AV equipment shared neutral

[Jason H]

AV guys on this job are saying that the 12/3 circuit I provided for 2 projectors and 2 drop down screens aren't working because of the shared nuetral. I split the circuit so that 1 projector and 1 screen are on each leg. Total load for each set is 15amps and protected on a 20ocd. I don't see how a shared nuetral for this equipment would keep it from working properly. Has anyone had this issue before?

 

[zbang]

What does "not working" mean here?

Assuming this is a properly-wired multi-wire branch circuit (MWBC, see 210.4), there shouldn't be any problem at all. After all, the neutral feeding the the panel is shared.

 

[LarryFine]

A/V guys are nuts when it comes to stuff like this. Ask them if it would be okay with the projectors on one circuit and the screens on the other.

I used a regular 50a 120/240 feeder and panel in my home theater, with some equipment on each leg. My system is dead quiet with no signal.

 

[tom baker]

Very nice, whats the small yellow cord on the bottom right of your panel?
Why the recp outlets instead of plug strips?

 

[Arester]

I always used dedicated neutral on commercial A/V installations. On larger jobs i.e. university A/V classrooms I provided small isolation transformers 120/240 to have independent supply. Reason - harmonic distortion on shared neutral is cauisng problems. In theory on 120/240V THD should canel - but that only theory. Older guy told me too do this, never had a problem. Cost more to troubleshoot and rectify.

 

[LarryFine]

Very nice, whats the small yellow cord on the bottom right of your panel?

It feeds the panel from the 12v DC switched trigger output of my preamp-processor. The output is only capable of 15ma, so the yellow cable energizes a reed relay, which energizes a 10a cube relay, which in turn energizes a 4-pole 20a contactor, all inside the panel.

Four of the eight 15a circuits are switched through this contactor, and supply the two power amps, the two sub-woofer amps, and a few other components, like the plug-in LV power supplies, so the power switch on the preamp-processor can control the entire system.

I used the G.E. sub-panel because the breakers being offset from the center provided enough room for the relays and the contactor to fit inside the enclosure. I thought the cable channel was a good way to keep the power cords neat and away from the signal cables.

Why the recp outlets instead of plug strips?

I would still have needed several receptacles, some circuits only feed single components, like the power amps, the projector, etc., and I still would have needed a separate strip for each circuit supplying more than one component, so there would be no real advantage.

Each power amp has its own dedicated circuit, and the sub-woofer amps (built into the main speakers) share one. The main amp is rated at 300w x 2, the center/sides/rear-speakers amp is rated at 200w x 5, and the two sub-woofer amps are rated at 300w each.

That's a total of 2,200 watts of continuous power. The system will play at full, realistic concert levels without breaking a proverbial sweat. I have replaced the preamp-processor in the picture with a Fosgate FAP-T1+ unit that has a wide-screen monitor on the front.

 

[LarryFine]

On larger jobs i.e. university A/V classrooms I provided small isolation transformers 120/240 to have independent supply. Reason - harmonic distortion on shared neutral is cauisng problems.

How would your shared-neutral 120/240v transformer be different from the utility?

 

[Jason H]

Apparently the circuit board on the projector lifts were bad. Replaced them yesterday and all is working fine. Thanks everyone.

 

[Arester]

As i said. "I always used dedicated neutral on commercial A/V installations." By dedicated I ment not shared.

 

[roger]

As i said. "I always used dedicated neutral on commercial A/V installations." By dedicated I ment not shared.

But as Larry points out, the service is utilizing a shared neutral, so regardless of dedicated to the equipment a shared neutral is involved.

Roger