derating group circuits

[Eighe]

I'm checking out the electric wiring in my new house trying to see if there is anything I need to get put right before I decorate and live in the house properly.

So, I've been reading up and I'm confused about derating grouped circuits.

There is trunking(1.5m long 50mmx25mm) in my kitchen with the cooker cable(10mm) and two ring mains i.e. 4 cables of 2.5mm.

The cooker cable has a 32A MCB has two ring mains(Kitchen and downstairs) also have a 32A MCB each.

Should these grouped circuits have been derated(e.g. x0.7) or does nobody bother with group derating in domestic instations?

What about the 2 meters of trunking(there are two truckings actually) where all of the cables from the consumber unit travel up the wall together? Surely this is circuit grouping and would mean that all the circuits also need derating?

It must be pretty similar to every other house in the country but I'm assuming that most house circuits are not derated?

Do I need to get somebody in to derate the MCBs(and suffer nuisance trips!) or is the installation typical and nothing to worry about.

I also have a similar concern about all the cables traveling from the consumer unit and into the ceiling space between joists where the hot radiator pipes flow. It must get pretty hot in there with nowhere for the heat from the pipes to dissapate. Should I get all the circuits be derated to account for this? or maybe I should just insulate\shield that part of the space bettween the pipes and the cables? Again, this is probably a common situation and I'm assuming that not everyone has derated to 20A MCBs?

Can anybody explain or reconcile the derating regulations, with what is found in typical house installation?

Thanks for any help!

 

[deadshort]

I would be focused on paying the mortgage +de-rates

 

[JohnW2]

It must be pretty similar to every other house in the country but I'm assuming that most house circuits are not derated? ... Can anybody explain or reconcile the derating regulations, with what is found in typical house installation?

I suspect that most people would prefer not to (try to 'explain or reconcile'...) - I would think that your observation about how common this is in domestic installations is probably correct!

Do I need to get somebody in to derate the MCBs(and suffer nuisance trips!) or is the installation typical and nothing to worry about.

If one wanted to de-rate the cables, it would often not be as simple as just 'de-rating' the MCB, since the de-rated current-carrying-capacity of the cable is not allowed to be less than the design current of the circuit - so replacement of the cable with larger might be necessary.

Kind Regards, John

 

[Eighe]

Yep, MORTgages, tell me about it!

Thanks for your reply John,

I see what you mean about the design current, but the more I read, the more this value seems like as a bit of a fudge and a moving target.

Cookers change(in this case from double oven to single oven), houses change hands, appliance use changes, especially when home ownership changes, I don't have a hot tob in the garden like the previous owners did!

Since the cables are already installed, the easiest(cheapest) thing to do seems to be to do what is required to 'respecify' the design current and get the MCBs changed down to 20A? Is 20A the minimum for a ring?

I suppose I could replaster(it is a mess) the corner of the wall the cables come down and separate the cables and clip direct under capping(where do you get capping for 10mm cable?!?) That would ungroup the circuits and I could do this myself at little expense.

But.... the same circuits would still be grouped at the consumer unit like every other house in the country! So this seems like a load of work for a derating regulation every sparks in the country just ignores! Arrrrhhhhh!

Does anyone have any thoughts on all the CU cables passing through the hot(main radiator supply pipes) section of joist? Is this also typical?

Would this justify a rewire of the CU into the next joist, or shall I just try and insulate the pipes?

I'm going a bit mad with this and I'd like some advice before I get a professional in. I'm not really trusting tradesmen at the moment and would appreciate some impartial advice!

Thanks!

 

[JohnW2]

Thanks for your reply John, I see what you mean about the design current, but the more I read, the more this value seems like as a bit of a fudge and a moving target....

In some cases/situations,yes, but there are some 'fixed' issues you are up against. For example, see below, ring circuits are only explicitly allowed if the current carrying capacity (CCC) of the cable is at least 20A. With loads such as showers, immersions heaters, other heaters etc., the load is defined and the CCC of the cable must be at least as high as that load.

Since the cables are already installed, the easiest(cheapest) thing to do seems to be to do what is required to 'respecify' the design current and get the MCBs changed down to 20A? Is 20A the minimum for a ring?

