Our company is planning to shift to a new site and I was tasked with finding out the required kVA power to install a PMT. I have the kW ratings, amperage, voltage and power factor for all machines and other equipment in use. Using this as a reference, I found the reactive power for all machines, then summed up the total kW ratings, summed the kVAR ratings, and used the power triangle to get the total real power.

Is this method correct? Is it 'accurate'? I was told it is incorrect and I would have to find the real power for the single-phase and three-phase machines separately, and divide the single-phase total power by 3 because it goes into three-wires. This is the first time I am dealing with a real-life design problem so help would be appreciated. If any other details are required please let me know.

  • 3
    \$\begingroup\$ In a real world situation like this, there should be a senior engineer around to help you learn this stuff or look over your work. If not, bail out of there. Being junior and the only engineer at a company is bad for the company, and bad for your career and growth. This company doesn't understand the value of engineering and is buying on price only. Again, get out of there. \$\endgroup\$ Apr 3, 2018 at 10:55
  • \$\begingroup\$ With regards to the actual machines I deal with at work, I do have a supervisor. However the company does assign me a lot of other engineering tasks which I have to do by myself (such as this, figuring out actuators best suitable for a purpose, etc). You make good points.. \$\endgroup\$
    – dnclem
    Apr 3, 2018 at 11:06
  • 1
    \$\begingroup\$ How can we verify your calculations if you don't disclose them? Without exact formulas you have used, I'm inclined to close your question as unclear. \$\endgroup\$ Apr 3, 2018 at 12:24
  • \$\begingroup\$ @DmitryGrigoryev I've uploaded my Excel file here: drive.google.com/file/d/1wSkJj-4ul2jBzKjFozf07fYp7TIrXR__ \$\endgroup\$
    – dnclem
    Apr 5, 2018 at 4:11

3 Answers 3


If you just add up the rating plate values for every piece of equipment in the building, you will end up with a figure that far exceeds what you actually use. That's because you haven't allowed for "diversity". In practice, nobody turns every bit of equipment on at the same time, nor is everything running at full power all the time.

You would be better off buying a clamp meter, and clamping it around each of the phases running into your current building in turn, to see what you actually use. Then add a bit of safety margin on top.


It is true that the single-phase loads are divided among the three phases. If they are not balanced, you need to determine the highest phase current.

If you have the data, you may want to do the calculation for both actual and nameplate currents. I assume that "load factor" in your example is actual/nameplate current or actual/nameplate kW. You need to determine if management wants to size based on actual or nameplate data or calculate both and present all of the data.

If some loads cycle on and off or the load varies cyclicly, you need to be aware of that and determine to what extent that should be considered.

If the records containing all of the load data are not well organized, you management may be expecting that you will put them in good order. Each branch circuit should have a list or "schedule" of connected loads. Single phase branch circuits should be identified by phase. The schedules should list all of the branch circuit breaker ratings. Search the internet for examples and detailed guidance.

If the records are already well organized, study them carefully and learn how they are organized and why.


I looked over your calculations. First some important things to note:

  1. Power Factor - Devices should be specified as leading or lagging PF (Since you show them all as positive, it is assumed every device is inductive.)

  2. Current Data - Using the full load current data (nameplate data) or typical test load values is okay when calculating the steady state of the system. However, you need to consider what types of induction motors you have in the system and what drives they use. If those current values are provided to you, they may be taking this into consideration already but you should consider that induction motors can draw 6X the full load current when starting. If you are starting devices such as this frequently, you should consider using those currents for higher bounds of system ratings.


Your single phase (sheet 1) and three phase (sheet 2) calculations are correct in the totals. I do not know why your power calculations (sheet 3) shows 1250 in the kW column. Always sum P's (Real Power) and Q's (Reactive Power) to create a final power triangle to calculate S (Complex Power) total.

S(1ph) + S(3ph) = (P1 + jQ1) + (P2+jQ2) = (S Total) For entire system

Since it is assumed you want balanced three phase loads you should spit the single phase devices across the three phases.

My S total was 1331.19 kVA with .752 PF lagging (before safety factors)


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.