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#140971 - 05/25/04 02:32 AM Ring circuits revisited again
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
This is something of an "old favorite" topic at ECN, but it's been a while since the last round of discussion. As we now have several more British members, and I've just replied to an e-mail query about rings, maybe we could go another round and get some fresh opinions.

ECN regulars will already know my opinion on this one......

Anyway, here's the brief reply I sent to the e-mail query, just for any new non-British members who are still not quite sure what we're talking about:

 Quote:
The ring circuit is a peculiarly British method of wiring, used only here in the U.K., in one or two ex-colonies, and to a limited extent in the Irish Republic. It is not an arrangement found elsewhere in Europe.

The British IEE introduced the concept of the ring circuit in the late 1940s, along with a new design of plug and receptacle ("socket outlet" in their terminology). The ring circuit soon came into widespread use and has been an accepted standard for wiring British homes ever since.

As the name implies, the wiring for a branch circuit starts from the distribution panel, runs to each outlet in turn, and then a final cable links the last outlet back to the panel. The hot wires from each end of the ring are terminated at the same fuse or circuit breaker terminal, and the two neutrals are similarly both connected to the neutral busbar. In the case of a domestic circuit where our equivalent of NM cable (Romex) is used, the grounding conductors are also paralleled at the appropriate busbar to form a ring.

In the original specifications, the cable size to be used was 7/.029, i.e. a conductor comprised of 7 strands, each 0.029 inch in diameter, and in domestic wiring a ring could serve any number of outlets within a floor area of up to 1000 sq. feet. When the IEE and manufacturing switched to metric in 1970, that became 2.5 sq. mm conductors and 100 sq. meters (1076 sq. ft.) area. A 2.5 sq. mm conductor is approximately 20% larger than #14 AWG.

The overcurrent protection for the ring is a 30A fuse, or a 30A (old) or 32A (new) circuit breaker. As an individual 2.5 sq. mm conductor is rated well below 30A, the integrity of the ring is essential for insuring that conductors are not overloaded.

Protection for individual appliances and cords is then provided by a tubular ceramic-bodied cartridge fuse fitted in each plug. The plugs and sockets are rated for 13A maximum, chosen to allow portable appliances of up to 3kW to be used. The plug fuses are 1" long by 1/4" diameter, and were originally available in ratings of 2, 5, 10, and 13A for use with different appliances. Some years later, the range was changed to 3, 5, and 13A, and in the latest specification, the IEE recommends the use of just 3 or 13A fuses, supposedly to simplify the choice for the layman. Other ratings are still available, however.

As well as receptacles, the ring can also incorporate a fused connection unit. This is basically just a device to allow a fixed appliance to be hard-wired into the ring, and has a removable carrier which takes the same type of cartridge fuse as used in the plugs. Fused connection units are typically used for a small wall-mounted heater, exhaust fan, and similar devices.

Specifications also allow a socket or fused connection outlet to be run as a "spur" off the ring, using a cable of the same size as those comprising the ring itself. Such a spur may be tapped at any existing outlet, or may be tapped at any other point on the ring with a junction box.

Note that in British wiring it is normal practice to wire lighting points on separate circuits to receptacles, so lights do not normally come into the ring-circuit picture, however it is permissible to use a suitably fused connection unit on a ring to feed fixed lighting if required. Such a method is normally found only where it is otherwise impractical to tap an existing lighting circuit, e.g. when wiring an extension.

The idea of the ring is that it provides a versatile arrangement capable of delivering a considerable amount of power to any outlet (over 7kW on a ring -- everything here runs on 240V remember). As the rules permit a
ring to serve an area up to 1000 sq. feet or thereabouts, in the early days, and indeed up until as recently as the late 1970s, it was quite normal to find a small or average-sized home with a single ring feeding all
receptacles. In more recent times particularly as kitchen loads have increased, the average house now has two ring circuits, sometimes three.

The arrangement certainly has drawbacks, however. Loads do not divide evenly around the ring, and a heavy load near to one end results in much higher current flowing through the short leg of the ring than the longer leg. Although the IEE considers it unlikely to happen in practice, it is certainly possible for heavy loads placed close to the end of a ring to overload the cables. There is also the danger that a broken connection somewhere can leave a heavy load on an undersized conductor. The problem is that a single break still leaves power at all outlets, but if the break is
near one end, the remaining leg of the ring can end up carrying the full current.

