Overloading Dimmer without blowing it up?

So, i'm thankful for saved passwords, because i haven't been here in forever and did NOT remember my password.

Anywho. We've got an EDI Mark VII rack in my school. Each dimmer pack is two 2.4KW channels. I've got about 2800 watts of lights (Some floods, and some Christmas lights) that will be running off of one of the channels... if i keep the dimmer below 80% (This should leave 200 watts of overhead on the dimmer), would i risk damaging the dimmer or tripping a breaker anyway? I've tried digging around, but i can't seem to find anything on doing this. We only have on channel available for doing this, as our entire light rack is at 75% capacity, except for this one circuit (which was reserved for stuff like this). I'm worried about tripping the breaker, or causing the SSR (which is apparently rated for 40 amps in these dimmers?) to overheat... the ambient in the room is about 55F, and the rack is only have full of dimmers, so they have plenty of airflow. It would be running in this state for about an hour a day for two days, and then it won't be used anymore. (School's awards days)

Thanks for any input. I understand this is a weird idea, and if it's completely retarded, please let me know. :)
 

65535

Active Member
As you cut power to the lamps you cut wattage exponentially, however inrush current could kick the breaker. However by design dimmers have circuit breakers on them that prevent damage to the dimmer, they're hard to hurt with over current because they just shut down.

It's not good practice but if you keep it below 90% or less you should be fine. Also avoid bumping that channel as the inrush current may kick it out. I would be very surprised if you couldn't find a way to double up instruments on another circuit somewhere. 1 and 4ch dimmer packs also exist which are easy to add on.
 
I would love to get a larger dimmer for this, but the issue with that is money, and the simple fact that this is the first time we have EVER needed more power than one channel could give us.

So if i understand correctly, if i limit the dimmer to something below 90%, i shouldn't have to worry about damaging the dimmer, even though the potential to overload it is there?
 

Les

Well-Known Member
Running an overloaded dimmer at a lower intensity is not a reliable method of keeping it from tripping, as the called for percentage (ex. 50%) is not linear to the dimmer's output (ex. 1,000w @ 50% = NOT 500w).

There actually has been much debate on this in the past, and I'm not sure where in the curve it happens, but the dimmer is generally under close-to full load much earlier in curve than you would think by simply looking at the percentages on the board.

Tripping a dimmer is not necessarily harmful to the dimmer, but it is risky. The entire system is much more vulnerable[-]. If you have long cable runs, your resistance increases meaning your actual current draw is above your 2800w lamp ratings[/-], so the room for error is much smaller. Running a dimmer at capacity is fine for a lot* of dimmers, but I would never run any dimmer above capacity because you will have no idea how much power you are actually drawing, or how long the breaker will hold.

*While ETC Sensor dimmers are rated for 100% load continuous, not all dimmers are equal. "Continuous" duty-cycle is usually defined as 3hrs or more, but some dimmers don't like anything above 80% load continuous. It depends on the build of the dimmer as well as the breakers (ETC Sensor uses magnetic breakers, FWIW).

http://www.controlbooth.com/forums/lighting-electrics/8015-quick-question-about-wattage.html
http://www.controlbooth.com/forums/question-day/9282-acceptable-put-4x-s4s-dimmer.html
http://www.controlbooth.com/forums/lighting-electrics/9449-dimmer-profiling-avoid-over-wattage.html
 
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It's mainly due to cabling and money. We have a ton of lights on 48 dimmers, and all of them except for this one have 3 750watt lamps on it. I'm planning on seeing if i can dump some lamps somewhere, but we would also need another adapter to connect the lights to our system. (The plugs are actually 30 amp outputs, so i'm not worried about that... getting from that to something that Christmas lights and cheap DJ lights will plug into is the issue.) I'm thinking of dumping one of the floods, and just eliminating the entire problem.
 
Thanks for the links. It's looking more like having to stay below 50% in order to make everything work, and at that point, i might as well just cut out 25% of the wash lights (1), and then run the rest at full. Thanks :)
 

JD

Well-Known Member
Only a 40 amp SSR on the Mark VII ? Wow, the old Mark V I had used two 63 amp SCRs, for a back-to-back capacity of 126 amps on a 2.4k dimmer. One day in the shop (please don't try this) I intentionally overloaded the heck out of one of the Mark V modules with the breaker bypassed. Yes, I was trying to see the failure point. I had 7kw on it before I gave up! (The dimmer never did.)

So anyhow, the rule of thumb is do not do it. I have seen 10 amp dimmers with only 15 amp triacs, so many do not have the reserve capacity. The 40 amp SSR has 40 amps as it's RMS rating, and a much larger inrush rating. So you are not going to pop it at 23.3 amps. You will however probably trip the breaker if it is on long enough. In fact, EDI used a pretty fast breaker and I had more problems with breakers on those packs then I ever did with toasted modules.

