A proper grounding scheme is the biggest issue to
address.
Phases are often wrongly blamed for other issues in the
system. There are different opinions about
harmonics and
neutral currents but the shortest answer is that it's usually only perceptible when there are other problems at
play. Realistically, the vast majority of sound systems out there are on 3-phase
power. A smaller slice of which are on single-phase, which minimizes
harmonics because in an ideal application the phases cancel. I'm not generally aware of any sizable systems that are driven off of a single
leg. It looks good on paper but in practice it's not a widespread trend because in order to perceive the difference as an audience member there are a dozen other stops you have to pull out ($$$,$$$.00) before that one pays off. Certainly in a case where you wouldn't be behind a shielded isolation
transformer, the results you can expect from hopping on an existing
leg with other loads on it that could include motors,
HVAC, lighting, the green room garbage disposal -- it's not the first priority I would make. If
FOH and convenience receptacles near the
stage are not also on the same
leg, what you do with the processing rack ends up being meaningless if all those other
power sources are not also addressed.
There's a deep rabbit hole you can go down but like
@FMEng, I'd say focus on the signal cabling for the new installation first, and worry about the existing electrical later if you perceive a problem.
I say that because the rabbit hole you can go down is deep and decisions are are of minimal cost impact in new construction can be incredibly expensive in existing installs. Will the mix
console and all other AV electronics also be
fed from the same panelboard or
leg just because you put the processing rack on the same
phase? Will there be any shielded isolation
transformer in place to reduce noise from the rest of the building electrical
system? Is the
HVAC system loud enough in the theater that paying $50,000 or $150,000 extra to kill a few dB from your audio
system's noise floor is going to make a difference? Are you driving a $750,000
speaker system with a $3,000
mixer? Are your
circuit conductors properly twisted in
conduit and did they avoid using MC cable when they installed the existing electrical?
I would say that on average, the majority of existing installs I've had to troubleshoot with noise issues end up coming down to a few different things:
- Improper grounding of signal cabling at patchbays or inputs of DSP/amps.
- Knicked signal cables that are coupling with the conduit ground.
- Untwisted speaker wires that are introducing lots of crosstalk between systems.
- The noise is actually coming from HVAC equipment but has been incorrectly attributed to the sound system.
Which
leg the AV
system is on or that the
system is on 3-phase
power tends not to be what makes my phone ring.