# Dimmer capacity



## SalvatoreDelorean (Apr 26, 2012)

I'm working in a college black box theater, and I have 24 dimmers to use to design an entire (reasonably large and complex) show. As you can imagine, it's pretty limiting and I'm hoping I can get creative with twofers. I have 2.4k Berkey Colortran dimmers and 1.8k circuits, and I'm working mainly with a bunch of Source Fours and 575w Fresnels. Is there any way to tell how many instruments I can twofer into a circuit before my dimmers burst into flames?
Thanks!


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## chausman (Apr 26, 2012)

What are the S4s lamped at?


Sent from my iPhone using Tapatalk


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## SalvatoreDelorean (Apr 26, 2012)

chausman said:


> What are the S4s lamped at?
> 
> 
> Sent from my iPhone using Tapatalk



575w/120v


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## Musicman56 (Apr 26, 2012)

It would appear that your circuit is going to be the limiter here. It can only hold 1800 watts. I've never personally seen a situation where a circuit can't hold as much as a dimmer, but it appears to be possible. Regardless, here's the math that you need to do to solve your problem. You have 1800 watts to work with and 575 watts per fixture (you can round to 600 for ease and as a threshold). 1800 divided by 600 is 3. So three 575 units is the max load you can put into your circuits, with 1725 watts being used. Now, I know a lot of electricians who use the 80% rule and only use 80% of their available wattage as a safety. Along that reasoning, you would use 2 units. The theory being that getting too close to the limit will trip the dimmers even though you don't use all of their potential. By that same reasoning, your 2.4k dimmers could run 4 lights technically and 3 by the 80% rule. But your circuits can't do 2.4k, so they are the limiting factor.

Hope that was helpful!


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## ptero (Apr 26, 2012)

Can you patch two circuits to a single dimmer? Then you could go to 4 lamps on a dimmer, by way of using two circuits plus twofers. We are tight on dimmers (who isn't?) and routinely load our 2.4k dimmers w/4x575w. These are old Strand CD80 2.4k 12-paks and they have a pair of 'outs' for each dimmer. It pushes beyond the 80% concept but has worked for years. Breakers don't trip and if it is shortening dimmer life that's over a long timeline. These paks have been at it since 1984 iirc. 

Good luck...


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## Musicman56 (Apr 26, 2012)

Ah, I was assuming a 1 to 1 hard patch, but if you can play with the hard patch then...


ptero said:


> Can you patch two circuits to a single dimmer? Then you could go to 4 lamps on a dimmer, by way of using two circuits plus twofers.


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## SalvatoreDelorean (Apr 26, 2012)

Musicman56 said:


> It would appear that your circuit is going to be the limiter here. It can only hold 1800 watts. I've never personally seen a situation where a circuit can't hold as much as a dimmer, but it appears to be possible. Regardless, here's the math that you need to do to solve your problem. You have 1800 watts to work with and 575 watts per fixture (you can round to 600 for ease and as a threshold). 1800 divided by 600 is 3. So three 575 units is the max load you can put into your circuits, with 1725 watts being used. Now, I know a lot of electricians who use the 80% rule and only use 80% of their available wattage as a safety. Along that reasoning, you would use 2 units. The theory being that getting too close to the limit will trip the dimmers even though you don't use all of their potential. By that same reasoning, your 2.4k dimmers could run 4 lights technically and 3 by the 80% rule. But your circuits can't do 2.4k, so they are the limiting factor.
> 
> Hope that was helpful!



Thank you! That's a huge help and that makes a lot of sense. If I overloaded the circuits but kept it under 2400 watts, would I just trip a breaker or would I damage the circuits/dimmers? And yeah, it's 24 circuits hard-patched into 24 dimmers.


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## Musicman56 (Apr 26, 2012)

You don't want to overload the circuits. That's bad news (that's how electrical fires are caused). If you overload them you'll screw up the cabling before it gets to the dimmers. You are sure that they are rated less than the dimmers though? Could the circuits be older than the dimmers? Anything under 2.4k isn't gonna hurt the dimmers though. In this configuration, you max is gonna be 1.8k no matter how powerful your dimmers are. 

