Radial Circuit in Garage - Change to Ring?

[EFLImpudence]

I guess my statement on "can it be 20/25A" meant "on existing 2.5mm".

Yes, 2.5mm² has a capacity of 27A.

I thought a ring was required to allow for higher loads (up to 32A) on 2.5mm

It is because there are two cables which therefore could carry 54A if they were wired in parallel but only 32A is allowed.

as larger becomes expensive / cumbersome.

Whilst 4mm² (capacity 37A) is larger and more expensive, it is much nicer to work with.

Totally appreciate that it could just be a radial (as in, open ended).
So it's ok then to add another radial, using standard 2G sockets, with 4mm and do 20A on that?

As said, with 2.5mm you could have 25A MCB.
With 4mm², 32A MCB.

Yes, cooker, power-shower, lights(?) are all radial.

Yes, everything is apart from the peculiar anachronistic British socket ring circuit.

Why are rings pointless today?

Because the regulations are still written with rewireable fuses in mind which require larger cables than the same rating of MCB so, unless the sockets actually are all arranged in a ring right back to the consumer unit, then there is little point.

I know the argument that should there be a cable break then the user won't know and the 2.5mm could be taxed beyond 16A designed and therefore a fire hazard.

As, said it has a capacity of a very conservative 27A - so near 32A that it would rarely matter - for 16A you could have a radial of 1mm² T&E.

 

[JohnW2]

It is because there are two cables which therefore could carry 54A if they were wired in parallel but only 32A is allowed.

Indeed -and, as you know, the reason 'it is not allowed' is that the total load current in not necessarily (not usually) shared equally between the two 'legs' of the ring. In the absence of any restriction on the placement of sockets, a total load of 54A 'plugged in' close to one end of the ring could lead to appreciably more than 27A flowing in the 'short leg'.

However, if one installed a radial which literally had two 2.5mm² cables ('Method C', and not 'grouped') in parallel (i.e. both cables going to all sockets) then the total permissible load would be 54A (hence protection with, say, a 50A MCB would be compliant) - although those with limited ability to think might well be unhappy because this is not one of the examples of an acceptable circuit mentioned in BS7671!

As, said it has a capacity of a very conservative 27A - so near 32A that it would rarely matter - for 16A you could have a radial of 1mm² T&E.

Indeed - a 16A radial could, indeed, be wired in 1mm² cable ('Method C'), were it not for the fact that the regs require at least 1.5mm² for a 'power' circuit.

Kind Regards, John

 

[crappy]

Really enjoying the information folks. Thanks!

 

[SUNRAY]

However, if one installed a radial which literally had two 2.5mm² cables ('Method C', and not 'grouped') in parallel (i.e. both cables going to all sockets) then the total permissible load would be 54A (hence protection with, say, a 50A MCB would be compliant) - although those with limited ability to think might well be unhappy because this is not one of the examples of an acceptable circuit mentioned in BS7671!

Kind Regards, John

A perfectly acceptable circuit design. I'm a little surprised you say parrallel feeding is not mentioned any more.
A number of years back I encountered a ring circuit in a village hall enabled by a 4 pole contactor/sound level meter and one pole had failed. To cut a long sequence of events short I and the installers/approved electricians agreed the best solution was a parallel fed 2P contactor with 3A FCU spur (To power the meter) was the best solution.

I understand the hesitancy to accept a parallel fed radial socket circuit as you describe.

 

[JohnW2]

A perfectly acceptable circuit design. I'm a little surprised you say parrallel feeding is not mentioned any more.

That's not quite what I said. BS7671 has a reasonable amount say about conductors in parallel in general, particularly in relation to their protection against overcurrent.

The issue to which I was referring is that a circuit such as I mentioned does not appear (as 'guidance') as one of the examples of final sockets circuits given in Appendix 15 of BS7671 - and 'those with limited ability to think' seem to assume that anything which does not appear in that guidance is 'not permitted'. Another example of that is a 4mm² unfused spur from a ring final.

I understand the hesitancy to accept a parallel fed radial socket circuit as you describe.

Why do you 'understand the hesitancy' - just because it is 'unfamiliar', perhaps? Whether there would often be any particularly good reason for wanting such a hypothetical "50A radial" is a different matter - two 32A/4mm² radials (or two 32A/2.5mm² ring finals) would presumably usually what would be used.

Kind Regards, John

 

[scousespark]

Evening folks,


I have a radial circuit in the garage which is fed from a single 16A breaker through 2.5T&E. It has 6 x 2-gang sockets on it. CU is in one corner of the garage and the final socket is the opposite corner. Total run is around 30m I estimate.

The garage CU is fed from the house from a 40A MCB over 6mm armoured and into the CU which has a 63A ELCB.


I want to put some more sockets on the "blank" wall and easiest would be to have another run from the CU through to the new sockets (I will add 3 x 2 Gang) and then on to the end of the radial.

Can I simply do this and change the radial to a ring and the 16A to a 32A?


