Three Phase

[kovacika]

I was looking for a basic explanation on three phase power, and some of the equations that go in to figure amperages. At my college we have 48 2.4k dimmers (etc sensor) in the small theater tied into a 100amp breaker (1 24 pack), and a 60 amp breaker(2 12 packs). By my count 48x 20 (960) amps is way more than 160 amps, so i was wondering how this worked.
Thanks

[soundman]

My knowlege of 3 phase is not what I would like it to be so take everything I post with a grain of salt.

I think that the 100 amp breaker is 100 amps per phase makeing it 300 amps total. This is still less than the required 480 amps needed by 24 20 amp circuts. The reason it works is because the dimmers are never all fully loaded and all on at the same time.

[SHARYNF]

The amperage side of power is pretty straight forward

In the US, you have single phase, 120 volts, that is typically limited to about 50 amps, using a twist lock, or 60 or 100 amps using a bates connector, RARELY is this level of amperage used in a dimmer at 120 volts, since the wire size is pretty large.

For most flexibility a lot of the dimmer packs have what is called single phase but is really split phase, this allows for connection to a electrical service at the residential or small business level. Here a 100 amp service is connected with a dual pole breaker, each leg has 100 amps, and you want to use a 4 wire connection. Usually this maxes out around 100 amps for each phase. If you have a split phase dimmer and you connect it to two phases of a three phase service you need to be aware that you only will have 208 volts, so the wattage will be lower.

A lot of the dimmers have the ability to use three phases, so that then each leg has 100 amps, giving you more capacity. As you get up to larger and larger dimmer systems, you wind up not supplying total amperage but instead doing a design based on typical use.

In most cases on a portable basis you will see 100 amp three phase, 200 amp three phase, and up from there 400 and 600. At these levels you looking at huge cables so it is less common for a temporary wired in situation.

Sharyn

[SteveB]

Originally Posted by kovacika

I was looking for a basic explanation on three phase power, and some of the equations that go in to figure amperages. At my college we have 48 2.4k dimmers (etc sensor) in the small theater tied into a 100amp breaker (1 24 pack), and a 60 amp breaker(2 12 packs). By my count 48x 20 (960) amps is way more than 160 amps, so i was wondering how this worked.
Thanks

Instead of bringing in the power on one huge wire, the utility company splits it across 3 wires (as stated by Sharyn, some smaller systems use 2 phase, or single phase w/ 2 wires) . Thus your 100 amp electrical panel probably has 3 wires at 100 amps EACH - or 300 total amps. Ditto the 60 amp panel - 3 wires = 180 amps, for a total of 480 amps.

The electrical codes do not require you to provide power as connected load - which is a sum of the capacity of the breakers times the total number of breakers. Given that theatrical dimmers are usually not running all at full at the same time, nor are they all loaded at max. capacity, you can provide Demand power, which is whatever the user figures to be a reasonable capacity given the number and wattages of the loads.

FWIW, Connected load for the 48x2.4kw's is 960 amps divided by 3 feeders = 320 amps per phase. You have 160 amps per phase which most would consider too small a feeder as it essentially downrates your dimmers to 50% capacity. ETC sends out their 96 racks with a 600 amp breaker, which is roughly 2/3 connected load and which is typical demand power.

SB

[Chris15]

Hopefully this will fill in a gap or two. The reason behind three phase is not just to spread the load between three current carrying conductors. If that was the sole purpose, then they would all be in phase with each other. Power is generated in three phases, each 120 degrees out of phase with each other. I am not sure why, but engineers etc. have determined this is the most efficient means of generating power. When the load on all three phases is equal, then the neutral current is zero. The neutral rests at the centre point of the triangle between the three phases. This means that there is one less wire that needs to be run when power is distributed. (This being based on Australian 3 phase, which I believe is star configuration).

[SteveB]

Originally Posted by Chris15

Hopefully this will fill in a gap or two. The reason behind three phase is not just to spread the load between three current carrying conductors. If that was the sole purpose, then they would all be in phase with each other. .

Well, no, the reason IS to reduce wire size. It is much more difficult and expensive to increase wire size to carry larger loads.
The METHOD used is to tap a generator at 3 locations along the winding so as to get maximum efficiency out of a single generator. If you wanted to combine 3 generators to produce current all in phase, you could, it's just very wasteful.
Note that you can get single and dual phase generators. Think about units designed for the construction industry or for home applications, which have a single, or 2 taps off the windings These are smaller capacity units as compared to the generation systems used by utility companies, or by portable power generation systems as used by location movies, etc..
Note also that along with the efficiency of better utilization of the generator, comes the advantage of powering 2&3 phase motors, which are essentially generators in reverse.
FWIW, Wikipedia has decent articles about power generation and distribution
SB

[SHARYNF]

I think you will see it came down to economics, where copper wire was definitely an issue, and also motor efficiencies, A three phase motor can produce more horse power on a total lower current draw. Since the motor is rotational the three phases allow for power to be delivered to the motor in three pulses (sort of this is a simplification) so it is like turning a wheel once, twice or three times per rotation, the three phases reduces the time when the rotational force is not being applied to the motor

Here is a link which better details it
http://www.woodweb.com/knowledge_bas...ase_power.html

Sharyn

[ship]

ESTA about six months ago did a proposed standards paper on something like "Proposed draft regulation on single conductor feeder wire." Not sure of the exact title. Very good read, studied it in depth and was writing up a ... urr we have problems.. paper for the boss based upon what was proposed based on very sound reasoning, verses how we do it.

If you have access to ESTA proposals, it's a good read and goes into a bit of study into the double neutral concept and other stuff sufficient in detail, but not written on the doctoral level.

ah' here it is:
Draft BSR E1.18
Standard for the Selection, Installation, and Use of Single-Conductor Portable Power Feeder Cable Systems for Use at Less than 601 Volts Nominal for the Distribution of Electrical Energy in the Entertainment and Live-Event Industries
Ep/2002-7003r10.2
For 2006 ESTA Spring (USITT) Meeting

© 2006 Accredited Standards Committe E1, Safety and Compatibility of Entertainment Technical Equipment and Practices, and its secretariat, the Entertainment Services and Technology Association. All Rights Reserved.

[kovacika]

[I knew we were a little underpowered (all the lights at full=no lights+tripped breakers). So if im correct this is whats happening
>>>>>>>>>>>>--------100amps------
100 amp breaker--------100amps------ dimmer receives 300 total
>>>>>>>>>>>>--------100amps------

Would this mean if I opened up the panel the breaker would cross three buses/phases (buses being the metal rods running verticly in the box, attached to the power from the pole), or does the breaker just split one bus/phase into three? maybe like a three pole breaker? (Im trying to compare to what basic house boxes ive seen). The reason i ask is because the 100 amp breaker takes up 3 spaces on the left side of the breaker panel. Its also a box maybe 15, 20 years old.

[Chris15]

Steve, I did say that cutting wire size was not the ONLY reason. The efficiencies of poly phase motors noted by Sharyn show why 3 phase is better than 3 independent phases. Though thinking about it, you are more correct than I initially gave you credit for. Wire size and economics are the principal cause.

Kovacika, a three phase breaker is just a 3 pole breaker, with those poles joined so that if one phase trips, they all switch off. If you look inside, there are a couple of options. Many a switchboard will have a three way central busbar that goes ABCABCABC so that breaker 1 is phase A, breaker 2 phase B etc.I suppose it could also be such that there were a wire to a busbar of each phase, but I am not aware of this.