Of bigger concern is that running the SSRs above the "dimmer's" rating will prematurely age them. It is the heat cycle over the years that catches up with them.
 

DuckJordan

Well-Known Member
I'm still a little confused, your use of terms is misleading.

can you put a diagram out of how you have connected the fixtures?
Do you really need that much light?
What use are the Christmas lights?

Honestly if you are using 48 2.4kw of power theres an issue.
 
I'm pretty sure it's a 40 amp SSR. It MAY be more, though. I've heard a few things, with 40 being the minimum. 7KW is impressive. That makes me feel better about the SSR. I guess i'll try it and see where the failure point is, and then judge from there. If i can get most of the way to full, i'll go for it. If i can't, then i'll just run 3 washes and some Christmas lights.
 

Les

Well-Known Member
Thanks for the links. It's looking more like having to stay below 50% in order to make everything work, and at that point, i might as well just cut out 25% of the wash lights (1), and then run the rest at full. Thanks :)

You're right about that logic. Due to amber drift, your lower quantity of wash lights @FL will be much brighter than all of them running at 50%. Is there a way you can lamp them down? For example, (and back to the phenomenon of amber drift), a 500w lamp at FL will have a higher color temperature and appear brighter than a 750w lamp faded down to the point where it is drawing 500w.

I guess this also ties in to the question of: Do you need lots of intensity in one place, or is there a lot of area to cover? You say "wash lights", so I am assuming that you need coverage. If so, lamping down would be a good option without sacrificing coverage, if that option is available to you.
 
I can't really come up with a diagram.

The way the system was installed is with each dimmer output having 3 connections on the stage. We NEVER use all of the lights at the same time. We have two sets of gelled lights and then white, with most of the lamps being 750 watts. It's not 2.4KW, but it's close enough that there's really not any room left on the dimmers.

The Christmas lights are for effect. They're wanting them to be on dimmers so we can turn them off for a few parts of the show, at which point the wash lights will also be off. The wash lights are going to actually be hitting the floor in front of the stage, again, for effect.

EDIT: To elaborate on this, each dimmer is driving 3 of the same colored lights. The plugs are set up in such a way that we end up with a Gel-Gel-White-Gel-Gel-White-Gel-Gel-White pattern of lights from 3 dimmers.
 
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I'm needing coverage. Lamping down might be an option, but i'll have to check to see if we have any smaller lamps. I'm thinking i could get away with 3 lights hitting the area in front of the stage, though. The more i think about it, the more i'm liking the idea.
 

JD

Well-Known Member
If you have long cable runs, your resistance increases meaning your actual current draw is above your 2800w lamp ratings, so the room for error is much smaller.

Not on resistive loads such as conventional fixtures. The longer the run, the greater the resistance, the lower the current draw.
Example: A 1000 watt lamp rated and running on 100 volts will draw 10 amps. The running resistance of the lamp is equal to 10 ohms.
Add a really long run of cable (lets say 2 ohms worth) and the resistance in the circuit is now 12 ohms.
A 12 ohm load on 100 volts is 8.33 amps.

That being said, switch mode ballasts will auto-compensate for a lower voltage by drawing more current. In that case, draw will increase due to length. Just not true with conventionals.
 

Les

Well-Known Member
I'm needing coverage. Lamping down might be an option, but i'll have to check to see if we have any smaller lamps. I'm thinking i could get away with 3 lights hitting the area in front of the stage, though. The more i think about it, the more i'm liking the idea.

Good to hear! Keep on keepin' on -- solving problems like this makes for good designers and master electricians!
 
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Les

Well-Known Member
Not on resistive loads such as conventional fixtures. The longer the run, the greater the resistance, the lower the current draw.
Example: A 1000 watt lamp rated and running on 100 volts will draw 10 amps. The running resistance of the lamp is equal to 10 ohms.
Add a really long run of cable (lets say 2 ohms worth) and the resistance in the circuit is now 12 ohms.
A 12 ohm load on 100 volts is 8.33 amps.

That being said, switch mode ballasts will auto-compensate for a lower voltage by drawing more current. In that case, draw will increase due to length. Just not true with conventionals.

Thanks for clearing that up, JD!