C

The simplest option is to extend the radial from socket 6 through to sockets 7,8 and 9 (given you were planning on making this a ring)

 

[scousespark]

Indeed -and, as you know, the reason 'it is not allowed' is that the total load current in not necessarily (not usually) shared equally between the two 'legs' of the ring. In the absence of any restriction on the placement of sockets, a total load of 54A 'plugged in' close to one end of the ring could lead to appreciably more than 27A flowing in the 'short leg'.

However, if one installed a radial which literally had two 2.5mm² cables ('Method C', and not 'grouped') in parallel (i.e. both cables going to all sockets) then the total permissible load would be 54A (hence protection with, say, a 50A MCB would be compliant) - although those with limited ability to think might well be unhappy because this is not one of the examples of an acceptable circuit mentioned in BS7671!
Indeed - a 16A radial could, indeed, be wired in 1mm² cable ('Method C'), were it not for the fact that the regs require at least 1.5mm² for a 'power' circuit.

Kind Regards, John

The ring is protected by a 32A breaker and the cable rated at 54A (due to doubling up the 27A rating for the single cable). If the load is 50A and there is a break in the ring, the MCB trips after about 1 hour. In your design, the breaker would never trip. Do you think running 54A indefinitely through a conductor rated for 27A would be a fire risk. I doubt many of us could say every ring they have tested has been intact. In a recent thread you posted that you would prefer sockets set up to only have 1 conductor per terminal to prevent broken conductors, so doubling up seems to counter your thinking.

 

[ericmark]

There are three limits to ring final or radial.
1) Current carrying capacity of cable.
2) Volt drop on cable.
3) Ability of the protective device to operate within the prescribed time with a short circuit.

The current carrying capacity has already been talked about. Of the remaining two, 3) is easiest so I will start with that.

A MCB is a combination of two devices, a thermal and magnetic trip, the thermal part is slow, and although it will trip in fullness of time, 40 amp on a 32 amp trip could take an hour. But more to point is with a short circuit it may not trip within the prescribed time. So the magnetic trip is fast, typically 0.01 second. But many items have an in rush when they switch on, so the magnetic trip is so many times the rating of the thermal trip.

So a B = 3 to 5 times, C = 5 to 10 times, and D = 10 to 20 times. So for a B32 MCB to trip within the time allowed with a short circuit it needs 160 to flow. So the impedance (resistance is called impedance with AC) using ohms law needs to be 230/160 = 1.4375Ω we allow 5% for safety so 1.365625Ω and we can either use inquiry, or measure, or combination, in the main we measure it with a loop impedance meter.

Now to 2) the volt drop, not easy as you need to work out the load, there is no hard and fast rule, but in the main we with a ring final assume 20 amp at centre and 12 amp even spread, this 20 amp with a 20 amp MCB on a radial is often also used at the end of a radial, as a result we normally look at 106 meters for ring final, and 32 meters with a 20 amp radial both using 2.5 mm² cable.

We are allowed 6.9 volt “lighting”, and 11.5 volt “other” and with a garage you already have some volt drop on the supply to the garage.

Very few times is the volt drop really an issue, fluorescent lights have been known not to strike, and I had problems with a shrink wrap machine, some old radios would produce a mains hum with excessive volt drop, and refrigeration units with single phase motors have been known to stall, this is why nearly every freezers says do not use an extension lead.

As an electrician we must follow the rules, last thing we want is to have to do some thing FOC because we did not follow the rules. As to how closely you need to follow the rules is really up to you, your not likely to take yourself to task as volt drop too high.

Even the 1.36Ω ELI, (earth loop impedance) as long as RCD protection is included, does it really matter? We can debate this, but with a TT installation 200Ω would pass, so although technically should be considered, I am sure many DIY people ignore it.

A B32 and a C16 will allow same in-rush. Also both require same ELI.

Personally I like the ring final, don't forget the word final, a ring circuit is very different, that allows a section to be isolated while maintaining the supply to all points. For a 100 amp supply we need around 0.35Ω but often it is a lot lower, unless work being done on the supply ring, and this has posed a question, if the supply is 0.25Ω should the limit be 1.26Ω to allow for it raising to 0.35Ω when the supply is being worked on?

Today with RCD's on all circuits it does not really matter, unless some thing stops the RCD from working, like DC with a type AC RCD. But are you really worried, likely not, however it does show why we use ring finals.

The cheap (£50) plug in testers pass at 1.9Ω one needs to pay around £200 to get a tester to measure 1.36Ω

 

[JohnW2]

The ring is protected by a 32A breaker and the cable rated at 54A (due to doubling up the 27A rating for the single cable). If the load is 50A and there is a break in the ring, the MCB trips after about 1 hour. In your design, the breaker would never trip. Do you think running 54A indefinitely through a conductor rated for 27A would be a fire risk. I doubt many of us could say every ring they have tested has been intact

I thought I was clear in indicating that I was talking about a (hypothetical) 2 x 2.5mm² (in parallel) 50A radial circuit?

You are obviously right that if one of two cables in parallel breaks, then that could put parts of the wiring at risk, if the pair of cables were protected by an OPD with a rating equal to double the CCC of each individual cable. That is an inevitable problem with 'cables in parallel' and the reason why such an arrangement probably has no place in many installations, certainly not domestic ones.

