Ring Circuits and Current Carrying Capacity theory?

[Mrdjc]

I've got a question.

Say you where to create a ring circuit which is 50M long from end to end

The cable is 2.5mm2 T&E.

Since each socket is being supplied with TWO cables, does that make the current carrying capacity be subject to 5mm2?

The max cable load for a single strand of 2.5mm2 T&E for 25M is 27A (According to the datasheet)

How does a ring circuit affect current carrying capacity? Would it be 2x 27A?

4mm2 is 37A max and 6mm2 is 47A max according to the datasheets.

Averaging this out would be 42A for 5mm2. But yet 2x27A is 54A.

So, the question is, what kind of theory do you apply to calculating maximum current for a ring circuit?

I tried searching online but I can not find any official or scientifically backed information on this.

 

[ban-all-sheds]

How does a ring circuit affect current carrying capacity? Would it be 2x 27A?

No.

So, the question is, what kind of theory do you apply to calculating maximum current for a ring circuit?

There isn't one, really.

BS 7671 says that you may use 2.5mm² cable on a 30/32A ring final circuit serving BS 1363 accessories (A key feature of which is that they are fused) provided the cable as installed has a current carrying capacity of at least 20A and provided no individual section of it will be overloaded for long periods.

I tried searching online but I can not find any official or scientifically backed information on this.

Are you allowed ring finals in Slovakia?

 

[JohnW2]

How does a ring circuit affect current carrying capacity? Would it be 2x 27A? So, the question is, what kind of theory do you apply to calculating maximum current for a ring circuit?

It is impossible to apply any precise theory to the calculation unless one knows exactly what loads will be conected to the ring circuit and, in particular, where they are connected.

What you have to understand is that, unless a socket is right in the middle of the ring, the current will not be split equally between the two 'legs' of the ring from the socket back to the supply, but will split in proportion of the impedence of the two paths back to the supply (the impedence being a function of the lengths of the paths). That's why the current carrying capacity of the ring is not 2 x 27A.

For example, if you had a load connected just 1m from the end of a 51m ring circuit, about 98% of the current would flow along the shiort leg and only about 2% along the long leg. This means that, in the most extreme of possible situations, nearly all of the current flows along the one cable with a CCC of 27A. UK regulations allow a 2.5mm² ring final circuit to supply a maximum of 32A, on the basis that the very extreme situation I have described is very unlikely to arise in practice - but the point is that the current in one leg of the ring could easily be appreciably more than half of the total the circuit can supply (i.e. 32/2 = 16A).

Hope that helps.

Kind Regards, John.

 

[Mrdjc]

Thanks guys, that helps a lot.

I was just curious as to how the ratings work since your effectively doubling the CCC. (Granted, its all relative to distance for each leg to load).

The local electrician here had never heard of ring circuits, he went off and did some research and supposedly the law had just changed.
As a result Ring circuits are now allowed, and also recommended over radial circuits for domestic rooms.

When I first started looking at what was already present in the building I am working on I was shocked. A radial circuit in my eyes is a spur on spur on spur on spur, added over time without any regard as to whether or not the first leg of the section is up to it!) which they seem to do here.

None of the appliances here are fused, unlike the BS1363 plugs.
Add to that the lack of RCD, or independent earth (Or earth bonding)
On each socket the PE is simply connected to the N, right back to the "consumer unit" (Read: wooden box with 1950's style screw in fuses, with aluminium wiring) and there its simply connected over onto the Supply PE-N. 3x 63A fuses in case you have a short anywhere in the whole system is rather scary considering there is no RCD protection at all.

I think I'll simply limit the rooms to 20A, and where more is required I shall split the room into two different zones. (E.g. Kitchen/Dining room)

That way the MCB or RCBO (Depending on zone) shall pop long before the maximum current of the cable is met.

Thanks again guys.

 

[ban-all-sheds]

As a result Ring circuits are now allowed, and also recommended over radial circuits for domestic rooms.

You poor devils.

Introducing that where it's never been known before can only end badly.

A radial circuit in my eyes is a spur on spur on spur on spur, added over time

Then your eyes are not seeing properly.

Which would be the spur(s) on this radial circuit?

without any regard as to whether or not the first leg of the section is up to it!) which they seem to do here.

Why should the first section not be up to it?

None of the appliances here are fused, unlike the BS1363 plugs.

