Dumb electricity question

[tonyvision1111]

I feel I should know this better, but I just don't.

I have a 200amp disconnect in a theater. 5 wire; 3hots, neutral, ground.

I have my distro and dimmer tied in.

My total wattage draw is 14,840watts from leko, led, moving lights. All of them running at 120v.

The math says thats a total of 124 amps being drawn.

Does that mean I only have 76 amps available for guest power (or really 56amps safely), or does the 3 phase nature of the disconnect give me more head room? I've always been confused as to how to look at this correctly.

Can anyone help?

Thanks
Tony

 

[theatricalmatt]

It depends on whether the loads are evenly distributed across all three legs. It's possible (though unlikely) that all of your power is drawing off only one leg.

When it doubt, meter it!

 

[tonyvision1111]

For sake of argument, we're balanced. It's a nice Lex Distro with each socapex circuit feeding the same number and type of light. The dimmer is also balanced. What is my headroom? I find so many different answers as I search around.

 

[SteveB]

I feel I should know this better, but I just don't.

I have a 200amp disconnect in a theater. 5 wire; 3hots, neutral, ground.

I have my distro and dimmer tied in.

My total wattage draw is 14,840watts from leko, led, moving lights. All of them running at 120v.

The math says thats a total of 124 amps being drawn.

Does that mean I only have 76 amps available for guest power (or really 56amps safely), or does the 3 phase nature of the disconnect give me more head room? I've always been confused as to how to look at this correctly.

Can anyone help?

Thanks
Tony

The easy math says your load is 41+ amps per phase (14840 divided by 3 (phases) = 4947 watts divided by 120 volts = 41.22 amps), IF balanced across 3 phases, but there's rarely that kind of precision on the load distribution, so figure potentially 50-60. Then assuming a 20% derating of the feed service, so 160 per phase available, thus you have headroom of at least 100 amps per phase.

 

[BillConnerFASTC]

SteveB, Why are you derating temporary cord and plug connected loads?

Also seldom are all circuits on at full. Seems like you've got a lot of headroom.

 

[SteveB]

Are not all OCD's required to be derated if not rated for continuous loading over 3 hours use ?


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[JD]

The question has multiple answers depending on the show.
The main 200 amp x 3 OCD should be derated for continuous loads to 160 amps per leg. BUT- Lighting loads are anything but continuous!
When you strike up the lamps on the movers, there will be a temporary swell on the load, which will then level off to their operating load, and if they have a good Power Factor, their load will be predictable and as close to continuous as you can get. The LED fixtures will take an inrush when first powered up even if they are black. After that, their draw will be pretty linear dependent on brightness. The conventional fixtures will be lopsided toward the low end of the curve, but basically will only draw a few percent at black, and close to 100% when full up.
So, basically, if everything was turned on and left at full power and your draw was running at 124 amps per leg, AND the load was balanced, you would have a headroom for running a continuous draw of 36 amps per leg. 36 x 3 = 108 amps total.
BUT this just is not the real world! If the loads are transitory your headroom is FAR higher. For example, if your duty cycle is less than 50%, then you have almost 100 amps per leg available, provided that load has about the same duty cycle.
The only way to pin things down is to meter it in a real world setting.

 

[SteveB]

JD, where are you coming up with 124 amps per leg ?. The OP is saying his TOTAL wattage draw is 14,840 and is distributed across 3 phases.

Am I missing something (wouldn't be the first or last time !)


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[Scenemaster60]

As has been said above, you most likely have a LOT of headroom in this system. Each leg of a 120/208 volt, 200 Amp 3-phase Y distribution can safely handle 200 Amps of 120 load for short periods of time an 160 Amps of 120 volt load continuously. While I'm sure your load isn't perfectly balanced I bet that's it not severely lopsided either. In my opinion you could safely add another 300 amps of load (100 Amps to each leg) and never come close tripping a breaker or blowing a fuse. That's being conservative. I personally wouldn't hesitate to add 120-130 Amps of connected stage lighting load to EACH leg if I needed to.

 

[BillConnerFASTC]

In the days of piano boards, you basically had to connect 100% of the load. Generally I don't think cord and plug connected loads are ever considered "continuous" but then again a local authority can require anything.

