Max. floor area for ring main?

[Penners]

I have a feeling that there is a specified maximum floor area that can be served by a single 30A ring main. Can anyone please tell me what it is?

Is there a maximum number of sockets that can be included in a single ring?

It'll be interesting to see what Fred Drift can make of this topic. :|

 

[MdB]

100m^2 and no.

(though I would wait for Zebra to comment as she's an expert)

 

[middi]

Last time my ( qualified !!) electrician friend fitted new electrics I asked him how many sockets should be (legally) fitted on a 30amp ring main.

He said 8 double sockets with an option of 2 " spurred off" singles ( the extra two sockets an extension off of the doubles an not in the 30 amp ring), is the norm in the average residential home.

I was going to be awkward and get ready for a fight but never went ahead and mentioned that I might want to plug 16, 3 kw electric heater, all at once in the 8 double soclets

Middi

 

[agbagb]

The latest regs for new instalations point to radial circuits.

20 amp breakers, 2.5mm2 cable, rule of thumb up to 50m2, and as many sockets as you like
30 amp breaker, 4.0mm2 cable, rule of thumb up to 75m2, and as many sockets as you like

(these assume no runs through or under insulation)

The reason being, if you have dogy conection somewhere on the ring you might never know about it until half of it is loaded up to 30A and the 2.5mm2 starts working as "in wall" or under floor heating.

You could split an existing ring in to two radials each on their own 20A breaker.

(PS. I'm no expert)

 

[MdB]

He said 8 double sockets with an option of 2 " spurred off" singles ( the extra two sockets an extension off of the doubles an not in the 30 amp ring), is the norm in the average residential home.

Norm possibly though 8 sounds rather low. My understanding that the regs don't give a limit and that you can have as a single socket spurred off each single socket on the ring and the same for doubles. Again - not an expert though a lot of the experts aren't either...

that I might want to plug 16, 3 kw electric heater, all at once in the 8 double soclets

Which would of course cause the fuse to trip when the third one was switched on.

 

[Lime]

The regulations do not specify a maximum number of sockets nor the maximum area that a ring circuit can supply.

Put simply:
It is up to the designer of the ring circuit to evaluate what loads the circuit is likely be asked to supply in order to calculate the maximum number of outlets for that ring. If more outlets are needed they will require an additional ring circuit.
Additionally the ring circuit must comply to the regulations concerning safe disconnection time in case of a fault (either L-N or L-E). This limits the length of cable a ring may be constructed with because the longer the cable the greater its resistance (impedance). The designer is required to ensure the impedance of the cable is low enough to ensure safe disconnection.
The design and type of MCB or RCBO will also influence these calculations.

In general the rule of thumb of a 100metre area holds good in most cases using 2.5mm T/E cable with a 30A MCB type B, any larger area is better served with 4mm T/E cable.

 

[Penners]

Many thanks, everyone, for those replies.

The reason for the question is that we're planning a modification to our downstairs layout, to move the washing machine and tumble dryer to a different location. This will involve a modest extension to the downstairs ring and the addition of two new sockets. However, given the very modest footprint of our Georgian cottage, I don't think this will be a problem whether the regs specify 100m^2 or not.

With one dissenting voice (Middi's mate!), the concensus seems to be that a maximum number of sockets is not specified. This seems to make sense, because it seems to me that a larger number of sockets doesn't necessarily imply a larger electrical load on the circuit*. Surely a family will use the same number of electrical appliances, regardless of how many sockets they have. More sockets simply offers them a wider variety of places to plug in the same appliances. The existence of spare sockets is surely not an encouragement to buy more gadgets!

* Does a properly installed socket introduce any resistance into the circuit when it's not in use?

 

[Zebra]

The 100m sq floor area is mentioned in the latest regs, but as a historical limit, or a guideline for a standard circuit, rather than as a hard and fast max. The number of sockets on a ring is not mentioned in the latest regs as far as I can recall, and if it were, it would similarly be a guideline rather than a rule. As you say Penners, more sockets does not mean more load, it just means more options. (There's no resistance on a socket when not in use unless it has a neon indicator.)
What agbagb says is not entirely correct if it implies that rings are no longer acceptable, because they are. What Lime says is correct, the Regs simply insist that you mustn't overload the cable, must ensure the disconnection times are achieved etc etc. How you achieve this is down to the designer of the circuit, and everything else is a recommendation for how you might achieve this.
However, as Lime says, there is a given limit on circuit length, which is dependent on the type of circuit breaker or fuse that you have. For example on a 32A circuit breaker or RCBO using 2.5mm sq cable your max circuit length of a ring is 106 metres, but this is because of volt drop rather than load (and is a rule which is often ignored, because volt drop doesn't generally cause a problem in the UK, because the transmitted voltage is higher than it is nominally supposed to be anyway). What type of circuit breakers or fuses do you have?
I don't think you would have a problem in doing the modifications that you want, given that you are simply relocating the load rather than adding any more. If you are going to put in new sockets for the washing machine and tumble dryer though, do keep the extra sockets on the ring rather than install them as a spur, because washing machines and tumble dryers are quite heavy in terms of load, and you wouldn't want all that on a spur.
(Remember of course the rules about work in a kitchen having to be notified under Part P!! :wink: )

 

[Penners]

Many thanks for that tutorial, Zebra.

