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?
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?
- TL;DR
- Do all people who use SWA unequal-tees need to downrate current protection on the thick legs to match that which is needed by the thin legs?
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