ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Ring circuits UK style.

This isn't permitted by the Regs. If the new wiring were tapped onto the existing ring near its mid-point, there wouldn't be a problem.

If the tap-point of the "figure 8" was close to one end of the ring, however, the short leg would carry the bulk of any heavy load connected to the new outlets with the possibility of an overload.

It's sometimes easiest to make the physical layout almost a "figure 8" while keeping it a single ring electrically.

e.g. Cut into the existing ring and terminate each end into one of two adjacent junction boxes. Then bring the two ends of the new wiring into each box. This also makes it easy to run the new wiring then shut off power just long enough to make the two j-box connections.

>If the tap-point of the "figure 8" was close to one end of the ring, however, the short leg would carry the bulk of any heavy load connected to the new outlets

I don't understand how this is any different from a longer ring. To me, they appear electrically similar. If anything, the figure eight would produce a better balance.

Can you provide any rough numbers to back up this assertion?

Yes, if the new ring was spliced in a figure-8 form to the exact mid-point of the old ring, then obviously the total load on the new ring would always be evenly distributed around the two halves of the old ring.

The problem arises if the new ring were joined onto the old ring close to one end. Go back to the example I used before: Existing ring of 120 ft. with a outlet placed 12 ft. from one end of it.

Now assume that a new ring feeding several outlets in the extension is tapped into the existing ring at that outlet in a figure-8 form (albeit a very lop-sided figure-8!).

OK, if the new ring is feeding just a TV, table lamp, etc. there's not going to be any problem. But if the new outlets are a kitchen or utility area, they could have a heavy combined load. Let's say that 6kW is connected to the new ring, giving a current of 25A.

Obviously that 25A will divide around the new ring, but as far as the old ring is concerned, it represents a 25A load at a point just 12 ft. from on end. Now suppose that the existing outlet at that 12-ft. point is feeding a 1200W load, bringing the circuit total up to the maximum of 7200W or 30A.

We now have a 30A load divided between a 12-ft. cable and a 108-ft. cable. The result is just 3A in the long leg and 27A in the short leg, which is greater than the cable rating. Obviously if the tap point was less than 10% from the end of the ring, the imbalance would be even greater.

I'll concede that in most situations an overload is unlikely, but it is possibl.

This perhaps highlights a general point about designing a ring which is seldom mentioned in text books: For best current dstribution, design the heaviest loads either into the mid-section of the ring -OR- put heavy loads likely to be on simultaneously at opposite ends of the ring to balance the current as much as possible.

Bearing in mind that in many kitcens the ring feeds a 3kW washer, 3kW dishwasher, etc. this can be quite significant. I would still rather run separate, dedicated branches for such loads.

>Yes, if the new ring was spliced in a figure-8 form to the exact mid-point of the old ring, then obviously the total load on the new ring would always be evenly distributed around the two halves of the old ring.
But that was never a criterion.

>The problem arises if the new ring were joined onto the old ring close to one end.
This remains to be demonstrated.

>Existing ring of 120 ft. with a outlet placed 12 ft. from one end of it.
Now assume that a new ring feeding several outlets in the extension is tapped into the existing ring at that outlet

This is really no difference at all whether the new ring is spliced into the old ring and it remains closed or if the old ring is widened into a new longer ring.


>Let's say that 6kW is connected to the new ring, giving a current of 25A.
Obviously that 25A will divide around the new ring,
"Divide" in some sense. Naturally this doesn't mean a perfect split.

>as far as the old ring is concerned, it represents a 25A load at a point just 12 ft. from on end.
Which is perfectly acceptable, right?
I mean there is no problem to the ring is 25 A were loaded at that point, right?

>Now suppose that the existing outlet at that 12-ft. point is feeding a 1200W load, bringing the circuit total up to the maximum of 7200W or 30A.
Okay.

>We now have a 30A load divided between a 12-ft. cable and a 108-ft. cable.
True.

>The result is just 3A in the long leg and 27A in the short leg, which is greater than the cable rating.
Simply amazing.

I challenge your calculation as being bogus numbers that you made up from merely the relative lengths.

Obviously any wire carrying the 27 A would heat up, its resistance would rise, and the current would favor the cooler but somewhat longer path. This would tend to equalize current in both directions. I don't have an equation for this. But the result will be much, much better than 27:3.

>Obviously if the tap point was less than 10% from the end of the ring, the imbalance would be even greater.
It is not obvious to me.

>I'll concede that in most situations an overload is unlikely, but it is possible.
I imagine that if you put a 30 A load on a 25 A cable, overloading just might be possible.

