Reference

Terminals and markings

What the letters on the terminals mean — L, N, SW, O, I and the rest.

Shelly has no single standard for terminal markings. The same letter means different things across product families, and the same function can be labelled differently depending on which device you're holding. This chapter goes through what's actually printed at the terminals, verified against Shelly's official wiring diagrams on kb.shelly.cloud — not what the name "should" mean.

Which set of terminals your device has is tied to its generation. If you're not sure which one you have, see Which generation do you have? first.

Mains power: L and N

These two exist on practically every mains-powered device and mean the same thing everywhere: they supply the device's own electronics with power. On multi-channel devices like Shelly Pro 4PM there are often several L terminals (one per channel) but only one shared N.

The relay load circuit: I and O

This is the marking most often misunderstood. On single-channel relay devices like Shelly 1, Shelly Plus 1 and Shelly 1 Gen4, I is not a logic input the way a sensor input would be — it's the load circuit's input terminal, i.e. where the power to the load enters before passing through the relay. O is the load circuit's output terminal, where the load (the lamp, the motor) connects.

Verified on Shelly 1, Shelly Plus 1 and Shelly 1 Gen4: all three have L, N, I, O and SW. The I terminal lets the relay switch a completely separate circuit (e.g. low voltage, or on a different fuse) from the one powering the Shelly module itself.

Note: Shelly 1PM Gen4 (same family, but with power metering) does not have an I terminal according to the verified wiring diagram — it only has L, N, O and SW. So the same letter I isn't even used consistently within one product family.

Multiple channels: O1, O2 …

Devices with more than one relay number their outputs:

Verified on Shelly 2.5, Shelly Plus 2PM, Shelly 2PM Gen3, Shelly 2PM Gen4 and Shelly Pro 4PM.

Switch inputs: SW, SW1–SW4, S1–S4 — and why "I1–I4" is NOT correct

This is where Shelly's own products disagree with each other, and it isn't purely a generation thing:

Important to know: despite the product name Shelly i4 / Shelly Plus i4 — which reads naturally as "four inputs labelled I1–I4" — the verified wiring diagram shows the terminals are actually named SW1, SW2, SW3 and SW4. We found no official Shelly device where the terminals are actually marked I1–I4. If you see that label somewhere, it's worth double-checking against that specific device's own wiring diagram — we can't confirm it from Shelly's KB.

Shutters: ▲ ▼

Most two-relay devices (Shelly 2.5, Shelly Plus 2PM, Shelly 2PM Gen3/Gen4) can run in shutter mode, but they simply reuse O1/O2 — the wiring diagram for these does not use arrow symbols in the terminal marking. O1 drives the up direction, O2 the down direction, but the terminal itself is printed O1/O2, not with arrows.

The arrow symbols ▲ (up) and ▼ (down) are verified to appear on Shelly Pro Dual Cover/Shutter PM, which has four output terminals in total — one per direction per channel:

The difference comes down to relay count: a device with only two relays (like Shelly 2.5) can only drive one shutter, so O1/O2 is enough as a label. A device with four relays can drive two shutters, and then the arrows are needed to distinguish direction from channel.

DC power: +, −, GND, 12V/24V

Several devices can be powered with low-voltage DC instead of mains, and the terminals for that aren't fully consistent either:

The symbol is the conventional ground symbol for DC ground/negative — the same meaning as GND. On devices that carry it instead of the text GND, that's where the negative wire connects.

Shelly Plus Uni — its own world

Shelly Plus Uni is a low-voltage module and doesn't follow the terminal pattern above at all. Instead of numbered screw terminals it has colour-coded leads, where each colour is a fixed function. Here is each terminal individually, verified against Shelly's KB:

Good to know: the Plus Uni is powered either via VAC1/VAC2 or via +5VDC/GND — not both at once. The OUT1/OUT2 outputs are solid‑state (they pull to GND), not potential-free relay contacts, so they only handle low voltage (≤30 V DC) — never mains. A good reminder to always read the wiring diagram for your specific device.

The dimmer's terminals

Verified on Shelly Dimmer Gen3 and Shelly Dimmer Gen4 (identical marking on both):

The dimmer supports wiring without a neutral for loads above 20 W; otherwise a bypass module is required for smaller loads. Note that the dimmer's switch inputs are marked S1/S2, not SW1/SW2 — the same pattern as the Pro line, but different from Shelly Plus i4.

Summary by device

A common misconception: I1–I4

Despite the name Shelly i4 / Shelly Plus i4, the input terminals are marked SW1–SW4, not I1–I4. On Shelly the letter I is reserved for the load circuit's potential-free input (see The relay load circuit above), not for logic control inputs. If you see I1–I4 anywhere, it isn't Shelly's own marking.