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Old 11-17-2010, 07:00 PM   #1
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3-Phase Load Balancing Question


We use a lot of 208/120v three-phase in our data center. Almost all of our loads are 208v/20a single phase (it's actually phase-to-phase, but single-phase is what they call it...).

I'm trying to keep the loads relatively balanced, but some recent adds, moves, and changes have left me with a 115a/100a/80a load on one distribution unit. Our UPSes don't really like this (although they will handle it), so I'm trying to put nine new 20a circuits where they will help balance things out.

Question: Is there anything tricky about load balancing on a three-phase system? If I put a 40a load on phases B-C, will that show up as 115a/140a/120a? Or as 115a/120a/100a? Or will that depend on how current is measured in the distribution unit? (AFAIK, it's three CT transformers on the panel feeds.)

I'll probably be finding out empirically in any case, but a little fore-knowledge will help plan the best locations to land the loads...

Thanks in advance...

Bill

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Old 11-17-2010, 07:35 PM   #2
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3-Phase Load Balancing Question


To order to get the three phase balanced out this part is best done by electrician they will have proper equiment to verify it and the other reason is that due you are in commercal location and this will have to be done by electrician.

And also many case you may run into MWBC { multi wire branch circuit } on three phase system it can get tricky if not addressed properly.

Again I will empized this one get a electrician to deal with it.

That is the only safest way to do this due there are too many thing can go wrong if someone unqualifed to work on three phase system.

Merci.
Marc

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Old 11-18-2010, 06:09 AM   #3
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3-Phase Load Balancing Question


Take all of your 208 volt loads. Try to balance those among the AB, BC, and CA connections.

The following does not compute the current draw on the 3 legs but add up the amperage ratings of the 208 volts in the fashion that if a 30 amp load is connected across A and B and a 40 amp load is connected across B and C then B has 70 points.

Then take all of your 120 volt loads. Try to balance those among the A, the B, and the C connections, but put more 120 volt loads on the legs with fewer points computed in the previous paragraph.

This should produce a balance that is close enough for most purposes.

Quote:
Is there anything tricky about load balancing on a three-phase system? If I put a 40a load on phases B-C, will that show up ...
I don't have the math handy but the calculation is the same no matter what kind of transformer was used to deliver the 120/208 volt 3 phase current.

Incidentally if you have nothing but one 208 volt load turned on and it is drawing 30 amps from legs A and B then there are zero amps flowing on leg C.

Another method: Use a clamp on ammeter to measure the current flow on the A leg, the B leg, etc. If one leg is much higher or much lower than the other two then rearrange your loads.

In a workshop or other situation where most of the loads are cord and plug connected, you just have to let things average out. Here you arrange the receptacles going down the workbench: AB, BC, CA (208 volt) and A, B, C (120 volt) alternating.

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Almost all of our loads are 208v/20a single phase (it's actually phase-to-phase, but single-phase is what they call it...).
Correct. The terminology referring to the connection is phase to phase. Taking power from any two phases of a 3 phase system you have a single phase source.
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Last edited by AllanJ; 11-18-2010 at 06:41 AM.
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Old 11-18-2010, 07:32 AM   #4
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3-Phase Load Balancing Question


Thanks, guys. I've been working with the electrical engineers on the design of our new power systems for the past few years. And I can build a better distribution whip than most of the electricians I've worked with...

Previously, I'd never paid too much attention to load balancing, figuring that random placement would result in a more-or-less even distribution. That seemed to be true, but now that we're on the new system and we've been moving, removing, and changing a lot of loads around and I'm trying to be a bit more anal.

The "A" and "B" side UPS systems (750kVA each) feed several Liebert PPCs (480-208 step down with three panels) with full current/voltage monitoring, etc. I can even track THD and K-factors. (Not that I do...I just can.) So I'm usually fiddling around with the Fluke, measuring current draws and figuring out what needs to go where. (And current draws in a data center full of IT equipment is a whole 'nother ball game...)

Anyway, thanks, and I'll give it a shot and see what I can do...

Last edited by slickwilliam; 11-18-2010 at 08:15 AM.
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Old 11-19-2010, 07:27 AM   #5
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3-Phase Load Balancing Question


One other thing to take into account with both single phase and three phase systems is power factor. This is more likely to be an issue in an industrial setting where there is a lot of motorized equipment (power tools, etc.) Power factor is compensated for by installing something (typically a capacitor) across the line (AB, A-neutral, etc. as needed) In some cases the capacitor must be in the form of a capacitor bank where portions are switched in and out as more or fewer power tools are in use.

I don't know whether electronic gear creates a power factor issue.

Usually your power company should be contacted when you are discussing power factor; they will advise you whether power factor correction is suggested, strongly recommended, or required.
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Last edited by AllanJ; 11-19-2010 at 07:30 AM.
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