That's true in almost the entire world - why was it news to you?

Add to that the lack of RCD, or independent earth (Or earth bonding)
On each socket the PE is simply connected to the N,

AIUI that was (is?) a common practice in Eastern Europe.

I think I'll simply limit the rooms to 20A

How?

That way the MCB or RCBO (Depending on zone) shall pop long before the maximum current of the cable is met.

How will you know that?


Does what you plan to do, and how, comply with the laws and regulations there?

 

[Space cat]

Averaging this out would be 42A for 5mm2. But yet 2x27A is 54A.

That's because, among other things, the current carrying capacity of a cable is not a linear function of the cross sectional area. Look at it this way, if you double the linear size of a cable, the cross sectional area gets four times bigger - and so you might think that it could carry four times the current. Wrong! The surface area, through which all the heat has to escape, has only doubled.

If you compare the ratings of cables of increasing size, you'll see that the area goes up faster than the current. Two separate cables of 2.5 sq mm will carry twice the current of a single one but a single 5 sq mm cable won't.

 

[bernardgreen]

and so you might think that it could carry four times the current. Wrong! The surface area, through which all the heat has to escape, has only doubled.

If you compare the ratings of cables of increasing size, you'll see that the area goes up faster than the current. Two separate cables of 2.5 sq mm will carry twice the current of a single one but a single 5 sq mm cable won't.

Wow, ring the church bells, a well informed person.....education works.

Sorry for the humour but I had to explain just that to an "electrician" on Saturday only it was then four times 2.5mm and one times 10mm.

 

[Mrdjc]

Thanks for the insight spacecat.

Banallsheds, I'm not sure what you are hoping to accomplish with your questions, but they aren't very relevant to the original question.

I find them rather.... argumentative and confrontational to be honest with you.

But just to see where this will go, I'll take the bait and answer the questions you've presented me with.

Introducing that where it's never been known before can only end badly.

The UK had it introduced at some stage too, did it end badly there?

Which would be the spur(s) on this radial circuit?

Let's take a closer look at the definition of a spur:
An angular projection, offshoot, or branch extending out beyond or away from a main body or formation

In your picture, from A -> D, D ->E & B->F I would consider a spur. It branches off the "main" line.

Why should the first section not be up to it?

Imagine you create a single radial circuit, Let's say in your picture, from A to C, and use 2.5mm wire.
You plug in a Fridge at A, a TV at B and every now and a electric toaster at C.
Fridge at 200W, TV at 400W and electric toaster at 1500W. That's a total of 2100W.

The Distance from Consumer Unit to C is 25meters.

You decide to add a "Spur" from B to F which is 15meters , 2.5mm and plug in a 2KW heater.
You are now consuming 4100W over that same cable between the Consumer Unit and A. All still fine and well.

You decide to add another "Spur" from A to D which is 20M long, and powers a water pump for the garden periodically, which is 1500W.

You're on 5600W now, and exceeding the maximum current allowed for the leg between the Consumer Unit and A.

See where I'm coming from?

That's true in almost the entire world - why was it news to you?

I never said it was news to me, I am merely pointing out that that is the case here.

I think I'll simply limit the rooms to 20A

How?

By using a 20A RCBO or MCB? That way it will trip long before the maximum allowed current of the cable used is reached.

How will you know that?

2.5mm2 has a maximum CCC of 27A. Using a ring circuit as others have stated you are permitted 32A total according to BS. 20A falls a long way below this. This is how I will know this.

Does what you plan to do, and how, comply with the laws and regulations there?

Yes it does.

You might want to be more sensitive in the way you communicate with people in the future on forums, you have no clue who might be sitting on the other end of what kind of knowledge they possess or background they come from.

 

[JohnW2]

In your picture, from A -> D, D ->E & B->F I would consider a spur. It branches off the "main" line.

BAS's point is that he could have re-drawn exactly the same arrangement in a manner which would almost certainly have caused you to call different things spurs. Basically, he's just trrying to 'be clever', but the underlying poiunt is valid - that, whist a radial circuit may branch (and even branch more than two ways from one point in some situations), one cannot define what is 'a spur' and what is the 'main circuit'...

... consider a supply to socket A, which then branches to sockets B and C (maybe with other subsequent ones). Which (of A->B and A->C) are going going to call the "main" line and which the spuyr. The answer to that question is totally arbitrary.