Could you maybe see how others have managed to do it in the past? If you really end up needing more power than you think your space can safely provide you with, I would suggest looking into renting dimmer packs from your local shop.


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## ruinexplorer (Apr 27, 2012)

Are you positive that the lamps are 575w/120v? HPLs are often 575w/115v. This will alter the number of lamps on a circuit. Also, take into account that lamp filaments are a non-Ohmic load and will thus act differently depending on how warm the filaments are.


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## JD (Apr 27, 2012)

Just to be clear, what is the limitations on the circuits caused by? A 2.4kw dimmer will have a 20 amp breaker on it and the circuit should be 12 gauge wire (or better if there is cable de-rating involved.) 1.8kw implies a 15 amp limit. This can be due to one or more of the following:

Secondary 15 amp breakers.
Insufficient wire gauge for the application.
Connectors rated below 20 amps.
Total system de-rate based on available supply.

Depending on why there is a de-rate on the circuits, you may be able to get an electrician to find a work around.


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## hobbsies (Apr 27, 2012)

I'd imagine it's from using edison plugs.


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## SalvatoreDelorean (Apr 27, 2012)

Thanks, that makes sense. I figured overloading the circuits couldn't be a good thing. I have no idea why or how our circuits are rated less than the dimmers, it does seem incredibly stupid. It's a pretty sketchy converted space that has all sorts of weird quirks like that, and it seems to have been built by people who had no idea what they were doing. I'll track down the old Master Electrician and ask about the limiting factor of the circuits. They're twist-lock, not Edison, so I'd guess it's either insufficient wire gauge or lack of power supply. Like I said, it's a sketchy system that doesn't make a lot of sense.


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## TimMiller (Apr 27, 2012)

Then how do you know you are limited at 1.7k? If the connectors and wire are rated for 20a you should be ok. Next you have to find out how many amps you have to play with feeding the rack you may have to play with your cueing so that you do not trip the main. I worked at a venue that had an array of 2.4k 6k and 12k dimmers but all of the larger dimmers were split down into 2.4k circuits. Everything was hard patched. It was also possible to overload the mains so you had to keep in mind what your loads were and what was on each leg.


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## hobbsies (Apr 27, 2012)

I would just confirm you're using 12/3 circuits. If you look at the tiny writing on the cable, look for something that says 12/3 or 14/3. Should also say something like 600v soow. The EEs on this forum can tell you what it should say in completion.


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## Clarkwg3 (Apr 28, 2012)

This thread has my head spinning, and I deal with just about every issue discussed on a daily basis. I'm gonna try to clean-up the previous posts to help make sense of it. I realize I'm opening my work to constructive criticism.

2.4Kw Dimmer = 20Amp = 12AWG(American Wire Gauge) SO, SOO, SOOW(standard) cabling = 10AWG circuit wiring for full load at 20A outlet; if the circuit wiring is 12AWG then 80% de-rating MUST apply(It's National Electrical Code Requirement based on length and gauge).

I think this is what your situation has been discribed as:

2.4Kw Dimmers, 12AWG circuit wiring = 15A per circuit; hard patch spider board allows twofering multple Stage circuits to be supplied from a single dimmer. If correct; then you have two places to watch your wattage. First place is the wattage per stage circuit(1750w) The second is 2400w per dimmer at the Spider patch(make sure to use 12AWG twofers, 10AWG would be better here)

Has the squiggly line been straightened?


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## derekleffew (Apr 29, 2012)

Clarkwg3 said:


> ...2.4Kw Dimmer = 20Amp = 12AWG(American Wire Gauge) SO, SOO, SOOW(standard) cabling = 10AWG circuit wiring for full load at 20A outlet; if the circuit wiring is 12AWG then 80% de-rating MUST apply(It's National Electrical Code Requirement based on length and gauge). ...


