Under floor heating over 3kw

Sb8389 · Sep 20, 2019

Hi guys
Being to a job today where the builder has installed some electric under floor heating
Total area is 20.5 m2
The mats are 150w per m2
The total load is just over 3kw.
He has first fixed the wiring and checking it he has used a socket wired off the ring main to spur off to a switch spur where has then wired a 1mm t&e to the thermostat direct In a dot and dab wall so clipped direct.
I know the CCC of a 1mm refC is around 16amps so well within the limit.
In my opinion i would have recommended it being installed on its own circuit as I know it’s not good practise wiring it off the ring main.
Do you foresee any future issues with the install as I may be overthinking it but don’t want to have issues further down the line as I am making the final connections.

Thanks

Deleted account · Sep 21, 2019

I am presuming the builder did the design and cable calcs since he did first fix it’s up to him to sign on the cert that he did the design.

He has also done the testing by attaching a plug and plugging it in.

If you want to second fix and notify the work with his details filled in the design and testing boxes fine, But i would walk away from this one.

My concerns:

Over 3KW so over 13amps. Cannot be on a 13A FSU.

Any load 2KW or more should have its own dedicated circuit.

The good old bang test. Switch it on without any testing.

Unless I have seen first fix and been involved in first fix I won’t finish a job. In this case I would like to see the location of thermostat and the underfloor heating mats before being put down so I could clarify the actual load.
[automerge]1569022264[/automerge]
To add to the above I would have probably split it into 2 on separate circuits.

Just had a thought. Is the programmer rated at a maximum load, I think most are.

Pete999 · Sep 21, 2019

Sb8389 said:
Hi guys
Being to a job today where the builder has installed some electric under floor heating
Total area is 20.5 m2
The mats are 150w per m2
The total load is just over 3kw.
He has first fixed the wiring and checking it he has used a socket wired off the ring main to spur off to a switch spur where has then wired a 1mm t&e to the thermostat direct In a dot and dab wall so clipped direct.
I know the CCC of a 1mm refC is around 16amps so well within the limit.
In my opinion i would have recommended it being installed on its own circuit as I know it’s not good practise wiring it off the ring main.
Do you foresee any future issues with the install as I may be overthinking it but don’t want to have issues further down the line as I am making the final connections.

Thanks

Advisable to supply loads @2KW and above from a dedicated circuit, coming from a spur off an, RFC is not the way to go, bad move call him back and get him to do the job properly.

telectrix · Sep 21, 2019

should have been split into 2 mats, each with it's own stat fed off 2 x FCU on a dedicated 20A radial.

bill01803 · Sep 21, 2019

Some of the controllers are rated at 16A so would be ok on a 16A radial

telectrix · Sep 21, 2019

bill01803 said:
Some of the controllers are rated at 16A so would be ok on a 16A radial

but a 13A FCU isn't. unless you're going to use the 16A MCB for fault protection. overload protection is not required for a fixed load such as this.

bill01803 · Sep 21, 2019

Why do you need a fcu a 20A dp will do for isolation

Sb8389 · Sep 21, 2019

The stat is rated at 16amp so can carry the load. He has now first fixed all cables so is awkward to see his routes but is pretty self explanatory as his stat is only half a meter from his spur that he’s installed. He has laid the accolade mats he told me so they are 0.5mx6m he has laid 7 of these. Although he did tell two of these he cut to 5m so brings the square meter age down a touch but my calcs still come in at pretty much 13amp.
I did look at the potential for a new circuit but now all plastered and customer does not want disruption. I understand it’s bad design but other than potential to blow the fuse although if he is telling me the truth it would be borderline it’s full capacity. I don’t see any overload issues with the cable size he has used really, is there any other issues I should be mindful off that I have not thought off.

westward10 · Sep 21, 2019

I wouldn't certify anything I have not had 100% involvement with.

HappyHippyDad · Sep 21, 2019

Sb8389 said:
The stat is rated at 16amp so can carry the load. He has now first fixed all cables so is awkward to see his routes but is pretty self explanatory as his stat is only half a meter from his spur that he’s installed. He has laid the accolade mats he told me so they are 0.5mx6m he has laid 7 of these. Although he did tell two of these he cut to 5m so brings the square meter age down a touch but my calcs still come in at pretty much 13amp.
I did look at the potential for a new circuit but now all plastered and customer does not want disruption. I understand it’s bad design but other than potential to blow the fuse although if he is telling me the truth it would be borderline it’s full capacity. I don’t see any overload issues with the cable size he has used really, is there any other issues I should be mindful off that I have not thought off.

Sb I think you'd be crazy to sign off this work. A 3kW load on a 1mm cable spurred off a ring, there's not really anything else to say. I would never put my name to that. Try not to feel pressured by the builder. It needs to be on a dedicated circuited 16A MCB with 2.5mm cable (possibly 1.5mm).

The 13A fuse will not blow at 13A it will keep going for ever at 13A, which means that 1mm cable will be constantly (while the heating is on) taking it's full load and that's assuming it's CCC is 13A in the stud wall, it could be less.