Kind Regards, John.

 

[ban-all-sheds]

The UK had it introduced at some stage too, did it end badly there?

Yes.

We've had it for several generations of DIYers and electricians and it is still going wrong all the time.

From your own experience you have electricians there who don't know what a ring final is or that they are now allowed. You tell me what's likely to happen when they are asked to add new outlets to one.

In your picture, from A -> D, D ->E & B->F I would consider a spur. It branches off the "main" line.

Completely wrong.

Sockets A B D & E were the ones originally installed when the circuit was first put in.

Imagine you create a single radial circuit, Let's say in your picture, from A to C, and use 2.5mm wire.

First, most important, and possibly show stopping question:

With this 2.5mm² cable is In ≤ Iz?

You plug in a Fridge at A, a TV at B and every now and a electric toaster at C.
Fridge at 200W, TV at 400W and electric toaster at 1500W. That's a total of 2100W.

The Distance from Consumer Unit to C is 25meters.

You decide to add a "Spur" from B to F which is 15meters , 2.5mm and plug in a 2KW heater.
You are now consuming 4100W over that same cable between the Consumer Unit and A. All still fine and well.

You decide to add another "Spur" from A to D which is 20M long, and powers a water pump for the garden periodically, which is 1500W.

Why do you keep mentioning the lengths?

I haven't checked VD, but that didn't seem to be your concern, nor have you said what your Ze and maximum EFLI is.

See where I'm coming from?

Not really, assuming that the installation hadn't been done by an idiot.

I never said it was news to me, I am merely pointing out that that is the case here.

OK - I just can't work out why.

By using a 20A RCBO or MCB? That way it will trip long before the maximum allowed current of the cable used is reached.

Shouldn't trip - Ib should be ≤ In - you aren't supposed to use an OPD as a load limiting device.

2.5mm2 has a maximum CCC of 27A.

How will you know what it has where you're installing it?

How will you know that your EFLI is OK for the device?

Using a ring circuit as others have stated you are permitted 32A total according to BS. 20A falls a long way below this. This is how I will know this.

You're going to have a 20A ring?

Why?

Is it allowed there?

Does what you plan to do, and how, comply with the laws and regulations there?

Yes it does.

How do you know that?

you have no clue who might be sitting on the other end of what kind of knowledge they possess

Indeed not.

Which is why I asked questions about what and how you knew things.

Would you rather I just made assumptions about what you know and what your background is?

 

[ban-all-sheds]

BAS's point is that he could have re-drawn exactly the same arrangement in a manner which would almost certainly have caused you to call different things spurs. Basically, he's just trrying to 'be clever'

No - I'm trying to make the point that talking about a radial circuit being "a spur on spur on spur on spur, added over time" as some sort of criticism is rubbish.

 

[Paul_C]

... consider a supply to socket A, which then branches to sockets B and C (maybe with other subsequent ones). Which (of A->B and A->C) are going going to call the "main" line and which the spuyr. The answer to that question is totally arbitrary.

And with the way that a circuit such as that depicted should be wired, it really doesn't matter which section, if any, you care to define arbitrarily as the "main" leg. So long as the fuse/MCB rating is equal to or less than the current rating of all cables which form part of the circuit (including that first run from CU to A in the diagram), then there's no problem. Typically for the U.K., that would be 2.5 sq. mm cable on a 20A MCB.

The problem with a "spur from a spur" arises with the peculiarly British ring circuit due to the fact that the protective device is allowed to be rated higher than the current-carrying capacity of the cable.

 

[JohnW2]

No - I'm trying to make the point that talking about a radial circuit being "a spur on spur on spur on spur, added over time" as some sort of criticism is rubbish.

Fair enough - you're obviously entitled to make that point. However, even if unintentionally, you were also making the point I mentioned - which, once understood, makes the other one rather moot!

As you're saying/implying, there is no problem with, and no reason for critising, a radial circuit, no matter how many branches or extensions it may have, since the cable will, throughout, be rated to take the maximum permissible (per OPD) current of the circuit - which is obviously very different from the situation with a spur from a ring (unless the spur is wired in cable with at least double the capacity of the ring cable - which is probably never going to happen).

Kind Regards, John.

 

[Paul_C]

You're on 5600W now, and exceeding the maximum current allowed for the leg between the Consumer Unit and A.

