Theatrical Dimming Circuit Calcs

[slamppin]

Is any type of load diversity allowed for raceway carrying multiple circuits for theatre lighting? These circuits are typically loaded to 50% per circuit and never utilize all at once. Applying 310.15(B)(a) to this situation seems a little extreme.

 

[480sparky]

There's no exception.

The last theater dimming system I did spec'd #8's anyway, and no multi-wires. So a lot of 1" was used.

 

[mxslick]

Say what?

Say what?

<snip>These circuits are typically loaded to 50% per circuit and never utilize all at once.

I'd love to know what theatres you're working at where that statement is true. :grin:

Every live venue I've been at, (so far) the lighting loads have always been horrendous and used at or near max dimmer channel (circuit if you prefer) capacity.

Oversized feeders and branch circuit wiring has always been the norm (I've seen #8 used on a 2400 watt rated dimmer channel..wasteful, but sure has a lot of reserve). (edited to add: Maybe it was one of 480's installs. :grin

And remember that in live performance work, set designers and lighting designers don't care a whit about Code or electrical design, they want it to look good no matter what it takes.

I could easily see a well-planned, properly utilized system being safely calculated at say 70-80%, but never 50%. That is too close for comfort..an elaborate production can easily use a lot of luminares running for extended periods of time.

The combination of all those loads, constantly shifting from one circuit to the next, lots of potentially flammable scenery and props, combined with a venue potentially holding a lot of people..I wouldn't want the liability of designing anything that didn't assume 90-100% continuous loading for at least three hours at a time.

 

[boater bill]

The max ot the control consoles are typically set for 95% to extend the lamp life. But when the scene calls for the lighting to be full on, it needs to be full on.
I just spec'd and installed a NSI 12 channel, 1200 watt per channel dimming rack. It required 60 amps 208 V single phase. I had #2 run for the main power feeds and #12 to each light bank. There is no diversity accounted for. Overkill, maybe, but I do not want problems.
Some churches I have designed have 96 channels at 2400 watts each! Big power requirements.

Good luck

 

[slamppin]

I understand your concerns for underating and just as a matter of credibility, we are lighting system integrators specializing in theatrical applications such as Disney, Universal Studios, performing arts centers (high schools, colleges,commercial), House of Blues, Hard Rock Live, and large resorts around the world. I have also seen a lot of "circuit limited" installations that constanly force a system's branch wiring to push the limits. When all is said and done, the total power needed (assuming no liomitations of infrastructure) is fairly consistant between venue types. That being stated, a venue requiring a 400 amp 3 phase dimming system feed could have any number of 2.4k branch circuits depending on design. In our case, we typically design load each of these circuits with a 575 watt load and only provide 1 outlet which would encourage a tech to grab the next closest circuit instead of a 2 fer. This automatically forces our quantity of dim circuits to increase. Others may figure each at higher loads reducing the total amount of branch circuits. The total load remains the same. The later would result in a less expensive installation partly due to the derating rules. These rules assume (without diversity) that if you increase the amount of current carrying conductors in a raceway, you are also increasing the heat. In the above scenario this is generly not true.
I don't know, it's something that has been bothering me for a while and I was hoping that someone out there had found something I couldn't. :-?

 

[dbuckley]

I'd love to know what theatres you're working at where that statement is true.

Thats the modern way, with "dimmer per socket"

In the good old days a typical local theatre might have 36 channels of dimming and a patching arrangement feeding maybe 80 sockets. Lamps would be paralleled onto dimmers up to (and often beyond, I've seen 4K on a 2K channel and the breakers never tripped) the maximum load of the dimmer. Today that same venue might have 120 channels of dimmers, feeding 120 sockets. Most dimmers would have loads of 1K or less, and many dimmers would actually be unused.

Even though the venue has about three times as many dimmer channels, the incoming supply to the stage dimmers would probably not have been upgraded, and ultimately that limits the total load; You could never get 120x2K out of the supply.

Scarily, with the labour and parts costs of patch panels, and the relative cheapness of dimmers, going dimmer per socket is not nearly as scary as it sounds, in economic terms. It also makes the rigging quicker, as no patching means one less job to do.

 

[mhulbert]

You guys are forgetting the one HUGE exception that the NEC allows for theatrical lighting: when using wireways, you do NOT have to derate. To take advantage of this, many theaters have 2.5x2.5" wireways going up to the grid and pipes. I've never compared the budget of this vs conduit, but quite a few contractors seem to think it was more effective to go the wireway route.

See 520.6 (2001 NEC)

Mike