X3 Immersion heaters on a single radial

[RedSpottedSev]

Hi,

Some help please.

We have moved our boiler heating system to the loft. The new system in total has x3 immersion heaters, 3000kw each.

 

I would like each of the immersion heaters to be controlled by a smart fused spur (SM FSTWIFITU

WiFi Controlled 13A Fused Switched Spur - Tuya Compatible)

 

I was wondering if a radial 10mm 2core twin earth will be safe to supply them all, if it was protected at the consumer unit with an RCBO.

 

What is the best way to wire in the spurs from the 10mm cable? Do we daisy chain off the back of the line side of each spur with the 10mm cores?

 

This will all be done and checked by the electrician, however I just want to make sure the plumber and builder have the first fixes all set up right.

 

Thank you

 

Thank you

[JohnMo]

Although 3kW is a 13A draw, you would really need 16A switching for long term reliable operation.

 

Why do you need 3 immersion heaters?

[ProDave]

I very much doubt you will get two 10mm cables into one of the terminals on one of those.

 

Is this a (home made?) storage boiler?  the commercial ones will have one high power feed to a control box and typically use a contactor to switch each element.

[Mike]

As an alternative, consider the Shelly Pro 4PM for the remote control functions, used to switch 3 x 25A contactors (1 for each immersion). If they won't fit in your main consumer unit, fit an auxiliary one in the loft.

[RedSpottedSev]

Thank you all,

 

It is a custom build. Main hot water cylinder has x2 (top and bottom), another in a buffer tank.

 

On first glance the shelly looks spot on. I will certainly have a read. I can't run any additional cables down to the main consumer unit as all conduits full and decorating done in the rooms that would need new chases.

 

So I assume I am going with a mini auxiliary consumer unit in the loft to house the Shelly. Will the 10mm cable be ok to power it if all 3  immersions if they come on together?

 

[Nickfromwales]

20 minutes ago, RedSpottedSev said:

Thank you all,

 

It is a custom build. Main hot water cylinder has x2 (top and bottom), another in a buffer tank.

 

On first glance the shelly looks spot on. I will certainly have a read. I can't run any additional cables down to the main consumer unit as all conduits full and decorating done in the rooms that would need new chases.

 

So I assume I am going with a mini auxiliary consumer unit in the loft to house the Shelly. Will the 10mm cable be ok to power it if all 3  immersions if they come on together?

 

It’s down to the length of the cable, for voltage drop, and things like grouping factors, if run in a bunch with other cables quite tightly packed aka “contained” (trunking and conduit etc).

 

You can hopefully run a 50a MCB in the primary CU and then fit 3x 16a RCBO’s in a secondary CU in the attic. Those each feed a Shelly, and then you use heat resistant flex from Shelly > immersion.

 

You should use bootlace ferrules for the flex’s Link to make the best possible connections. 

 

How often do you think all 3 will be running? This is an all electric house with no heat pump?

 

2 hours ago, RedSpottedSev said:

What is the best way to wire in the spurs from the 10mm cable? Do we daisy chain off the back of the line side of each spur with the 10mm cores?

You can’t do this because of the high rating of the supply breaker.

 

32a max to a single socket or double socket or fused spur, fed locally via 4mm2 radial or 2.5mm2 ring.

 

For eg, I swapped my electric shower for a thermostatic unit when I fitted a gas combi. The 40a MCB then feeding the 10mm2 cable got reduced to 32a (RCBO) and a BFO (60a) junction box went on the end of the 10mm2 cable where I made a new utility area. I came out of the JB with 2.5mm2 as a ring main, picking up 2x 1G sockets for the washer and the drier, and another for the boiler spur.

 

You have to reduced the available current locally, or at source, to match the lowest current carrying cable in the chain; so you can’t have a 40/50a breaker directly feeding sockets or spurs. 

[JohnMo]

14 minutes ago, RedSpottedSev said:

shelly looks spot on

Wouldn't want to have my heating system reliant on so called smart stuff, use something that has internal control for time and temp, that can be accessed remotely. Keep it simple. Then should you loose internet, or home assistant switches off or doesn't want to play, it all runs regardless. I dabbled and nearly had my head on a spike.

