Short small diameter wires with 16A fuse?

[OneMoreFarang]

I am working on the electrical installation in my condominium.

I use 2.5sqmm cable to the power outlets and 16A breakers.

 

I know the rule that the cable must be big enough to withstand the current so that the breaker does its work before the cable fails. With 2.5sqmm cable and 16A breakers that should be fine.

 

But what if I add small Shelly device, which uses just a few mA, in between? Can I use i.e. 1sqmm wire to the Shelly device? These 1sqmm wires would be within an electrical box in the wall. The Shelly devices will be connected to an electrical switch. But there is no load (in my case). The Shelly devices have a WLAN receiver and that's what I use.

 

If there would be a shortcut in the Shelly, then maybe the little i.e. 1sqmm wires would fail before the breaker switches off. But then again, I am pretty sure there are even smaller wires within the Shelly, so if there would be an internal shortcut, then they would fail internally.

 

My question is, is it ok to do the following. Use 2.5sqmm wires to the power outlet, cut them and add small wires to the Shelly?

This is a sample picture of what I want to do.

The Shelly devices are not mission critical. For me it doesn't matter if they would be off if the power to the power outlet would be off for an hour or two for whatever reason.

I don't use the relay in the Shelly.

 

I know that technically this will work. My question is, if a good electrician would do that. Or would he use 2.5sqmm wires to connect the little Shelly devices? 

 

 

 

 

[degrub]

It depends on how you wire it and what the worst case load is down those smaller wires.
From what you have described, this looks the same as if you have plugged a device into a wall plug. The wire going to the device has to be adequate for the  current passing through so that the insulation does not degrade and start a fire. For example , plugging in a stereo amplifier. The wiring to the stereo box is adequate for the nominal  load, yet the stereo may have fuses and other current limiting devices to  protect internal circuitry and possibly prevent overloading the wiring to the stereo ( not sure about the last part as i have never looked closely at the design).

 

What you cannot do is put the low current wire in series with the 16 amp breaker. 
 

I don’t know the local electrical code says specifically about what you want to do with that hardware. In a junction box of adequate volume for the heat generated ( device and insulation max T ) - yes, possibly the Shelley needs to be in free air instead of sealed up in a junction box ?

So, you will have to wait for @Crossy  to comment on what would be allowed.

[Muhendis]

I would fit one of these in-line fuses in the live wire.

https://shopee.co.th/กระบอกฟิวส์-5x20-mm-ใส่ฟิวส์หลอดแก้ว-In-Line-fuse-holder-i.561759750.18751433394?sp_atk=114860b4-86b4-4e45-a220-9da377b67d84&xptdk=114860b4-86b4-4e45-a220-9da377b67d84

There is a possible fire hazard in the "Shelly" (what's that?) if an internal component should fail.

Does the "Shelly" have any internal protection?

 

[OneMoreFarang]

Thanks for your replies so far.

 

A little info about the Shelly device, there any a couple of them.

 

They are mostly used to put behind existing electrical switches to allow home automation. When installed instead of an existing switch then the switch can be used like before, or the Shelly can be switched per WLAN for home automation.

 

In my case I don't replace an existing switch. I only use the Shelly devices behind new switches to get the signal over WLAN if something should be on or off. And with that information in Home Assistant I switch LED lights, etc.

 

Here is some more information about them.

 

https://www.shelly.com/en/products/shop

 

 

 

 

[sometimewoodworker]

Oh the fun to be had with unfused plugs. The British plug version avoids all problems like this just add a 3A fuse and Robert is you mother’s brother 

[KannikaP]

14 minutes ago, OneMoreFarang said:

And with that information in Home Assistant I switch LED lights, etc.

I have many RGB WiFi controlled LED bulbs, operated by Tuya app, or my Amazon Fire TV, or my Google Assistant. Who needs Shelly?

My Xaomei Air Filter device has it's own App, as do my Air Cons via Airconnet devices.

[Muhendis]

43 minutes ago, OneMoreFarang said:

Thanks for your replies so far.

 

A little info about the Shelly device, there any a couple of them.

 

They are mostly used to put behind existing electrical switches to allow home automation. When installed instead of an existing switch then the switch can be used like before, or the Shelly can be switched per WLAN for home automation.

