Custom floor pockets power- theatre stage

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ODonnell

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Branson, MO, USA
I have a project for our theatre in Branson, MO. We are installing two new floor pockets on the stage which will require a connection through a fire-rated enclosed box to the actual electrical floor box which houses a 38-pin connector, 2-19 pin connectors for multicables for scenery, several other individual stagepin and twist lock connectors, all for dimmers and 3 or 4 20A circuits for constant power, PLUS, my problem, a 30A 3-phase, 5-wire, 208V(PPPNG), used for power for motorized scenery.
Most of these are no problem. the multicables cover the ratings needed for connections and the others are served by SO cable, all from the patch panel (which has the conduit runs from the dimmer patch bay and the breaker panel) to the actual electrical floor box (just under 3 feet long, 18 inches or so deep), in order to be able to easily swing the floor box all the way out of the stage to work on it. My issue with the 30A 3-phase is the limitation of the SO cord, when derated due to the 4 current carrying conductors, leads me to believe in order to correctly connect between the patch panel and the floor box, which all must be flexible, but cannot really be flexible conduit as the entire floor box actually swings completely over for service, and the space is very tight, i will need to go up to 6 gauge SO. I have this covered, but not in 5 conductor, only 4 conductor. We were thinking of running the ground parallel with the 4-conductor 6gauge SO. This way we have the ground wire with the cable, just not in the cable. The total distance this has to run would be 6 to 8 feet from the patch panel, clamped at the exit, through a small (6 inch) pipe, then clamped again at the electrical floor box entrance. That ground would be coming off a ground buss where all the grounds for each pipe feeding the patch panel land, including that for the 30-3phase.
Does anyone see any issue with this? Does anyone see any (cheap) alternative to needing 6-gauge SO to cover the 30-amp 5wire connection?
I wish there was more out there covering theatres. While we do have a small section and a nice load diversity rule we can follow for dimmers, there are always these special types of configurations for theatre that do not really have any direct answers. And the derating of SO cable is really, really harsh...
 
ODonnell said:
I have a project for our theatre in Branson, MO. We are installing two new floor pockets on the stage which will require a connection through a fire-rated enclosed box to the actual electrical floor box which houses a 38-pin connector, 2-19 pin connectors for multicables for scenery, several other individual stagepin and twist lock connectors, all for dimmers and 3 or 4 20A circuits for constant power, PLUS, my problem, a 30A 3-phase, 5-wire, 208V(PPPNG), used for power for motorized scenery.
Most of these are no problem. the multicables cover the ratings needed for connections and the others are served by SO cable, all from the patch panel (which has the conduit runs from the dimmer patch bay and the breaker panel) to the actual electrical floor box (just under 3 feet long, 18 inches or so deep), in order to be able to easily swing the floor box all the way out of the stage to work on it. My issue with the 30A 3-phase is the limitation of the SO cord, when derated due to the 4 current carrying conductors, leads me to believe in order to correctly connect between the patch panel and the floor box, which all must be flexible, but cannot really be flexible conduit as the entire floor box actually swings completely over for service, and the space is very tight, i will need to go up to 6 gauge SO. I have this covered, but not in 5 conductor, only 4 conductor. We were thinking of running the ground parallel with the 4-conductor 6gauge SO. This way we have the ground wire with the cable, just not in the cable. The total distance this has to run would be 6 to 8 feet from the patch panel, clamped at the exit, through a small (6 inch) pipe, then clamped again at the electrical floor box entrance. That ground would be coming off a ground buss where all the grounds for each pipe feeding the patch panel land, including that for the 30-3phase.
Does anyone see any issue with this? Does anyone see any (cheap) alternative to needing 6-gauge SO to cover the 30-amp 5wire connection?
I wish there was more out there covering theatres. While we do have a small section and a nice load diversity rule we can follow for dimmers, there are always these special types of configurations for theatre that do not really have any direct answers. And the derating of SO cable is really, really harsh...
Click to expand...
Forum member SceneryDriver may have something to contribute, or if he does not jump in you may want to try PMing him.
 
