Maximum cable run for ringmain

Hi,

Can anyone give me a clue ?

I am trying to work out how to specify the re-wiring of the kitchen ( 2 ring main circuits) assuming consumer unit with 32A MCB can the total cabling length (out from consumer unit, once around the kitchen and back to the consumer unit) be 60metres?

I am told that the maximum cable length for a ring main with MCB as protection is 50metre??

Cheers

G
Reply to
blue
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On 2 Mar 2007 10:03:04 -0800, "blue" mused:

The maximum area a 32A ring main wired in 2.5mm cable is 100m2, how you'd do that with 50m I don't know.

As long as your kitchen isn't huuuuge, or your run from the CU to the kitchen anythig ridiculous like 250m then you'll be fine.

Reply to
Lurch

No it's 84m. On both PME and TT-S systems.

Reply to
Ed Sirett

Thanks, I could find lots of references to 100m2 (which would imply a theoretical room 100m x 1 m = run of 202 metres) but nothing I could understand about total run lengths.

Reply to
blue

The 100m2 is the floor area the ring serves, not the area enclosed by the cable loop.

It is a recommendation only, and applies only where there is no specific data about the power requirements in the area. If you know what the power requirements in the area are going to be, then use that to work out the maximum area a ring circuit can cover.

Reply to
Andrew Gabriel

Agreed, assuming a 32 A Type B MCB. For other situations Table 7.1 in the On-Site Guide is your friend.

TT-S? ITYM "TN-S" - only a typo I'm sure.

Reply to
Andy Wade

Thanks Chaps, briefly - house has solid ground floor, so means running all services in either under the 1st floor floorboards OR taking everything up into the loft space, along the loft & back down again. So chanelled into walls. Since all the plaster is shot & has to be replaced, whilst the floorboards are fine seems logical to spare the floorboards. But hence my question about the maximum run, which would be (approx) 3m up the wall to to the loft, 13m along the loft, 5.5m back down to the kitchen = 43m then once around the kitchen 14m = 57m.

May I impose on your kindness with supplemental questions, please? : I am assuming that I can specify standard twin-and-earth 2.5mm for ringmain, 4mm for cooker and 1.5mm for lighting ? Channeling into wall : enclosed in plastic conduit, covered by earthed metal cap ?

the complicated run is up from consumer unit into loft, where a whole bunch of cables will be close together. I have found some metal conduit internal 225x22.5mm which would do to take the general household circuits (4 x rings & 3 x lighting) but I am thinking to specify the 'kitchen' circuits to be run in individual conduits (1 x

4mm and 2 x 2.5mm).

Is there anything there that rings alarm bells?

Finally, do you think I am allowing enough circuits?

3 lighting : 1 upstairs, 1 downstairs, 1 for hallways & stairs. 4 'general' rings : 2 for upstairs (3 bedrooms) and 2 for downstairs ('parlour' and sitting room) 2 'kitchen' rings' = 1 for heavy appliances (dishwasher, washing machine, dryer) and 1 for 'above the worktop' appliances 1 cooker

comment gratefully received

Che

Reply to
blue

The lighting sounds fine. Watch conductor size for volt drop if the route is tortuitous and long I doubt the need for two rings for bedrooms. The loading from 3 bedrooms is unlikely to merit this. Its also unlikely you need two rings for parlour and sitting room. The kitchen is the likely place for heavy loading - washing machine, dishwasher, dryer etc. A dedicated ring for these is likely going to be fairly well loaded. You might be better running two 4mm radials with 32A protection each. Again watch volt drop over the length. It might be neccessary to use 6mm at least for the run from CSU to kiltchen. Also allow for volt drop on the cooker circuit and a if you use a control with a socket it must meet 0.4 second disconnection times with Earth Loop Impedance (5 seconds if no socket).

Don't forget to run in any supplementary bonding for the bath/ shower rooms while routes are accessible. 4mm minimum or 2.5mm if protected along entire runs. Watch grouping factor where applicable and insulation where applicable. Have you got the Regs and On Site Guide. Also worth look at the "Green Book" An Electricians Guide to the Building Regulations which is helpful in avoiding problems with holes or notches in joists etc, penetration of walls, chase depths and so on.

