3-phase Amps to Wattage for Generator

[barkster28]

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

 

[TurbineGen]

First, was the computer room running at full capacity (ie all systems, lights, ect. running when you did this measurement? If so you can go ahead and calculate your 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.

 

[barkster28]

Yes, full load and is 480V. So I just use the highest value? How do I balance the load, I know that the whole circuit is converted to single phase as currently we are not using any 3-phase but we are wanting to add a 3-phase a/c thus the calculation. Thanks for the help.

 

[barkster28]

PS: why multiply by sqrt3?

 

[ScottyUK]

You have considered what happens to the temperature in the server room when the A/C plant shuts down during the power outage? So many times this is overlooked: a genny which is too small to run the A/C is specified, so the A/C is left without power. Then the customer gets grumpy because their expensive UPS shuts down on over-temperature and dumps the load.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

You should also find a way to come up with a far better balance between phases (or correct a typo). Generators don't like signficant imbalances.

 

[barkster28]

any ideas as to what can cause that, the only thing it does it comes from the generator/city power to a step down transformer? is it because we are going from 3-phase to single phase?

 

[jghrist]

It's because you have more load connected to two phases than to the third phase.

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.

 

[waross]

Back up and start over. Check the maximum load on the UPSs. Consider that some types will go on bypass but continue charging. That may give you a load that is about double the nominal rating of the UPSs. Whatever, determine the maximum load that the UPS can draw.
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

 

[BigIanH]

You also need to be aware that most generators will only accept a load step of 40-50% of their rating in one hit.

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"

 

[barkster28]

Ok, thanks for all the great info definitely helps a ton. Thanks!

 

[davidbeach]

You also need to be aware that most generators will only accept a load step of 40-50% of their rating in one hit.

Not true in my experience. You may not like the voltage dip, but I've seen 1250kW machines go from no load to 1250kW of load in a single block and get it picked up; took several seconds to get back to speed and voltage and the sky got rather dark, but it worked. Smaller machines can do it much easier.

 

[Pikoplatt]

Quote by Waross: "Even at that size, when the A/C starts, the UPSs may react due to the frequency dip and/or the voltage dip"

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.

 

[waross]

True; Feeding the rectifiers and charging the DC bus should be a no-brainer.
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