Load

[pithpal]

Hi
I am doing a load survey for an office building for emergnency generator. The building is stuffed with computers and lot of unbalance as I can see when we measure current, here is an example:
Ia- 27.7 AMP
Ib- 9.25Amp
Ic- 15.25 Amp

120/208 V

I should go for load =Sqrt(3)* 208*27.7 cos(phi)

Now which current should I assume and what power factor .
One more thing to it, those duplex outlets, which are not being used right now, what load should I assign to them, as you know
each outlet is rated for (15 A, 120 V) and there are two receptacles in each outlet.

Thanks

 

[unclebob]

If possible, consider using a recorder (ACE, RPM, BMI...) witch measure both voltage and currents and calculate for you lots of electrical values such as:

power factor
displacement power factor
real, reactive and apparent powers...
harmonics up to the 50th...

Hope this helps I

 

[RonShap]

Use the highest current measured over an hour of busy working time. If you can't measure power factor, use .8 or so.
If you have unused receptacles, you could use 180VA (or some multiple of that you feel comfortable).

Add a fudge factor for future growth.

Generally don't plan to run the generator at 100%, 80% max is where I would consider.

 

[waross]

I select and install a lot of standby generators. At present I am installing a 40 KW set, There is a 60 KW set in storage to be installed next, and a third 50 KW set is on order, to be installed when it arrives. I have done the sizing on all of the above.
I always ask to see the power bills for the last 6 or 12 months or more.
For a large building, they usually show maximum demand each month, as well a KVARHrs. which you can use to calculate power factor.
If you get lucky, more information may be available. In my area the utility is using electronic meters on more and more commercial buildings.
I had a building owner request a download of the meter history from the utility.
The utility sent a floppy disk with;
WHr
KVARHr
KW
KVA
KVAR
KW
PF
Volts "A" phase
Volts "B" phase
Volts "C" phase
Current "A" Phase
Current "B" Phase
Current "C" Phase
The readings were at 15 minute intervals for the last 7000 15 minute intervals.
Check to see what information is available from the office (old power bills) and the power utility.

 

[pithpal]

Thanks Everybody,

I will procedd now with all the ideas and ask the client for billing and other details as suggested by you.

Before all this, based on your experiences,
what is the power factor of a lighting panel used for flourscent lighting. Please advise a value which I can consider.

What power factor should I consider for a panel hooked to a computer rooms.



Thanks

 

[waross]

I'd use a power factor of 80%. That is an almost universal rating on 3 phase diesel generators in the size range you will be using. If the actual power factor is higher than this, no problem.
If the power factor is lower, in fact if it is lower than 90% you should be considering capacitors to avoid utility penalties on your power bill. This will also raise the power factor that the generator "Sees".
Remember, with computer loads, the neutral must be full size or larger.

 

[pithpal]

Thanks, A silly question flashed in my mind.
When we have a 2 pole, 30 Amp breaker on a 120/208 V 3 phase 4 wire system , the power consumed by that load will be
This is what I think.= 30*208*cos(phi)

Am I correct?
Thanks

 

[dpc]

It will not be 30 A, but whatever the actual load current is. Also, keep in mind that a 30 A breaker can only carry 24 A on continuous basis. Above that, it may trip.

 

[ScottyUK]

Fluorescent lighting can vary enormously. Cheap and nasty iron ballast luminaires with no PFC capacitor will be lucky to get much above 0.3PF. Reasonable quality iron ballast types will be better than 0.7PF. HF electronic ballasts should be better than 0.9PF, but watch for harmonics from these small loads: the harmonic current per luminaire may seem trivial, but in an office complex containing hundreds or thousands of luminaires it becomes significant. Loads containing high levels of low-order harmonics can upset your generator's AVR unless you specify the nature of the load to the supplier of the generator and contractually stipulate that the generator shall be stable when supplying a load with your harmonic profile. Harmonics will already be significant from the load you described initially - take measurements if you can.


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Your body might be a temple. Mine is an amusement park...

 

[pithpal]

Thanks for the answer
I beleive , You guys missed my question:
What i want to know is is
the power formular ok or I am wrong:
as a 2 pole 30 Amp breaker has line to line voltage of 208 v and phase current of 30 A(Suppose it is )what will be the phase angle, between Ia or Ib as if the breaker for phase A and B.
The formula is V(Ph)I(Ph)cos (phi)
while I am using V(L-L)I(Ph)cos(phi).

Please advise now.

Regards

 

[ScottyUK]

Assume the line voltage V_AB is your reference for phase measurement. The current is shifted in phase relative to the voltage by the nature of the load. This relationship between voltage and current is inherent whether you use a phase voltage or line-line voltage, so the formula is correct either way. I_A = -I_B, so the phase angle between these currents is 180[°]. Draw out the vector diagram if you want to check.


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Your body might be a temple. Mine is an amusement park...

 

[waross]

I think your reference to phase angle is confusing. If you are using a two wire load, it is a single phase circuit. Single phase power is volts times amps times power factor.
Power factor is the cosign of the angle between the volts and the amps.
The volts will be 208 as you stated.
Another thing that is a little confusing is the 30 amp breaker. Use the actual amp draw of the load, not the breaker rating.
Use the power factor of the load, not the cosign of the phase angle between phases.
If the load is 22 amps at a power factor of .78 then the Watts are;
208 Volts x 22 Amps x .78 Power factor = 3569.28 Watts.
I agree completely with scottyUK on the power factor of the lights. I wanted to say the same thing but I didn't have the exact figures at hand.
Thanks scottyUK