Dimmer capacity

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.
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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.
 
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.
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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?
 
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.
 
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?
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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.
 
Clarkwg3 said:
When I stated the mandatory 80% derating I was thinking standard breaker, not full load breakers.
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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.
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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.
 
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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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
 
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.
 
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
 
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
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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.)
 
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.)
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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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