Ground floor ring main, 120 square meters.

[Ivor Littlunn]

Good evening lads, in a bit of a quandary? Running a new ring main downstairs in the house we are renovating and the floor area is a little over the requirements for a 32amp breaker.
There's around 25 sockets in total and the washer, tumble dryer is in a separate utility room to the kitchen. There is a spare way (32amp) in the split load CU so am I best doing two rings and dividing the ground floor?

Cheers,

 

[flameport]

Floor area isn't a regulation, although it does appear as a recommendation in a number of documents.

Of more relevance is the total length of cable for the circuit, and whether a ring circuit is the best option.

 

[Ivor Littlunn]

Are you suggesting that two x 4.0mm radials would be a better option?

 

[JohnD]

the washer, tumble dryer is in a separate utility room

give the futility room its own radial.

Having 25 sockets doesn't matter when almost all of them will be unused, or have trivial loads like phone chargers, table lamps and TVs in them.

 

[flameport]

Are you suggesting that two x 4.0mm radials would be a better option?

I'm suggesting that someone actually considers what would be an appropriate design for the particular circumstances, rather than just assuming one particular type of circuit is the correct choice.

 

[EFLImpudence]

the floor area is a little over the requirements for a 32amp breaker.

Consider the silliness of the area recommendation -

Does the cable run straight around 10m.x 10m. (40m.) or 25m.x4m. (58m.) or 33m.x3m. (72m.) or
zig-zag all over a smaller area (could be any length).

 

[sparkwright]

I seem to recall a ring circuit should be 73m or less in cable.

It would be recommended to have two ring circuits downstairs anyway, perhaps the kitchen on it's own.

Ring circuits are sometimes frowned upon, but I think the advantages outweigh the disadvantages.

 

[JohnW2]

Consider the silliness of the area recommendation - ....

Indeed. However, I suspect that the recommendations (which may once have been regulations?) in relation to floor area were probably not so much about cable length but, rather, were based on the assumption that the likely electrical load (indeed, the likely number of rooms involved) would be vaguely related to the total floor area involved. That's obviously no better than a very rough rule of thumb, and one really has to consider the actual situation on a case-by-case basis to get something more approaching a realistic estimate of 'likely load'.

Kind Regards, John

 

[JohnW2]

I seem to recall a ring circuit should be 73m or less in cable.

There are some uncertainties/doubt about the process by which he calculates it, but eric frequently quotes 106 metres as the maximum length of a 2.5mm² ring final circuit - that, I think, being based on voltage drop considerations (which, if one goes by the 'recommended' max VDs in the regs, is often/usually the limiting factor, rather than Zs).

Kind Regards, John

 

[EFLImpudence]

Ring circuits are sometimes frowned upon, but I think the advantages outweigh the disadvantages.

Only where the advantages outweigh the disadvantages.

 

[ban-all-sheds]

That will be in La La Land?

 

[ericmark]

Because with a ring for volt drop you work on 20 amp load at centre and 12 amp even spread you can use 106 meters of 2.5 mm² cable they normally work out better than radial which still has 20 amp at end and rest even spread. Use 4 mm² on a ring and you can have 179 meters of cable. With 6 mm² 274 meters of cable and still be within the 11.5 volt drop limit.

With industrial premises running 6 mm² in the loft with junction boxes for each sockets dropping down the wall in 2.5 mm² was a way around the problem, with house more likely to use two rings splitting side to side as that reduces the cable required more than up/down split. But it is volt drop due to cable length which is limiting factor, a radial is down to 32 meters of cable at 20 amp and using 2.5 mm² so that would not help at all. (42 meters with 16 amp)

Note:- The 106 meters limit was quoted at a IET lecture, I tried to work this out and even with the full calculation using a correction factor I could not get the figures, so at next IET lecture I asked the question, this was where I was told about the 20 amp centre 12 amp even spread so you calculate at 26 amp when working out the volt drop. After this I wrote an excel program for my phone, when changing phone I re-wrote it in java script so I can now quickly pop in the figures and get results.

