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

 

[UNG]

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.

The most reliable method to size MICC cable would be to look at the gland and / or pot for the size this will identify if it is a metric or imperial cable

 

[paul-bristol]

I think if it has been operating since the 1970s without catching fire, that might answer your question...

 

[Jock from the north]

Back in the day for the first 2 years of my apprenticeship all I did it seemed like was make off pyro ends .all council buildings had it as a must especially for corridor lighting etc ,in fact for everything.when I think of the cost involved material and labour no wonder theve got no money .would love to watch a spark change an old board full of micc into rcbos etc. But if they were made off perfectly then no moisture should have seeped into pot .I did change a consumer unit in a council flat that was wired in micc and ir results were all well over 100 mega ohms.great cable to work with and has the advantage of carrying more than your average load

 

[johnnyuk]

Back in the day for the first 2 years of my apprenticeship all I did it seemed like was make off pyro ends .all council buildings had it as a must especially for corridor lighting etc ,in fact for everything.when I think of the cost involved material and labour no wonder theve got no money .would love to watch a spark change an old board full of micc into rcbos etc. But if they were made off perfectly then no moisture should have seeped into pot .I did change a consumer unit in a council flat that was wired in micc and ir results were all well over 100 mega ohms.great cable to work with and has the advantage of carrying more than your average load

Funnilly enough when trawling the web to try to find answers to my above questions, I'm sure I found a cost study by an MICC manufacturer saying that MICC was cheaper to install (I think maybe just from a labour perspective) than PVC twin and earth. Not quite sure how they managed to prove that!

 

[johnnyuk]

Thanks everyone for the help on my original question. @Tom256 can I just check my understanding. Are you saying that "if" no overload protection is required (I am saying it isn't as there is no way to amend/extend the underfloor circuits under the concrete screed) then the cable size only needs to carry more than the design load ie. more than 23.3a?

Wouldn't that mean under a fault condition the cable would fail instead of the 30a breaker?

To explain a bit more - the original circuit was originally protected by a 30a rewireable BS3036 fuse. The protective device has since been switched for a 32a BSEN60898 and it's this that I'm trying to understand is accepable or not.

@UNG unfortunately the glands are buried in the walls so I can't see them. However I have measured the cores with a caliper and am 99% sure they are 2.5mm. Is there any other way of telling if this is light duty or heavy duty and does it make a difference anyway?

Sorry for all the questions.

The other thing I noticed is that if I look at the trip curves for a 20a type b 60898 MCB (or at least what I think is the right one), then it doesn't look like it would ever trip even with constant load at 23.3a. Am I right about this and if so is there any reason I couldn't fit a 20a mcb?

 

[brianmoooore]

. However I have measured the cores with a caliper and am 99% sure they are 2.5mm.

Do you mean 2.5mm2 or 2.5mm diameter? One is twice the area of the other.
It's a fixed load,so there's no problem. 2.5mm2 T&E is permitted on a 32A MCB (spur off of a RFC) if it feeds a load of less than 20A.

 

[johnnyuk]

Do you mean 2.5mm2 or 2.5mm diameter? One is twice the area of the other.
It's a fixed load,so there's no problem. 2.5mm2 T&E is permitted on a 32A MCB (spur off of a RFC) if it feeds a load of less than 20A.

Hi brian. I mean 2.5mm2.
The load is 23.3a though so more than 20a
Sorry if I've misunderstood something.

 

[johnnyuk]

Hi brian. I mean 2.5mm2.
The load is 23.3a though so more than 20a
Sorry if I've misunderstood something.

Oh I should add I think this is a radial circuit. Hang on let me check

 

[johnnyuk]

Pretty sure it's a radial circuit - one circuit for each zone of the underfloor heating.

 

[Jock from the north]

Funnilly enough when trawling the web to try to find answers to my above questions, I'm sure I found a cost study by an MICC manufacturer saying that MICC was cheaper to install (I think maybe just from a labour perspective) than PVC twin and earth. Not quite sure how they managed to prove that!

I can't see how that would be possible.it should only take you 3 minutes to terminate a twin socket [ring] in t and e but stripping ,screwing on pot , sliding on seal ,testing at 1000v as we used to do,visual inspection of pot for shards of copper then compound ,then test again before final crimp [its been near 20 years since I made of a micc pot just going by memory]and then your sleeving and final test .not to mention pot box clamps or glands instead of fastfix boxes .maybe there study meant the duration of cable .I don't know would be interesting to see what everyone else thinks