Rings are funny animals, since they actually violate the usual requirements for cable protection. However, there is specific provision in the regs to be allowed, if certain conditions are satisfied - one of which is that the CCC of the cable must be at least 20A. You mentioned a de-rating factor of 0.7. If you applied that to 2.5mm² cable, you would get a CCC of only 18.9A, which would therefore not be acceptable for a 32A ring (and the regs speak explicitly of nothing other than 32A rings). You could have the ring split into two radials, each with 16A MCBs, but that would be rather restrictive.

I suppose I could replaster(it is a mess) the corner of the wall the cables come down and separate the cables and clip direct under capping(where do you get capping for 10mm cable?!?) That would ungroup the circuits and I could do this myself at little expense. ... But.... the same circuits would still be grouped at the consumer unit like every other house in the country! So this seems like a load of work for a derating regulation every sparks in the country just ignores! Arrrrhhhhh! ...

It's hard to know what I can/should say. As I implied before, I think that a quite high proportion of domestic installations would have to be revised if the 'de-rating' regulations regarding grouped cables were strictly adhered to!

... Does anyone have any thoughts on all the CU cables passing through the hot(main radiator supply pipes) section of joist? Is this also typical? ... Would this justify a rewire of the CU into the next joist, or shall I just try and insulate the pipes?

I don't fully understand - do you mean that the cables go through the same hole(s) in a joist as do hot pipes,or is their wood between cables and pipes?

I'm going a bit mad with this and I'd like some advice before I get a professional in. I'm not really trusting tradesmen at the moment and would appreciate some impartial advice!

As above, I think there is a substantial divide between what is commonly done and what should be done. A lot depends upon how 'pragmatic' you and your electrician are!

It would be interesting to hear the views of others.

Kind Regards, John

 

[Eighe]

Hi John, I'm so grateful for your patient and pragmatic explanations in this and many other posts I've read.

So it derating the rings in not an option as 32A is the only option!

None of the rings have any fixed equipment, only plug sockets.

The 10mm cable only goes to the cooker(with a plug socket on the isolator)

I think I'll fix them all to the wall with double cable width separation and then replaster. It is a mess anyway.

Regarding the temperature derating;
All the circuits exit the CU, go up the wall into the ceiling between two joist. Travelling along parallel to joists in the same 2.7m long void are the radiator pipe so the void is pretty hot and there is nowhere for the heat to escape.

All the circuits cross through the joist, but there is about 50cm of each circuit in the hot void(wide joist separation)!

There is at least 5 cm of clearance between any pipes and cables and no cables go through the same holes as the pipes.

Do you think this situation typical of domestic heating\electric installations, or is something I should get rectified before I fit the flooring and tidy up the wall!

I'll crank up the heating and measure the temperature of the cables in the void.

Thanks again.

 

[JohnW2]

Hi John, I'm so grateful for your patient and pragmatic explanations in this and many other posts I've read.

Thank you, and you're welcome.

So it derating the rings in not an option as 32A is the only option!

There are always 'other options' if one is capable and knowledgeable enough to argue that a circuit is satisfactory 'from first electrical principles' - there is no compulsion, per se, to comply with the things which are mentioned in the Wiring Regulations. The issue about a standard ring is that cable with a CCC which may be as low as 20A is being protected by a 32A MCB, which normally wouldn't be allowed (if it weren't a ring). The specific dispensation in the regs, however, allows it. If the cable CCC (after de-rating) were a bit less than 20A, one could not use that specific dispensation, but it shouldn't be too difficult to produce a sound electrical argument that, by analogy with the "32A MCB + 20A CCC" standard ring, a ring protected by a 20A or 25A MCB would be equally safe - but I've never heard of anyone doing that!

The 10mm cable only goes to the cooker(with a plug socket on the isolator)

That cable, per se, is not an issue, since it is already way 'OTT' in terms of size - without any de-rating, even a 4mm² cable would be adequate with the 32A MCB. With 10mm² cable, you would have to de-rate it by a factor of 0.5 before it it was not suitable for a 32A MCB.

Regarding the temperature derating; All the circuits exit the CU, go up the wall into the ceiling between two joist. Travelling along parallel to joists in the same 2.7m long void are the radiator pipe so the void is pretty hot and there is nowhere for the heat to escape. ... All the circuits cross through the joist, but there is about 50cm of each circuit in the hot void(wide joist separation)! ... There is at least 5 cm of clearance between any pipes and cables and no cables go through the same holes as the pipes. ... Do you think this situation typical of domestic heating\electric installations, or is something I should get rectified before I fit the flooring and tidy up the wall!

