Please review my math on power consumption

Hello Roldar,

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kw = kva * p.f

By your estimation your load is 35 kw so kva = 35/0.8= 43.75

Now your major load is AC, so i would suggest a 50 kva 3 phase gen set with loads of Airconditioners equally balanced on each phase.

This way you will not have the voltage dips which are encountered when A/c kicks in and out.

Cheers.

Chaterpilar thanks for the info.

We're running a 3 phase unit now (240 V 50 Hz, 30kVA). I've been able to reduce, replace, or drop power to get us at the genny's 25.5kW rating (assuming a PF of .85).

Now I've been looking for info about tapping into each phase individually to balance the load as you suggested. But when I look all I find is colour codes and info about the wiring itselft, nothing about how to tap into each phase individually. As it stands now the power all goes into a big breaker box, so I'm assuming (and I may be wrong) that the box either converts 3 phase to single phase or it just drops phase 2 and 3 and runs on the first phase. Where can I find that info? My onsite resources consist of me and what I can find on the internet.

The power goes from the genny to a breaker box that appears to be completely bypassed (wires tied to the same side of the bus bars) then to a secondary breaker (but all the wires in and out are the same colour). After the office closes and I can poke around without fear of a 240V zap I'll figure all that stuff out.

I was making it much more complicated than it is, but correct me if I'm wrong. 4 wires coming out, 3 hot 1 neutral. We tap leg one into 1/3rd the load, leg two into 1/3rd the load, and leg 3 into 1/3rd the load.

how close to perfect do these loads need to balance out? I'm trying to get all the constant use A/C units figured out and then the constant load for the PC's, then the seldomly used A/C units last. But does this need to be a zero sum balance?

For some reason I was stuck in the electronics lab with big oscilloscopes and the 3 phase diagram not the acutal wires and stuff.

Hello Roldar,

Can you check the amperes drawn in each of these "hot legs" using a clampmeter ?

Strictly speaking they should not be more than 10 % of each other.

Fo example.. A=10 amps, B=10amps, C= 9amps is ok.....but A= 20 amps B= 4 and C= 5 amps is not OK.

I feel your load is high for the generator you have...and it looks like a classic case of your office and your business growing fast....but gennie is same.

Also please check the voltages A-B B-C C-A and post...

Cheers.

The electricain is now an hour late. I hope he has a clampmeter, but I have never seen one in country (not saying there aren't any just a rare find, I've only seen 4 voltmeters). If he doesn't ahve one we'll send out someone to beg, borrow, or buy one. I'll post the results however they turn out.

a 50kVA unit is out of our price range, in fact I think another 30kVA unit to run in tandem with the current one is out of or price range. But that leads me to another question. Do gensets need to be the same rating to be paired up? Or could I run a 20kVA unit with our current 30kVA unit? Evertying I've found online for pairings shows the same unit (make, model, and output), but those are also usually manufacutures websites listing how awesome their products are.

Hello Roldar,

Clampmeter as shown in the image.

For industries and factories it is ok, but for offices of your size, i will not suggest synchronising gensets.

I have not seen anybody parralleling 30 kva gensets...other members may differ.

Cheers.

Dear,

Please, keep in mind that a genset should normally be loaded upto 80% of its maximum rating. This is done to take care of the current surges caused by some equipmemnt starting during the running of the genset taking a higher starting current than what is required during running. The rating mentioned on the genset is maximum. Therefore, care is to be taken not to run it to its maximum capacity.

In the event the connected load increases by addition of some more equipments, the new genset capacity should be chosen in such a way that it is never run below 50% of its rating. It is not in the good health of a genset to run it on a very low load.

S.K. PRUTHEE

Hello colleagues,

for the avoidance of wet stacking a generator should be loaded atleast 30% of its rated capacity not 50%.

Good day.

Roldar,

I agree that you should not try to put the outputs of two generators together (this is called paralleling to us electricians). It requires some costly equipment to do correctly and can easily be done incorrectly (you will notice many threads on CR4 posted by people who have problems with this). Much easier would be to have a means of carrying part of the load on one and part on another; fancier would be to have a transfer switch which could switch some of the load onto a second generator when the first one gets overloaded.

Pay attention to the comments about % load on the generators. Their engines are going to last longer and need less maintenance if they are loaded to 80% or more of their rating (compared to 30-50%) because the engines will be at or near their designed operating temperature.

From the information in your posts I assume that you are NOT an electrician, but that you have very good technical knowledge and a healthy respect for electrical safety.

Usually people who are calculating the electrical load for a building will include typical values (in units of watts per square meter or square foot area, based on many years' experience) for the general lighting and receptacle loads. Then they will add the power loads such as air conditioning, computer servers, and similar. Your method of calculation is not "orthodox", but still is close enough to the ways I would use. Anything you can do to increase energy efficiency is good.

In your power panel, if it has circuit breakers (usually in one or two columns with handles that move to left or right) with wires attached to these breakers, and has the power wires from the generator attached either to a breaker or to the bus bars, then I can describe how the power is divided. The top breaker is on the "A" phase, the next on "B", the next on "C", then the 4th through 6th repeat "A", "B", "C", and so on until the bottom is reached. Two-pole breakers or three-pole breakers therefore automatically attached their wires to different phases. To see how your load is theoretically divided, make a chart with one column for the loads, and three for the phases. Then starting from the top and going to the bottom you would mark the phases that loads

are connected to. Add each of the three columns and you can see how well it is balanced "on paper". This is what all engineers do. (IMPORTANT--the above description assumes 3-phase power, if single phase, only use "A" and "B".)

Regards--JMM