Not exactly. Article 310 of the 2011 NEC specifies exact conductor ampacity, which is dependent on many factors, including: ambient temperature, temperature rating of conductors, number of current carrying conductors, pipe fill, etc. I don't see anything in that section regarding conductor length or voltage drop. Can you cite the "80% de-rating factor"?


Clarkwg3 said:


> ...(make sure to use 12AWG twofers, 10AWG would be better here)...


I don't think I've ever seen a #10 AWG tufer in my life. We discussed under-sized two-fers here: http://www.controlbooth.com/forums/lighting-electrics/22174-awg-ampacity-two-fers.html .


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## SalvatoreDelorean (Apr 29, 2012)

Clarkwg3 said:


> 2.4Kw Dimmers, 12AWG circuit wiring = 15A per circuit; hard patch spider board allows twofering multple Stage circuits to be supplied from a single dimmer. If correct; then you have two places to watch your wattage. First place is the wattage per stage circuit(1750w) The second is 2400w per dimmer at the Spider patch(make sure to use 12AWG twofers, 10AWG would be better here)
> 
> Has the squiggly line been straightened?



Do you mean I can bypass the stage circuits and patch directly into the dimmers and get more wattage out of them?


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## epimetheus (Apr 29, 2012)

Clarkwg3 said:


> 2.4Kw Dimmer = 20Amp = 12AWG(American Wire Gauge) SO, SOO, SOOW(standard) cabling = 10AWG circuit wiring for full load at 20A outlet; if the circuit wiring is 12AWG then 80% de-rating MUST apply(It's National Electrical Code Requirement based on length and gauge).
> 
> I think this is what your situation has been discribed as:
> 
> ...



No. Not sure where you're getting this 15A stuff or the mandatory 80% derating, as it's not in the NEC. A 2.4kW dimmer is good for outputting 2.4kW. Dimmer racks use 100% rated breakers, so the 80% derating applied of normal thermal breakers does not apply. 12AWG circuits are good for 20A in most situations, with the exception of the rating factors Derek mentioned.


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## Clarkwg3 (Apr 29, 2012)

When I stated the mandatory 80% derating I was thinking standard breaker, not full load breakers.

What is the derated load at the end of the 12AWG stage circuit(100 feet plug to dimmer)? Is not 2400watts. Next lowest device rating is 15A or 1750watt. I suspose one could use 20 amp plugs and label them for the maximum allowable wattage at that plug.


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## kicknargel (Apr 29, 2012)

Wow, this thread has some bad, some good, and some overly complicated and confusing information. I don't think we're all talking about the same things. Sometimes CB discussions digress into abstract code arguments rather than trying to solve the OP's questions. 

First, when you say "circuit," I take it to mean hard-wired permanent circuits from the dimmers through conduit, etc, to some kind of outlets or pigtail in the grid. Or are we talking about SOOW or other temporary cabling? Second, on what information are you basing the 15a circuit rating? Third, photos always help us stay on track.


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## SalvatoreDelorean (Apr 29, 2012)

kicknargel said:


> First, when you say "circuit," I take it to mean hard-wired permanent circuits from the dimmers through conduit, etc, to some kind of outlets or pigtail in the grid. Or are we talking about SOOW or other temporary cabling? Second, on what information are you basing the 15a circuit rating? Third, photos always help us stay on track.



That's exactly what I mean by circuits, and that information comes from the former technical director of the theater. I met her only briefly before she moved and I'm not able to contact her, so I'm not sure why they're 15 amps or what the limiting factor is. I'll see if I can take some photos when I'm in there later.


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## DuckJordan (Apr 29, 2012)

If you have a dimmer rated for 20 amps, Cable rated for 20 amps, and a connector rated for 20 amps, You don't have a 15 amp circuit, You may not be able to get the full 20 amps but its definitely not De-rated.


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## SalvatoreDelorean (Apr 29, 2012)

DuckJordan said:


> If you have a dimmer rated for 20 amps, Cable rated for 20 amps, and a connector rated for 20 amps, You don't have a 15 amp circuit, You may not be able to get the full 20 amps but its definitely not De-rated.