I thought it was clear that I was not in any way advocating such a hypothetical circuit. I was merely using it as an example to explain one reason why ("in normal service"), a 2.5mm² (Method C) ring final had to be protected by a 32A OPD (rather than a 54A/50A) one (i.e. because of potential unequal loading of the two legs of the ring). You are going one step further than that (in relation to this hypothetical circuit) by not only changing the discussion from radials to rings, but also by considering fault conditions ('broken' conductor') as well as "in normal service" conditions - which is fair enough.

However, it terms of your comments about ring finals, we need to remember that the regs only require cable with a CCC of 20A. If that is protected by a 32A OPD, then, if the ring is broken, that could allow a current of about 36A toflow indefinitely, and a current of about 46A for about 1 hour - which may or may not represent a (fire or anything else) risk for a cable with a "CCC" of 20A.

In other words, I'm not sure that (as you seem to imply) the regulations regarding ring final circuits intend the circuit to 'be safe' even in the presence of a broken ring. Let's face it, a 32A 2.5mm² ring final with a broken ring becomes two 32A 2.5mm² radials - which are not allowed by regs, presumably because they would not be considered to be 'safe'.

In a recent thread you posted that you would prefer sockets set up to only have 1 conductor per terminal to prevent broken conductors, so doubling up seems to counter your thinking.

As above, I was in no way advocating such a hypothetical circuit (even if it would probably be technically compliant with BS7671), but was merely using it to explain a point.

Kind Regards, John

 

[securespark]

Indeed - a 16A radial could, indeed, be wired in 1mm² cable ('Method C'), were it not for the fact that the regs require at least 1.5mm² for a 'power' circuit.

It was not always thus.
When I did my 15th, we did a theoretical immersion heater circuit design using 1milli.

It was only recently the 1.5 minimum rule for "power" cables came in.

 

[JohnW2]

It was not always thus. When I did my 15th, we did a theoretical immersion heater circuit design using 1milli. It was only recently the 1.5 minimum rule for "power" cables came in.

Oh, thanks - I didn't realise that.

That therefore seems to put paid to my speculation that the prohibition of 1.0mm² for "power circuits" may have been a hangover from the days when everything was BS3036 fuses, so perhaps (i still haven't bothered to do the sums!) 1.0mm² was inadequate for sockets circuits

... which, of course, makes one wonder even more why this "1.5mm² minimum" came about!

Kind Regards, John

 

[Harry Bloomfield]

Why are rings pointless today?
I know the argument that should there be a cable break then the user won't know and the 2.5mm could be taxed beyond 16A designed and therefore a fire hazard.

I'm not sure why you might think a ring might be 'safer'. More available current means more risk generally, slight though it is.

Nor do I subscribe to the theory of rings being pointless, they had and still have their place as an optional configuration for providing sockets. Radial and ring both have the good and bad points. I would think carefully though, as to whether the current capability ring might even be necessary in a normal garage. One point that has not been mentioned is the capability of the supply to the garage.

I have a large garage, with an attached workshop area and lots of sockets. I run stick welders, MIG welders and site transformers all on a 16amp MCB. Normally never any heating, I never normally need to run more than one item at a time, trips are very rare.

 

[ericmark]

Even electricians get it wrong, my charge hand in Sizewell fitted the 16 amp extra low voltage sockets in the vehicle garage, and wired them as a 32 amp ring, with 16 amp commando sockets, he seemed to have completely missed the idea that with a ring final you have fuses in the plugs.

He was a domestic electrician who was completely out of his comfort zone working in a commercial environment. Seems he knew the regulations, all you ever heard was 16th edition says this or that, but reminded me of a robot, if told this is the way to do some thing, that's the way he did it, no thought involved.

 

[Harry Bloomfield]

He was a domestic electrician who was completely out of his comfort zone working in a commercial environment. Seems he knew the regulations, all you ever heard was 16th edition says this or that, but reminded me of a robot, if told this is the way to do some thing, that's the way he did it, no thought involved.

Yep, you need to be a good 'end to end' thinker for many things. I once came across a charge hand electrician, who had a flat battery on his car. I always carry jump leads, so offered help. He had it in mind because you connect batteries nose to tail in a torch, you connected car batteries +ve to -ve to jump start them. He absolutely insisted he was correct, so I put my jump leads back in the boot and left him to it.

 

[crappy]

One point that has not been mentioned is the capability of the supply to the garage.

Garage is fed from house, 6mm buried armoured. Mcb in house is 42A and feeds a CU with earth leakage device.


As for is this enough? Unless I am welding, the 2.5hp compressor kicks in and I've all the heaters on and the electric car (which I don't even own) is being charged on the charge point I think I'll be OK
On the radial in question, I've only the compressor, some battery chargers (doing trickle) and odd power tool. When I get a welder it might be like you - seldom trips.


At the risk of prolonging the very interesting dialogue, there is no - that I can see - earth spike in the garage which is about 50m away.
Obviously the armoured has the armour which is earth but is that all that is needed? You don't have to earth outbuildings? Risk of some odd ground loops if you do?