New member, straight in with a question

[Ampo]

Hi all, new on here. Been watching for a long time but hoping someone can help on something I've drawn a blank.
Been asked to supply temporary 4 x 63a 3 phase supplies from a genny. All the info I have is that they are pulling about 61a on each so close to the top. I need to work out kw/kva of genny but can't remember the calculation for this. If single phase I would just 61×230(around 14kw).
For 3 phase I can't remember if I just times this by 3 or do 61x400. All feeds are running motors so are balanced and on the way I will be taking off some small single phase which shouldn't unbalance thing too much. Any help appreciated
Cheers

 

[telectrix]

basically you use 61A x 3 x 400A = 73,200 VA or 73kVA.

 

[Ampo]

Thanks for such a quick reply! Am I right in thinking then I'll need a 292kva genny total or 233kw using x 0.8?

 

[David M]

GENERATOR AMP RATINGS - THREE PHASE EXTENDED
3 PHASE AMPERES - 80% POWER FACTOR*

kW Times 1000 divided by (Volts*1.73) *.80

This chart approximates the amperage of a generator based on the size of the generator and the load on the generator at 100 percent of capacity. Please note that this table is intended to be used as an estimator is not an exact representation due to various factors that can increase or decrease amperage. Elevation, temperature, fuel quality, age, wire size and length and other factors can decrease the amount of power a specific installation can generate.

Here is a link to a chart that gives you the information you require.
Generator Amp Ratings Chart, Three Phase Extended Custom - https://www.generatorjoe.net/html/genrate.html

 

[telectrix]

Thanks for such a quick reply! Am I right in thinking then I'll need a 292kva genny total or 233kw using x 0.8?

no..... 73kVA total . (as you have stated the phase current, i've not considered PF).

 

[Spoon]

Ok... Am I just being really thick.
The OP requires 4 feeds of 61A (3 phase)... Isn't that 244A?
According to a genny calc site that would be about 170kVA

If these are all running motors then the starting of the motors may affect the max amperage needed.

 

[telectrix]

i was wrong earlier.missed out the √3 . not enough beer yet. answer is 42kVA.

Amps (A) to kVA conversion calculator - https://www.rapidtables.com/calc/electric/Amp_to_kVA_Calculator.html

 

[David M]

Depends on whether its 4 x 3phase loads @ 61A per phase or not.
in which case it would be 170kva

 

[David M]

i was wrong earlier.missed out the √3 . not enough beer yet. answer is 42kVA.

Amps (A) to kVA conversion calculator - https://www.rapidtables.com/calc/electric/Amp_to_kVA_Calculator.html

You need to start drinking a bit earlier Tel.
That's a handy site I've used it plenty of times. I even let the apprentice in on it but then realised my mistake... He should continue learning to do it the hard way like we had to.

 

[Ampo]

This is all simpler than I thought so I think I may be over complicating this in my head. Is it as simple as saying 61a x 230 = 14kw then x 3 for 3 phase (42kw) and then I want 4 so 168kw total (bit more for Kva) am I there or have I done this wrong

 

[Spoon]

Been asked to supply temporary 4 x 63a 3 phase supplies from a genny

Please can you clarify this comment. What is it you actually need?
See post #6 & post #8

 

[telectrix]

that's about right. i forgot that you'd got 4 supplies, so 42 x 4 = 168kVA. if the 61A current is a given, then forget about PF or kW-kVA. that's already allowed for if the 61A is correct.

 

[Ampo]

Please can you clarify this comment. What is it you actually need?
See post #6 & post #8

Sorry yes the actual need is 4 separate 3 phase supplies at 63a so its 42kw for each 3 phase supply times 4. Thanks for your help guys. I had the answer originally and then completely over thought it. Back to basics

 

[telectrix]

can't beat basics:

oopps, thought you said basques

 

[Spoon]

If the genny is powering 3 motors, that "are pulling about 61a" each, how are the motors controlled (DOL, inverter) and do they all start at the same time?
Would you not need a bigger genny for the start-up of the motors?