help me understand three phase electricity
January 17, 2010 11:35 PM   Subscribe

So the place I work is about to grow and needs more power in the building for more people and a very large serverfarm upgrade. I am a bit unclear on a few topics, mainly related to three phase vs single phase power.

So, currently we have two power lines coming into the building, a three phase 200A line and a single phase 400A line that goes into four single phase 100A meters. We are looking to replace it all with a single three phase 800A line.

My main issue is understanding what sort of increase we would actually get from this. At one point we were looking at a 1200A line, and thinking that would double our effective power (600A to 1200A), but after briefly talking to a few electricians and electrical engineers, they think that 1200Amps is way too much and that we will be getting much more with 800Amps than we understood.

Question #1. What is the effective % increase from a 600A ( 200A three phase and 400A single phase), to a 800A three phase?

Question #2. So lets say we have a 200A three phase 120/208V line. And we have a server that takes 2A at 208V. How many servers can I power? 100 or 300? Or would it be somewhere in the middle? Obviously this is a simplistic example, not taking into account overhead or anything else, I'm just trying to understand. This is where I am mixed up, some people have said that you have 200A to work with, but then others say that since three phase has three legs, each leg carries 200A, so you effectively have 600A.

Thanks for the help!
posted by anonymous to Technology (7 answers total) 2 users marked this as a favorite
Since this decision will have long reaching and expensive consequences, I would pick up the phone, swallow your pride, and call back the electrical engineer familiar with the project and ask him exactly these questions. I wouldn't ask the electricians.
posted by benzenedream at 1:51 AM on January 18, 2010

You need to worry more about what you can cool than you need to worry about how much power you can feed in.
posted by iamabot at 1:51 AM on January 18, 2010 [1 favorite]

You need an electrical engineer who knows what your power requirements are and what the current / future feeds are. You need an electrical engineer to design the power system properly for you. Any answers you get on here are not good enough. This will cost you not very much compared to the trouble you can land in by improperly designing this. Call you local utility's engineering department, or look for a consulting engineering company who works with the utility, they will guide you in the right direction.
posted by defcom1 at 2:04 AM on January 18, 2010

Yes, you probably need someone more qualified than us.

A small amount of background. I don't know much, but here is what I know.

AC power is more like a waveform than power gushing through a pipe. A single phase 120v line consists of a neutral, which is zero. The "hot" is an alternating current at 120 volts in reference to this neutral. It alternates at 60 hz. So every second, the wave goes positive and then negative. Usually, at least in residential, your power comes in as two phases. On the second leg, the phase is reversed. It goes negative then positive. So in reference to neutral, its voltage is also 120. Because the phases are 180 degrees opposite of each other, if you measure the voltage of the two phases in reference to each other, you end up with 240v. This works out well for residential, because half the house can be powered off of one phase, and the other half from the other, and any 240v appliances can be powered from both.

If each leg can supply 100 amps, you have 200 amps of power available.

In three phase power, those phases are offset by 120 degrees. You can power 110v devices off of any one phase to neutral. If each phase is 100 amps, you have 300 amps of power.

Now, the question you need to ask is what's the voltage of each leg of the three phase. If each leg is 208v, you can power three 100 amp 208v devices off of that 300 amp service. Or one 300 amp device. I think.

Good luck.
posted by gjc at 4:25 AM on January 18, 2010

Power is voltage times current. So the amperage of your new switch gear is only half the story. That said, everyone else here is right, you should find a few good consultants. You need to go talk to a electrical engineer about what kind of power you will really need. You need to consider if you need a UPS, and possibly a back up generator. You also need to talk to a mechanical engineer to figure out how you plan to cool all of this stuff down.
posted by ihadapony at 4:40 AM on January 18, 2010

Best answer: While I agree that you need an electrical engineer to do the actual design work (and I am not one of those, to be clear), it still doesn't hurt to understand it at a high level.

If you are used to single-phase power, where a 1 amp load across the two supply wires results in a 1 amp current in each of those wires, you might expect a 3-phase supply to provide you 3 times the power. Now you can have three 1-amp loads connected between the three legs of the supply (A to B, B to C, C to A) and still have 1 amp drawn from each leg, right?


Notice that we have actually got one end of TWO loads connected to each leg of the 3-phase supply. For example, leg A has the "A to B" and "C to A" loads connected to it. And the currents drawn by those loads add up. It's a little tricky because we are adding up out-of-phase sine waves, but if you work through the math, you find out that the current in the supply legs will be 1.73x the current flowing through the three loads.

So, our three 1-amp loads result in 1.73 amps flowing in each of the supply legs. That means although we have 3 loads instead of 1, each can only draw 1/1.73 times the current, so we get 3/1.73 times the power from a three-phase supply that we could have gotten from a single-phase supply with the same current rating.

3/1.73 = 1.73 (it's just the square root of 3!). And this is how we get the rule of thumb, which you can find elsewhere on the web, that a 3-phase supply provides 1.73x the power of a single-phase supply at the same voltage and current.

So, armed with that conversion factor, let's see what your "single phase equivalent" is now, and what it would be after the proposed upgrade.

Now: 400A x 1.0 (single phase) + 200A x 1.73 (three-phase) = 746A "single phase equivalent"

Upgrade: 800A x 1.73 = 1384A "single phase equivalent"

Your proposed upgrade will give you 1.85x the available power you have now. So there's question #1.

Question #2, how many 2A 208V single-phase servers can you run from a 200A 208V three-phase supply? It's not 100 and it's not 300 either. You need to use that 1.73 conversion factor again.

You will be able to get 200/1.73 amps x 3 "circuits" out of this supply, or 115.6 amps x 3 circuits. Divide by 2 amps per server and you get 57.8 servers per circuit. Multiply that by 3 circuits, and you get about 173 servers (not surprising, there's that 1.73 again, just times 200 amps/2 amps per sever). So there's question #2.

Of course you would not load up the circuits to 100% of theoretical capacity like that, but that should give you a good idea what's going on with 3-phase power.

When you are connecting 3-phase loads they're usually specified in terms of the actual current drawn from the 3 legs of the supply, so no fancy math is needed to add those up. But you still only get 1.73x the available power, they're just measuring the current at the supply terminals rather than in the 3 internal circuits in the device.
posted by FishBike at 8:25 AM on January 18, 2010 [2 favorites]

Anonymous, the firm I'm with specializes in exactly this kind of thing. MeMail or e-mail me and I can explain more, but didn't want to sound like a shill, just thought maybe I could help.
posted by geoff. at 11:29 AM on January 18, 2010

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