tech and heavy iron

1. The contactor (for on/off control)

A contactor is essentially a heavy-duty industrial relay. It consists of two entirely separate electrical circuits:

• The coil (low power) — an electromagnet that needs very little current, under 1 A, to activate.
• The main contacts (high power) — heavy metal lugs designed to handle 63 A or 100 A loads.

Instead of switching your appliance directly, the USL-Relay acts as the trigger. When you press On in the UniFi app, the relay sends a small signal to the contactor's coil. The coil magnetises and loudly slams the heavy main contacts shut, allowing the full current to flow to your appliance.

2. The automatic transfer switch (for changing power sources)

If you want to use the UniFi app to switch a single load between two different power sources — swapping a building from grid to generator, say — you cannot simply use two standard contactors. If a software glitch or a mechanical fault causes both contactors to close at the same time, the grid and the generator will violently collide and cause a catastrophic short circuit.

You need a device with a mechanical interlock. The most cost-effective way to get one is a motorised automatic transfer switch (ATS). It takes two massive inputs, Source A and Source B, and provides one output. Its internal motor physically throws a heavy switch block left or right, so it is mechanically impossible for both sources to be connected at the same time.

The relay never carries the load. It only tells something much bigger what to do.

Scenario A — simple on/off with a contactor

First, the relay needs its own DC supply

This is the step people miss. The USL-Relay is not mains powered — it takes 12–48 V DC into its VDD and GND terminals. In a consumer unit the tidy answer is a small DIN-rail 24 V DC power supply, a Mean Well HDR-15-24 or similar, 230 V AC in and 24 V DC out, fed from the same 6 A control MCB that feeds the coil circuit.

The relay's outputs are entirely separate from its own power. O1 and COM are a volt-free dry contact, which is exactly why a relay running on 24 V DC can switch a 230 V coil circuit.

The low-voltage control circuit (the coil)

Buy a contactor with a 230 V AC coil, meaning the electromagnet triggers on standard mains voltage.

1. Power the relay

   Feed the DIN-rail 24 V DC PSU from a dedicated low-amp breaker — 6 A is plenty — and run its output into VDD and GND on the relay.

2. Feed the contact

   Run a 230 V live from the same 6 A breaker into the COM terminal of the relay.

3. Relay to coil

   Run a wire out of O1 and connect it to the A1 terminal on the contactor.

4. Neutral to coil

   Connect a standard neutral wire directly to the A2 terminal on the contactor.

5. Close the circuit

   When the relay closes its dry contact, 230 V flows from COM through O1 into A1, powering the electromagnet and completing the circuit at A2.

The high-voltage load circuit — 2-pole or 4-pole

The main power feed for your heavy appliance does not touch the UniFi relay at all. It goes strictly through the contactor's main lugs, labelled 1/L1, 3/L2 and so on, and out through 2/T1, 4/T2.

North American variant — 120/240 V split-phase

The control principle is identical, but four things change on a US installation:

• Coil voltage. Specify a 120 V AC coil, not 230 V, and feed the relay from a dedicated 15 A or 20 A branch circuit.
• Which conductors get switched. A 240 V load is fed by two hot legs. Both hots go through the two poles; the neutral is not switched and runs straight through to the load.
• Form factor. US installations use definite purpose (DP) contactors rather than DIN-rail modular units, and they step in 30/40/50/60/75/90 A ratings — a 60 A DP contactor is the practical equivalent of the UK 63 A part.
• Terminal markings. DP contactors label the main lugs L1/T1 and L2/T2. The coil terminals are often unmarked quick-connect spades rather than A1/A2, so check the datasheet before you land the control wires.

Sizing, and the extra rules for EV chargers

Cable and breaker sizing follows the load, not the relay. As an indication for a 63 A single-phase load: 16 mm² on the load circuit, 1.0 mm² on the 6 A control circuit, and 25 mm² supply tails. An immersion or hot-water element wires identically on a smaller circuit — typically a 16–20 A MCB and 2.5 mm² cable. These are starting points only; resize the breaker and cable to the actual load and confirm by design.