I realize this, but I didn't say that I have 20 amp cable. I was told the circuits were rated for 15 amps. The connectors are standard twistlock, rated at 20 amps so I'm assuming it's insufficient cable gauge that's restricting ampacity. Is there a way to test the ampacity of the circuits besides plugging in instruments until something explodes? Is that something I can do with a multi-meter?


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## DuckJordan (Apr 29, 2012)

or just look at the cabling, Most electricians I know wouldn't ever put a higher rated connector on a lower rated cable. The reason is, ITS AGAINST CODE! Now I'm not saying it couldn't of gotten tampered with, but in all likely hood the old TD was probably wrong.


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## 65535 (Apr 29, 2012)

SalvatoreDelorean said:


> I realize this, but I didn't say that I have 20 amp cable. I was told the circuits were rated for 15 amps. The connectors are standard twistlock, rated at 20 amps so I'm assuming it's insufficient cable gauge that's restricting ampacity. Is there a way to test the ampacity of the circuits besides plugging in instruments until something explodes? Is that something I can do with a multi-meter?



Your best bet is to lockout/tag-out the power and pull a receptacle and check to see what kind of cable is run, check a few different locations, it all should be the same but you never know.

Considering how big of a code violation running undersized cable on a circuit is I highly doubt it's truly limited to 15A, if it is you really need to get someone to put in for a swap to 15A breakers on your dimmers (not sure you could even get that done.) 

The alternative would be to estimate the cable run measure open circuit voltage and measure voltage of a known load say 1Kw. Calculate the voltage drop and resistance of the circuit and you should be able to roughly estimate what gauge wire is run.


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## TimMiller (Apr 30, 2012)

I think every dimmer install I have ever seen they use 12awg on 20amp circuits. Only time I have seen 10 awg used is when the wire is aluminum.


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## epimetheus (Apr 30, 2012)

Clarkwg3 said:


> When I stated the mandatory 80% derating I was thinking standard breaker, not full load breakers.



Most dimmers use 100% rated breakers.


Clarkwg3 said:


> What is the derated load at the end of the 12AWG stage circuit(100 feet plug to dimmer)? Is not 2400watts. Next lowest device rating is 15A or 1750watt. I suspose one could use 20 amp plugs and label them for the maximum allowable wattage at that plug.



Yes, it is 2400 watts, assuming that the circuit has been properly designed, i.e. all proper derating factors applied. NEC mandates that the branch circuit protection for a circuit prevent the circuit from be overloaded. Therefore, if the OP truly has a 15A circuit (for whatever reason), then there must be a 15A breaker somewhere on that circuit, otherwise the installation is in violation of the NEC.

Also, when you design a circuit you don't pick the wire gauge, then determine it's ampacity based on how it's installed. You select your desired ampacity, then determine what gauge wire you need to get that ampacity once all derating factors have been applied. The only exception might be if the installation wasn't done to code to begin with. Then you might bring it into code compliance by lowering the selected ampacity of each circuit, and installing the proper branch circuit protection based on this lowered ampacity.


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## JD (Apr 30, 2012)

If it is 2.4k dimmers, 20 amp breakers, 20 amp connectors, and 12awg wire, then there are only a couple of reasons there would be a system de-rate.

One would be the number of wires ran through a given conduit. (The more current carrying conductors in a given cable or conduit, the more the conductors get de-rated.) 

The other reason would be if the system was de-rated due to feed issues. This gets a little gray because what they would be doing is dividing the total feed by the number of circuits. In a theater setting, it is assumed that all the dimmers will not be fully loaded, and the demand is calculated by the expected load of the inventory you have. (Subjective and changeable.) In this scenario, the individual dimmers really are not de-rated, only the total load. It would be easy for some department director to misinterpret this and take that number and divide it by the number of dimmers and declare that "X" dimmers should not have more than "Y" watts on them. 

