Electrical Diversity in Lighting Circuits without drop cables

[TuckerD]

I recently read an article in Fall 2010 edition of Protocol by Steve Terry (@STEVETERRY) titled Understanding the hidden electrical diversity in entertainment lighting systems. Electrical diversity is something I knew absolutely nothing about until I read the article but it left me with some questions. I understand what the diversity requirements mean for drop cables but I'm not sure what sort of diversity restrictions I should be following in the system I typically use.

As far as I know our distribution system is like this:

• We have some dimmer racks with 2.4kw (120v) capacities. They are fed from a large breaker and have a diversity restriction of 41%. This gives my whole system on the stage a limit of 41% (right?)
• From there there is some conduit going to the stage and connecting to all sorts of circuits, those circuits come through boxes like the following
  • Pigtail and outlet boxes
  • Long connector strips

Are pigtail boxes considered to be a drop cable? They only have three conductors in them, what kind of diversity restrictions does this create if any?

From there the power might flow directly into the cable for a fixture or it might connect to a long extension cable that is restricted to 15A @ 120v or a cable restricted to 20A @ 120v. If I were to split that out and power three conventional fixtures totaling a load of 2000w am I exceeding some diversity restriction I missed along that circuit (assuming the cable was a 20A cable and all of electrical load bearing devices were being used past their restrictions? I could describe them in more detail if someone things I should)

I ask because the article only addressed drop cables at the "Rialto" and I am not sure if anything in my circuit counts as a drop cable and only the conduites between blocks should have more than three conductors in them. Unfortunately I do not have a copy of the NEC and I have no money to purchase one (is there an online resource I can use?) that I could look for the answers in.

Thanks in advance for taking the time to consider this,
-TuckerD

 

[derekleffew]

Some/all of your questions may be answered in this thread: http://www.controlbooth.com/threads/electrical-diversity.22932/ .

...Are pigtail boxes considered to be a drop cable? They only have three conductors in them, what kind of diversity restrictions does this create if any? ...

Are the "pigtail boxes" fed via rigid EMT (conduit) or flexible multi-circuit cable? I suspect you meant three circuits, rather than conductors in them. Pipefill tables in the NEC determine quantity and size of conductors in a given conduit.

Diversity, electrical does not apply to single-circuit cables, or circuits run individually, as there's nothing to diversify. The size of the hot and neutral wires inside the conduit should be coordinated with the size of the load and the OCPD.

The latest version of the NEC is available for online-only reading, with registration, for free at http://www.nfpa.org/codes-and-standards/free-access .

 

[MNicolai]

Diversity, electrical does not apply to single-circuit cables, or circuits run individually, as there's nothing to diversify.

You can correct me if I'm mistaken, but I seem to reminder the exception to your statement being true is that diversity does apply to single-circuit cables which are bundled together or run in parallel as these cables will heat one-another.

 

[TuckerD]

The boxes are fed via rigid EMT. I meant are the pigtails considered drop cables. They only have three conductors in them (they are a three wire cable). Each box has 6 circuits in our system. Thanks for your help guys, I'll have to do some more reading. Like I said, this is the first time I have been exposed to the idea of electrical diversity and actually seen any of the content in the NEC.

EDIT: Thanks for linking me to that thread. It didn't really answer any of my questions, I understood the article pretty well I just didn't know how electrical diversity related to circuits without drop cables or even if there was any mandated diversity for those circuits. It was worth reading though. Thanks for helping answer my questions.

 

[DavidNorth]

The drop cables are the multiconductor cables that combine more than one hot and neutral into a single jacket. If you have wire in conduit from the dimmer rack to the pigtail, and those wires are appropriately sized, have the correct number of neutrals and are in the right size conduit, then you have no diversity issues on that circuit. I am assuming that this is the case in your facility.

Now, a 41% diversity rating is going to refer to how much you can load your system and the cool thing is that you ahvesome options here. In very general terms, this menas that you can:

- Load every dimmer to 41% of its capacity and drive all the dimmers to full without tripping the main breaker
- Load 41% of the dimmers to full capacity and drive those specific dimmers to full without tripping
- Load the dimmers at will and with various levels set, not trip the main breaker as long as 41% of the feed [or less] is maintained.

Now, your long connector strips may be different. If they are fed with SO cable, depending on the cable conductor size and construction, you might be limited to 50% diversity on those specific circuits, which in reality, everyone ignores. What happens when ignoring this? Well, the cable gets warm and soft, in extreme cases, and ages prematurely thereby causing replacement in the future depending on how often and how heavy the cable is used.

It is rare that SO cables in permanent facilities get used at greater than 50% diversity, but on tour and in TV studios, they may be heavily loaded. Just think, if you usually put 10A or less on each circuit in that strip, then you're good.

Make sense?

 

[TuckerD]

Yeah David, That helps a lot. I understood the 41% diversity at the dimmer rack and what that means for my system. I just wasn't sure about what it all meant for the rest of power distribution. I'll have to look at our specs and equipment to see what our connector strips have to say, but they are fed via conduit like our pigtail boxes so I think that what you said about those applies to our connector strips as well.

 

[JD]

At the risk of oversimplifying things, in this case maybe it would be a good place to start-
When a theater lighting system is designed, it is understood that there will be many more circuits and lights then would be used at any given time. Think about it. In any given scene, you may be using about 10% of the resources. Now, that is a very ambiguous number of course. After all, we can all think of scenes that may require less than 1% or more than 50%. Still, if we were lighting a gym for a sports game, we would be thinking along the lines of 100%. That is not the case in theater. If we were to design a theater system to handle 100% of the load the cost would skyrocket. The challenge in the design is to estimate within reason what percentage of the total capacity of the equipment will actually be used at any given time. That calculation looks at what the dimmer capacity is AND what the lighting inventory is. As such, the subject needs to be revisited each time inventory is changed. If a theater that had 120k in fixtures adds another 50k in equipment, the demand estimate would change.
With the estimate in place, the electrical layout is designed and installed. At that point, within reason, the inventory can be used without further need to calculate.

 

[TuckerD]

Yeah. I understand that. This was about curiosity, I'm not facing a design where I am actually worried about anything.