 

In my case I don't replace an existing switch. I only use the Shelly devices behind new switches to get the signal over WLAN if something should be on or off. And with that information in Home Assistant I switch LED lights, etc.

 

Here is some more information about them.

 

https://www.shelly.com/en/products/shop

 

 

 

 

OK so the in-line fuse is the right way to go and the fuse needs to be rated for the current of the light plus background power of the Wi-Fi switch.

I am personally content to hike over to the switch to operate it or better still get my missus to do it. ????

[OneMoreFarang]

2 hours ago, KannikaP said:

I have many RGB WiFi controlled LED bulbs, operated by Tuya app, or my Amazon Fire TV, or my Google Assistant. Who needs Shelly?

My Xaomei Air Filter device has it's own App, as do my Air Cons via Airconnet devices.

Who need Shelly?

People who want something like Home Assistant which works 100% local without internet. If you want to invite Google or Amazon in your home, then feel free to do that. I won't! 

Another advantage of Home Assistant is that everything can be integrated. It is not necessary to have different apps from different suppliers. 

But that is another discussion which has nothing to do with this thread. 

[OneMoreFarang]

1 hour ago, Muhendis said:

OK so the in-line fuse is the right way to go and the fuse needs to be rated for the current of the light plus background power of the Wi-Fi switch.

I am personally content to hike over to the switch to operate it or better still get my missus to do it. ????

There is no current for the light at that location. There is only the "background power of the Wi-Fi switch". The cable for the light is somewhere else.

 

I can also hike over to the switch to operate it or better still get my missus to do it. That is one option, because the Shelly is behind the "normal" switch. And additionally, Home Assistant allows me to do it automatically (depending on the time or if a person is in that area, etc.) or I can do it on my phone or tablet, etc. 

[Crossy]

What do the manufacturer's instructions for your Shelley device say?

 

I suspect that the device itself will have internal protection (fuse) even if it's just a fine PCB track that will fail in the event of an internal short.

 

In reality short lengths of "undersize" wire will still open your 20A breaker before the wire itself gets hot enough to be a problem if you do get a short-circuit.

 

 

[OneMoreFarang]

17 minutes ago, Crossy said:

What do the manufacturer's instructions for your Shelley device say?

 

I suspect that the device itself will have internal protection (fuse) even if it's just a fine PCB track that will fail in the event of an internal short.

 

In reality short lengths of "undersize" wire will still open your 20A breaker before the wire itself gets hot enough to be a problem if you do get a short-circuit.

 

Thanks!

I don't see any information about that in the "manual". My question is also more general for any device which needs little power, not just the Shelly.

I see on the box is written: Power consumption < 1.2W and other details but nothing about the connection wires.

[bluejets]

The device itself is pretty well covered above but.....what is your intended load via the internal relay...??

[Fruit Trader]

5 hours ago, OneMoreFarang said:

The Shelly devices will be connected to an electrical switch. But there is no load (in my case). The Shelly devices have a WLAN receiver and that's what I use.

Shelly device with powered electronics only, no relay load.

 

All brand name devices like Shelly and Sonoff have some kind of internal protection at the electronics supply circuit. Inline fuse, fuseable resistor or similar. 

 

As already mentioned, very short lengths of 1mm cable will easily carry your breaker trip current although an electrician following his code would likely shout foul.

[OneMoreFarang]

13 minutes ago, bluejets said:

The device itself is pretty well covered above but.....what is your intended load via the internal relay...??

I don't use the internal relay at all.

For people who replace an existing switch it makes sense to use that relay. In my case the power connection is only for the internal processor and WLAN (< 1.2W). I use the information which I receive per WLAN in Home Assistant. The power connection to the LED lights is independent from this one. 

[Crossy]

For those who are interested: - 

 

https://engx.theiet.org/f/wiring-and-regulations/29739/adiabatic-plots

 

[Muhendis]

17 hours ago, Crossy said:

I suspect that the device itself will have internal protection (fuse) even if it's just a fine PCB track that will fail in the event of an internal short.

I wouldn't rely on that unless it actually had words to that effect in the spec.

 

17 hours ago, OneMoreFarang said:

In my case the power connection is only for the internal processor and WLAN (< 1.2W).

