Polypool 410 Goes Smart: Shelly 1PM Mini Gen3 Plug Extension (Wi-Fi + Power Meter)

Ever wished your “regular” plug could magically become a smart plug… without a chunky external box? 😊
This project does exactly that: I took a very common Italian DIY-store plug (Polypool 410) and redesigned the entire outer shell to neatly fit a Shelly 1PM Mini Gen3 inside.

Result: a compact plug extension that can:

• switch loads over Wi-Fi

• measure power/energy (W, V, A, kWh)

• keep everything tidy in a single enclosure

What’s original / what’s reused

To keep the critical parts solid and reliable, I reused from the original Polypool 410:

• the 3-pin plug insert (metal pins assembly)

• the original screws

• the blue cable clamp / strain-relief clip

And I replaced:

• the two original plastic shells → with new 3D-printed clamshells shaped around the Shelly module

BOM (what you need)

Printed parts

• 1× Front shell

• 1× Back shell

Reused parts (from Polypool 410)
https://www.obi-italia.it/p/1827500/spina-salvaspazio-10-a-2p-t-uscita-cavo-a-90-nera

• 3-pin plug insert / pins assembly

• screws

• blue cable clamp (strain relief)

Electronics

• 1× Shelly 1PM Mini Gen3
  https://www.amazon.it/Shelly-Mini-Wi-Fi-Interruttore-Intelligente/dp/B0CRRT6HCB

Wiring

• mains cable suitable for your country and current rating

Printing notes

• Prefer tougher / more heat-resistant materials (PETG / ASA / ABS / PC-blend).

• Use multiple perimeters and solid top/bottom for stiffness.

• Check fit: tolerances vary by printer and material.

Assembly tips

• Route wires through the internal channels before closing the shells.

• Make sure strain relief is doing its job: cable pull must not reach the terminals.

• Ensure no conductor can be pinched by the seam during closing.

• Tighten screws evenly (don’t overtighten plastic).

Known limitations / Safety notes (read before building)

• This is a DIY enclosure. It is NOT a certified electrical product.

• The Shelly 1PM Mini Gen3 is rated 8A, but for a sensible safety margin I recommend:

  • Max continuous current: 6A

  • At 230V → ~1380W max (6A × 230V)

  • At 110V → ~660W max (6A × 110V)

• Avoid high inrush / heavy loads (examples: heaters, kettles, hair dryers, toasters, large motors, compressors).

• Monitor temperature on first real use: if the enclosure gets noticeably warm, stop and investigate.

• Use appropriate cable, proper terminals, and do not leave exposed copper outside clamps.

• If you’re not fully confident with mains wiring: don’t build it. Ask a qualified electrician.

⚠ MEGA DISCLAIMER (SERIOUS STUFF)

DANGER: MAINS VOLTAGE (110–240VAC) CAN KILL YOU AND CAN CAUSE FIRE.

This is a DIY hobby project shared for educational purposes only. It is NOT a certified electrical product and it is NOT guaranteed to comply with any safety standard (CE/UKCA/UL or others).
By downloading/printing/building/using this design, you accept all responsibility and all risk.

Do NOT build or use this if you are not qualified to work with mains electricity.
If you are unsure: consult a licensed electrician.

Before any use:

• verify correct wiring for your local rules (L/N/PE)

• ensure protective earth continuity (PE) where applicable

• inspect insulation and that no wire can be pinched or pulled out

• check for loose screws/terminals and any sign of overheating

• test under load in a safe, supervised environment

Files are provided “AS IS” without warranty.
I am not responsible for any damage, injury, loss, or legal issues resulting from use or misuse of this design.