HTH

Reply to
cynic

Hi Cynic, (I like your handle)

Thanks very much. Really helpful. Just to impose some more....I've added some bits below

-- I think the lighting circuits are ok as these will virtually follow the same pathway as the old wiring (which seems to be a single circuit for the whole house)

-- I know, it does sound overkill doesn't it? but the layout of stairs, bathroom, and the solid ground floor means that effectively the house is in two parts 'front' and 'back' (with the kitchen as a separate issue owing to likely loads)

-- 4 or 6mm from csu to kitchen junction box and then drop to 2.5mm for the 'local' part of the circuit ?

-- actually being put in place 'against future requirement'

-- I'm glad you mentioned that. Is it better to use plastic type pipes for water? Was mentioned to me that was much safer than copper piping with earthing?

-- This is something I didn't understand I read up some bits but I'm non-technical so, whoosh! straight over the head. was an issue that was worrying me a bit, as maybe I would try to specify something that isn;t possible to do.

Have you got the Regs and On Site Guide.

-- sadly not!

Also worth look

-- I found most of this in the bldg regs (after a lot of reading & cross-referencing) but I am praying that I can avoid any tearing up of (any more) floorboards & (any more) holes in joists!

Cheers

Che

Reply to
blue

You may care to think about radial circuits then. A domestic multi outlet 2.5mm radial with 20A breaker can serve 50 square metres of floor area. The loading is not normally a problem for bedrooms or living areas

Right

Hepworth say so As long as any bonding is in place and connected it should be safe either way

See what ebay have to offer

The green book is a condensed version of the bits you need. Your local library might be a good place to spend an afternoon.

Reply to
cynic

Library it is then !

Very very many thanks for your help

Vheers

G
Reply to
blue

On 4 Mar 2007 00:26:00 -0800, "blue" mused:

Standard where? Standard, as in common in the UK, would be 2.5mm for

32A ring main, 6mm for 32A cooker circuit and 1.0mm for lighting.

Whatever you want, assuming it is run in the prescribed zones.

Why all the conduits?

Yes, but you'd probably get shirty if I said that I thought it was your competence so I won't say anything.

I'd say that sounds reasonable for a 5 bedroom detached haouse of reasonable size.

I can't imagine that.

Reply to
Lurch

On 4 Mar 2007 02:03:37 -0800, "cynic" mused:

Check the tables in the regs. You could probably get away with 32A radial circuits run in 4mm if they are about 3m long.

Reply to
Lurch

4mm twin and cpc volt drop tabulated is 11mV per Ampere per metre run. At 32A load and 4% of 230 volts( = 9.2 volts) permitted volt drop that works out to approximately 26 metres. Where did your 3metres come from Stuart? (Maybe you missed a zero off your post?)
Reply to
cynic

Sorry, yes TN-S or TN-C-S (PME).

Reply to
Ed Sirett

On 4 Mar, 08:26, "blue" wrote: //snip// but I am thinking to

err, yes if not the alarm bell, then a definite warning whistle

  1. Avoid 4mm FTE cable the CPC (the 'earth' conductor) is too small in cross-section area for most purposes. Effect is to severely curtail the length of a 4mm^2 circuit. If you want to use it you need to DIY the sums for circuit length as there is only limited information for radial circuits in the On-Site Guide (table 7.1.1) and none at all for ring circuits. You'd do better to use 6mm^2 for cooker circuits, though technically you should check the cooker rating first. [ie cabling should be specified to meet the load required by installed equipment; not t'other way round]
  2. Inserting cable in conduit generally reduces the maximum rated current - see on-site guide table 6D1 - compare ratings for ref method
1 with ref method 3 for instance - reduction is around 20% More normal is to clip cable directly to a wall and bury under plaster. You can channel out blockwork or brick a little if plaster depth is inadequate. Some think it good practice to cover the cable with metal capping to protect against minor damage such as drawing pins being stuck in the plaster.

You also do better to run cable horizontally or vertically from fitting to fitting - see OSG for permitted routes. That keeps cable runs shorter, and incidentally reduces volts drop in the circuit which reduces the amount of elec energy - paid_for_by_you - wasted in the cable.

  1. beware of cable bunching which also may reduce cable rating. Again see OSG

HTH

Reply to
jim

On 5 Mar 2007 11:11:10 -0800, "cynic" mused:

Maybe I should read the tables in the regs then?

Reply to
Lurch

Thanks, It looks like I'm going to spend some time in the library. I've got a lot to think about

Cheers

Che

Reply to
blue

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