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Author Topic: Electrical wiring for church sound system (former public school)  (Read 18148 times)

Mike Sokol

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Re: Electrical wiring for church sound system (former public school)
« Reply #20 on: May 05, 2015, 11:42:10 am »

The USA has a lot of grandfathered electrical power. In fact, you'll find K&T (Knob & Tube) wiring in many pre-WWII buildings here. That's really dangerous since in lots of cases all the wiring was black, and they often switched the neutral wires for overhead lights and such. It's the wild west for electrical power in the states which is why we need to be so careful when running any new lines.

Also, while I see High-Leg Delta power pretty often in old industrial buildings here, I wonder if that ever even existed in GB. Can you clarify that, Steve?

For newbies here, check out http://en.wikipedia.org/wiki/High-leg_delta

Also, in the last year I found several outlets with reversed ground and neutral wires. This is not easy to detect with simple testers, but will cause all sorts of ground loop hum, including something I call GLID (Ground Loop Induced Distortion), where bass notes that draw a lot of current from the AC power line induce temporary hum that modulates the bass notes. 
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D Edgcumbe

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Re: Electrical wiring for church sound system (former public school)
« Reply #21 on: May 05, 2015, 05:06:07 pm »

Some pics of the main splitter, the lighting splitter, L.P.C. disconnect, and L.P.C. (current lights/recepts).
As you can see, no bonding conductors (other than some local recent additions).
Looks like neutral bonded in main splitter and so far neutral looks isolated thereafter.
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D Edgcumbe

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Re: Electrical wiring for church sound system (former public school)
« Reply #22 on: May 05, 2015, 06:06:56 pm »

If you will be pulling new wire in conduit twist the hot and common, then pull that twisted pr with a normal straight safety ground

You're aggravating my orderly and ease of pulling wires in tendencies lol
But I know what you mean, they twist cat5 and other cables for a reason
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Stephen Swaffer

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Re: Electrical wiring for church sound system (former public school)
« Reply #23 on: May 05, 2015, 06:33:50 pm »

As Mike mentioned some jurisdictions (and actually the NEC as the minimum standard) allow EMT/Rigid Conduit and the armor of AC (but not MC) cable to serve as the equipment grounding conductor or safety ground.  The main issue I see in your situation is the lack of bonding jumpers around concentric knock outs on the boxes.  If you have a 100 or 200 amp fault that needs to blow a fuse in one of the panel disconnects, those little tabs on the concentric KOs might blow first leaving a hot chassis/panel.

My thought would be to install a grounding bar in the panel you intend to supply AV from-then all grounds (or technical grounds  if you decide to go isolated) can be connected to that one point so that you have a common ground potential for all AV equipment.  Optionally, you could either run a copper EGC (inside or outside the conduit) or install bonding jumpers/bonding bushings around any concentric KOs as insurance that a fault would not use your AV gear as a path back to the neutral if a conduit connection failed.

Two different purposes-grounding bar to keep potentials the same-EGC bonding to guarantee a good fault path.
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Steve Swaffer

Jonathan Johnson

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Re: Electrical wiring for church sound system (former public school)
« Reply #24 on: May 06, 2015, 08:02:36 pm »

I'm guessing this is old enough that it doesn't maintain separate grounds and neutral through the various sub-panels. If it doesn't have a neutral bus bar insulated from the box, then getting a quiet ground for audio could be a challenge. Since you're an electrical apprentice, see if you can open up a panel and take a picture of the ground and neutral bus. If all the ground and neutral wires are jammed in the same bus bar, then we need to do some thinking on how best to proceed.

This may be a case where, if you're not planning to rip out the existing service and completely replacing it, you might be better off just adding a subpanel exclusively for FOH, stage power, A/V, and remote audio systems (such as in overflow rooms). That would be a way of getting a "clean" ground.

And by "clean" I mean where all sound-related ground potentials are equal, which is the real goal in eliminating ground loop noise.
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Mike Sokol

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Re: Electrical wiring for church sound system (former public school)
« Reply #25 on: May 06, 2015, 08:21:02 pm »

This may be a case where, if you're not planning to rip out the existing service and completely replacing it, you might be better off just adding a subpanel exclusively for FOH, stage power, A/V, and remote audio systems (such as in overflow rooms). That would be a way of getting a "clean" ground.

And by "clean" I mean where all sound-related ground potentials are equal, which is the real goal in eliminating ground loop noise.

