Clarification of the purposes for the multiple “L” terminals on Shelly 1PM, 2.5 and Shelly 4 PRO

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There have been frequent questions about the L/L1 terminals on the Shelly 1PM, the two terminals labeled L on the Shelly 2.5, and the four terminals, L1/L2/L3/L4 on the Shelly 4 PRO.

First it is important to understand that for all of these devices, the multiple L terminals are simply connected internally. Even on the Shelly 4 Pro, they must be externally connected to the same phase/circuit. None of these devices are compatible with any sort of connection involving more than one phase or circuits that come from different breakers/fuses.

Why have multiple terminals that are just connected internally? Doesn’t this make it redundant? Can we use them like a wire nut or Wago?

The key to understanding every one of these questions is a matter of the electrical unit called amperes, or amps, for short, and usually abbreviated as simply A as in 16A.

Every device in your house has some maximum amount of current, measured in amps which it is expected to use under normal circumstances. If the only rating you find on something like a light bulb is watts rather than amps, you can convert. This will be explained later. Once you have the amp ratings of all of the devices that are connected on a given circuit, you can simply add them to find out the total amount of current which will be used. A 2A computer power supply and a 0.2A USB charger plugged into the same power strip amounts to 2.2A. The same math holds for everything that is connected in your walls.

Wires and connectors also have ratings in amps. The wires in your walls must be of sufficient size to carry the amount of load that you put on them, using the math described above. In the USA it is typical to have 15A and 20A wiring. The current in the US is higher because the voltage is lower, which will also be explained when we talk about the conversion for watts.

The connectors on the Shelly devices, like everything else, have a maximum rating. Each individual screw terminal cannot accommodate any current higher than 16A. This is how some of the Shelly products can carry a 16A rating with just a single L and N connector as input.

The complications about using multiple “L” connectors on the Shelly devices discussed here stems from this same math. If each output of the Shelly 4 PRO is used to power a device drawing 9 amps, there would be a total of 36 amps being supplied. Given you actually have it connected to a circuit capable of this high current, using wires that are properly rated as well, connecting just a single “L” and “N” to screw terminals rated at 16 amps would be a problem. Carrying all of that load, the terminals themselves would heat up. So the Shelly 4 has L connectors to accommodate the total current which might be drawn by 4 connected devices on the O1-O4 outputs.

The same logic applies to the Shelly 2.5. The total rated current, 2 x 10 amp per channel equals 20 amps, which exceeds the rating of a single screw terminal at just 16A. Any combination where the two connected devices might exceed 16A demands that both of the “L” connectors be connected to line power outside of the Shelly enclosure, even though they will be electrically connected via a screw terminal or Wago connector to a single wire in the wall. It’s a matter of the weakest link a chain. Each wire that connects to the Shelly should be the proper rating for the circuit, so it could alone carry enough current, but the screw terminal is that weakest link. Doubling up is the answer. It’s the safest way to wire your Shelly 2.5, so it’s what is recommended.

On a Shelly 1PM, a single connector is sufficient to power the maximum load that the Shelly relay can output. Therefore, unlike the other devices, the Shelly 1PM having multiple connectors is a matter of convenience and, up to a point, can be considered similar to a Wago or wire nut connection. Its primary purpose is to make wiring a switch between “L” and the “SW” input easy. In terms of powering anything else, it is only up to a point specifically because, once again, those screw terminals are rated for 16A. If a 20A circuit in a US home is wired into a Shelly 1PM and then leaves again from the junction box via a connection made to the spare L connector on the Shelly, it creates a hazardous situation. A device capable of drawing more than 16A could be plugged into an outlet downstream from the junction box and overload the screw terminal on the Shelly 1PM.

The same logic holds for “creative” uses of the additional L terminals on any of the devices described here.

The safest way is to always connect the L terminals as additional power inputs on all 2.5 and 4PRO devices. On the Shelly 1PM, don’t use the second L connector other than for a switch unless you really understand the implications described in this document.

Now to the question about converting watts to amps. To figure the current (amps) used by a device that is only rated in watts, you simply need to divide by the supply voltage. In a house wired with 110 volts, a 100 watt light bulb will use a maximum of just over 0.9A. To make the math easier, rounding down to 100 volts is a conservative effort, making this calculation an even 1A. Rounding up is the safer direction, since we are always concerned with maximums. If you are unsure of whether you have 110 vs. 120 or 220 vs. 240, use the smaller of the two numbers in your calculations.

This following is part of a conversation from the Shelly support page where the possibility of wiring many circuits in series, using the L connectors as both input and output was discussed. Note that this discussion specifically called out the limitations of a maximum amount of current. If you don’t know for sure, or the wiring would leave the equation unknown, like any situation where a wall socket is involved, this practice is not recommended.