That's a simple problem of trying to connect too much load to the circuit though, not one caused specifically by the particular "branching" arrangement depicted.

If the cable ran in a single line as CU - A - B - C- D - E - F, then the leg from CU to A would still be called upon to carry the same amount of current with the loads given in your example.

 

[Mrdjc]

The UK had it introduced at some stage too, did it end badly there?

Yes.

We've had it for several generations of DIYers and electricians and it is still going wrong all the time.

From your own experience you have electricians there who don't know what a ring final is or that they are now allowed. You tell me what's likely to happen when they are asked to add new outlets to one.

Don't hire cowboys that have no clue and randomly connect things without looking.

Completely wrong.
Sockets A B D & E were the ones originally installed when the circuit was first put in.

In the house in question, there was a "Spine" down the middle of the house from which all the outlets were sourced. Additions had been made to this circuit. That's why I referred to them as "Spurs" off an original circuit.

Because I didn't use the correct terminology for your liking, you now have a stick up your rectum. Solution: Remove stick from rectum.

First, most important, and possibly show stopping question:
With this 2.5mm² cable is In ≤ Iz?

In is 20A, As already stated in the previous two posts I've made.
Iz for my configuration (If I were to be using only ONE 2.5mm² cable) Will be between 20A and 27A, I do not know exactly which figure to apply as there are various environments that the cable will run through (Ceiling with insulation, Conduit in rendered wall etc)

In any case, In ≤ Iz.

Why do you keep mentioning the lengths?

Lengths of cable under load can have a huge influence on voltage drop, which in turn influences current load in the cable.

I haven't checked VD, but that didn't seem to be your concern, nor have you said what your Ze and maximum EFLI is.

As mentioned in my post, there is no current earth in the existing setup, it is tied directly back onto the Neutral at the outlets, I have no idea what the value of Ze is as it currently is non existent.

The maximum EFLI I do not know either, I do not know which type of fuses for consumer units are standard here, so I do not have any data sheets in order to calculate maximum EFLI.

Not really, assuming that the installation hadn't been done by an idiot.

Try 3 or 4 idiots over a span of 50 years without any legislation covering it.

OK - I just can't work out why.

Well I don't know, something must be wrong with your head if you do not understand why I would mention that we do not have fuses on plug here like the UK does, after someone mentioned that 32A is permissible for a 2.5mm² ring circuit provided that fuses on plugs are fitted.....

Shouldn't trip - Ib should be ≤ In - you aren't supposed to use an OPD as a load limiting device.

Ib is up to 20A
In is 20A
There for... Ib ≤ In

How else do you propose to limit the current in a circuit which you do not wish to exceed 20A?

Analyse the wording: OverCurrent Protection Device.
In dum dum speak: "Too Much Current Switchy Offy."
You wish to LIMIT the LOAD on a circuit to 20A.. Makes perfect sense to me, but apparently you have a different take on it, I'd love to hear it.

How will you know what it has where you're installing it?
How will you know that your EFLI is OK for the device?

By using the tables provided in the IEE 16th edition to compute it?
By finding out what type of fuses are standard here, and calculating the required EFLI. Then measuring to see if they are within limits?

You're going to have a 20A ring?

Why?

Is it allowed there?

Yes, for one room.
Because the limiting factor is the 2.5mm2 wiring in ring, and it requires adequate protection. 20A is sufficient for a bedroom. (And it sure as hell beats 63A per phase protection which is currently there.)

How do you know that?

Who are you, my father or teacher, your starting to act like I need to satisfy your questions for some obscure reason.

Because it has been discussed with the electrician who will check over the work, commission it and sign it off.

Which is why I asked questions about what and how you knew things.
Would you rather I just made assumptions about what you know and what your background is?

I had a simple question about computing CCC for a ring circuit. Turns out there is no simple answer, there is only the guidance from the BS.

That's a simple problem of trying to connect too much load to the circuit though, not one caused specifically by the particular "branching" arrangement depicted.

If the cable ran in a single line as CU - A - B - C- D - E - F, then the leg from CU to A would still be called upon to carry the same amount of current with the loads given in your example.

True,but the difference between radial and ring is the loading via two cables, giving two different paths to reach the same location.
The radial was never designed to have so much added onto it.

Granted, the ring could have the same done to it, but its a lot more logical to me to have zones with rings than having a "loose end" somewhere you can tie into.