If indeed the feed is undersized for whatever reason, the bigger concern is phase balance so that your load does not fall primarily on one or two of the phase legs, with little load on the second or third. A quick and easy way of doing that would be to proclaim all of the dimmers at a lower value. As ST would remind us, a properly designed system can be run way out of balance with no ill effects. If it is a feed issue, then we can assume something changed since install and it is now a compromised system for some reason.


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## SalvatoreDelorean (Apr 30, 2012)

So it turns out I have 15 amp cable, 15 amp connectors (which seems weird because I thought all twist-lock connectors were rated at 20 amps, but 20 amp breakers. That seems super dangerous and sketchy, but like I said, this theater's pretty janky. Thanks for all the input, but I think my question was answered. My instruments are lamped at 575w, so I'll just limit it to 3 instruments on a circuit. I feel like I can do plenty with that. Thanks for all the help though! That really helped me understand the limitations and precautions of my system


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## avkid (Apr 30, 2012)

SalvatoreDelorean said:


> 15 amp connectors (which seems weird because I thought all twist-lock connectors were rated at 20 amps,


 L5-15 is rated at 15 amps, and many are rated higher.


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## derekleffew (Apr 30, 2012)

SalvatoreDelorean said:


> ... 15 amp connectors (which seems weird because I thought all twist-lock connectors were rated at 20 amps, ...


You thought incorrectly. See the chart at the wiki entry Twist-Lock.


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## 65535 (May 1, 2012)

Having a 20A breaker on the dimmers with 15A cable and receptacles is a pretty big issue. The 15A connectors don't de-rate the circuit without a breaker that matches them. They will happily flow 20A and risk overheating, same thing for the wire. The breakers need to be swapped.


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## SalvatoreDelorean (May 1, 2012)

Yeah, I've been poking around and some of the connectors are melted and fused together. It doesn't seem particularly safe, I think I'm going to have a chat with the facilities people about some repair/upgrade work


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## Chris15 (May 1, 2012)

SalvatoreDelorean said:


> So it turns out I have 15 amp cable



On what basis was this determined?


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## JD (May 1, 2012)

SalvatoreDelorean said:


> Yeah, I've been poking around and some of the connectors are melted and fused together. It doesn't seem particularly safe, I think I'm going to have a chat with the facilities people about some repair/upgrade work



Twist-lock connectors sometimes melt even at much lower loads. If the contacts in the female have lost tension, or there is enough oxide on the male pins, they can burn up on a single 575 watt load. 

Now, the other issue is more troubling. By 15 amp, do you mean that the wiring is 14 awg? If you are running #14 on 20 amp breakers, you have a serious violation. Changing the breakers alone may not get you up to code. Don't have my book handy, so ST may correct me, but my understanding is that public theater space requires a minimum #12. Your portable cables must also be #12 SO, but there are some exceptions. (Ex- SJ on two-fers, whip cables on fixtures, etc.)


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## 65535 (May 1, 2012)

JD said:


> Twist-lock connectors sometimes melt even at much lower loads. If the contacts in the female have lost tension, or there is enough oxide on the male pins, they can burn up on a single 575 watt load.
> 
> Now, the other issue is more troubling. By 15 amp, do you mean that the wiring is 14 awg? If you are running #14 on 20 amp breakers, you have a serious violation. Changing the breakers alone may not get you up to code. Don't have my book handy, so ST may correct me, but my understanding is that public theater space requires a minimum #12. Your portable cables must also be #12 SO, but there are some exceptions. (Ex- SJ on two-fers, whip cables on fixtures, etc.)



A shady memory and possibly questionable sources, but I believe the requirement is for service duty (S designation no Junior service) is required for theatrical applications involving temporary portable cabling, the gauge is subject to circuit capacity.


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## JD (May 1, 2012)

65535 said:


> A shady memory and possibly questionable sources, but I believe the requirement is for service duty (S designation no Junior service) is required for theatrical applications involving temporary portable cabling, the gauge is subject to circuit capacity.



SJ is allowed on two-fers and breakouts with a length limit. S (or SO) is required for the rest. ST = STEVETERRY.

See - http://www.controlbooth.com/wiki/Cable+Types+S+SO+SOOW+SJ


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