[Nickfromwales]

1 minute ago, JohnMo said:

Wouldn't want to have my heating system reliant on so called smart stuff, use something that has internal control for time and temp, that can be accessed remotely. Keep it simple. Then should you loose internet, or home assistant switches off or doesn't want to play, it all runs regardless. I dabbled and nearly had my head on a spike.

Simple fail-safe is to bridge L&N from the Shelly in(s) & out(s) in a local 20a DP switch (the type normally used to turn an immersion on / off anyways) and no heads go on any spikes

 

If we do an UVC in an attic, I always fit the 20a switch in the living area somewhere, with a neon, and then the lady of the house just pokes that with her finger vs your left eyeball. 

 

 

[RedSpottedSev]

Thank you,

I need to study a little to fully appreciate what you are describing. Especially to save my head from spikes and such...

 

No heat pump, but set up for it and a large solar array that is likely to come pending planning etc..

 

@Nickfromwales there are x2 10mm cables running side by side, a distance of 8m from main CU to attic. Nil else with them. Only to be used for heating, maybe future vent/aircon. Nothing connected to them currently. At most maybe power a smart hub if we get signal drops across the house/outside.

 

Instead of 10mm to Shelly PM4, then from there to 3 immersions,  you are suggesting 10mm to auxCU, then x3 RCBOs then shelly (I assume a different model) then immersion. Is it so that they don't all go off if PM4 fails?

 

 

[RedSpottedSev]

I think I understand now that the shelly doesn't take on the job of the MCBs after it, so the x3 MCBs would be needed eitherway.

[JohnMo]

I think I would step back a little 

 

First decide what your running logic is for the immersions. Are they all switched together, one heating at a time etc 

 

Then test is that the right logic to take - is too simplistic or just way to complex to make sense for anyone else to try and fix it etc. will save you energy, will things fight each other etc 

 

Then when that is all making sense, plot the best control approach and from there how best to configure the wiring.

[Nickfromwales]

1 hour ago, RedSpottedSev said:

Thank you,

I need to study a little to fully appreciate what you are describing. Especially to save my head from spikes and such...

 

No heat pump, but set up for it and a large solar array that is likely to come pending planning etc..

 

@Nickfromwales there are x2 10mm cables running side by side, a distance of 8m from main CU to attic. Nil else with them. Only to be used for heating, maybe future vent/aircon. Nothing connected to them currently. At most maybe power a smart hub if we get signal drops across the house/outside.

 

Instead of 10mm to Shelly PM4, then from there to 3 immersions,  you are suggesting 10mm to auxCU, then x3 RCBOs then shelly (I assume a different model) then immersion. Is it so that they don't all go off if PM4 fails?

 

 

Yes

 

12 minutes ago, RedSpottedSev said:

I think I understand now that the shelly doesn't take on the job of the MCBs after it, so the x3 MCBs would be needed eitherway.

And yes.

 

MCB’s aren’t usually used these days, as RCBOS offer better protection. 

[Nickfromwales]

5 minutes ago, JohnMo said:

First decide what your running logic is for the immersions. Are they all switched together, one heating at a time etc 

3 channels of Shelly’s is ample control and completely flexible. 

[Mike]

2 hours ago, RedSpottedSev said:

I think I understand now that the shelly doesn't take on the job of the MCBs after it, so the x3 MCBs would be needed eitherway.

Correct, the Shelly only switches the contactors. You need an RCBO & contactor for each immersion, plus another RCBO to protect the Shelly itself.

[Nickfromwales]

1 hour ago, Mike said:

Correct, the Shelly only switches the contactors. You need an RCBO & contactor for each immersion, plus another RCBO to protect the Shelly itself.

I thought the Shelly’s could be bought with switching capacity for direct connection to the immersion. 

 

 

[Nickfromwales]

4 hours ago, RedSpottedSev said:

there are x2 10mm cables running side by side, a distance of 8m from main CU to attic. Nil else with them. Only to be used for heating, maybe future vent/aircon. Nothing connected to them currently. At most maybe power a smart hub if we get signal drops across the house/outside.

Just put a 4th RCBO in the aux CU and run it to a double socket so you can plug stuff in downstream. Easier and cheaper to get this done in one hit if you’re getting a sparky out.