>For best current distribution, design the heaviest loads either into the mid-section of the ring
That sounds silly to me.

> -OR- put heavy loads likely to be on simultaneously at opposite ends of the ring to balance the current as much as possible.
Again, this is silly.
If the ring can withstand a heavy load at each end, then why can't it withstand a heavy load at one end when the load at the other end is not operating?

(What you are saying could make sense for an American 240 V system in terms of minimizing current on the GC.)

>I would still rather run separate, dedicated branches for such loads.
I also would if the ring is not of sufficient ampacity to haul the current.

Regardless, I don't see how interconnecting reasonably short rings can place an overload at a point on a given ring if the total load or the second ring would be permitted at an outlet on the first ring.

Not exactly true. Connecting the new wiring so that it becomes part of the existing ring will increase the overall resistance of that ring. The extra load on the new wiring will then be farther along the ring than if it were joined figure-8.

>>as far as the old ring is concerned, it represents a 25A load at a point just 12 ft. from on end.
>Which is perfectly acceptable, right?
I mean there is no problem to the ring is 25 A were loaded at that point, right?

You're getting close to the crux of the matter now. At this loading the short leg of the ring would be carrying 22.5A (starting cold & accepting that the resistances will change slightly as the cable warms up).

The cable ratings per current (1992) IEE Regs. are:
Clipped direct (i.e. surface run), 27A
In wall in trunking, 23A
Buried in thermal insulation, 18.5A

So even this far, it could be borderline. And to be frank, I think the 27A surface rating allowed these days is too high anyway. (This wire is about 20% greater CSA than your #14.)

The IEE in general figures that it's unlikely for two 3kW loads to be plugged into the same twin socket.

OK, now look at a secondary ring spliced figure-8 at the same point on the old ring. As this new ring will be feeding outlets spaced apart, and maybe even in different rooms, it's considered far more likely that two heavy loads (e.g. 3kW each) could be connected simultaneously.
As far as the old ring is concerned, this is equivalent to two 3kW loads at the same outlet at that 12-ft. point, which we've already seen is borderline at best. And that's before we've allowed for the extra 1200W available which could easily be on the original outlet.

I'll admit that a similar situation could arise with a "legal" system. Example: The same original ring with a spur at the 12-ft. outlet to another single socket. Any combination of 2 x 3kW plus 1 x 1.2kW on these three sockets would give the same result of putting the whole 7.2kW load at the 10% point on the ring. Again, the IEE considers it unlikely that this would happen, though of course it is possible.

>I challenge your calculation as being bogus numbers that you made up from merely the relative lengths.

Do you agree that the proportions are correct when starting with cold cables?

>Obviously any wire carrying the 27 A would heat up, its resistance would rise, and the current would favor the cooler but somewhat longer path. This would tend to equalize current in both directions.

True. Perhaps the argument comes to just how much the temperature rise would affect the balance. Does anyone have a temperature coefficient formula for Cu conductors to calculate this?

>>Obviously if the tap point was less than 10% from the end of the ring, the imbalance would be even greater.
>It is not obvious to me.
How so? The resistance in the short leg would be reduced and that in the long leg increased by the same amount.
The initial imbalance would have to be greater, so are we just back to how much the heating effect would change this?

>If the ring can withstand a heavy load at each end, then why can't it withstand a heavy load at one end when the load at the other end is not operating?

Maybe I phrased my point about this rather badly. Bearing in mind the above points about the short "home legs," what I meant was that if one heavy (3kW) load has to be connected close to one end, a second similar load would be better on the opposite side of the ring rather than very close to the first.

Best OVERALL balance around the ring is achieved with the load distributed as evenly as possible around it.

>Regardless, I don't see how interconnecting reasonably short rings can place an overload at a point on a given ring if the total load or the second ring would be permitted at an outlet on the first ring.

I hope I've shown that an overload is theoretically possible with a "legal" system. A secondary ring with several outlets just makes it more likely to occur if the point of contact between the rings is at one end of the primary ring.

I agree though, that if the secondary ring feeds just a few sockets in low-load ares, it is extremely unlikely.

>Connecting the new wiring so that it becomes part of the existing ring will increase the overall resistance of that ring. The extra load on the new wiring will then be farther along the ring than if it were joined figure-8.
In the U.S.A. we wouldn't see that as a good thing.

>At this loading the short leg of the ring would be carrying 22.5A
My question was whether a figure-8 was allowed for an extension. If the ring is overloaded at any point, it is overloaded regardless of how the loads are attached.

