IEE Ring Main Cable Size

[slogical]

Hi,

Apologies if this has been posted previously. I am currently working on the outline plan for rewiring a 3 bed semi and have worked through the calculations for cable size but have clearly missed something as the resulting cable sizes are far too large. I understand the standard ring main cable size would be 2.5mm^2 or 4mm^2 depending on load and whether run through thermal insulation.

Having calculated the required MCB rating of 32A, if corrections are applied for running in thermal insulation (0.89) and grouping 3 cables (0.7) this gives a requirement of 51.36A - using the IEE cable size tables this requires at least a 6mm^2 cable depending on installation

- this can't be right.

Firstly, should I be factoring in the grouping if the cables are only being grouped for short run in conduit through joists and walls. Secondly, I must have made a mistake somewhere of not applying some other factor for it being a ring main - can anyone advise where I have gone wrong?

Thanks in advance, Simon

[ac1951]

Not sure where you've gone wrong but don't forget that a ringmain wired using 2.5mm twin/earth actual provides a total conductor size of

5mm to each socket on the cct.

Andy

[Andy Wade]

It's not. The as-installed rating required for a standard ring is 20 A, not 32 A - see Reg. 433-02-04. This is subject to a check that the likely load distribution won't cause long-term overload in any part of the circuit, compared to the actual as-installed rating (Iz) - this is only likely to be a problem if you have a kitchen or washroom at one end of a ring.

Applying your factors, the It rating needed is, by coincidence, 32 A - which would mean 4 mm^2 cable. With a bit more thought and re-arrangement you could probably get that down to 2.5. The Cg factor you're using is for bunched cables; can you not use the single layer figures with T&E? Can you space the cables where they go through the insulation, so that the the Cg and Ci factors don't apply simultaneously?

How short is short? I wouldn't worry too much about 50 mm through a joist, but 300 mm through a wall is a different matter.

[John]

I fully admit to not being an electrician so obviously I'm already donning my flameproof jacket :o)

However, I do know four different electricians (as a teenager I could always earn extra by labouring for any of them at weekends) and when rewiring a

3-bed semi such as the OP is doing, none of them ever did any calculations for anything. It was always 1.5mm T&E for lighting (splitting into upstairs lights and downstairs lights with a 5A fuse/6A MCB for each) and 2.5mm T&E for ring mains (splitting into upstairs ring, downstairs ring and kitchen ring with a 30A fuse/32A MCB for each) as long as each ring didn't serve a floor area of more than 100sq m.

I got the impression that it was a sort of "rule of thumb" if you like that (in domestic situations like 2 and 3 bed houses) that 2.5mm T&E was used for ring mains but from what I'm reading here, that no longer applies does it - if it ever did?

John

[Owain]

2.5mm is the *minimum* for a 32A ring circuit wired in T&E or conduit singles, or 1.5mm in MICC ('pyro'). It is not guaranteed to be compliant, but a smaller cable size may not be used even if it were compliant.

This is the difference between a professional and an amateur job - an amateur can take his time and do the job properly.

Owain

[Dave Plowman (News)]

I'd say if the calculations show 2.5mm is inadequate in the average house you want to go to two rings for that area anyway.

[David Hansen]

On Mon, 12 Nov 2007 05:36:26 -0800 someone who may be slogical wrote this:-

For what distance? A short length of insulation means that the heat can escape on either side, a long length means the heat can't escape.

Grouping factors only apply if the cables are heavily loaded. If they are not then they don't matter for grouping. Typically the cables in lighting circuits can be ignored for grouping, though this depends on individual circumstances.

Both are explained in the usual guides.

[ac1951]

Andy, Could you explian the issue here ?

This is subject to a check that the likely load distribution won't cause long-term overload in any part of the circuit, compared to the actual as-installed rating (Iz) - this is only likely to be a problem if you have a kitchen or washroom at one end of a ring.

thanks Andy

[John Rumm]

A 32A circuit can notionally supply over 7kW of power. However one needs to ensure that the current loading on any individual cable does not exceed the cable's rating. In the case of a ring circuit, the current is carried by two conductors in parallel, the proportion of the current split between them will be dictated by the ratio of the resistances of the cable runs. So for example, a 30A load at one end of the ring would overload the shorter cable leg since it would end up taking the bulk of the current, whereas three 10A loads positioned at the start, middle and end would not load any one leg at more than 15A.

So basically with ring circuits, distributed loads spread about the ring are ok, but big concentrations of load at one end of the other are not so good.

[ac1951]

Ok, understood thanks for the clarification..

[Andy Wade]

Thanks for answering that, John. There's also an IEE /Wiring Matters/ article on the subject here:

[Tony Bryer]

ring at home probably contravenes - there's about 12m of cable one side of the kitchen, 50m the other. What's irritating is that not so long back I was moving cables around and it would have been easy to split it into two.

Having said this, I think the IEE paper you cited is rather pessimistic in terms of today's appliances - washing machine and dishwasher each drawing 2.5kW for a solid half hour, so I think it more a theoretical problem than real.

[David Hansen]

On Tue, 13 Nov 2007 12:32:34 GMT someone who may be Tony Bryer wrote this:-

>Having said this, I think the IEE paper you cited is rather

It is certainly pessimistic in terms of "old" appliances. My washing machine is over 15 years old. However, the heating element (nominally 3kW) is not on for half an hour solid, even for a 95C wash. It may be on for as much as 10 minutes, perhaps even 15 minutes, but no more.

Of course this is an "old" machine with hot and cold fill. What one of the "modern" "improved" washing machines, with cold fill only reduced water consumption and a puny 2.something kW heater, does in the way of heating I have no idea, but I suspect that the heating element is on for at least as long as in my "old fashioned" washing machine.

Dishwashers contain rather less water than washing machines, so the heater is on for rather less time in one go. However, there may be several hot parts of the cycle and thus the heater is on for longer overall.

[Andy Wade]

If clipped direct conditions apply for the 12 m run it's probably compliant. At 32 A full-load the current in the short leg will be approx. 50/62 * 32 A which is about 26 A. The clipped direct rating for

2.5 T&E is 27 A.

Its when the short leg is run in thermally insulated walls etc. that this becomes an issue.

And how often, in practice, do you switch them both on from cold at the same time?

[slogical]

Thanks everyone - very helpful - hopefully building control will be happy!

Cheers, Simon

[Mike Clarke]

Our 2 year old cold fill washing machine consumes about 2.25 KWh on a 90 deg full size wash. I think it's a 3 KW heater so I'd guess the heater's on for about 35 to 40 minutes.

The quite elderly slimline (9 place setting) dishwasher uses just over 1 KWh for the heavy duty cycle. I think it's a 2.5 KW heater so that'll be about

25 minutes.

[ctkenney24]

With respect. You have 3 bed semi and probably gas central heating. Allowin g for the diversity factor and that you will no doubt not be using 3kw elec tric fires in every room (lol), you only need 2.5 cables, except for the co oker the electric shower and possibly the immersion heater. Unless you are going into cannabis production in which case you need to speak to the Natio nal Grid plc..

[John Rumm]

I spose he might have done it in the 9 years since he asked...

(and for those that are interested, what he was missing was the cable need only take 21A for one leg of a ring)