Electrical Diversity

[Shawncfer]

So I just got done reading Steve Terry's article:

Electrical Diversity

But theres something that I want to make sure I completely understand here.

Steve gives an example: “Why, 32 twenty amp loads, of course. I’ve got a 20 amp dimmer feeding each circuit, the connector strip and the gridiron junction box are fully rated, and I have 3600 amps of power feeding the dimmer system. In fact, the light plot for my next production calls for 32 - 2000 watt fixtures on the first electric, so I’ll have an extra 400 watts per circuit.”

And then Steve begins to explain why its wrong. But I want to make sure I understand why this is wrong.

In one way the electrician is right because each circuit will be 20 amps and 2400w. But the thing that makes him wrong is the fact that the 2 SO cables running from the grid to the raceway will have 32 conductors and 4 ground and the 32 conductors means that they can only have 60% of ampacity according to table 520.44 correct? So therefore each circuit can only be loaded up to 12 amps or 1440w. Is that right? And so if for whatever reason I absolutely needed every dimmer on that electrical to be loaded to 2000w each (is my terminology correct by the way?), I would need to run individual cable from the grid to each circuit? Because even if I had a 4 conductor multi-cable, then each dimmer could only be loaded to 1920w each right?

And Also, as far as the temperature rating in the same table, the amperage listed under each table is the amount of amps that size wire can be loaded to before overheating correct? However I know not to exceed the amperage listed on the far right of the table under "Maximum rating of overcurrent device". This is just more of a theoretical question.

Sorry for so many questions, I just wanted to make sure I understood this right.

 

[JD]

Yes, that is correct. The more conductors you have in a cable, the less current you are allowed to carry. The reason is mutual heating. The same gauge conductor will have many different capacity ratings depending if it is "open air" (such as on a pole outside), buried, or grouped with other conductors. When grouped, the number of other conductors matters as each will add to the overall heat generated in the cable.

Unlike the general copper wire capacity, the grouping derating has to do with heat damage to the insulation on the wire in the cable.

 

[xander]

Correct me if I am wrong, but I was under the impression that Electrical Diversity also took into account the fact that--in theater/live events--not every fixture is used at the same time. This is part of the reason that these installs are legal. You can, theoretically, load all of your circuits with 2kw based on the assumption that you will not ever be turning them all on at the same time.

-Tim

 

[Shawncfer]

Tim,
From what I understand of that article you're right. But I've worked at a school before that had 24 circuits on the first electric. They had 20 S4s for front lighting areas 11-20. And the other four were gobo wAshes. All S4 750w. Now this would be okay right? Because 60% of 24 circuits times 2.4kw each is 34560w spread over the whole electric and I'm only drawing 18000w making it okay right? And I know some of y'all will assume that they will never be fully loaded but the same stage is used for band concerts and they usually turn all areas, color washes, and gobo washes to full. Is that okay still?

 

[Shawncfer]

Oh and was I right about the whole "if each circuit is loaded with 2000w" each, then you mean need to run a separate cable for each circuit right?

 

[STEVETERRY]

Correct me if I am wrong, but I was under the impression that Electrical Diversity also took into account the fact that--in theater/live events--not every fixture is used at the same time. This is part of the reason that these installs are legal. You can, theoretically, load all of your circuits with 2kw based on the assumption that you will not ever be turning them all on at the same time.

-Tim

That is correct.

ST

 

[STEVETERRY]

Oh and was I right about the whole "if each circuit is loaded with 2000w" each, then you mean need to run a separate cable for each circuit right?

If each circuit is loaded to 2000W and you wanted to turn them on all at once, you could break up the drops into more cables with fewer current-carrying conductors each. You could also use a larger gauge wire. To do what you describe would not necessarily require a separate cable for each circuit, unless you wanted to be completely free of the 520.44 50% diversity requirement, in which case separate cables (not in contact with each other) would do it.

ST

 

[Shawncfer]

So I have another question about this same article. Does table 520.44 apply to touring rigs? I know that bigger rigs these days use mostly movers and/or LEDs, and if they do use conventionals they rarely fully load them. But for the few times they do (if they ever) would table 520.44 apply to them or would they need to refer to table 400.5 because typically every circuit would be used for a touring rig?