What I see from this is that a ring really doesn't buy you double the ampacity except for a load at a point that is equidistant from both ends of the ring.

> it's considered far more likely that two heavy loads (e.g. 3kW each) could be connected simultaneously.
What is considered does not alter reality.

>As far as the old ring is concerned, this is equivalent to two 3kW loads at the same outlet at that 12-ft. point, which we've already seen is borderline at best.
The fact continues to be that heavier wire should be used or there should be complete separation.

>I'll admit that a similar situation could arise with a "legal" system.
My point exactly. There is really no new hazard with a figure-8 that wouldn't exist in a larger single ring.

>Do you agree that the proportions are correct when starting with cold cables?
Sure. I'd even agree that they might be worse than you state.

>Does anyone have a temperature coefficient formula for Cu conductors to calculate this?
Sure. But that is a side issue. Nothing will eliminate the overload except sifficiently sized conductors.

>what I meant was that if one heavy (3kW) load has to be connected close to one end, a second similar load would be better on the opposite side of the ring rather than very close to the first.
My point being that the ring is assumed to withstand this. So it should also withstand the load at one end being on while the load at the other end is off. As far as current goes, that's the same as adding a sub-ring close to one end.

>I hope I've shown that an overload is theoretically possible with a "legal" system.
Yes.


[*] What you've shown me is that a ring topology has no safety advantage over a linear topology or the American star topology.


[*] U.K. ring systems may be operating way over conductor ampacity just because of incorrect assumptions about the load distribution; whereas the NEC protects ungrounded conductors based on their so-called actual ampacity.


[*] The system is premised on the idea that more resistance is better. I have never thought of balancing electrical loads by increasing the length of the path back to the source (RF loads are a different story).
Even if I had learned nothing else, your discussion with me got me thinking outside the old box.

Thank you!


[This message has been edited by Dspark (edited 09-29-2001).]

Well, I'm glad it's got everyone thinking about different arrangements.

>>Connecting the new wiring so that it becomes part of the existing ring will increase the overall resistance of that ring.
>In the U.S.A. we wouldn't see that as a good thing.
No arguments from me there. Higher resistance = higher voltage drop = more power loss.

>My question was whether a figure-8 was allowed for an extension.
Whatever the rights or wrongs of it, the short answer according to the IEE is "No."

>What is considered does not alter reality.
Wow, I think we're finding more points to agree on now!

I agree that what the IEE considers "normal" or "reasonable" loads is often in conflict with reality, and the ring system is not completely foolproof. When I say "It's considered that" I'm just passing on what the IEE publications quote as the reasons; I don't necessarily agree with all of them.

>The fact continues to be that heavier wire should be used or there should be complete separation.

For foolproof ring circuits fused at 30A, yes. But the next size up is much bulkier & harder to work, so I'd rather just forget the whole ring concept altogether.

>My point exactly. There is really no new hazard with a figure-8 that wouldn't exist in a larger single ring.

Well, I can still see the IEE's point of view to a degree: That with a sub-ring with several sockets there's more chance of a larger load than a single spur soket.

But, thinking about it, it would be possible to wire a compliant ring with three outlets within the first 12 ft. of ring, each with a spur to a single outlet elsewhere. Given the wrong loading, this could even be worse than a three-outlet sub-ring tapped figure-8 at 12 ft., so I do see your point.

>

[*] U.K. ring systems may be operating way over conductor ampacity just because of incorrect assumptions about the load distribution;

I think that sums the situation up quite well. As I said, it's not foolproof.

If you look back at earlier threads on rings, you'll see I said that personally I don't like them. Thanks for making me think hard about it again and remind myself of one of the reasons why!

Everyone else is remarkably silent on this topic. Or were they just waiting to hold our coats?!

That last one is probably a British expression!

Had you been fond of rings, it might have been a different outcome.

why jump in when you are doing so well?

boy reminds me of US power distrobution... underground cables feeing transformers are set up in loops so to speak... heres an example: Bø ~---TX---TX --TX--TX--~ Bø all transformers can be fed from 2 places when theres an open in one segment.. theres usually one transformer thats a "normally open" so the loop gets fed from one side to the normally open, and the other side from another riser, should a segment fail, they just move the normally open and feed the transformers more on one side, fewer on the other.. power is still on though. we also call transformers fed from one source radial feeds... im interested in british power distrobution, and ive often wanted to work in england as a utility worker, or even a regular electrician.. id do it, if i could find some good food.. lol tell me pauluk, whats the house service? a single leg of 240 to ground or what? how do they meter it, and whats a usual service amprage size?

-m