3-phase power question WRT theater lighting

So long-story-short preface: I got a(nother) new job, and today I saw something that I have a question about.

It was theater lighting, and the individual wires for the lights had black, white, and green sheathing. The cable sheathing itself was labeled “12/3.”

To me, 12/3 usually means 12/3 WG, black/red/white/green. I would have called it 12/2 with ground. So my curiosity asked the guy I was working with (not an electrician, but a lighting designer) and he said well, it might be different because it’s 3-phase. 3-phase, said I? Why is it 3-phase? But I guess what I saw makes sense, because in that case there would be 3 live conductors, but then I was thrown off again because one was green, which is as far as I know generally understood to be a ground.

I can possibly have my next questions answered tomorrow if I can track down the right person for probably way longer than they have to spare, but I know we have both theater people and electrical experts here, so:

1- If it actually is 3-phase power, why do lights (power-hungry, large, and many; granted, but in a small auditorium) need 3-phase power?

1.5- If it is 3-phase, wouldn’t the 3rd conductor be colored something beside green? Red, for example? Or is that standard?

2- Could it be single phase power pulled off of a 3 phase supply?

3- Why do theaters use 3-phase power? I have a basic understanding, so I think that maybe it is just better for a power-hungry lighting setup. I was working in a high school theater, though, so I wouldn’t think it would be that much of an issue. My theory based on that is that the fixtures themselves are standardized to be used in much larger venues where 3-phase would be necessary; so even smaller spaces install 3-phase in order to use the same fixtures since they are more readily available.

Just downloading some thoughts on notes I took today, now pardon me while I go Google some more shit :smiley:


Is it just three insulated wires?

If it’s just three insulated wires (and nothing else), I don’t see how it could be three phase. Because (at a minimum) three phase requires four wires: phase A, phase B, phase C, and safety ground.

If it’s just three wires, then maybe it’s 240 V or 120 V with a safety ground.

One way or another, a power cable must have a safety ground.

Flexible service cords count the ground, fixed cables do not. If the cord is labeled something like SOOW 12/3 it will have W/B/G, all individually insulated and supplying one phase, neutral, and ground. Lights have no use for three phase, so you can supply three separate circuits off of one 3 phase drop.

Three phase allows for less wire - 4 wires gives you three circuits - and theatres tend to have a lot of motor loads, which are more efficient on three phase.

Thought you might show up, so thanks :slight_smile:

Yeah, just 3 insulated wires. It’s not 240, because the plugs/recepts are just normal 120v 20A.

Ah! Yup, that’s it. I have no experience with flexible service. I will check for the SOOW tomorrow, but I’m pretty sure you nailed it there. That makes so much sense, thank you!

It will almost definitely start with S, OO is oil resistant, W is water resistant, and J is “junior” which is not as heavy duty.

@FinsToTheLeft gave the correct answer: flexible cordage counts the ground in its numbering scheme.

So it looks like it’s regular ol’ 120 VAC single phase, with a hot, neutral, and ground.

Thanks again; that explains a lot. Once again, the Dope has come through!

At one time you could purchase 12/3 or 12/2WG. The 12/3 all wires have insulation around the copper wire. 12/2 WG has the black and white insulated and ground is a bare copper wire. Green or bare copper wire is always ground.

Three phase wire will have 4 wires. Black, red, blue and green.

  1. I do not think they make 3 phase lights. The light would need three elements.
  2. YOu do get single phase power from a 3 phase power supply. In three phase power there are 3 hot leads, a neutral and a ground. The voltage from any hot lead to neutral will be 120 volts. The voltage between any teo hot leads will be 108 volts.
  3. 3 phase power is used because it is simple. Single phase transformers, and power panels have to be larger along with the wires going to the power panel have to be larger.

I would think all lights in a large theater and a small theater will be single phase 120 volt lights.

It’s not uncommon for a theater to have a 3 phase supply coming into the building. 3 phase is common in all sorts of buildings, especially apartment buildings and business parks. For large buildings and large loads, the power company doesn’t want you loading only a single phase. They would much rather have you balance out the load between all 3 phases.

Just because the building is 3 phase doesn’t mean that 3 phase is wired all the way out to every light. Each lighting circuit will be single phase. You just need to balance the lighting circuits so that overall the loading on each phase is roughly equal.

To expand on this a bit, some years back I tore down a lighting control unit to clean it out (the group I belonged to was rehabbing a theater that hadn’t been used for a while). The unit had 24 dimmers and was fed by 480v three-phase, broken down as follows:

  • Dimmers 1. 4, 7, 10, 13, 16, 19 and 22 were attached to leg A;
  • Dimmers 2, 5, 8, 11, 14, 17, 20 and 23 were attached to leg B;
  • Dimmers 3, 6, 9, 12, 15, 18, 21 and 24 were attached to leg C.

I suppose it would have been simpler to attach 1-8 to A, &c, but the goal was probably to achieve something resembling balance among the three legs. Anyway, the output from each dimmer was (obviously) 120v single phase.

I want to go into business making 3 phase incandescent light bulbs. There’d be 3 filaments mounted close together. I guess I’d wire them delta, to save a fourth connect through the envelope. The benefit would be that flicker would be reduced because only one filament would pass through zero power at any given moment, and the individual filament flickers would be added 60 degrees out of phase (because they flicker at 120 Hz).

Who’s in???

Nitpick: if it’s a wye configuration, then yea… it’s very common to get single phase 120 VAC by connecting between any one of the three legs and neutral. Getting a ground-referenced single phase from delta is a bit more rare. You can get 120 VAC with a delta “high leg,” and you can get 240 VAC using a corner ground. But as mentioned, it’s much more common with a wye.

That’s a sure-fire way to get rich. You will be able to buy an island with the money you make from that patent. :sweat_smile:

Sure was SOW. Only one “O”, but yup.

Designers never want to admit that they don’t know. So they’ll make up something that they think sounds plausible.

But this guy was completely wrong.
Theatre lights are all single-phase, 108 or 120 volts nominal (less actually, since they are run on dimmers most of the time). About the only theatre lights that might be more are large follow spots, but they are usually at most 240V single-phase. And not likely to be used in a high school theatre.

Also, most theatre lighting is not incandescent any more. Maybe old equipment like might be in an old high school theatre. But newly-purchased equipment is nearly all LED nowadays. Uses much less power, produces less waste heat, lasts longer, and some can even change the color output.

Also, power is not usually a problem in a theatre – they were all designed for banks of incandescent lights, and most are way over wired for modern lights. Possibly the building & theatre power panels might be 3-phase, but it will be broken down into single-phase 120/240 volt power for individual wiring circuits.

Again, another spot-on (if you will excuse the pun) reply.

The fixtures we are installing to replace the old are indeed LED. One we looked at pulls 180W. The incandescent ones (or similar) they are replacing pull 700W.

I believe it was mentioned above, but the power coming in to the theater power system is 3-phase, and everything downstream of that it regular old 120. I’m thinking OttoDaFe’s description is accurate. There is a whole bunch of large power stuff just to the right of the dimmer cabinet, which must be (I assume) what is used to separate out the legs for the dimmers.

In my guy’s defense, he wasn’t trying to have the answer; just spitballing based upon something the PM had told him :slight_smile:

You can say it’s on par with the best.

Ugh, the puns are going to flood in…

I’m just glad that I got that, TBH.

Just as long as they don’t leko out.
Yeah, I was involved in theater lighting for a long time.