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Originally posted by Dspark:

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.

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.