That unit seems vey small to have any isolation between AC and processor. Does it have any conformancy spec numbers on the back like UL for example?

[Muhendis]

I just had a look at the video of the similar looking "Shelly" device and notice there are a number of things missing in the writing on the front of the unit of the OP device. 

Chinese Copy?

[Adumbration]

The photo of your shelley device has it rated at 16A also.  Why are you not just running 2.5mm squared cable to the device instead of 1mm?

[sometimewoodworker]

1 hour ago, Muhendis said:

I wouldn't rely on that unless it actually had words to that effect in the spec.

Few, if any, devices have enough detail for you to find a fuse-able resistor marked as such in the specs, many won’t give you a circuit diagram. You need the ability to recognise what one looks like. You can get an idea if you watch BigClive on YouTube as he regularly reverse-engineers PCBs

[OneMoreFarang]

2 hours ago, Muhendis said:

That unit seems vey small to have any isolation between AC and processor.

I don't think it needs any isolation. There is no "user output" like USB. This unit is built to be in an electric box in a wall and nobody will touch it or any wire coming out of it.

 

 

[Muhendis]

28 minutes ago, OneMoreFarang said:

I don't think it needs any isolation. There is no "user output" like USB. This unit is built to be in an electric box in a wall and nobody will touch it or any wire coming out of it.

 

 

Brilliant.

That CE mark says it all plus the other glyphs.

I can safely say that with those conformancy markings that device should not catch fire.

Also in the tear down section of the video, I think I caught sight of a pcb mounted fuse.

 

39 minutes ago, OneMoreFarang said:

I don't think it needs any isolation.

It is double insulated according to the triangle with the circle and tick mark inside. So from the viewpoint of electric shock safety everything is fine.

With regards to isolation, what I was referring to is the internal circuitry between AC mains and low voltage DC supply for the processor etc.

[Muhendis]

2 hours ago, sometimewoodworker said:

Few, if any, devices have enough detail for you to find a fuse-able resistor marked as such in the specs, many won’t give you a circuit diagram. You need the ability to recognise what one looks like. You can get an idea if you watch BigClive on YouTube as he regularly reverse-engineers PCBs

Agreed.

I would be looking for something like "internally protected" in the sales blurb and/or the specs.

[TimeMachine]

As long as you do the math and are aware of the risks its ok. But problems arise when someone else takes over the property . They will not be aware and asume that line can handle 16 amps. Then they will face overheating and possible burning. 

 

[Crossy]

33 minutes ago, Muhendis said:

With regards to isolation, what I was referring to is the internal circuitry between AC mains and low voltage DC supply for the processor etc.

 

I'll bet it has none, just a simple capacitor dropper, at these power levels they work well.

 

Of course, it means that the internal electronics is potentially at mains voltage, but with no interface to the outside world it matters not.

 

Note that this unit may be rather cleverer as it's dual AC/DC power supply (there are some dead clever PSU chips around these days), but I bet it's still not actually isolated.

 

PMP20182 - Ultra-wide Input Range (10.8V to 264V AC or DC) Bias Power Supply

 

https://www.electronicsdatasheets.com/manufacturers/texas-instruments/reference-designs/PMP20182

 

[Fruit Trader]

32 minutes ago, Muhendis said:

It is double insulated according to the triangle with the circle and tick mark inside.

 

 

https://www.eess.gov.au/rcm/regulatory-compliance-mark-rcm-general

 

 

 

 

[Fruit Trader]

Mains supply isolation requirement for home automation switching devices is mostly decided by the the relay contacts state. If the contacts are dry (volt free) then good mains isolation at the PCB is desirable.

Example below Shelly with dry contact isolation. The OP has model without dry contacts. 

 

[Muhendis]

1 hour ago, Crossy said:

just a simple capacitor dropper, at these power levels they work well.

Agreed but.................

 

1 hour ago, Crossy said:

it's dual AC/DC power supply

Could be done with a dropper resistor and zener plus all the usual ac-dc bits.

[sometimewoodworker]

7 hours ago, Muhendis said:

Agreed.

I would be looking for something like "internally protected" in the sales blurb and/or the specs.

Again in my experience you won’t even get that, though many PCBs are in fact protected.