Theatrical lighting circuit de-rating

[frank osuna]

When applying 2002 NEC Article 520, to performance theater stage lighting, where the number of conductors in a raceway exceeds 30: Paragraph 520.6 headlines "Number of Conductors in Raceway". 520.6 goes on to specify that the stage lighting branch circuits be sized for voltage drop. And indicates deration for number of conductors exceeding 30 by Article 310.15(B)(2)(a) is not required by exclusion of 366.6 (applicable to auxillary gutters) and 376.22 (applicable to wireways).

In my theater under construction I have a wireway at the catwalk level with (10)4" conduits down to the dimmer rack 20 feet below. The wire count is 216 phase, 216 neutral, 23 grounds for a total of 480 conductors. The wireway fill is 9%. By the NEC paragraphs above the conductors are not required to be derated due to number of conductors. And because the distance does not exceed voltage drop requirements the conductor does not need de-rating for voltage drop either. Fill on the 4? conduits (raceway) is 11%. But the wording of the code does not specifically state the exception to the wireway can be applied to the conduit (raceway). The only way it seems to apply its intent is that the Heading of 520.6 says RACEWAY. To me it appears an oversight in the code. I think I can win the argument with the inspector is if I can convince him that heat dissipation in conduit (raceway) is the same as heat dissipation in wireway. Do you believe that to be true?

 

[jschultz]

Re: Theatrical lighting circuit de-rating

by definition, conduit is not the same as metal wireway. So you should check it over with the inspector before it gets installed.

 

[frank osuna]

Re: Theatrical lighting circuit de-rating

Thanks for the reply. The inspector on one day agreed that there appears to be a glitch in the code and just wanted a letter from us. Then 2 weeks later he is on the fence. But by then the conduit is in. So all I need is something to push him off the fence. We are at half the 20% wire fill the code requires to allow more than 35 conductors. Is heat dissipation in wireway that much diffent than conduit?

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Frank,
If the heat problem is the same for conduits and wireways, why is that we must start derating at the fourth current carrying conductor for conduits, but not until the thirty-first current carrying conductor for a wire way? As far as your question, I read 680.6 opposite from the way you read it. I see the section saying that the rule in 376.22 that permits you not to derate until the 31st conductor does not apply to your application. My interpretation is that you must derate starting with the fourth current carrying conductor for both conduits and wire ways. However the last sentence of 520.6 is poorly worded and not clear.
Don

 

[dereckbc]

Re: Theatrical lighting circuit de-rating

Don 680.6, huh? Sure you didn't mean 520.6?

I agree with Don that the 30 conductor limitation of 366.6 and 376.22 shall not apply.

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Dereck,
Yes, that should have been 520.6. I don't know where the 680 came from.
Don

 

[frank osuna]

Re: Theatrical lighting circuit de-rating

Don,
Let's break this problem into 2 aspects to reduce the complexity. In this reply I will only address the conductors in WIREWAY and AUXILIARY GUTTERS.

LOOKING AT AUXILIARY GUTTERS FIRST: The installation satisfies 520.6 in that the conductor fill in my application is 9% (well below the required 20%). By the last sentence of 520.6 the conductor limitations of 366.6 are eliminated.

Looking at 366.6: It states that the derating of conductors is not required if less than 30 conductors are installed. But 520.6 eliminated the restriction on number of conductors in its last sentence.

THEREFORE: My interpretation of 520.6 is that derating of 480 conductors in Auxillary gutters (366.6) is not needed because the fill does not exceed 20%.

The same logic results when looking at 520.6 for wireways (NEC 376.22) ie NO DERATING IS REQUIRED FOR THE 480 CONDUCTORS IN THE WIREWAY.

Do we agree to this point?

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Frank,
No, we do not agree. The general rule in 310.15(B)(2) is that when you have more than 3 conductors in a raceway or calble you have to derate. I read the 30 conductor rule in the wireway article as an exception to the main rule in 310. When you take away the exception, which is what 520.6 is doing (in my opinion), then you have to derate strating with the 4 current carrying conductor.
Don

 

[ed downey]

Re: Theatrical lighting circuit de-rating

I Believe Don Is Correct In The Way He Is Looking At This According To The 2002 Code.
But Looking Through The 1996 Code It Specifically States That Derating Is Not Required For Wireways & Gutters {362-5 (NEC 1996)}.
I am Now Curious Why This Change? Where People Abusing This Rule Or Was It Deemed Safer After This Way.

Anyway You Look At Frank's Problem I Don't Believe The Code Ever Intended For A Conduit To Be Considered A Wireway.

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Frank,
Ed's post got me looking at the 99 code. The main rule in 362-5 limited the number of current carrying conductors to a maximum of 30 in any cross section of the wireway. It goes on to say that derating is not required for these 30 current carrying conductors. Exception #1 says that if you derate per 310-15(b)(2)(a), then you can exceed 30 current carrying conductors as long as you do not exceed 20% fill. The rule in 520-6 says that the 30 conductor limit does not apply. This means that you are permitted to exceed 30 current carrying conductors, but it doesn't really say anything about the derating of these conductors. I don't know the intent of the code here and my set of code documents does not go back far enough for me to research the intent. I have the ROPs and ROCs back to the '84 code cycles but this wording was in the '78 code. I guess that you could read the code words as saying that derating is not required for this application, even when the number of current carrying conductors exceeds 30. I wonder if the thinking on this issue is, 1) that not all of the conductors will be fully loaded at the same time and 2) they will not be fully loaded for extended lengths of time?
As to the conduit issue, there is no code provision not to derate. Why don't you just install wireway down to the dimmer bank?
Don

 

[iwire]

Re: Theatrical lighting circuit de-rating

With Ten, 4" conduits you could use #6 THW rated at 65 amps derate .35 to 21 amps and this would fit easily in your raceway 89 #6awg THW fit in 4" EMT. You could push this out to #2awg THW and get up to a 40amp rated circuit. 49 #2awg THW fit in 4" EMT, (bring the wire lube) that would get you 490 total conductors. I assumed EMT it will change if RMC.

