CB Calculations for Underfloor Heating

[MartK]

Hi All,

I've taken a look at a consumer unit today and noticed that the previous electrician wired bathroom underfloor heating using a 20A circuit breaker. There are a range of spare CB's and having made my own calculations, just wanted to check what you all thought.
According to previous receipts supplied by the customer, the underfloor cable is 'Prowarm' 150watt loose cable, 64m in length, laid with a spacing of 5cm. From what I can find out on their website, such a length of cable will amount to 640w, therefore requiring no more than 3A. This doesn't sound right for underfloor heating but i'm missing why the previous electrician used a 20A CB.

Any thoughts?

Many thanks

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[Leesparkykent]

Yes it’s 10 watt per linear meter. The previous electrician sounds like he knows what he’s up to and done a good job. What’s your issue?

 

[MartK]

Thanks for your reply. The previous electrician seems to be very good from what I have seen so far so it's always nice to learn from experience.
I have little experience in UFH and wanted to understand his thought process in going past the spare 6s and 16s, and connecting into a 20A.
Its a 24 way board so lots of spares!

 

[The Ghost]

So no FCU intervening between mat and MCB, so how did he join up the cables?

 

[davesparks]

I've taken a look at a consumer unit today and noticed that the previous electrician wired bathroom underfloor heating using a 20A circuit breaker.

From what I can find out on their website, such a length of cable will amount to 640w, therefore requiring no more than 3A. This doesn't sound right for underfloor heating but i'm missing why the previous electrician used a 20A CB.​

Why were you looking at the CU and circuit in question? Are you there to do work on it?

What is the rating of the fuse in the sfcu?

 

[MartK]

Yes there's an FCU between CU and UFH with a 13A fuse.

Helping a family member as the CU isn't labelled.

 

[Baker1988]

what size is the cable suppling it? the mcb in the board is there to protect the cable suppling it to the fused spur. also if its a fixed load and not going to pull that much then there is no problem

 

[Toneyz]

I would fit a 3A fuse in the spur.
We should not be encouraging the OP to be opening accessories or consumer units and exposing him to live parts to see what size cable it is. Maybe the OP can take a picture of the consumer unit so we can tell if RCD protected or a copy of a certificate.

 

[davesparks]

Yes there's an FCU between CU and UFH with a 13A fuse.

Helping a family member as the CU isn't labelled.

I’d probably have fitted a 5A fuse rather than 13A but I see no issue with the circuit based on the info you have provided.

 

[MartK]

According to the schedule of test result, the circuit is using a 2.5mm conductor and is protected by an RCD.

Thanks all. From all the helpful replies it seems that I do not need to be overly concerned.

Summarising the replies above, its the 13A fuse which is protecting the UFH which would maybe benefit from a lower rating; the size of the CB is almost irrelevant?

From what i can remember from my science class over 20 years ago(!!!), 640w / 230v = 2.78A. So why a 20A CB?!

 

[Rpa07]

The MCB as has been stated, protects the feed cable and its installation method up to the drop in protection accessory which in this case is the fuse spur which can benefit from a 3/5 amp fuse because of the load of the UF heating element.
2.5mm radial 20amp MCB is an industry standard.