Chris Lewis wrote:
According to HorneTD :

Chris Lewis wrote:

[Nod to Tom: Given that it's a branch circuit, the available short-circuit current should be well within the

interrupt-rating of the breaker.]



That would depend on the location of the fault in the branch circuit
would it not. Given that this is a new installation there is a very low
likelihood of an inadequate withstand rating.


All true, but the likelyhood of exceeding withstand ratings is sufficiently
low that I wouldn't worry about it. For example, making a reasonable
presumption that any dead shorts would be at least 10 feet of wire
away from the main, the down and back resistance of 20' of 12ga would
limit the available fault current to no more than 3750A, and probably
considerably less due to resistance in the main feed. Which is below
even the worst breakers (I think).


If you really want to
test the rough wiring then rent a MegOhmMeter.


By far the best - even detects crimped/damaged insulation that isn't quite
yet a short. Question for you: do you test new circuits with one?
Do residential electricians commonly do it? Should they? Does "professional
practise" say they should?

It's worth remembering that electrical codes theoretically bury the wiring
deep enough that nailing/screwing wall covers shouldn't reach it, and
damaging a cable with a staple/strap should be visually obvious and remedied
before attempting to power up.

The only time I've had dead faults on new circuits have been:

1) A carpenter dead-ended a complex circuit by wire-nutting the
hot and neutral together. Shoulda checked first that he had
done what I instructed (capped off the wires separately).
A bit of a puzzle until we finally found it. No damage...

2) a defective piece of romex straight off the roll - it
was laid in a raceway and _couldn't_ have been damaged
during installation.

3) I trusted my grandfather to follow wiring instructions... ;-)
[long time ago, on a fuse panel, _way_ out in the boonies on
a Sunday - no fuses within 4 hours drive.... Developed
the lightbulb trick just before we ran out of fuses...]

No I usually don't use a megger on new residential work. I test it
clear with a meter prior to closing any brakers though. Megging is
comonly done on large feeders with large available fault currents. It
is rarely done on residential branch circuits.
--
Tom H