The never ending topic of power distro

[beltsvillecrucib]

Hey all,

I've searched the forums and read a good bit, but have a few questions that I need answered before I get a full grip on three phase wye and how its distributed.

For example's sake lets say we have 400 amp 3 phase, that is 400 amps per phase. My Cam tails are all hooked in and feeder is run to a moving light power distro. Now from my understanding between hots the voltage is 208V. So we can pull 208V power from Hots A>B and B>C and C>A? Also, how many amps of power can we pull? If we had a big enough power distro could we essentially draw the full 1200 amps?

Example two: Same power ratings and all except this time lets say we have a 200 amp distro ( so thats pulling 600 amps, 200 amps per phase) after the distro we run feeder to our dimmer rack. Am i correct in the assumption that the dimmers will have 600 amps (200 amps per phase) before anything goes haywire?

Thanks all,

Max

edited for grammah

[ship]

Hard to answer the question in not answering by way of liability in saying yes or no.

Yes to the second paragraph though one does not combine into 1,200 Amps when speaking of three phase power. It's what our minds calculate but not what one speaks of professionally because it is not.

Third paragraph, going haywire... if one's load is balanced between phases than your rating should be safe. This given no CamLoc type plugs and a multitude of other things were not bad on the way to your power supply.

What's the voltage and what's the metered load? Always check before and during. If possible even at the dimmer/distro end and supply end. Lots of other factors, say your distro is 300' away from the supply, what's the voltalge drop? Voltage drop in a VxA=W way, and given W is a constant, should it drop it drops voltage and rises amperage signifigantly in a melting of cable type of way the circuit breakers might not notice.

Lots of factors in answering your question properly. This beyond training, eductation, books and experience. If not sure, don't.

Also there is the 80% factor. If over 80% of capacity you had at best be really careful.

[Chris15]

Also agreeing that a yes / no answer would leave us all a little too liable for my liking.

If your loads were phase to neutral, then you could just add all the current draws up, but bearing in mind that your 400A / ph or whatever is breakered at 400A / ph not 1200A total. Not sure how one factors the current rating when drawing between phases...

Also bear in mind that you made some mention of movers. Movers = discharge lamps. Discharge lamps = inrush currents. So you couldn't draw a constant 400A / ph because your inrush currents would kill the breakers.

Bear in mind those things ship mentioned as well, feeder voltage drop and such. But I am going to have to disagree with you ship. VxA does not equal a constant W, because V = IR so P = V2/R or I2R. R will stay a constant, so the filaments will drop less current when running at a lower voltage. They'll also have reduced output, and all the other stuff like longer life and lowered colour temp because of it.

Regardless, you'll have the potential to be contending with substantial power losses as heat in your feeder cables...

[soundlight]

Also, keep in mind that some movers use 208V (two out of three legs) power.

[Chris15]

Originally Posted by soundlight

Also, keep in mind that some movers use 208V (two out of three legs) power.

Which is where the comments in the original post about active to active come in. Still doesn't mean I know how to add up their current draw...


Just jump on a plane and come across to 230V +10% -6% land...

[beltsvillecrucib]

Alright thanks guys. Don't worry, I won't actually be touching any three phase any time soon. I just wanted to make sure my general understanding is right on. So that a 400 amp three phase system, with each phase fully and properly loaded, in a completely ideal world, has the capacity to carry 1200 amps TOTAL (400 amps per phase).

I think I got it. Oh and with voltage drops, I had enough fun with that in physics this year; I figure I'll give my mind the summer off before I start having fun with them again!

[SteveB]

Couple of thoughts and a partial reply to the other comments.

One of the reasons for 3 phase power is to distribute the loads across 3 separate wires, as opposed to having to provide one huge (in this case - 1200 amp) wire, or sets of wires. The beauty of A.C power distribution is you can do this, as opposed to DC, much to Thomas Alvin Edison's consternation. Here's a decent Wiki explanation = http://en.wikipedia.org/wiki/3_phase_power
So in reality, on a 3 phase power distro @ 400 amps per phase, you really do have 1200 amps at your disposal.
Now what you do with it, and how you use it is a concern for the applicable codes as well as common sense.
Thus others have mentioned the 80 percent power loading factor - which essentially means you should play it safe and not expect the 400 amp, 3 phase service to provide 1200 amps of continues power, 24/7. This leads to overheating and breakers/fuses blowing. Loading to 80% gives you headroom in the event it's a TV/Film shoot, as example, with everything sitting full on for 17 hrs. on a hot day. If memory serves, I don't think the applicable US Nat'l Code for theatrical dimming systems requires an 80% power factor, as such systems (theater dimming systems) have loading that is considered temporary, as well as non-permanent in terms of wiring.

You should also factor in in-rush currents of devices such as arc lamps (Moving Light lamps) as well as motors starting up, that momentarily can exceed their normal usage rating. Note however that most circuit breakers and fuses are designed to sense these short overloads and to not trip right away. Again, common sense dictates that when loading a power distro with ML's, design in some headroom.

Steve B.

[JD]

One last little thing to confuse you... Power factoring! Although more of a problem with motors then lights, it comes into play whenever you are dealing with a non-resistive load. Basically, with a resistive load, the highest current is drawn at the peak of the waveform. This would be 100% factor. With any reactive load (Ballast) the highest current is not drawn at the same time as the highest voltage. A poorly ballasted lamp may have a PF of 50%. Here is a crude example: 100 watts at 100 volts will be drawing 1 amp if it is a 100%PF. The same 100 watt lamp (or fixture with a ballast) would draw 2 amps at 50% PF. Just to really drive you nuts, the lamp is still drawing 100 watts, it's just there is a phase lag between peak volts and peak amps! Most ballasts contain capacitors that counter the lag and increase the PF to better than 90%. Electronic ballasts are not 100% PF either due to the AC-DC converter that front loads the waveform. (They are close ~ 94%) So, what does this all mean? Not much to us in the stage lighting business, but for architectural lighting it becomes a big factor! The TEN one-hundred HPS fixtures may draw 20amps, not 10, even though they are only using 1000 watts! This is not to be confused with inrush, or with the extra wattage the ballast uses up, this is all about phase lag. Bottom line is that things run hotter and draw more current when the PF is less than 100%. There are now rules in the US about minimum PF on indoor installed lighting. All ballasts should list the PF on them. However, with much equipment coming in from China and other such places, and theater lighting not considered "installed", you may want to read the PF ratings on your ballasts! By the way, listings are sometimes done as single digits, 1 = 100% etc. http://en.wikipedia.org/wiki/Power_factor_correction

[Chris15]

Most architectural work these days down here will spec minimum 95% PF...

But the crux of all this is that if you are drawing power through an ohmic resistive load, then yes, you could pull up to 1200 amps from a 3 phase 400A service. Ergo you could get 288kW assuming ideal conditions, 240v supply and no voltage drop anywhere... like that'll ever happen...