If I understand you correctly (that the pipes and cables are simply running in the same space between two joists), I would say that was extremely common - and not something I've ever heard of anyone 'taking into account'. Obviously the greater the distance between cable and pipes, the better. It's when cables are 'wrapped around' hot pipes (yes,it has been known!) that one has to start worrying

Kind Regards, John

 

[Eighe]

I measure the temperature of the cables beneath the pipes in the hottest joist void(the one with the parralell pipe in) and even with the heating on for 7 hours full blast the wires got no hotter han 24 degrees C. I measured this a the closest point to the heating pipes(30mm between pipe and cable) with an electronic thermometer and also checked it with an infra red one.

So I don't think that the ambient temperature under the floow where the cables run is a problem!

Thanks again for your patient advice John.
I'm beginning to think that most of the regulations are pointless, since qualified sparkies are ignoring them!
It seems there is a big differene between safe and compliant.
Safe is easy to achieve and compliant requires a pointless rebuild!

Sorry it has taken so long to get back to this topic, it took me ages to be able to turn the heating on as there was half a tonne of laminate flooring stacked up in front of a couple of the radiators! I could reach the cock and had nowhere to move the flooring too and didn't want to cook it!

 

[Taylortwocities]

You need to check the cables when they are carrying a substantial load.

The parameters for cables, on which cable calculations are done, are at their specified operating temperature. For thermoplastic cables (ie general purpose PVC cables) the operating temperature is 70°C.

In a pure (ie passing your City and Guilds) cable calculation environment, the cable temperature is assumed to be 70°C.

In a domestic situation it isn't going to be anywhere near that figure so it isn't something that domestic sparks loose sleep over (eg grouped cables entering a CU).

Another view and attitude is taken in environments where the environemnt is less friendly.

 

[JohnW2]

I measure the temperature of the cables beneath the pipes in the hottest joist void(the one with the parralell pipe in) and even with the heating on for 7 hours full blast the wires got no hotter han 24 degrees C. I measured this a the closest point to the heating pipes(30mm between pipe and cable) with an electronic thermometer and also checked it with an infra red one. ... So I don't think that the ambient temperature under the floow where the cables run is a problem!

Indeed - and I'm not particularly surprised. 24 degrees is, of course, a perfectly reasonable/credible ambient temperature (even for totally exposed cables in free air) - so, as you say, it is not, in itself, an issue.

Thanks again for your patient advice John. ... I'm beginning to think that most of the regulations are pointless, since qualified sparkies are ignoring them! ... It seems there is a big differene between safe and compliant.

The regulations are, in general very cautious/conservative - hence 'very safe'. If one falls short of fully complying with them, one theoretically is creating a 'less safe' situation, but it sometimes may well be that the extent of that theoretical 'less safeness' (probably within the 'safety margins' built into the regs) is so small that many people would regard it as not being of practical importance. However, one cannot escape the fact that (just with speed limits etc.) 'regulations are regulations', and some people have no choice but to play things by the book!

Kind Regards, John

 

[JohnW2]

You need to check the cables when they are carrying a substantial load.

Indeed - if one wanted to determine their operating temperature. However, I think the OP was interested in the ambient temperature (with no load on the cables) because of the proximity of the hot pipes. Having demonstrated that it is a perfectly 'reasonable' ambient temperature, the standard CCC tables (which, as you go on to say, give the currents which are expected to result in an operating temp of 70°C) presumably apply. Indeed, the equation given in the regs for temperature-correcting calculations assumes an ambient temp of 30°C (as compared withthe OP's 24°C).

The parameters for cables, on which cable calculations are done, are at their specified operating temperature. For thermoplastic cables (ie general purpose PVC cables) the operating temperature is 70°C. ... In a pure (ie passing your City and Guilds) cable calculation environment, the cable temperature is assumed to be 70°C. ... In a domestic situation it isn't going to be anywhere near that figure so it isn't something that domestic sparks loose sleep over (eg grouped cables entering a CU).

Indeed. One of the shortcomings of the regs seems to be that, unlike the situation where cables pass through insulation, there appears to be no provision for reduced grouping factors when the length of the region of grouping is very small - it would seem inappropriate to have to increase cable sizes just because of grouping of cables for an inch or three where, for example, they enter a CU, if the remainder of the cable runs involved no 'grouping'.

Kind Regards, John