2.5mm MICC on 32A breaker

[johnnyuk]

Hi All

I was hoping someone could shed some light on a question about the setup of my underfloor heating.

I live in a 1970s block of flats with electric underfloor heating. There are 4 heating circuits in total with the original 1970s wylex consumer unit. 3 of the heating circuits are each protected by 16a wylex rewireable fuses.

The fourth circuit is a more powerful heating circuit which draws a maximum of 23.3amps (measured with clamp meter). This is protected by a 32a rewireable fuse.

As I understand it, the current carrying capacity of 2.5mm PVC twin and earth isn't high enough to have a 32a fuse on it. However my flat and the heating circuit in question is wired in the original 2.5mm MICC (Orange sheath. I measured the cores and it does seem to be metric 2.5mm) cable. I don't know exactly how the underfloor circuits are installed, but I would guess the MICC is laid between the structural concrete floor slab and a layer of screed.

I've been in a couple of the other flats in the same block with the original wiring setup, and they also have the same 32a rewireable fuse on this same heating circuit, so I am confident it is as-installed upon construction, and the construction quality of the block of flats is good (ie. not thrown-up modern construction).

My question is this - Is the current carrying capacity of 2.5mm MICC higher than 2.5mm PCV T+E and is it high enough to allow safe protection by a 32a breaker? I've also read the attached document about overload protection - see the last two pages - which comments a bit vaguely on the requirements where overload protection isn't strictly required. The logic as I understand it is that because this is a fixed underfloor heating system which cannot be extended, it will never draw more than it's design load of 23.3a (except I assume when in fault), so overload protection isn't required? Except I then lose track a bit as to what the author is saying in regards to the associated reuqirements for cable rating and protective device.

Anyway as you can see I'm a bit confused and looking for someone to shed some light for me. I'd be grateful for any advice as there's not much info out there on MICC that I can find.

Thanks all in advance.

 

[westward10]

I presume this is a 3OA rewireable fuse and not 32A.
For cable selection a correction factor is applied to the 30A of 0.725 which gives 41.27A.
The rating of a 2.5mm² MI cable is 31A (light duty) or 34A (heavy duty) so in either case you can argue it is undersized. You can argue that it is a fixed load being purely resistive and it should not overload.

 

[johnnyuk]

I presume this is a 3OA rewireable fuse and not 32A.
For cable selection a correction factor is applied to the 30A of 0.725 which gives 41.27A.
The rating of a 2.5mm² MI cable is 31A (light duty) or 34A (heavy duty) so in either case you can argue it is undersized. You can argue that it is a fixed load being purely resistive and it should not overload.

Apologies westward yes, 30A rewireable (the red one like the attached).

So are you saying that these circuits were incorrectly installed when new? It seems unlikely that the original builders (they built a lot of decent flats in my area and were well-known) would put in a circuit with undersized cables.

Does the "fixed load" issue help me or hinder here? Ie. does the fact that it shouldn't overload mean that it doesn't matter if the cable carrying capacity is less than the protective device rating, as long as the cable has a greater cable carrying capacity than the design load of 23.3a (Ib)?

Also are installation reference methods applicable for MICC cable ratings, or is it different for MICC?

 

[Tom256]

Apologies westward yes, 30A rewireable (the red one like the attached).

So are you saying that these circuits were incorrectly installed when new? It seems unlikely that the original builders (they built a lot of decent flats in my area and were well-known) would put in a circuit with undersized cables.

Does the "fixed load" issue help me or hinder here? Ie. does the fact that it shouldn't overload mean that it doesn't matter if the cable carrying capacity is less than the protective device rating, as long as the cable has a greater cable carrying capacity than the design load of 23.3a (Ib)?

Also are installation reference methods applicable for MICC cable ratings, or is it different for MICC?

If no overload protection required the design current as stated of 23.3 amps can be used rather than the 30 amp rating of the BS 3036.

Now, it has been a while since I designed with mineral protected by BS 3036 fuses but something in my mind is telling me the Cf factor need not be applied when using MICC cable. A situation unique to this wonderful cable.

The limiting factor was never damage to the cable but the temperature it could reach if exposed to touch and / or pvc coated as in this case