How many instruments per circuit?

[JLNorthGA]

I've mulled through the replies on my previous thread on how many lights is enough. This got me to thinking - how many lights per circuit?

I'm running D20 (2.4 KW) modules in a Sensor 3-48 rack. The distance from the Sensor rack to the catwalk raceway is fairly short - maybe about 30' to the start of the raceway (and another 30' or so to the end). That has 12 gauge wire - so I'm not worried about running two instruments per circuit if I use 575 W lamps. I'm comfortable running twofers in this situation.

The stage electrics are another issue. It is maybe 50' from the Sensor rack to the ceiling, another 40' to the terminal boxes, the length of the cable (35-40') down to the raceways and then the 40' length of the raceway. The wire is all 12 gauge. The total length is on the order of 130' to the start of the raceways.

I've got 750 W BTN lamps in the Fresnel instruments. At full power that's 1500 W on a 2400 W circuit. The circuit should carry the power required. Would it be advisable to run twofers on the stage electrics? My main concern is that the wire (and cable) from the trough on the wall to the stage electrics is maybe 30 years old. I did have it evaluated when the Sensor rack was wired in last year.

 

[SBC]

How many lights per circuit? Go back to the suggestions of building a rep plot. How many seperate systems / focus areas are you creating?

The install company should have provided you with oversight / training on your end circuit load. If replacing the homerun wire in the conduit was cost prohibitive when the new dimmers were installed, they could have installed lower capacity dimmer modules.
If, shoulda, coulda... How many seperate conduits run to the stage electrics and how many circuits are in each one?

At 1500 watts on the fresnels, you are only pulling 12.5 amps. Generally, not all loaded dimmers will be at full at all times. Of course that depends on the needs of each production moment to moment.

 

[JD]

Generally, 2.4kw is 20 amps. Most agree that there needs to be some headroom on a dimmer. For example, with 575w S4's loading 4 per circuit would be a load of 2.3kw. Good? Well, some feel there is a little too little headroom, and a better cap would be 3. In the same way, Three 750s S4's would produce a load of 2.25kw. Still a bit too close.

Dimmer breakers are rated (at least ETC) at 100% duty cycle, so even at full load they should not trip. If you have a mixture of 750 and 575, then setting a "house rule" of three would be a good idea. In the worst case scenario, you end up with 2.25kw loaded. This should be ok, but if your lights were ALL 750 watts, you may even want to limit that to two.

The "wear" element in the system is the SSR inside the dimmer. The less load, the cooler it runs, the longer it lives. ETC's can be fully loaded, but why not try to get as much life out of them as you can!

 

[derekleffew]

Two past pertinent discussions:
http://www.controlbooth.com/forums/question-day/9282-acceptable-put-4x-s4s-dimmer.html
diversity, electrical
.

 

[JLNorthGA]

The install company should have provided you with oversight / training on your end circuit load. If replacing the homerun wire in the conduit was cost prohibitive when the new dimmers were installed, they could have installed lower capacity dimmer modules.
If, shoulda, coulda... How many seperate conduits run to the stage electrics and how many circuits are in each one?

How many separate circuits? - therein was an original problem. The cable that runs to the stage electrics is a S25/12 cable - that is it has 25 conductors that are 12 gauge. Ground wire is separate. They originally had ganged neutrals running outside of the S25 cable. I now have 11 circuits on each bar with a separate circuit for the work lights. One conduit (~2") leads to each terminal box from the trough.

I generally have six focus areas on the stage (30' W x 20' D) and 4-5 on the apron.

 

[StNic54]

One of the most overlooked items in your circuiting are the connectors - are you using 20amp or 15amp connectors covering the distance between your dimmer and your light source? Remember two-fers aren't always 20amp, and your 15amp connector will burn out when it's overloaded - I see it most in using 1k scoops two-fer'd together. Also, I agree that the dimmer definitely needs a little headroom.

 

[BillConnerFASTC]

I put 4 - 750s on 3k dimmers for years; and never had a problem with strip 16 - 150 watt pars in strip lights on a single 2.4k. The scr pack in most dimmers is rated for 50 amps - maybe some no-names not so much (it's a requirement of our spec for all projects) and the breakers fully magnetic or nearly fully magnetic. Even 12 gauge wire is rated for much more than 20 amps. It's a non-safety issue unless, as noted, someone has scrimped on connectors or portable wire size.

Good planning or good practice? Harder to determine I think. Probably not great design for a circuit to be planned to be fully loaded for a long period of time - like permanently in a building. One show for a week of tech and weekend of performances - I wouldn't give it a second thought unless easier or cheaper not to. I'd never plan sky drop or cyc lights for a high school to be circuited fully loaded but not sure I would flag it as a significant flaw - just something I don't like but with no good engineering basis.

From a production design point of view - I'd rather have every unit on separate control so any unit can be a special or part of a system or wash, so lost of circuits. Until the move to LED and distributed dimming (what ETC calls Layers of Light TM) I figured basic high school was 3 racks - somewhere around 220-240 production circuits and remainder for house, work, utility, and non-dims. That's all changed.

Related, ETC folk recently previewed me on a presentation on Layers of Light focusing on the issues of moving to LED and the relative costs, impact on design, lumens, etc. Very cool - arrange for a presentation if you can. Based on a "typical high school" from their records and interviews and research - very informative. One lesson confirmed - with a theater consultant you get about a 50% larger system than with just and electrical engineer working with the architect and a sales rep.

