3-phase Amps to Wattage for Generator
3-phase Amps to Wattage for Generator
(OP)
I just amp'd our 3-phase service to one of our dedicated circuits to computer room at leg1(9.8)leg2(1.9)leg3(9.2) and wanted to size a generator for the service. Do I just take the highest amp reading and use that value to calculate the wattage(4700watts) or do I have to combine them or use a power factor? Never done any three phase. But I can't see all the stuff we have on that circuit only being 4700watts? Thanks






RE: 3-phase Amps to Wattage for Generator
It would be best if the three phases were balanced. If you cannot balance the the currents on the three phases, you must use the highest cureetnt to calculate your wattage. I'm guessing this is a 480 volt service correct? in that case you need a generator:
480 * 9.8 * sqrt3 = 8200 Watts minimum
I'd look for a 10 kW generator.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: 3-phase Amps to Wattage for Generator
RE: 3-phase Amps to Wattage for Generator
RE: 3-phase Amps to Wattage for Generator
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RE: 3-phase Amps to Wattage for Generator
RE: 3-phase Amps to Wattage for Generator
RE: 3-phase Amps to Wattage for Generator
You multiply by sqrt(3) because you are using line currents and line-line voltage. If you used line current and line-neutral voltage (277), you would multiply by 3.
RE: 3-phase Amps to Wattage for Generator
Then add three times the KVA of the air conditioner.
Even at that size, when the A/C starts, the UPSs may react due to the frequency dip and/or the voltage dip.
This should be big enough to accept some imbalance, but as David says, generators don't like current unbalances.
Root three:
277 Volts times 10 Amps, times three = 8310 VA
277 Volts times 1.73 (root three) = 480 Volts
480 Volts times 1.73 times 10 Amps = 8310 VA
Bill
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"Why not the best?"
Jimmy Carter
RE: 3-phase Amps to Wattage for Generator
Depending on how the load applies itself in a mains outage, (i.e. is it staged in gradually? or all starts back in one go?) would effect the genset size.
Rule of thumb, double the size of genset to cope with a heavy impact load.
Most genset manufacturers have "sizing" software available for free if you ask them nicely....
"Power is nothing without control"
RE: 3-phase Amps to Wattage for Generator
RE: 3-phase Amps to Wattage for Generator
RE: 3-phase Amps to Wattage for Generator
What is the response of the UPS to frequency/voltage dip? The Generator supply is powering the rectifier, which is recharging the battery and also powering the inverter.
RE: 3-phase Amps to Wattage for Generator
But then the brilliant programmers, who may have a tenuous relation with the real world and no field experience whatever start writing software control routines. Some UPS's will run happily from a generator, some choke on the frequency and voltage variations that are generally not noticed but quite common with gensets. Such a UPS will be continually and annoyingly flashing its lights and blowing its beeper to indicate that the incoming power is not pristine.
If you are going to add an air conditioner use a factor of 300% to allow for the starting current. If in doubt, go bigger rather than smaller. Over the years I have encountered about a dozen generator installations that would not start the air conditioning load connected to them. I replaced about eight of them with properly sized sets at a nice profit. (I was not responsible for sizing the original sets.)
By 300% I mean if the A/C needs 5kW, add 15 kW to the generator size. Even at that, even generator UPSs may beep and flash a little when the A/C starts.
Bill
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"Why not the best?"
Jimmy Carter