Who goes atrond their home and checks on plug and cord connected loads and the circuit's they are on?

 

[TheaterEd]

In the days of piano boards, you basically had to connect 100% of the load. Generally I don't think cord and plug connected loads are ever considered "continuous" but then again a local authority can require anything.

Who goes atrond their home and checks on plug and cord connected loads and the circuit's they are on?

You mean to tell me that you don't?

Side note, I just want to say I love when someone sets you guys off to figuring out electrical loads. I feel like I understand the theory pretty well, but it's good to read the practical application of it. Keep up the good work folks!

 

[BillConnerFASTC]

I had to figure it out for dimming systems and house lighting. Determining feeders for a dimming system - in the days of patch panels and prior to dinner per circuit - was easy since you could depend upon many dimmers being loaded to capacity - just add up dimmers. Dimmer per circuit was a major change because of extreme under capacity and no load. I'm sure I have some tremendously oversized feeds as do others. Pre LED and many non-dimmed loads, I came to base feeds on how many units could be reasonably hung and focused in the venue. One side observation was that engineer/sales rep designed systems usually fed a rack at 400 amps and consultant designed was 600. In any case, I have a lot of high school 3 to 5 rack systems with 800 amp feeds which was like 300 units or more when 750 and 1000 watts was common. But I had to justify it and, as the NEC requires, I did a panel schedule and assigned 750 or 1000 to each dimmer (but had voltage drop based on 1500 or 2000 per circuit). Now, with LED and distributed dimming, changing again. Still trying to resolve inrush and diversity while being practice, and worrying about one time events that bring in a lot of legacy gear. Doubling the feed for inrush for the whole theatre seems excessive, so are sequentially activated relays the answer?

If you want good answers, read what Steve Terry has written if contact him. Mitch Hefter and Ken Vanice are also great resources. I contact them frequently to be sure of some things.

And then how emergency has changed and complicated things. And now a school that wants to turn off feeds for summer to save on quiescent power and how to handle emergency. Maybe not completely awful since it all tends to make hiring a theatre consultant with specific experience much more important and justifiable.

 

[SteveB]

Generally I don't think cord and plug connected loads are ever considered "continuous" but then again a local authority can require anything.

?

One of the early examples I recall reading about what defines a continuous load, is a TV or Film set, which may well have their "temporarily" connected loads running all day. In some applications, such as a TV studio, probably 95 % of the wiring is conduit and raceways, it's only the last connection, or connections (if using two-fers or cable) that are temporary, in that the lighting system can have it's load moved around on the dimmers/power sources. An indoor film set may well all be temporary, portable racks, multi-cable, etc...

So I assume derating is required. I KNOW this has been discussed as the fact that a centrally located Sensor dimming system, with it's fully rated branch circuit breakers has been discussed as an advantage over a distributed system, whose feed source may be a commercial branch panel with non fully rated breakers. This discussion indicated the need to de-rate the breakers and the result of the reduced power available to a shoe-box dimmer, as example. These systems are powering "cord and plug loads".

 

[BillConnerFASTC]

I have no safety concerns with a four way smart bar plugged into a 20 amp circuit with 4 575s plugged into the bar. No problem with 2000 watt hot plate on my buffet either.

Is it better design if especially for TV and film and architainment where the load accounting for some being dimmed is planned to be on continuously, without even a break, to derate - probably. Not sure its a code breaker however. And not probable in theatre in my experience. And I don't spend a lot designing for Easter Sunday or the Christmas pageant either.

Regarding the op, doesn't sound continuous or frequent and wondering if his stuff is all at max capacity when guest gear is also.

And based on averages, buildings I consult on are probably overpowered.

 

[derekleffew]

I feel I should know this better, but I just don't.
I have a 200amp disconnect in a theater. 5 wire; 3hots, neutral, ground. ...
Can anyone help?

Tony, I think your question has been answered, but just to be clear, a 200A, 3Ø 120/208VAC Wye connection company switch provides 200 amps PER leg, so you effectively have 600A available at 120V.
In my opinion, the switch need not be subject to derating when dimmable loads are connected, as the odds of all dimmers at full for over three hours, constituting a continuous load, are virtually zero. I think 2011 NEC 520.27(C) agrees: "... maximum load that the switchboard is intended to control ... ."

