In article .com,
wrote:
I have a bit of a dillema with my bathroom lighting. I am trying to
install a new light above the mirror but i just can't seem to get it to
hook up properly. The existing light worked and I was positive I saw
what was already hooked up and did the same with the new light. I have
black to black and white to white, but still nothing works. The concern
that I have, and which is more than likely the case, is that I am
attaching the wrong wires. There are three sets of black and white
wires comig out from the back of the lighting panel. If you picture the
octagonal box facing you there are to sets of wires on the far left
(which is closest to the wall with the light switch) one above the
other, and then the last set of black and white wires are located down
towards the bottom right corner of the oct. box. THe white wire from
the bottom right one is left open whereas the other white wires have
been merretted together. My problem lies with the black wires. Since
there are three of them exposed I have tried connecting the black wire
from the new light to each one of them and still no luck with power.

There are now 9 combinations to try, and it is still possible to
overload a white wire somewhere in your house if you use a white wire
other than the one associated with the proper black wire (assuming more
than one white wire works).

If any wires are of color other than what is usually used (sometimes
indicating a hot-neutral reverse - I do see this sometimes), it gets even
more complicated. You will need a multimeter to identify hot, grounded
(neutral), and not-connected-to-anything wires. Hot wires have close to
120V with respect to ground. Neutral wires have no voltage with respect
to ground, except can have a little if current from something else
flows through a part of that circuit. Neutral wires have close to 120V
with respect to hot wires. Wires not connected to anything ideally have
no voltage with respect to either hot wires or ground, although you
usually get some low voltage readings with respect to hot, ground, or
both.

Anyone else doing such a job in the future, when seeing wires going
nowhere of same color as wires being disconnected, should put tape or some
other marking on the wires that were used.

===================

Is the new fixture fluorescent? If so, you need a ground wire going to
the green wire in the fixture or to the metal fixture frame. Using the
neutral for this will work but feels to me like a code violation and
probably increases the risk of a shock hazard from wire or connection
failures.
Fluorescent bulbs trying to start can have the electric field
distribution within the bulbs affected by grounding of the fixture. This
sometimes (not always) affects starting of fluorescents.

- Don Klipstein )