Most British electricians seem to accept the ring as "a good thing," although personally I do not like the arrangement and would like to see it abandoned.




[This message has been edited by pauluk (edited 05-25-2004).]

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#140972 - 05/25/04 10:32 AM Re: Ring circuits revisited again
Dave55 Offline
Member

Registered: 05/08/04
Posts: 666
Loc: Crystal Lake, Illinois, USA
Thanks, Paul. I haven't seen this before and spent a bit of time going through old posts.

After review, the ring circuits seem to have some down-sides, especially that a break in the ring wouldn't be noticed without testing at the distribution panel. It would also seem a nightmare when tapped into by an uneducated homeowner or handyman (same as anywhere). Problems also, I suppose if done cheaply, with no consideration for kitchen appliances. Same situation in the U.S. where many general purpose outlets are on a 15 amp, 120 volt circuit and the vacuum cleaner won't run without tripping the circuit breaker!

The only part that really bothers me is that heavy loading on the ring near the distribution panel (causing an over-load on the conductor) "isn't likely". I think it may be more acurate to say that WHEN the conductor bursts into flame it "isn't likely" to burn your house down, because they're constructed of brick. I also feel sorry for you for the small electrical boxes. I like a lot of room for the wires and devices.

What are your feelings on the use of 240-volts over 120-volts for general purpose outlets? It seems to me that voltage drop might be less of a concern, but if you think of voltage as force, a shock might seem like you were getting kicked by a larger horse. I've heard that electricians who work casually on live wiring (and get away with it at 120-volts) are much more likely to die on higher voltages.

Dave

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#140973 - 05/25/04 04:21 PM Re: Ring circuits revisited again
Ever Ready Offline
Member

Registered: 03/27/04
Posts: 40
Loc: Scotland
I am actually an advocate of the ring circuit and prefer it to the radial type circuit, both systems have their pro's and cons, the one thing I do not like about radial circuits is that a lot of the radial circuits have gone on and on and on with more sockets being added as required, the main problem with a radial circuit in my opinion is that the cable from the distribution panel to the first outlet carries the whole current for the circuit which in a lot of cases can cause it to run a tad warm .

I realise that a ring circuits integrity can be breached if one of the ring conductors develops an open circuit but on the whole I have not found this to be a frequent occurence, another problem with people adding to a ring is they think that they can add as many additional sockets to the ring as they want especially on a spur.

The current rating for a BS6004 2.5 mm cable can be anything from 19.5 amps to 27 amps dependant upon the method of cable installation. I see a lot of houses with a lot of sockets on the ring although a lot of these sockets serve low current devices, eg can openers,blenders etc although there are quite a few high current devices eg, washer/dryers,toasters, kettles etc.

On the subject of 240 volt v 120 volt I prefer 240v for domestic use as it keeps the current lower and therefore cable size smaller than it would need for 120v.

Although voltage can kill you indirectly by burning you etc it is actually current that will kill you as a direct result of an electric shock, eg people have fallen on railway power lines (25kv) and died as a result of the burns they received from the high voltage.

Edited to correct a spelling mistake

[This message has been edited by Ever Ready (edited 05-25-2004).]

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#140974 - 05/25/04 04:48 PM Re: Ring circuits revisited again
aland Offline
Member

Registered: 05/20/04
Posts: 187
Loc: United Kingdom
Mmm Pauluk I can have some sympathy with you on your loathing of the ring circuit.
I seem to remember in the 1970's there was an idea banded about (London I think) to use a system where a ring main was wired all around a flat (sorry appartment) and the lighting for each room was spured from the ring. Dont know if it was ever put into practice. I tend to look at each ring main application on its merrit, I have seen some horrendous ring circuits running into what appears to be miles of cable. One problem I find is that lots of sparks only use these damnd plug in testers with lights on to check to see if they have a L,N,E at each outlet.Then its testing finished. With 2.5mm single strand cable there is a great tendancy for conductors to get damaged whilst stripping and even partialy cut through with terminal screws and then broken when being pushed back into box's (have done it myself)without noticing. The only way to be sure that the conductors are sound is by continuity testing end to end of the ring and R1+r2 and saddly it does not get completed in a lot of cases. I wonder how many circuits are out there that are not rings connected to 32amp breakers D type in a lot of cases.
My own preference is for 4mm cable on a radial circuit I must confess. I think 4mm stranded cable is mutch more user friendly. Mind you there's still the need to do Ri+r2 to prove continuity and polarity.

