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I have a 140 kVA, 440 V 3-phase 50 Hz generator running an aquaculture farm in Indonesia. There are 100 staff on site with the usual loads like freezers, light bulbs, TVs and water heaters etc.

Also there are a number of 3-phase pumps that run more or less continuously.

I am running well over 100 amps on all phases on a normal day. I thought this was very close to overloading the generator, but I then discovered that there is a difference in kVA calculation from amps depending on whether it is line-to-line or line-to-neutral. I would like to make sure I understand this distinction.

If I understand correctly, I can calculate the load in amps for the single-phase gear as "line-to-neutral" as follows:

I(A) = 1000 × S(kVA) / (3 × VL-N(V) )

And the 3-phase loads as line-to-line:

I(A) = 1000 × S(kVA) / (√3 × VL-L(V) )

The product of each can be added to give the total load, correct?

And the reason I am not overloading the genset is because although the amperage for line-to-neutral would be too much, I have line-to-line loads as well, correct?

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You have the correct calculation for total kVA. However you must also consider that you must not exceed the generator's kW rating or the line current rating for any individual phase. It is also not good to have too much imbalance among the phases.

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  • \$\begingroup\$ Thank you, I am aware of the importance of phase balance, however in my experience the variability in load day to day and hour to hour inevitably there are periods of phase imbalance. Can you give me a practical guide line on how much difference I can get away with? \$\endgroup\$
    – chris alyui
    Commented Feb 26, 2021 at 3:34
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    \$\begingroup\$ I think the primary issue is voltage imbalance caused by current imbalance. For single-phase loads, that would be seen as circuits experiencing a little higher or lower than nominal voltage depending on which phase they are on. Most loads should be ok with +5%/-10%. However fully loaded 3-phase motors would have a problem with that much voltage imbalance. I found one source that recommends voltage imbalance usually not more than 1%, but 4% might be ok some of the time. Ultimately, imbalance causes a higher operating temperature and higher operating temperature means shorter motor life. \$\endgroup\$
    – user80875
    Commented Feb 26, 2021 at 4:07

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