Are many potential uses of SWA **UNEQUAL** tee connectors illegal?

[kesterlester]

It seems to be easy, these days, to buy fittings (often containing resin) that allow someone to tee into a pre-existing SWA cable. Some of these are equal tees, and some are designed so that the "new" branch cable has a smaller diameter than the main "through" SWA which was broken into.

I'm wondering how those latter UNEQUAL tees are not made useless by regulations in 99% of cases.

Surely as soon as a thinner cable comes off a thicker one, the up-stream over-current device (which originally protected the thicker SWA downstream of it) would be incapable of protecting the cable in the new (thinner) tee from overcurrent damage as a thinner cable usually has a lower current rating.

I know there may be appropriately sized current limiting devices at the downstream end of the new thin cable, and they could be designed to limit over currents in the new tee under non-fault conditions. But they won't help if a fault occurs between them and the tee which is able to draw more than the thin cable's capacity but less than the thick cable's.

This leaves me feeling that the only legal use-case for such tees is where the original "thick" cable does not need to carry currents as large as its diameter would make you assume (i.e. it's too big) and so it can have its overcurrent protection downrated to a level which matches what the thin tee needs. But such a situation ought to be rare, because it's unlikely that the original cable would have been chosen to be fat unless there were a good reason for it being fat ....

Or am I missing something?

 

[davesparks]

Those joints are used for many types of cable and many types of installation, they are not only used in installations subject to BS7671.
They are also likely used in multiple countries following their own wiring regulations and practices.

 

[SparkyChick]

As you suggest, there is another reason for picking a larger cable... voltage drop.

One of my clients has an incoming supply cable that runs for maybe 150 meters (I'm not sure of the distance but it's somewhere in this ballpark)... the main cable is I think 90mm CSA, I suspect, but cannot prove 100%, there is a tee off joint on the large cable which branches into a 16mm CSA cable. The large goes on to the barn, the small one goes to the house.

Both are adequately protected by the suppliers fuse but the larger cable is necessary to ensure the voltage drop is within acceptable limits.

 

[Julie.]

Don't see any problem with this.

You fit overload protection at the receiving end on the smaller tee, and providing the Zs at both receiving end tees is within the fault current protection max Zs at the feeding end - all is good and compliant.

(Subject to voltage drop limits, current carrying capacity, adiabatic capacity etc, just like any other circuit)

Where/what is the problem?

 

[kesterlester]

As you suggest, there is another reason for picking a larger cable... voltage drop.

One of my clients has an incoming supply cable that runs for maybe 150 meters (I'm not sure of the distance but it's somewhere in this ballpark)... the main cable is I think 90mm CSA, I suspect, but cannot prove 100%, there is a tee off joint on the large cable which branches into a 16mm CSA cable. The large goes on to the barn, the small one goes to the house.

Both are adequately protected by the suppliers fuse but the larger cable is necessary to ensure the voltage drop is within acceptable limits.

OK - fair enough. I can see/agree that avoiding voltage drop on long cables would indeed frequently lead to them being installed with much larger diameters than their current draw alone would require -- and so smaller branches could be entirely reasonable and would fit within the same low overcurrent protection as would have been specified for the thicker cable. Thank you.

 

[Pretty Mouth]

I know there may be appropriately sized current limiting devices at the downstream end of the new thin cable, and they could be designed to limit over currents in the new tee under non-fault conditions. But they won't help if a fault occurs between them and the tee which is able to draw more than the thin cable's capacity but less than the thick cable's.

As you mentioned, overload protection can be dealt with at the downstream end. Fault current can be dealt with by the upstream protective device. Does the thinner cable meet adiabatic limits, disconnection times etc? If so, then it is considered protected against faults.

It seems the regs don't consider faults that would mimic overload conditions, perhaps they just don't happen

 

[littlespark]

On a much smaller scale, the idea of a busbar chamber..

You have a thick cable supplying the copper bars… then several smaller cables branching off.