Is it only running motors or do you envision a portable dimmer pack plugged into it? The neutral isn't counted as current-carrying in many three-phase circuits.
/mike
 
n1ist said:
Is it only running motors or do you envision a portable dimmer pack plugged into it? The neutral isn't counted as current-carrying in many three-phase circuits
Click to expand...

My thoughts, too. Does the actual connector have a neutral? If not, then don't run one :). Otherwise, it's pretty unlikely that the neutral would count as a current-carrying conductor anyway. I think you're OK with 10/5 or 10/4.
 
Diversity of loads comes into play with your 3ph receptacle, or at least I would argue it does. If this is for operating automated scenery or possibly chain motors, you'll need something approaching your full rated current (and probably much less) for very short times, usually less than 60 sec. I can't imagine even 10ga cable heating up appreciably during that time. If you have purely line-line loads, there are three effectively current-carrying conductors. There will be no current on the neutral if you're running 3-phase scenery automation motors, or chain motors.


If there's a chance a dimmer pack could be connected at that location, you're still ok:
1) if your three phase dimmer loads are evenly balanced, there will be no appreciable neutral current; thus, only three effectively current-carrying conductors.
2) worst case, if you were to load only one phase to max amperage, you'd still have only two effectively current-carrying conductors - the loaded phase and the neutral.

Upsizing to 8ga cable is arguably prudent, if for no other reason than physical durability. 6ga ridiculous! Good luck landing the 6ga wire on the terminals of the receptacle.

You fundamentally can't overload the cable; the breaker will protect the circuit. I'd appeal this; common sense should prevail.

You could always install a 15A breaker, and swap it later ;)

SceneryDriver
 
If the dimmer packs use phase controlled switching and do not include PF correction, there may be significant neutral current at third SSA and higher triplen harmonics. These components will add in the neutral without cancelation.


Sent from my XT1080 using Tapatalk
 
n1ist said:
Is it only running motors or do you envision a portable dimmer pack plugged into it? The neutral isn't counted as current-carrying in many three-phase circuits.
/mike
Click to expand...

In the past it has served as a small dimmer panel load and/or power distribution for several things on sets. I would expect that at any time there might be a load on the neutral.
 
GoldDigger said:
If the dimmer packs use phase controlled switching and do not include PF correction, there may be significant neutral current at third SSA and higher triplen harmonics. These components will add in the neutral without cancellation.
Click to expand...

That was true in the previous generation of dimmer racks, where back-to-back SCR's were used for phase control. Modern dimmer racks use IGBT's to switch the AC cycle OFF where desired. This results in MUCH lower harmonic content on the neutral. It's much quieter electrically too; the Noise Boys never did like it when the Lighting Department injected loads of 60Hz hum into nearby mic lines :)


SceneryDriver
 
SceneryDriver said:
Upsizing to 8ga cable is arguably prudent, if for no other reason than physical durability. 6ga ridiculous! Good luck landing the 6ga wire on the terminals of the receptacle.

SceneryDriver
Click to expand...

This is so true! I just checked the male connector for the 30A 3-ph 5-wire twist lock that would need to mate to the female in the floor pocket. It has a wire range of 14-8.
This all brings me back to the reading of the tables that deal with SO type cable and the derating of SO with 4-6 current carrying conductors. As I read it, I would need to provide a 6-gauge 5-wire (4 current carrying conductors, Table 400.5 (A) (1) Column A, 45 amps* .8= 36, which is the smallest to cover the 30A breaker for this receptacle.
If anyone sees a way around this I would be grateful. I am also thinking a call to Hubbell to ask them.
 
ODonnell said:
In the past it has served as a small dimmer panel load and/or power distribution for several things on sets. I would expect that at any time there might be a load on the neutral.
Click to expand...

But the main purpose seems to continue to be planned to be for 3-phase motors on the set, perhaps with added 120V loads in various shapes and sizes.
 
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