While we're talkin' 3?

[George Stolz]

Question: I have been taught that when calculating the service for a structure served with 3?, you must take the nominal voltage and multiply it by the square root of 3.

For example, a calculated load of 450,000 VA in a 277/480 3? system, would look like this:

450,000 / (480 x 1.73) = 541 amps

In light of the other thread, it seems to me that the magic mysteries of 3? would only take place for genuine 3? loads. Line-to-neutral, or two-pole line-to-line loads shouldn't benefit from the "x 1.73" voltage correction, should they?

A load calc for a 3? system should be an addition of single-phase loads and three-phase loads, calculated separately, it seems to me.

But I'm probably missing something.

[ September 24, 2005, 11:14 PM: Message edited by: georgestolz ]

 

[bob]

Re: While we're talkin' 3?

George
Your thinking is not out of line. What happens is that the single phase load is "assumed" to be balanced across a 3 phases and the magic of
1.73 takes affect. Example if you have 30 kw of single phase lighting and you connect 10 kw from phase to neutral, its the same as having a 30 kw 3 phase load.It is difficult to balance the load equally. However when all load is considered
together you will likely have phases balanced
close enough.

 

[rattus]

Re: While we're talkin' 3?

George,

Another way of looking at this:

First note that

Iphase = Iline
Vphase = 277V

Papp = 3*Vphase*Iline

Iline = Papp/3Vphase = 450KVA/3*277 = 541.5A

 

[George Stolz]

Re: While we're talkin' 3?

I'm not clear.

In my mind, what's good for a circuit, is good for a feeder, is good for a service.

Can I assume that I can downsize the conductors in a circuit because it happens to be three-phase? Can I naturally assume that 208V is actually good for 360V, so a 60W load rated for 208 (bear with me) will actually draw half the amps I expect?

 

[iwire]

Re: While we're talkin' 3?

George, the engineered prints we get have individual panel schedules for each panel in the building.

The panel schedules have the the entire panel planed out by the engineer as to where each branch circuit will go on the panel. Also included will be the VA for each pole of the panel. At the top will be the total VA for each of the 3 phases and neutral.

By doing this the engineers can see how to place the single phase loads to balance the panel.

Now they can add up all these loads as they pass through distribution panels and finally to the service.

What I am saying is I think your right, if you need dead on accuracy you need to start right at the beginning and add it all up. No magic, just a lot of calculations.

However if you assume a fairly balanced loading you can 'wing it' and come out OK.

The more panels you have connected to the service the more likely it will be balanced. In most buildings you would have to go out of you way to make the service not balanced. You would have to do something off the wall like not using phase C for any single phase loads.

One of our customers has 1600 amp 480 services in it's many locations, typical loading is about 700 - 800 amps per phase and only 50 to 100 amps on the neutral. The 100s of line to neutral circuits and dozens of single phase line to line circuits end up balancing each other out.

 

[charlie b]

Re: While we're talkin' 3?

Originally posted by georgestolz: Can I assume that I can downsize the conductors in a circuit because it happens to be three-phase? Can I naturally assume that 208V is actually good for 360V, so a 60W load rated for 208 (bear with me) will actually draw half the amps I expect?

I think you are looking at this backwards, and in that respect you are in good company. It is a common way to get confused. In all discussions of electrical quantities, the one thing that is common to all discussions is power. Power is power. On the other hand, current is not current, and voltage is not voltage. In three-phase systems, you have ?current at a phase angle of zero degrees,? and you have ?current at a phase angle of 120 degrees,? and you have ?current at a phase angle of 240 degrees,? and that makes all the difference in the world.

You are not ?downsizing? conductors because it?s three phase. You are sizing the conductors on the basis of the current that the circuit will draw. Here?s an example: Consider two separate 208 volt, 5000 VA loads.

The first load is single phase, and has a current of 24 amps. Those 24 amps are flowing from the source (one phase) along one wire, through the load, and back to the source (a different phase) along another wire. In each of the two wires, you would measure a current of 24 amps. You pick a #10 for this load.

The second load is three-phase, and has a current of 14 amps. Those 14 amps are flowing to and from the load on three separate wires, and you can measure 14 amps on any of the three. You pick a #12 for this load.

What you are seeing is a single phase having to use a larger wire than a 3-phase load, even though both draw the same total VA. The rest of the story is that the second load needs a third wire to transmit that same amount of power, and that the voltages in the three wires are separated from each other by one third of a cycle. That makes their physical behavior very very different, in the same sense that the behavior of a gas powered lawn mower is different from that of an electric toaster. There is no reason to expect them to be the same.

 

[George Stolz]

Re: While we're talkin' 3?

Okay, I'm clearer now. It'll probably get clearer as it sinks in.

Thanks guys.