I would agree. You don't need a perfect ground, just the same ground for all interconnected A-V gear.
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Stephen Swaffer

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Re: Electrical wiring for church sound system (former public school)
« Reply #26 on: May 06, 2015, 11:29:09 pm »

The neutral in the panel pictured is isolated from the box-the box is grounded by the conduit.  I have yet to see a panel that doesn't have an isolated neutral (of course that does not mean they don't exist)-almost always you have to intentionally bond the neutral and ground in a box if that is desired.  So in the case Mike mentioned where all the grounds/neutrals were bonded the extra work was likely actually undoing what was done during install!

My perception is that there was a time period when boxes and conduit where the EGC system.  Over time, the industry realized that this was less than adequate and so now we almost always use an actual EGC and bond it.

It appears to me that this install was intended to have the neutrals separated from the ground-its just that the ground is not what we are used to/like to see.  Actually appears to be a well done install-even if a bit dated in execution.

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Steve Swaffer

Mike Sokol

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Re: Electrical wiring for church sound system (former public school)
« Reply #27 on: May 07, 2015, 08:05:22 am »

I have yet to see a panel that doesn't have an isolated neutral (of course that does not mean they don't exist)-almost always you have to intentionally bond the neutral and ground in a box if that is desired. 

Think K&T era fuse boxes and such. My own house, built in 1923, still has some K&T wiring in a few of the rooms I've haven't upgraded yet. It had a 60's era (I'm guessing) service panel upgrade to circuit breakers with a common neutral/ground bus bar.

My parent's house was built in 1961 and it had fused panel with ungrounded, 2 banger outlets, even though there was EGC wiring installed. There was a common neutral/ground bus bar in that box, and the "ground" wires from the outgoing circuits were in the same bus hoes as the neutral wires. About 10 years ago I put in a new 200 amp panel with circuit breakers and an isolated neutral bus, which of course I bonded to the box since this was an entrance panel.

My neighbors house where I grew up was a converted sheep barn with depression era electrical power. I think there was originally a 30-amp service which has been upgraded to a 50-amp service for the stove. It had a separate meter for the electric hot water heater since the POCO at the time charged different rates for heating water or lighting the house. Yup, all K&T with screw in fuses and not a EGC wire in site. There's a neutral bus bonded to the back of the box.

I think the separating factor may be the advent of circuit breaker panels vs. fused panels. Sometime in the 60's (at least in Maryland) they began installing circuit breaker panels with an isolated neutral bus. Before that it was fuses and a common ground-neutral bus bar. And there was a transition where they ran branch circuits with 14-2 plus ground wiring, but still used 2-banger ungrounded receptacles. I do know that some of the churches I've been in over the last 30 years have really old WWII and earlier wiring, and lots of DIY guys have "upgraded" the wiring. That's why I don't trust anything until I get a look at it myself and run a few measurements to be sure.   
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Mike Sokol
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Stephen Swaffer

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Re: Electrical wiring for church sound system (former public school)
« Reply #28 on: May 07, 2015, 08:52:41 am »

I agree in not trusting-my comments on the OPs situation are based on what I can see in the pics he posted.

Yes most small resi fuse boxes are the common neutral/ground-but that is usually the least of the problems in that situation.

It is interesting the difference in resi vs commercial wiring.  Another difference that may exist is our areas.  Our church building was built in the 1860's and had a major renovation/addition in the early 1900's-but according to insurance maps in the State Library was not converted from gas to electric lighting until the 1930's.  What appears to be the "original" electrical wiring from that era is all rigid conduit.  What I think was the original fuse box has been gutted and a breaker interior cobbled in, so I don't know about grounding/neutral separation at that time.  Long way around to say, I rarely see K & T in commercial in this area-if old wiring is still there I usually see rigid conduit because of the time frame this area was electrified.
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Steve Swaffer

D Edgcumbe

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Re: Electrical wiring for church sound system (former public school)
« Reply #29 on: May 11, 2015, 07:03:56 pm »

So this is where things seem headed (drawing enclosed).
They seem agreeable with the suggestion of installing a subpanel where there may be furnaces installed, for the furnaces, possibly A/C, and other new circuits for receptacles and stage lights.

Some things mentioned or Ive seen:
Isolated grounding.  Is this big deal or really not effective?  I can't control whether the equipment will be isolated from the building.  I doubt they will go for doing it anyway without something rather convincing.
Sound circuits on same phase.  Is there something to this?
Number of circuits/receptacles. 2 circuits, 4 receptacles at front and back quite sufficient?

As it sits, it looks like 12/3 AC90 (BX) would be ran from the subpanel to pick up a couple circuits at front.  The subpanel fed from splitter/disconnect in elec. room.
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ProSoundWeb Community

Re: Electrical wiring for church sound system (former public school)
« Reply #29 on: May 11, 2015, 07:03:56 pm »


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