However you are the designer, and you could decide that 32 amp would be even spread in the house your building or because of heavy equipment in kitchen there is a 26 amp load in the centre. It is not written in stone or a regulation it is just something designers use as a rule of thumb.

Very like the area is a rule of thumb, if you fit sockets around sides of a 100 m² block, it would need around think the old figure was 82 meters of cable which was the old limit when the volt drop was 4% but with the rules on drilling beams one now uses more cable than the old system so the sq meter rules does not work any more.

Using a loop impedance meter line - neutral one can in theory work out the volt drop, so if incomer is 0.35Ω then maximum any point on the ring is 0.94Ω. However with any standard loop impedance meter you get variations each time you read it, so one could not be 100% that the volt drop is exceeded under 1Ω, and I have never noted anyone to date measuring volt drop.

There has been a problem where solar panels have locked out due to over voltage so the DNO has reduced the voltage to 230 volt nominal rather than the 240 volt which was historical set, this lower voltage has resulted in some fluorescent tubes failing to start, so volt drop is more of a problem today than some 10 years ago, however I would suspect there are very few ring finals which are every tested.

 

[winston1]

There has been a problem where solar panels have locked out due to over voltage so the DNO has reduced the voltage to 230 volt nominal rather than the 240 volt which was historical set, this lower voltage has resulted in some fluorescent tubes failing to start, so volt drop is more of a problem today than some 10 years ago,

I thought this "change" was to do with harmonisation with Europe rather than poorly designed solar panels. But as the actual voltage has not changed and there is no intention to do so how does it help solar panels and how has it effected florescent tubes?

 

[JohnW2]

There has been a problem where solar panels have locked out due to over voltage so the DNO has reduced the voltage to 230 volt nominal rather than the 240 volt which was historical set, this lower voltage has resulted in some fluorescent tubes failing to start, so volt drop is more of a problem today than some 10 years ago, however I would suspect there are very few ring finals which are every tested.

I think winston is right. I don't know if there are any long-term plans but, to the best of my knowledge, no DNO has so far reduced the voltage they are supplying as a result in the 'on paper' reduction in nominal voltage from 240V to 230V.

I have no idea whether this is the case, but part of their reticence to do this may be that so many of the loads these days are essentially 'fixed power' or 'fixed energy' ones, so that reducing the voltage supplied would increase the average supply current required, putting more demands on their distribution networks.

Kind Regards, John

 

[ericmark]

I noted when the voltage reduced for harmonisation for many years in real terms it stayed the same, however where I live there has been a row of houses with solar panels fitted, not long after they were fitted the voltage dropped from around 248 volt to 232 volt at my house, and the old kitchen fluorescent lamp failed to start, it was always on the slow side, but that 16 volt was enough to tip the balance. I replaced the 65 watt tube with a 28 watt LED as I could not find any replacements, and the 58 watt tubes work but not for long.

Before the drop in voltage I am sure the 254 volt limit was hit on times, this would cause the solar panels to lock out, it is all part of the safety design so in the event of a power failure the solar panels will not feed into the supply making locked off supply lines live. If there is a cable brake according to demand at the time the voltage could go up or down, with multi houses with panels fitted it could maintain the supply unless both under and over voltage causes them to auto disconnect. So now the DNO actually get complaints if the voltage is too high as well as too low.

It has been talked about many times, should you work on the supply at the time of the install, or allow for voltage or loop impedance to alter over the years? When looking for a loop impedance of say 1.44Ω with a incomer of 0.20Ω should you allow for this to alter in the future to 0.35Ω which is seen as the DNO's limit and be actually looking for 1.29Ω to allow for future changes? At one point I would have said yes, however in real terms the supply loop impedance depends on the max supply amps, so with a 60A fuse the supply loop impedance can be higher than with a 100A fuse or with commercial a 500A fuse.

We can maybe start the debate again?