"m_corbelli" wrote:
I did check all the outlets with a tester and got the green light, so no
neutral/ground issues. The wiring is copper....

That sort of check doesn't mean much though.

If you think about how an electrical service is set up, it
might be easier to test it. Starting at the transformer on
the pole outside, your home is supplied with two circuits.
When you plug in a device it is essentially in series with
the entire loop all the way back to the transformer.

That loop is the key to testing it.

Transformer

^ ^
| |
| outlet |
+---||---+ --- Dimming never seen
| |
| outlet |
+---||---+
| |
| outlet X --- Fault
+---||---+
| |
| outlet |
+---||---+ --- Dimming intermittent
| |
| outlet |
+---||---+ --- Motor plugged in here

That is a very very simplified diagram. If you put a
tester into the top outlet (closer to the transformer
than the fault), it will *never* show a problem. But,
if you put the tester into any outlet lower than the
fault, it *might* show a problem.

There will be no problem if either the resistance of the
fault or the current going through the fault is too
small. What causes your dim lights is the voltage drop
across that fault. Voltage = Current * Resistance.

So, the reason the tester won't see a problem is that it
does not draw enough current itself to make the problem
manifest, and it will show a problem *only* when
something else is triggering it. Motors cause lights to
dim because they draw a huge amount of current when they
start. The "fault" in that case could simply be that
none of the wiring is large enough to handle that much
current! In that case the fault is distributed all along
the entire circuit.

In any case, the lights dimming only happen where lights
are plugged in *after* the fault location. And then
only when enough current is drawn through the fault to
make it manifest.

So you could test it by putting a lamp into any of those
outlets, wait until you get dimming on other
lights... and *then* check to see if that lamp is dim
too, and from that determine which side of the circuit
from that lamp the fault location is on.

The first place you want to check is right at the
entrance to your electrical system, to determine if this
fault is in your house wiring or is part of the
utility's service. (It could easily be that your pole
transformer is overloaded, and the motor causing it is
an air conditioning unit in a neighbor's house!)

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)