Dave, It does not matter how big the horse is if he kicks you it still hurts, my phylosophy has always been to treat every circuit as live even if you know that there is no supply connected that way your gona live longer. We dont want to be talkin about dyin it ain an option. What voltage drop are you allwed on your systems?

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#140975 - 05/25/04 07:20 PM Re: Ring circuits revisited again
Dave55 Offline
Member

Registered: 05/08/04
Posts: 666
Loc: Crystal Lake, Illinois, USA
I'm not in electrical design, Alan, but it's generally considered fine to have a voltage drop up to 2%, with 5% as a maximum. I have never had an inspector mention it though. It's possible that a large house could have a circuit (radial) well over 100 feet long (15 amp, 120 volt) that would be used for a vacuum or carpet cleaner (in an upstairs bedroom furthest from the distribution panel). Although not required, I run 12 gauge wire for general purpose outlets (for 20 amp circuits). I also oversize the wire often for motor loads.

I realize it's the amps that kill you, but it's the volts that push it through, so to speak. I generally handle wires as if they're live even when I know they're not, and use 600 volt gloves much more since reading safety posts on the internet!

Dave

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#140976 - 05/25/04 11:23 PM Re: Ring circuits revisited again
Ever Ready Offline
Member

Registered: 03/27/04
Posts: 40
Loc: Scotland
 Quote:
I generally handle wires as if they're live even when I know they're not


Wise move Dave, I was taught to treat every wire as if it were live, I believe if you follow that rule you should minimise your risk of electric shocks,it would not be the first time that a "dead" circuit became live while you were working on it.

 Quote:
I realize it's the amps that kill you, but it's the volts that push it through.
I have always thought of voltage as the pressure pushing the current through the circuit.

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#140977 - 05/25/04 11:54 PM Re: Ring circuits revisited again
aland Offline
Member

Registered: 05/20/04
Posts: 187
Loc: United Kingdom
Dave,As a matter of interest how many outlets are you allowed to put on your radial circuits ?
Alan

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#140978 - 05/26/04 04:40 AM Re: Ring circuits revisited again
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
One aspect of the modern ring-circuit usage that I find curious is the apparent lack of thought that goes into planning them in many cases.

The days of wiring with only one ring for the whole house might be behind us now, but how many times do you see two rings, one for each floor? In the average house that means that the upstairs ring is feeding nothing more than a few low-power devices such as electric blankets, radios, bedside lamps etc. while the downstairs ring still has all the heavy loads on it.

Note for our American friends: There is no specific requirement here to run washing machines, dryers, etc. on dedicated circuits. That means that they often end up connected to a general-purpose ring circuit.

Lip-service is paid to the modern increase in kitchen appliances, and I've seen suggestions that this could make it appropriate to run an entirely separate ring to supply the kitchen. If you do that, you still have all the heavy kitchen loads on one ring, but then you have two other rings only lightly loaded (probably not even any heaters in a modern centrally-heated house).

Dave,
The cramped boxes are one of my frequent compaints about typical British wiring. American methods are far superior in this respect.

On the 120 vs. 240V issue, I think the U.S. probably has the best of both worlds: You have 240 available for heavy loads, but with the advantage that no point is over 120V to ground, making it that much harder for somebody to get zapped at the full 240V.

All of us in Britain grew up with supplies of 240V (or thereabouts) to ground, so I guess we don't really consider the higher potential too a big deal. It can kick though! The worst shock I recall ever getting was when I was about 13 and a holder on a chain of fairy lights disintegrated as I was holding it. One wire came off the center contact of the holder, leaving me with the end of the wire in one hand and the shell of the holder in the other. 240 hand-to-hand isn't pleasant!

