TV equipment: AV circuits on same phase

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malachi constant

Senior Member
Location
Minneapolis
Engineer here, designing a city hall. In the council chambers the cable-access TV folk are asking for all AV equipment receptacles to be fed from same phase in same panel. it is a double-tub panel that feeds office and other areas, there is plenty other equipment fed from this panel to balance out phases.

I've never had a request like this and want to make sure I'm not missing anything. These are maybe a dozen 20A/1P circuits. The only thing I can think of is the long built-in furniture where the council sits is fed from a couple conduits in the basement below. There are five circuits feeding this area, my guess is I'll have to derate them if they are all on the same phase. I'm having trouble making sense of the derating in 310 and am not entirely sure it goes into effect here, can anyone help out?

Thanks!
 

dkidd

Senior Member
Location
here
Occupation
PE
You will need a dedicated neutral for each circuit. Each hot and each neutral will need to be counted as a current carring conductor.
 
IMNSHO, they're asking for it because they don't know any better. It's perfectly acceptable to use all three phases, they probably don't understand the possible grounding problems that can occur with bad system design, and it would be their bad design, not yours. (Every TV station I've even worked in uses all three phases, and uses them indiscriminately.)

This practice has been discussed here before, should turn up in a search or two.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I have a 50a, 120/240v sub-panel in my HT closet, with some equipment on one phase and some on the other.

My system is dead quiet with no signal, and I even have (gasp!) signal and power cables in the same stud bay.

Correct grounding layout is more critical than common phasing, but what they're asking for certainly won't hurt.


Added:
Yup, that's what we do. And, not one IG circuit in the house. ;)
Actually, most circuits in most houses are IG. ;)
 
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defears

Senior Member
Location
NJ
The problem with putting everything on one phase and IG circuits is, someone did this before and it solved their problem. But they do not understand that they just masked the real problem. Like a neutral to ground bond downstream.

A properly bonded premise wireing system will have no problems with equipment. Unless some AV guy bonded both ends of a sheilded cable, like a mic. Or faulty equipment.

But if they pay for it, I'll be happy to install 1" rigid with 5 grounds to your sound system.;)
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It just adds to the cost with little real benefit IMO.

But, if that is what they want, I would give it to them rather than have them blame you down the road for whatever problems they encounter.
 

dbuckley

Senior Member
The phase the stuff is on doesn't matter, but it should all be from the same panel, and more specifically, from the same ground bar.

If you have recepticals from different panels then there is a small potential difference between the grounds (maybe only a few millivolts) and that's enough to get circulating ground currents which make noises through (poorly configured) sound systems, and hum bars on tellys.

So, the AV people don't know why they're asking for all the recepticals to be on the same phase, but they have experience that having everything on one phase delivers a cleaner AV system. But its the ground that makes the difference, not the phases. But they don't know that.
 

grich

Senior Member
Location
MP89.5, Mason City Subdivision
Occupation
Broadcast Engineer
...Actually, most circuits in most houses are IG. ;)

Ten years ago I was working with a new small-town radio station in wood construction, so romex was allowed. The electrician ran all the studio equipment circuits as home-runs to the panel...commented that it was a poor-man's IG. :)

...A properly bonded premise wireing system will have no problems with equipment. Unless some AV guy bonded both ends of a sheilded cable, like a mic. Or faulty equipment...

...So, the AV people don't know why they're asking for all the recepticals to be on the same phase, but they have experience that having everything on one phase delivers a cleaner AV system. But its the ground that makes the difference, not the phases. But they don't know that.

Toss in some consumer gear with unbalanced audio connections, which sounds possible in the OP's case, and it won't be hard to make trouble.

My full-time job is two floors of equipment fed from three different SDS's, yet with proper design, it works.

If they really want no noise, you could try to upsell them on balanced power. :grin:
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Toss in some consumer gear with unbalanced audio connections, which sounds possible in the OP's case, and it won't be hard to make trouble.
Of course, there are exceptions to every generalization. I have only unbalanced interconnects in my 'consumer' system.
 
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