It is difficult to say 'no', but sometimes we have to. I don't want to be thinking back over my career worrying if I've installed anything dangerous.

Deleted account · Sep 21, 2019

I think the builder needs 3rd party certification on this one.

Edit: not from you.
[automerge]1569069647[/automerge]

bill01803 said:
Why do you need a fcu a 20A dp will do for isolation

Because he has spurred from a ring final.

davesparks · Sep 21, 2019

Sb8389 said:
The stat is rated at 16amp so can carry the load. He has now first fixed all cables so is awkward to see his routes but is pretty self explanatory as his stat is only half a meter from his spur that he’s installed. He has laid the accolade mats he told me so they are 0.5mx6m he has laid 7 of these. Although he did tell two of these he cut to 5m so brings the square meter age down a touch but my calcs still come in at pretty much 13amp.
I did look at the potential for a new circuit but now all plastered and customer does not want disruption. I understand it’s bad design but other than potential to blow the fuse although if he is telling me the truth it would be borderline it’s full capacity. I don’t see any overload issues with the cable size he has used really, is there any other issues I should be mindful off that I have not thought off.

How has he joined 7 mats together?
How did he cut the mat down? This is not normally possible.
3kW is too much load for an SFCU to handle for the long periods that heating will be on for. There's more to this than straightforward maths, experience shows that an SFCU will burn out because of this.

I would strongly advise that you refuse to connect and advise the end customer that this situation could be dangerous.

The builder has carried out the work so they are responsible for the electrical safety and part P notification etc. Unless you are registered for third party sign off and have been involved with this from the design stage then technically you would be breaking the law to sign it off.

Sb8389 · Sep 22, 2019

I do agree

davesparks said:
How has he joined 7 mats together?
How did he cut the mat down? This is not normally possible.
3kW is too much load for an SFCU to handle for the long periods that heating will be on for. There's more to this than straightforward maths, experience shows that an SFCU will burn out because of this.

I would strongly advise that you refuse to connect and advise the end customer that this situation could be dangerous.

The builder has carried out the work so they are responsible for the electrical safety and part P notification etc. Unless you are registered for third party sign off and have been involved with this from the design stage then technically you would be breaking the law to sign it off.

He has laid 7 0.5x6m mats and taken all cables back to a single joint box although he did say he cut a couple mats down to 5m so not 100% on the square meters he, he has then run a 1mm from that point to the stat and a 1mm supply from the switch spur to the stat. I don’t really have any intention of signing it off, im getting hassle now as it would mean quite a bit off mess to install a new supply although I have explained the situation.

Deleted account · Sep 22, 2019

Sb8389 said:
I do agree

He has laid 7 0.5x6m mats and taken all cables back to a single joint box although he did say he cut a couple mats down to 5m so not 100% on the square meters he, he has then run a 1mm from that point to the stat and a 1mm supply from the switch spur to the stat. I don’t really have any intention of signing it off, im getting hassle now as it would mean quite a bit off mess to install a new supply although I have explained the situation.

As well as not signing it off I’d encourage you not to do anything to the circuit that would implicate you as the last person to work on that circuit.

Don’t be lumbered with liability.

Also write a report on the works undertaken so far, stating it was not you who carried out this work.

A**e covering time.

Sb8389 · Sep 22, 2019

I’m back there on Thursday so going to try and convince them of the potential issues going forward and to install a new circuit, from memory it could be wired in conduit on the outside straight into the garage where the consumer unit is. 2-3 hours work just for peace of mind All round is worth it.

Strima · Sep 22, 2019

Minimum cable CSA for other circuits is 1.5mm unless wired in flex. I’m not at home at the minute so can’t give the reg number.

I would get the builder and customer there at the same time to explain my concerns and solution then let them make a decision.

If signing this off you are taking responsibility for the circuit and the UFH mats, how do you know he hasn’t damaged them on installation? Wouldn’t be the first time someone has been blamed for damaged UFH.

Unless I knew the builder well I wouldn’t be signing it off.

Midwest · Sep 22, 2019

As an aside, hope the customer has deep pockets to afford the leccy bill.

Simon47 · Sep 22, 2019

Sb8389 said:
... he did say he cut a couple mats down ...

As Davesparks says, with some (many, most ?) systems, you cannot cut them down - and where you can, it may need special measures to maintain the integrity of the element.
Some "mats" are simply a wire system that's pre-laid out on a plastic mesh to make it easier to install. Some heater cables have multiple heating elements which can be cut in certain places. Others cannot be cut as the L&N wires are the elements, and are joined together at the far end - if you cut it then you need to re-join them and the resistance will be lower and the W/m higher than designed for.
So this alone should ring alarm bells. Make sure that you document everything that you were not involved in so they can't come back to you if there's any problems.

DPG · Sep 22, 2019

Maybe show the customer some of the comments on this thread.

ferg · Sep 22, 2019

Start off by walking away, slowly working up to a light jog culminating in a full blown sprint.

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