I know there was a recent question here on the subject. My question is
a) whether there are certain specific instances when it is permissible
to mix wire sizes (according to the NEC, anyhow), and b) what some of
those specific instances are.
I'm talking, of course, about downgrading the "last inch" of wiring, say
a short run to a device like a switch or light fixture. For example,
running a short leg of 14 ga. wire on a 20 A circuit. I'm pretty sure
such exemptions exist to the otherwise ironclad rule of sizing
conductors according to the "ampacity" of the circuit.
If you think about it, *all*
our circuits are actually wired that way,
or at least connected that way. In fact, it's way worse than that: a
typical 20 A outlet circuit has lots of devices plugged into it with,
say #18 cord sets (maybe even smaller). (Yes, I fully understand why you
don't want to have any #18 wire *inside*
your walls, thank you very much.)
I will say that in practice, I never do this (run smaller wire to part
of a circuit), since I'm not sure of these exemptions; even if I do
learn of specific cases, I may still never do so.
The phrase "jump the shark" itself jumped the shark about a decade ago.
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