Probably less costly now then changing to wireway, even after figuring out how to terminate the ovesized wire.

[ March 11, 2003, 12:54 PM: Message edited by: iwire ]

 

[frank osuna]

Re: Theatrical lighting circuit de-rating

Whoa! hold on thar.
Let's limit the conversation to the wireway and gutter. (thanks for for everyones input, by the way, it helped)

Don, I think we have a great suggestion, let's use the '99 as example of intent. You will notice that the 1999 NEC 362-5 indicates that the derating factors do not apply if the number of conductors are limited to 30. BUT THEN 362-5 EXCEPTION#2 SAYS: the 30 conductor limitation does not apply to theaters and similar locations.

I think the intent was clear. There may be 18000 light fixtures(in my case) but a maximum of 15% are on at any given time. That is still 66 watts per square foot on the stage!!! The total connected lighting load on the theater is 4400kW. But we have rated the transformer dedicated to the theatrical dimmer at 1500kW. ie There is a great deal of diversity in theatrical lighting and the code writers understood this.

I do not believe the code writers intended to create a new restriction. The entire format and layout was re-done with 2002. I think they mainly intended to re-phrase the code. And a crack has been revealed.

I think what trips us up is that the de-rating table 1999 NEC 310-15(b)(2)(a) should say "except as allowed by 362-5" or something to that affect to tie it together. And likewise a similar statement in the 2002 code. That would go a long way to clear this up.

Do we have agreement that although the wording of 2002 is different but that the intent is that the 480 conductors are not required to be derated in the wireway and gutters?

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Frank,
It appears that was the intent of the rule, but the wording is so poor that I can't be sure.

Don

 

[coppel]

Re: Theatrical lighting circuit de-rating

Hmm. Seems like I am jumping in a little late into this. I've not been reading as many posts due to the new format (i find awkward).

I'm no expert to code, but I do alot of work in enterainment lighting & sound. Because of the "special application", many times things that are not allowed normally can be done, safely.

An example of this, similar to what was mentioned above. A concert touring lighting system may have (just picking numbers) 100 fixtures hung over stage, each with at least 1000 watt bulb in them. If all lamps were ON at full power, this would require around 1000A. However, only 1/3 - 1/4 will be operating at any time (lights flashing in sequence, ect), or ones that are on, may not be at full power, but maybe 1/2. So the rig may only pull ~400A maximum at anytime. Instead of using a much larger feeder & distro (sized as you would for an "installation", they can get away with one a little smaller. This equates to less work for the roadies to handle, and less weight in the truck(s).

My advice: Consult the manufacturer(s) of the lighting equipment, or the company the same were purchased from. I'm sure they have dealt with these issues more than once.

If not them, check a local lighting & sound / productions / rental company. Most large cities have "at least" one such company. They deal with code / inspectors / ect. constantly.

 

[jschultz]

Re: Theatrical lighting circuit de-rating

if you have the lights limited so that only say 25% of them can possibly operate at a time, then the number of current carrying conductors would be reduced. They would just be extra conductors in the conduit. Just a thought.

 

[frank osuna]

Re: Theatrical lighting circuit de-rating

Don, Coppel, jshultz;
thanks for everyones input. I believe we have determined that the intent of the code was to allow for the diversity involved.

Unfortunately manufacturers back away from making what they consider "engineering advice". We the engineers are supposed to satisfy code.

I believe the code writers intended to include conduit in their consideration. But due to the complexity of the writing have inadvertantly ommitted applying the same diversity to conduit as is allowed for wireway. I do not see that conduit would dissapate heat much differently than wireway.

Maybe I'll resort to taking the inspector out for lunch.
Thanks for everyone's input.

 

[iwire]

Re: Theatrical lighting circuit de-rating

Sounds like an expensive lunch. :roll:

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Frank,
If I were the inspector and you had approximately the same amount of surface area on the conduit as on the wireway, I would accept the conduit on the same derating basis as the wireway. In a normal application the amount of surface area will be much greater for the wireway for the same approximate number of wires because of the 20% fill limitation on the wireway and the 40% conduit fill. If you have a 2" x 2" wireway, the 20% fill is 0.8 square inches. The 40% fill area of 1.5" EMT conduit is 0.814 square inches. The surface area of the wireway would be 8 square inches per running inch and the conduit would be 4.71 square inches per running inch. I would expect that the heat is more readily dissipated when you have more surface area. I think that you indicated that your conduit fill was about 11% so you should have plenty of surface area.
Don

 

[iwire]

Re: Theatrical lighting circuit de-rating

Don do you mean if you where the inspector or the AHJ, and would you give the "special permission" in writing? Your statement seems like you are rewriting the code.

 

[don_resqcapt19]

Re: Theatrical lighting circuit de-rating

Bob,
Using the authority granted in 90.4, I'd permit this installation.
Don