I'm putting together a proposal for a new sound and lighting system for a client, and I want to suggest a sub-panel near the electrical equipment on the stage. The lighting system will draw 16,000 watts, and the sound system will draw very close to that at full power. It's pretty much evenly divided among 20-amp circuits. The building has plenty of service (I recall someone saying 3-phase 600 amp).

So ... How would I spec it for the client's electrician if I wanted sound and lighting on different legs, using the same neutral, with possibly a "local" ground for the sound system? (Will the ground still have to be bonded at the main panel?)

(Or is >160 amps per leg not doable? The sub panel will probably be 80 - 100 feet from the main.)

Many thanks in advance!

-- RJ

 

You can run an isolated ground from the main panel. The bonding between nuetral and ground in the main panel cannot be removed. There is no connection between ground and nuetral in a sub panel.

 

When you say "using the same neutral" are you referring to the feeders for the panel, or the individual circuits? If the latter, I would advise against it. Many things in a lighting system, including flouresent or HID lighting (anything with a balast) and dimmer modules (definately a big concern for you) will induce harmonic currents on the neutral. Without going into a long lesson in power quality, I will say that harmonics will represent themselves as noise in your sound system. I would highly recommend running dedicated neutrals to all of the sound circuits.

 

Thank you for the replies.

I had pictured a four-wire system: Two hots, one neutral, and one ground (which I now understand must return to the main panel).

I was referring to the single neutral entering the sub-panel. Are you saying that that bus shouldn't feed both legs? That each hot leg should have its own neutral "home run" back to the main panel?

 

Rick; The numbers of "wires" in a system refers to the number of conductors that supply power to the connected loads. An Equitpment Grounding Condutor will conduct only Fault-currents and is not included in the number of conductors that describe a "system".

You are considering a 3-wire, 200 amp Feeder, which is a single-phase Feeder extended from a 3-phase, 4-wire system. If you added another conductor to the single-phase Feeder you would approx. "double" the Feeder's power capacity. The exact factor is 1.73 which is the square-root of 3.

If you wired two 3-phase, 4-wire, 50 amp Feeders, one Feeder for the lighting-load and one Feeder for the sound-system, the Neutral-currents for each would be in different Neutral conductors and you would "balance" the currents in the Service condutors.With a 3-wire, single-phase connection you would add 150 amps to Phase "A" ,150 amps to Phase "B", and Zero amps to Phase "C".

Is there any reason why you prefer all the liting circuits on "Phase "A" and all the sound-circuits on Phase "B"?

Good Luck & Enjoy the Experience!!!!!

 

You would be correct to have a single neutral on the "home run" (or, feeders, as it were.)

My reference was to the individual circuits from the sub panel to the lighting and sound equipment (branch circuits.)

As far as feeding the subpanel goes, PATT did already gave an excellent explanation, so I won't bother to repeat.

 

Sorry about the delay in responding.

The lighting system has to handle exactly 16,000 watts. The sound system is slated to have twelve 20-amp breakers, but its draw is roughly 16,000 watts at "rms" (average) power. (Meaning the transient spikes may be a bit higher -- on the order of 20,000 - 25,000 watts for milliseconds -- but the circuits also handle stage amps and processing gear in addition to the speakers' power amplifiers.)

From a stage (audio) grounding standpoint, it's much better to keep the lighting dimmers on a different feed than the sound system. But balancing the load for the neutral seems to be the main issue for the electrician.

Thanks again for the replies.

-- RJ

 

I can see what your digging at here. But, if you had eight of those sound circuits on one side of the bus and eight on the other you would still be in balance.

Incidentally, phase configuration is irrelevant to grounding. What you are concerned with here, as far as the sound system goes is "noise" in the system (harmonic currents as it were.) The best way to mitigate this is to run a dedicated neutral to each circuit and, possibly an Isolated ground. (a lot has been made of isolated gorunding, and it certainly has it's place. But the reality is your neutral is responsible for much more system "noise" than your ground.

However, you could use common neutrals on some multiwire circuits. As an example, if you put 1 sound circuit on A phase and 1 on B, they could share a neutral. I would advise (as I already have) against using common neutrals between a lighting and sound circuits (unless you like the sound of "60 cycle hum.") I would also consider an oversized neutral for an common neutrals you may run.

On another note, you could easily use a three phase system and maintain balance. As a rule, if three phase is available, use it.

 

Yes use Isolated receptacles if you are useing conduit (as it is a place of assembly) and have the electrician's pull a separate insulated grounding conductor to each receptacle that will be used for any sound equipment including any equipment that might be DI'ed on the stage. Keep many 1 to 1 isolating transformers (low level audio) available as when you DI a stage amp or some lead guitar players rack you never know how he has it wired this will prevent a ground loop from forming and also keep you from having to replace the stage snake to the stage because he was pulling power through the rack because of a lost neutral in some cord feeding it. I have done several clubs here in Indiana and also had training from Electro-voice, and we install a 3" PVC conduit from the stage to the sound booth to allow for changing the snake out even in the middle of a set. just leave a few rope's in it.
The 1 to 1 transformers that I keep on hand is nothing but XLR to 1/4" and 1/4" to XLR matching transformers. Both with male and female end's this can be configured in many ways to allow adapting to any hook up and you only need one to provide isolation, But keep set's of all four types as you will need them at one time or another. this is the most safest way to get rid of a ground loop hum. DO NOT ALLOW LIFFTING A GROUND! as many bands will try to do. The liability is too high to allow this for the cost of a matching transformer.

 

Thanks again. I now feel I can speak to the electrician without sounding like a complete idiot.

Hurk, all of the DI's we use have switches on them that lift Pin 1 after the xformer. I also have a couple of mic cables with pin 1 disconnected at the female end. I use them when a bassist insists on using the direct-out XLR spigot from the amp.

A band that recently played the club had amps with the ground pins broken off the edison plugs. A short demonstration with a volt meter between the amp chassis and a mic body made them go to the hardware store for new plugs.