Alexander Galkin wrote:

I wired my finished basement myself.
SNIP

When I switched off basement room receptacles circuit breaker while all
others except sub-panel which does not seem to affect the problem were on we
found there is 8 V power in that circuit. Switching off some of other
circuits reducing this voltage to about 2 V and only switching off all new
circuits brings disconnected voltage to zero as it should be.

If you were reading this voltage at the recept with nothing plugged in
on that circuit, then the 8V is perfectly normal reading and is due to
stray coupling between conductors.

Try the reading again with a lamp or other load plugged in.

Jim