Ring final area or length

[andyandy2]

Does the old rule of maximum size of 100 square metres floor area still apply, or is it now determined by maximum length of cable.

Also does anyone number the conductors in the c u (in a similar way to commercial boards)

 

[ricicle]

The rule was a guide to the number of sockets in a domestic situation that could be wired on a ring circuit 100m²
It wouldn't apply to a commercial or industrial premises or perhaps even in a domestic situation nowadays.
I personally would substitute it with proper load design. (and of course circuit impedance)

Nothing wrong with numbering wires to aid in identification

 

[ebee]

The rule still applies.
As do others.
If you look at the OSG you'd probably not install a ring whose length is more than 88 metres and best keep well below this length.
But you'd use your engineering judgement too and if you know or suspect high loading you'd modify the area served.
Similarly you'd avoid any method that does not help the ring to be loaded approx equally around the ring or alternatively most loading about the centre portion.
All these "rules" need considering when installing a ring

 

[1john]

You could also consider a radial or two, and forget half these 'rules'.

 

[ebee]

BOTH rings and radials have their own advantages and disadvantages

 

[1john]

True enough, although there are far more advantages with radials and far more disadvantages with rings.

 

[RF Lighting]

In your opinion.

 

[Risteard]

The rule still applies.

You sure? Take a look at the corrections made by the Corrigendum.

That said, I try to keep a ring final circuit to a max. floor area of 100m2, but I can't see how it can be claimed that it is a requirement anymore.

 

[ericmark]

Under the new volt drop rules with RCD protection I seem to remember 106 meters is the limit.

The calculations are worked out on 26A not 32A the idea being that 20A is drawn at centre and remaining 12A is even distributed around the ring.

Since 2.5mm cable is rated 20A so total 40A the volt drop per amp per meter is modified to allow for the under maximum draw on the cable.

All the formulas required are in the 17th Edition red book. I did once out of interest work out cable length using 4mm instead of 2.5mm but then decided it was likely in the future some one may extend with 2.5mm so did not proceed.

Although calculations say 106 meters I would not want to use the full amount. Should some one decide to test the loop impedance and prove it is not within the 5% volt drop allowed it is a lot of work to correct.

Historically 100 sq meters was considered as a rule of thumb method of keeping within the limits. In one housing estate the engineer visited first house and realised they were using beams with knock outs so the cable did not need to come 1/3 of way into room to move between beams and so he opted for up-stairs / down-stairs split. First house wired and tested all OK so set electrician to do rest in same way allowing a common "As built" plan to be produced for all houses.

However the builder part way through estate decided to do away with the composite beams and use wooden ones instead. Hence 1/3 rule came in for drilling beams. Three houses were wired before the first with new beams was tested and it was found ELI (Pre 17th Edition) was exceeded and three houses had 20A instead of 32A MCB's fitted to ring mains to comply. Side to Side split was used with remaining houses to reduce ring main length.

With a single house the electrician normally uses one drum of cable for each ring main and would never start a second drum but with an estate one uses part drums so easy to use extra.

As to if anyone works out the loop impedance to give volt drop and would snag due to volt drop I don't know. With RCD on all sockets the ELI is no longer the limiting factor it is the line/neutral impedance which means two 2.5mm wires not reduce to 1.5mm as with ELI but as far as I can make out only the ELI is measured and entered on paperwork which does seem a bit odd!

I would expect a reading of 0.8 ohms or better as Line/Neutral loop impedance allowing 0.35 ohms for incoming supply. I have only seen it tested once where a shrink wrap machine was failing and the manufacturer was brought in to find out why. Their engineer's first test was line - line impedance and he obviously knew the pass limit as he immediately said too much volt drop and sent us his bill. Our long standing request for a three phase loop impedance tester was suddenly granted!

 

[andyandy2]

Thanks for the replies, I had read 106 metres as mentioned by eric above, but was also told "as many sockets as you like within 100 square metres floor area"

 

[MalcofArabia]

True enough, although there are far more advantages with radials and far more disadvantages with rings.

Agree John biggest advantage of Radials is in the testing only R1 and R2 to do, not the ring tests as a whole.

Also can be quite a chew getting legs back to the board in domestic and in commercial length of runs can be a pain.

Think the idea of radials only will take on, and after all us old uns have gone to the great tea hut in the sky rings will be assigned to history.

 

[ericmark]

The idea of a radial with loads of fused spurs has long been used in industry. Often the radial is in the form of a track system with plug in fused spurs.

Fig 15A RING FINAL CIRCUIT ARRANGEMENTS, REGULATION 433.1.5 is main guide and the comment (Historically, limit of 100m² has been adopted.) makes one consider the rule of thumb methods are going out. To skirt 100m² takes 40m to start with not including drops and 1/3 rule. Split 100m² into 4 rooms and the length starts to increase.

By splitting the house side to side rather than up/down reduces the cable used and I have seen an estate wired that way. It also helps with circuit failure as until sorted extension leads go room to room not up and down stairs. However my son was dead against the idea as he rightly said people are use to up/down split and may assume this to be case in all houses.

Of course an electrician should test but it's the "should" word again.

What I would be interesting to hear is if any regard it now taken to volt drop and is it ever tested? ELI with RCD use is not longer a limiting factor it is the Neutral / Line Loop Impedance which now limits ring main length. However "Historically" the meters were provided with wandering leads and a plug in lead and with the latter many meters both set to prospective current and impedance measured between line and earth and did not take a Line/Neutral reading. So the Electrician would have needed to make a special lead to test Line/Neutral which would be dangerous if it ever got into wrong hands.

Temperature can also effect the readings and it would be OK if the circuit passed but if it failed by small margin what allowance if any should be made where the circuit prior to test was in use?

When I served my apprenticeship the old meters we had was wind up mega and volt meter and although ELI meters existed I did not use one until well out of my apprenticeship. They are now common place. Builders normally give a 10 year guarantee and so it has to be considered what happens if an estate built today is tested in 5 years time and it is found it does not comply with today's regulations? Can the owners insist the fault is corrected? We see how far back claims with insurance go every day on TV.

Personally I would not want to take the chance and would put my trust in to the no more than one role of cable rule of thumb much more than 100m² rule of thumb. Sorry now called Historically not rule of thumb!

Insurance is another thing. When I retire will any work done today be still covered by insurance or should I continue with payments when retired? Same for any other reason to stop trading.

 

[ban-all-sheds]

three houses had 20A instead of 32A MCB's fitted to ring mains to comply.

Were ring finals on 20A devices allowed before the 16th?

 

[Risteard]

three houses had 20A instead of 32A MCB's fitted to ring mains to comply.

Were ring finals on 20A devices allowed before the 16th?

Departures from the Regulations are permitted where they do not afford a lesser degree of safety than compliance with the Regulations.

 

[ericmark]

I do take your point that because of the 20A/12A split when working out volt drops the 20A ring will only be calculated at 6A less than a 32A ring. So it gives a mere 36 extra meters. However it was not my job and it was when volt drop was 4% not 5% so volt drop wise went from 85 meters to 113 meters and an ELI from 1.44 to 2.30 ohms so today with 17th Edition rather than 16th Edition it would pass with a 32A MCB.