Working out your output budget: O1 and O2

O1 and O2 are two identical, independently controlled dry contacts sharing a common COM. O1 is simply the first free output — there is nothing special about it. Each is rated 10 A maximum at 12–240 V AC or 12–48 V DC, roughly 2,300 W continuous at 230 V or 1,100 W at 110 V.

The combined figure is the one to watch. With both outputs in use, keep the sum under 16 A — 8 A each, for example. Switching contactor coils you are nowhere near that, since coil currents here are tiny. Both outputs are independently controllable in UniFi Protect, each with its own schedules and automations, so a single relay will happily run O1 for an EV charger contactor and O2 for a hot-water contactor.

Useful things to do with I1 and I2

The sensing inputs are easy to overlook, and they close a real gap: they let Protect know what actually happened rather than only what it asked for.

• Wire a volt-free auxiliary contact on the contactor to I1, so Protect can confirm the load really switched on — not just that the command was sent.
• Connect door or gate contacts, a tamper switch, or the volt-free output of an alarm panel or boiler stat to trigger automations.
• Use plain 2-core signal cable, keep the runs short, and only ever switch the input dry — never through a voltage source.

Scenario B — switching sources with an ATS

An ATS normally switches on its own when it senses a power cut. We can use the UniFi relay to trick it into switching exactly when you press the button in the UniFi app.

An ATS has main high-voltage lugs for the primary (grid), the backup (generator) and the output. It also has small, low-voltage sensing terminals, usually labelled AR/AN for primary sensing and BR/BN for backup sensing.

1. Wire the heavy cables

   Run the grid and generator power cables to the main lugs as normal, and the output to the building or load.

2. Intercept the sensing live

   Instead of wiring the grid's sensing live wire directly into the AR terminal, run it through the O1 terminals on your UniFi relay first.

3. On means grid

   When you turn the UniFi app on, the sensing circuit completes. The ATS motor senses the grid and physically throws the massive switch to the grid input.

4. Off means generator

   When you turn the app off, the relay breaks the sensing wire. The ATS suddenly thinks the grid has died, and its internal motor draws power from the backup side to loudly slam the switch over to the generator input.

North American variant — drive the remote test input instead

The sensing-line trick works on the generic dual-power changeover units, but it is the wrong approach for listed US transfer equipment. A UL 1008 transfer switch is built around a generator controller with 2-wire start, so faking a utility failure fights the controller rather than co-operating with it.

Use the terminals the manufacturer already provides. The ASCO Series 185 controller, for example, includes terminal provisions for a remote test switch — which is exactly the job the USL-Relay's dry contact is suited to: close the contact to command a transfer, open it to return to utility. No high-voltage sensing wire passes through the relay at all, and the switch keeps its listing.

Recommended parts — United Kingdom

Unless noted otherwise, these mount on a standard DIN rail enclosure and use 230 V AC coils or motors compatible with the USL-Relay.

63 A contactors — on/off

For loads up to 63 amps: standard EV chargers and sub-mains.

100 A contactors — on/off

For extreme loads up to 100 amps: commercial heating arrays and whole-panel switching.

100 A / 125 A automatic transfer switches — source swapping

For safely switching a whole building or heavy load between grid and generator. Build quality and transfer speed vary enormously in this category, and it is the one place in this guide where the cheapest option is a false economy.

Recommended parts — United States

US parts are specified differently: 120 V AC coils, definite purpose contactors instead of DIN-rail modular units, and UL listing rather than BS EN marking. Prices below are typical street prices and move around; treat them as a guide and confirm with your wholesaler.

40–60 A contactors — on/off

For EV chargers and heavy 240 V appliances. A 60 A DP contactor is the practical equivalent of the UK 63 A part.

100 A switching — on/off

Definite purpose contactors run out of headroom around 90 A. Above that you move to a lighting contactor or an IEC-frame contactor.

Automatic transfer switches

Read the UL 1008 warning above before you choose. The first option is cheap and unlisted; the other two are listed and will pass inspection.