 

[JLNorthGA]

One of the most overlooked items in your circuiting are the connectors - are you using 20amp or 15amp connectors covering the distance between your dimmer and your light source? Remember two-fers aren't always 20amp, and your 15amp connector will burn out when it's overloaded - I see it most in using 1k scoops two-fer'd together. Also, I agree that the dimmer definitely needs a little headroom.

I'm using 20 amp stage pin connectors.

 

[Focus]

Good dimmers do not need Headroom; its built into the design. A 20amp dimmer typically used devices rated for 40amps. 4-575s on on circuit is just fine, however it may not be useful or convenient. In a 120k Par rig 2-4 cans per dimmer usually is not a big deal, most LD are just using all of each color at a time. In theater, dance, or corporate events there may be more need to use each fixture individually, so one fixture per dimmer is Ideal. But, I digress. Wiring, connectors and dimmers, can be used near rated capacity without worry, but defiantly leave some headroom in reserve if using a generator though.

 

[JD]

Good dimmers do not need Headroom; its built into the design. A 20amp dimmer typically used devices rated for 40amps. 4-575s on on circuit is just fine, however it may not be useful or convenient. In a 120k Par rig 2-4 cans per dimmer usually is not a big deal, most LD are just using all of each color at a time. In theater, dance, or corporate events there may be more need to use each fixture individually, so one fixture per dimmer is Ideal. But, I digress. Wiring, connectors and dimmers, can be used near rated capacity without worry, but defiantly leave some headroom in reserve if using a generator though.

Must disagree on this. One thing 40 years of lighting has taught me is always leave headroom. ETC is rated at a full 2400 watts, 20 amps. Loading two 1k pars is not a problem. However, load up a set of 4559 ACL's (Which is a full 2400 watts) and you -might- have a breaker drop on cold inrush on the rare occasion. Remember, inrush is 5 times (some say much higher) than run current. The first rule is you want the show to come off in a reliable fashion with as few "technicals" as possible. ETC's are designed not to fail, and have capacities well beyond their rated capacities. Still, if SSRs were not being stressed in general use, we would not have had so many threads over the years on how to replace them

Once again, ETC is rated at 100% output load 100% of the time, so this is NOT a safety issue. For me it is just a matter of practice because:

The less you stress a dimmer of any capacity, the more life you will get out of it. And even more important, the less "surprises" you will have at showtime.

 

[Wood4321]

Must disagree on this. One thing 40 years of lighting has taught me is always leave headroom. ETC is rated at a full 2400 watts, 20 amps. Loading two 1k pars is not a problem. However, load up a set of 4559 ACL's (Which is a full 2400 watts) and you -might- have a breaker drop on cold inrush on the rare occasion. Remember, inrush is 5 times (some say much higher) than run current. The first rule is you want the show to come off in a reliable fashion with as few "technicals" as possible. ETC's are designed not to fail, and have capacities well beyond their rated capacities. Still, if SSRs were not being stressed in general use, we would not have had so many threads over the years on how to replace them

Once again, ETC is rated at 100% output load 100% of the time, so this is NOT a safety issue. For me it is just a matter of practice because:

The less you stress a dimmer of any capacity, the more life you will get out of it. And even more important, the less "surprises" you will have at showtime.

Sorry John,
I am going to have to disagree with you on this one.
I send my tours out on a regular basis with 4-575, or 3-750's on a single dimmer. These shows load in 120-300 times a year (Across 3 shows) , and perform around 1000 performances a year. I have probably replaced a total of 5-10 SCR's total, in the last 5 years. My dimmer racks, are also all original CEM Racks, so they are quite old. (Nice Job ETC!)

The bigger problem with running 4 fixtures on a circuit is the connectors, it doesn't take much for a screw to be loose, and a connector melt down during a show.
I don't normally have that problem however, since my fixtures are usually patched to multiple fixtures at the patch panel in the Rack.

 

[JD]

Sorry John,
I am going to have to disagree with you on this one.
I send my tours out on a regular basis with 4-575, or 3-750's on a single dimmer. These shows load in 120-300 times a year (Across 3 shows) , and perform around 1000 performances a year. I have probably replaced a total of 5-10 SCR's total, in the last 5 years. My dimmer racks, are also all original CEM Racks, so they are quite old. (Nice Job ETC!)

The bigger problem with running 4 fixtures on a circuit is the connectors, it doesn't take much for a screw to be loose, and a connector melt down during a show.
I don't normally have that problem however, since my fixtures are usually patched to multiple fixtures at the patch panel in the Rack.

To each his own. If everyone had the same opinion the world would be very boring place!

 

[DavidNorth]

Remember, inrush is 5 times (some say much higher) than run current.

With tungsten, we see 10-14 times inrush for the first 3 cycles. Also, turning off the lamps and letting them cool for 5 minutes is all it takes to get them back to that inrush level. See previous postings on whether or not to preheat before lamp check. Off topic and not to be discussed here.

On topic is the point that properly designed dimmers will have appropriate circuit protection, but you need to know what that is. Just because Sensor dimmers have breakers that are 100% rated and can handle the inrush doesn't mean all dimmers are made that way. See the thread above that Derek included. VERY good information in there.

Read that thread carefully as there is discussion of extra hard usage cable, which may be more of a limiting factor than dimmer breakers. Multiconductor SO cables have a derating defined by NEC Table 520.44. It looks like the OP should use a derating of 60%, which means limiting either the amount of load on each circuit or the use of the circuits on the cable.

Oh, and another comment on inrush.....some LED loads are upwards of 20-50x inrush on one cycle. Get good switchgear.

David