And I definitely second the above motion that you should obtain a true-RMS responding Amprobe (generic term) clamp meter (AND more importantly, proper training on how to use it). They can be had for as little as $100. Clamp Meters | ControlBooth

... Side note, I just want to say I love when someone sets you guys off to figuring out electrical loads. I feel like I understand the theory pretty well, but it's good to read the practical application of it. Keep up the good work folks!

So you like threads such as
What kind of cams are these? | ControlBooth
Power for rig3 | ControlBooth
Power for rig2 | ControlBooth
Power for rig | ControlBooth
Is it acceptable to put 4x S4s on a dimmer? | ControlBooth
Quick feeder question | ControlBooth
More Phase Questions | ControlBooth
and so on? It's been a long time since we've had one of those. If you want more (or have a suggestion for any QoTD), send a private message to any moderator and we'll see what we can do.

 

[JD]

JD, where are you coming up with 124 amps per leg ?. The OP is saying his TOTAL wattage draw is 14,840 and is distributed across 3 phases.

Am I missing something (wouldn't be the first or last time !)


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Not missing anything! Just reread post #1. He had listed 124 amps, but when you look at the load it's obvious that he was talking TOTAL load not load per leg. Which of course means he has tons of headroom. Teach me to PBC! (Post Before Coffee)

 

[porkchop]

As much as I love a good ole' theoretical math problem, it seems like it'd be a lot more practical to just turn everything on and use an RMS clamp multimeter to measure the loads on the feeder cable. That way you know what's actually there rather than depending on calculations.

 

[Dionysus]

I agree with porkchop and the others who suggested metering. As an electrician I have to say doing the math is great and is usually correct (or at least darn close) but nothing beats field measurement ESPECIALLY when talking about multiple phases. I've seen ALL KINDS of unexpected things on 3-phase power systems.

 

[SteveB]

I agree with porkchop and the others who suggested metering. As an electrician I have to say doing the math is great and is usually correct (or at least darn close) but nothing beats field measurement ESPECIALLY when talking about multiple phases. I've seen ALL KINDS of unexpected things on 3-phase power systems.

You need to know the math and the code issues BEFORE you do a design where you might be having power limitations. All good and well to own the true RMS Amprobe, only to find out you're over because you have too much gear.

A true story. The Lincoln Center Out-Of-Doors festival has been in residence at Damrosch Park next to the Met Opera for 30+ years. For probably 15 of those years the power feeder to lighting was 100 amps, 3 phase, 120/208. The festival temporary lighting system as designed (by the original LD - David Kissel) and installed had 72 - 1000 watt units, Pars and Altman 360Q's @ 1kw. So a load of 200 amps per leg. The console in the mid and late 80's was a 24 ch. TTI 2 scene. It was labeled along the top with the approx. amperage per channel. So, 1 = 20, 2 = 20, 3 = 20, 4 = 30, or whatever.

The idea was you counted the amps as you set levels. Really. You might have had to lower the FOH a lot in order to use the channels that had the upstage back blues.

One of the visiting dance companies the season I was assistant LD had an LD named Allen Lee Hughes. A very good designer, had been nominated for a Tony in 1983 for K2. A really nice gentleman (teaches now at NYU) . Allen shows up a few hours before show (no tech rehearsal, just run it at 8:30), with a PILE of pages of cues, all hand and custom written for THAT show for a 24 ch. 2 scene. Allen came prepared but he seemingly missed the memo about 100 amps, counting amps, etc.. . When I explained what the numbers were on the top row, his face went pale. Into his briefcase went the cues and Allen got to run his own show. He was seemingly good at math as we didn't blow a leg.

Moral is 'ya gotta do your homework.

 

[MikeJ]

A clamp meter is nice to have; you can get by with a cheaper non-true RMS meter, the waveform of power services will generally not be irregular enough to make a practical difference.

You do need to do the math beforehand to know things like; do I need divide my gear between 2 disconnects? Will the 200A power on stage right be enough, or should I bring 50 more feet of feeder to get the the stage left 400A disconnect? How big of a generator do I need? how much power will the bands floor package need? Is that stupid video truck that runs on single phase going to share my power?

Calculate first, meter to confirm.