Alan,
I understand your feelings about the 2.5 conductors in our cramped boxes perfectly. It's bad enough when you get two sets of cables on a ring passing through a 1" box, but if you find one where there's a third spur cable as well, it's incredibly tight. As Dave hinted, this is one area where I think the American methods are far superior to ours.

You mentioned 4mm being that much easier to work as its stranded. Can I also assume that you preferred the old 7/.029 over the mdoern 2.5 sq. mm for the same reason? Even though the overall diameter of the 7/.029 is slightly larger, I find it much easier to work with in old systems.

On the radial circuit issue in the States, I don't think the NEC sets any specific limit on the number of outlets per circuit. They have dedicated circuits for all large appliances, then at least two 20A "small appliance" branch circuits feeding kitchen/dining area outlets.

Everything else (receptacles and lights) is then wired on general-purpose 15 or 20A circuits on the basis of allowing a minimum 3 watts per sq. foot (residential).

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#140979 - 05/26/04 08:17 AM Re: Ring circuits revisited again
Dave55 Offline
Member

Registered: 05/08/04
Posts: 666
Loc: Crystal Lake, Illinois, USA
What is the area of these small boxes you are using? They sound extremely small. Do you have large devices like GFCIs?

The outlets and circuits here depend on the builder. Paul represented the NEC well. In this area (Northern Illinois), where the electrical power costs are some of the highest in the country, our ranges/ovens are gas. The furnaces, clothes dryers, and water heaters are gas as well.

A typical house here is wired very minimally to maximize the profit of the builder. They are usually two story homes (2-3000 square feet?). The circuits will be as follows:
1st floor lights (15 amp)
2nd floor lights (15)
1st floor outlets (15)
2nd floor outlets (15)
Bathroom GFCI outlets (often on different levels) (20 amp)
Sometimes a master bedroom circuit (15)
2 kitchen counter small appliance circuits (20)
Furnace (15)
Dishwasher (20)
Laundry (washer and gas dryer) (20)
Well (240 volt, 20 amp) if not city water
Sump (20)
Ejector pump-with basement bath (20)
Some also have spas in the master bedroom (20)

As a good design I'd run the general purpose outlets at 20 amp instead of 15, and add the following:

A third kitchen small appliance circuit (20)
Refrigerator/Gas Range (20)
Exterior outlets (20)
Individual bath GFCI circuits (20)
Basement workbench (20)
Garage workbench (20)
Entertainment center-we're serious about our TVs and movies here (20)
Office (20)

For this reason, I get no work from builders. I go in after the builder and give the people the electrical system they THOUGHT they were buying. It's very typical for people to want 4 or 5 ceiling fans for the bedrooms and other areas here also. There's probably more, but this is it, generally.

In other areas that have cheaper power rates, you would find electric ranges (50 amp, 240 volt) and electric clothes dryers (30 amp, 240 volt), electric water heaters (30 or 50 amp?, 240 volt) and electric heat (varies). They would all be on dedicated (individual) circuits.

Dave

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#140980 - 05/26/04 05:43 PM Re: Ring circuits revisited again
aland Offline
Member

Registered: 05/20/04
Posts: 187
Loc: United Kingdom
Paul,Yes the old 7/029 was great to work with as pvc cable, not so clever when it was rubber though it was realy bulky then and as sometimes hapens now you could get bad drums where you could not strip the dam stuff. Strip the outer sheath and the inner cores would strip to the copper straight away. Oh happy days!
Re the ring circuit syndrome,realy many of the problems come down to lack of forthought on the part of the installer / designer.
Another problem that I can see looming in some domestics is the home office equipment on the ring circuit all stuck on the RCD side of the split load unit. Some homes now have a considerable amount of IT equipment, not uncommon to see 2 or 3 pc's hooked up to networks scanners printer fax's modems all spread around the ring all dripping a few milliamps to E, not to long before it starts to pop the rcd at the mains in ome cases!

Dave,
Sounds mutch like the u.k. we dont work for builders either you finnish up in all sorts of trouble with extra's and variations because they underprice in the first instance to secure the work and then you get all the hastle from the client because they think its you thats trying to put the minimum of equipment in to cut costs. Then there's the patment issue, Noooow I dont think we will bother thanks.

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