(For some reason, that post didn't have a thanks icon under it).
[size=-2]...it does now though...[/size]
 
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Clifford

Active Member
The SPI-2 modules in the Mark VII rack should all have a minimum of 40A SSRs. I want to say some are considerably more, but I can't tell off the top of my head. All I have to reference is one conversation with someone who knew a little more about the units. What I can tell you, from experience, is that the breakers are pretty quick on the SPI-2 modules. (Students daisychained three R40s, 3+1 instead of 2+2.) The dimmers are pretty robust, and they'll take a lot of abuse, but usually the breakers will kick in before anything approaches the damage threshold for the dimmer.

The exception is that about 8 years ago some of our breakers were changed out, and I heard the other local venue with a Mark VII rack ended up doing the same for some of their units as well. I don't know if this was a recommendation from someone local or something handed down from EDI, but I can't vouch for the units with the newer breakers. I would assume they're similar, if not comparable to what was originally installed, but I'm not going to go overload them on purpose to find out.
 
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derekleffew

Resident Curmudgeon
Senior Team
Premium Member
... if i keep the dimmer below 80% (This should leave 200 watts of overhead on the dimmer), ...

Running an overloaded dimmer at a lower intensity is not a reliable method of keeping it from tripping, as the called for percentage (ex. 50%) is not linear to the dimmer's output (ex. 1,000w @ 50% = NOT 500w).
Two separate and equal factors at work here. The dimmer curve is not in your favor, as Les said above. The circuit breaker's trip curve is on your side. It could take hours for a 20A breaker to trip when loaded with 2800W (23.3A @ 120V). Then again, it may trip in only a few minutes. Ambient temperature and incoming voltage are also factors, as is voltage drop in the circuit.

In any case, it's not a good idea to intentionally overload a circuit, and depend on dimmer profiling or control console limiting the output. (Also, I'm fairly certain SteveTerry has stated the practice is not permitted by the NEC, but I can't find the citation.)
 

STEVETERRY

Well-Known Member
Two separate and equal factors at work here. The dimmer curve is not in your favor, as Les said above. The circuit breaker's trip curve is on your side. It could take hours for a 20A breaker to trip when loaded with 2800W (23.3A @ 120V). Then again, it may trip in only a few minutes. Ambient temperature and incoming voltage are also factors, as is voltage drop in the circuit.

In any case, it's not a good idea to intentionally overload a circuit, and depend on dimmer profiling or control console limiting the output. (Also, I'm fairly certain SteveTerry has stated the practice is not permitted by the NEC, but I can't find the citation.)

Here is what I said on this subject in my article on feeder sizing:

"One final note on feeder sizing: It is not permissible to factor the dimming of loads into the derating equation. Thus, you cannot turn on 800 amps of connected load with a 400 amp feeder under the theory that “we’ll never bring anything above 50%”. This is absolutely verboten, since a dimmer at 50% does not draw half the current of the same dimmer at full; it is not a linear relationship between dimmer setting and RMS current. In addition, reduced levels create harmonics which play havoc with an undersized service. The load schedule must consider the full power of any loads that will be energized. However, it is permissible to apply a demand factor based on the assumption that some percentage of the fixture inventory will not be plugged in for a given production".

The same line of thinking applies to the dimmer branch circuit as to the main feed. This is a matter of good practice, not NEC rules. Intentionally overloading a dimmer is not a "good practice" solution to the problem. If you don't mind a tripped breaker in the middle of a performance, or a failed dimmer due to overheating--have at it. But if you want to implement a robust system that will be reliable, find another solution. And there are many--smaller lamp wattages, less fixtures, more dimmers, etc.

All too often, we adopt the "what can I get away with" solution, and then we're surprised when it bites us in the ass. I suggest that it's easier to start with the right solution.

ST
 
Thanks for all the replies :)

The end solution is looking like 4 600 watt lamps on that circuit. I did everything i know to try and trip the breaker (Bumping, leaving it at full, and then cutting off, letting the lamps cool, and bumping again) and it didn't flinch. Even with an extra 750 watt lamp on the circuit (Comes out to 28 amps with the voltage going into the dimmer) it didn't trip, but i would NEVER run it for any length of time like that.

The room the rack is in is pretty cold. We keep it between 55-65 during the summer, and the heat stays off in the winter, simply to help the rack stay cool.

The lights shouldn't be on for more than 20 minutes at a time. 20 for the opening, and then 20 for the closing to wash the floor in a green light. I was originally thinking i could get away with just 3 lights, but we ran into issues with the green gels on the lights. The christmas lights are going to be on a cheap dimmer pack the theatre teacher here has, (Like a 300 watt pack) and i think one of the lights i have hung MIGHT be a 450 watt lamp, which would give me some headroom on the breaker. Even with all 4 600 watt lamps, the breaker shouldn't trip, correct? (We also have the rack hard limited to 90% output for the sake of lamp life.)
 

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