The ring in question is on a 32A MCB, protected by an RCD in the consumer unit. I certainly won't run a washing machine or dryer on a spur socket - they will both run from new sockets on the ring (2.5mm TWE).

The modification won't be in the kitchen - it's in a lean-to extension room outside the main house. The downstairs ring cabling will simply be diverted through the dining room wall into that extension and back in again. It's a very modest and straightforward matter.

Thanks again.

 

[Keith Bowman]

Many thanks for that tutorial, Zebra.

..........

The modification won't be in the kitchen - it's in a lean-to extension room outside the main house. ........

Thanks again.

Oh! Outside eh? I can hear the sharp intake of breath and the sucking of expert teeth from here!
Best wishes for a spark[l]ing New Year.

Pssst - want to buy some old black and red cable?

 

[Zebra]

Jolly good Penners. Do post some pictures tomorrow when you've done it won't you 8)

 

[MdB]

oh and make sure that you don't end up with a figure of 8 circuit when you modify the ring. It must stay as a complete ring with no shorts across the middle of it.

 

[Zebra]

Indeed. No shorts. Or trousers. Most dangerous...

 

[Penners]

No shorts. Or trousers.

In which case there definitely won't be any pictures (you'll be relieved to hear).

MdB - thanks, and I confirm that the ring will remain a proper ring. No fancy ice-skating manoeuvres.

Keith - no, not outside! The lean-to extension is a proper brick-built structure (and thanks for the offer of some "traditional" materials, however my needs in this regard are satisfied, thank you)!

 

[JohnB]

oh and make sure that you don't end up with a figure of 8 circuit when you modify the ring. It must stay as a complete ring with no shorts across the middle of it.

The kitchen extension in my last house had a separate ring. One day I turned it off to do some work, and wondered why it was still live. Then I discovered one end was connected to the wrong fuse :shock:.

 

[Owain]

The kitchen extension in my last house had a separate ring. One day I turned it off to do some work, and wondered why it was still live. Then I discovered one end was connected to the wrong fuse :shock:.

One of the advantages of the ring was that two existing 15A radials could be converted into a 30A ring by joining them together.

In some cases the ring was made but the ends were left on 2 separate 15As.

 

[Flyfisher]

If the proposed work is, effectively, just relocating sockets then I presume there should be no loading issues, given that no new applicances are likely to be added and the existing pattern of usage will remain the same.

However, a washing machine and tumble dryer used together on the same ring main will get close to the maximum current capability of a 30A circuit and making a cuppa with an electric kettle during laundry chores will easily exceed the circuit rating and cause the fuse/MCB to trip. All quite safe, of course, but not particularly convenient.

Unless it can be pretty much guaranteed that no other high-wattage device will be plugged into the same circuit, I would seriously consider installing a dedicated circuit for the washing machine/dryer sockets - which is what I did in our utility room that houses a washing machine, tumble dryer, dishwasher and assorted other sockets, including a frequently used electric kettle. All of that on a single ring would be a recipe for frequent trips to the consumer unit!

 

[Flyfisher]

The kitchen extension in my last house had a separate ring. One day I turned it off to do some work, and wondered why it was still live. Then I discovered one end was connected to the wrong fuse :shock:.

We once had a house with two ground floor rings. I needed to do some work on one of them but, removing the power fuses one at a time, I couldn't find the fuse that isolated the circuit. Somewhat confused, I removed ALL the fuses (which did isolated the power) and re-inserted them one at a time. Sure enough, the circuit in question was being supplied by TWO fuses!

After some investigation it transpired that a double socket had been installed between the two rings, effectively joining them together - nice!

Trust, but verify!

 

[MdB]

In some cases the ring was made but the ends were left on 2 separate 15As.

Please don't do this now...

 

[Penners]

I would seriously consider installing a dedicated circuit for the washing machine/dryer sockets

In an ideal world I would certainly do this, FF. However, the location of the appliances is at the opposite end of the house from the consumer unit. So, short of ripping up floorboards or tearing down ceilings (which I'm not busting to do!), it would be impossible to achieve this.

Given that the system has worked without problems so far (this being merely a relocation of the appliances on the same ring) I think we can live with the small risk of overload trips.