johndoe1234
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Given a 3-phase wye circuit (3 conductors and grounded neutral), with phase currents of L1=58A, L2=74A, L3=64A and N=12A, how does one compute the total current?
Calculating Total 3-P Amperage Given Individual Phase Amperages
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iceworm
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Whoops crossed posts - Deleted Q
ice
ice
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johndoe1234
Member
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For simple calculation at unity (or equal) power factor...
If you want to do a calculation with unequal power factor, let me know...
Thanks for the replies. I know the formula for calculating neutral current, as well as other basic conversion formulas.
More detail:
With a 65kVA UPS (with 80% derated max current of 160A), I'm trying to determine how close to capacity I am on a per phase basis, assuming balanced load (which of course I don't have in this example). I know from a total load perspective, I'm at about 55% (of 80% derated capacity). What I'm trying to determine is what the max amp per phase limits are with a max total current of 160A.
TIA for any replies.
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And you can calculate the line to line currents for each phase from the individual line currents, again if you know the power factors OR can at least assume that they are all equal.
Tapatalk!
iceworm
Curmudgeon still using printed IEEE Color Books
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The context always helps.
Since I don't have the mfg book, this is just a guess:
I would not look at the UPS as a generator with unbalanced phase currents. This is not a rotor/stator heating problem.
The phase output currents are through semiconductors. And they heat up pretty fast - like on the half cycle. I would guess that if any phase current is getting next to your proscribed limit - that's enough.
65kva is a significant UPS. If the mfg book doesn't cover unbalanced loads, suggest calling the mfg eng department.
ice
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I should think that 80% of your output current would be 144A, not 160A
But keeping with 160A, you have a max current of 160A PER PHASE, not 160A total.
iceworm
Curmudgeon still using printed IEEE Color Books
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I'm having trouble understanding exactly what you are trying to do? As others have already said, these terms don't have any meaning. Here is a suggestion on nomenclature:
Assuming 65kva, 208V Wye: Output current is 180A. Forget the "amps per phase". Forget the "total current". Those terms don't mean anything when discussing 3 phase.
For 3 phase balance load (fully loaded), put a clamp-on on each phase conductor, and each ammeter will read 180A. When you are asked what is the current? And you say 180A. You are saying the current is balanced and nominal 180A measured on each phase conductor.
For unbalance load, list the currents just the way you did, "phase currents of L1=58A, L2=74A, L3=64A and N=12A". That is perfect. There is no total current. They are vectors - if you add them, they better equal zero.
ice
ron
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Take the worst case (largest) phase current and use that to calculate 3 phase power as if all three phases have the same. Since the individual currents run through different transistors, it is not overall effective heating like a gen or transformer.
What's with this "derated" capacity???
If it's rated 65kVA, that's what you can power continuously (supposedly). Max current per line conductor is 180.5A. You can make allowances if you want... so why 80%?
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johndoe1234
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Thanks, Iceman, that's an answer I can live with. I have all the amp-clamp values. Thanks to others for taking the time to reply. Case closed.
What does it even mean?
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It does not mean anything. I would love it if people would stop using the phrase "total current."
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:thumbsup: Since power is not a vector, total power is completely meaningful, even though it is not necessarily the right way to check to see whether a given source can handle a given load configuration.
If different power factors on different phases are involved, on the other hand, total VA is also a somewhat nebulous concept.
iceworm
Curmudgeon still using printed IEEE Color Books
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I'll add:
amps per phase
surge (unless the discussion is about tidal bores)
ground loop (unless the discussion is about landing a tail dragger)
dirty grounds
110V, 220V
440V (unless the discussion is about U frame motors)
And dare I add split-phase (unless one is discussing motors)
surge (unless the discussion is about tidal bores)
ground loop (unless the discussion is about landing a tail dragger)
dirty grounds
110V, 220V
440V (unless the discussion is about U frame motors)
And dare I add split-phase (unless one is discussing motors)
ice
iceworm
Curmudgeon still using printed IEEE Color Books
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- EE (Field - as little design as possible)
Add: RMS power
ice
ice
Do you want to test it?
If you are pleased, please do it on my behalf.
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