Size Transfomers required for 1200 120 240 3 phase service

[Powerlineguy]

A small but welcomed reply would be most helpful in this matter!
According to my calculations, the size transfomers required for a 120 - 240 volt 3 phase service of 1200 amps (max rated load) should be three (3) 167 KVA transfomers!
Now if I consider 80% of full load (960 amps)I will wind up with appx a 133 kva rating for a set of transfomers. This will not be possible In as much as they don't make any pole mount transfomers in between 100 kva and 167 kva unless special ordered or in three phase padmount confuguration.
Anyway, I would like to check my figures to see if I am right! Thanks

 

[advidana]

Why are you putting in a 3 phase delta 120/240 system or do you want a wye 120/208 volt system?

 

[Powerlineguy]

120-240 3 phase delta,this is what they want for power for the new machine shop in the industrial park

 

[advidana]

If you are furnishing the transformers use the 3-167 kva transformers and fuse the primary and secondary per code.

 

[Powerlineguy]

What we do is supply the 1st transfomer and the customer pays for the other 2 in a standard 3 pot bank! If the 3 pot bank happens to be 240 - 480 or 277 - 480 then the customer pays for the entire bank seeing as we usually only carry 120 - 240 volt transfomers except for the extra replacements for the above!

 

[busbar]

You may want to review consensus standards for applicability.

IEEE Std C57.91-1995 Guide for Loading Mineral-Oil-Immersed Transformers, or
IEEE Std C57.96-1999 Guide for Loading Dry-Type Distribution and Power Transformers

 

[busbar]

In the US, every utility has localized practices, but a usual breakpoint for 240- versus 480-volt service is 30-60hp / 30-60kVA. Conductor and switchgear expense gets progressively more expensive for the lower voltage. A classic text on electrical design is D Beeman, Industrial Power Systems Handbook, McGraw-Hill, 1955.

Offhand, the relative expense of a 277- versus 240-volt transformer is probably negligible.

 

[margaret695]

Powerlineguy: How much of the actual load for the new machine shop is actually required to be at 120-240 3 phase delta? 20%, 30%, or ?

The lighting and HVAC stuff and receptacles for maintenance and office type usage would probably most likely be okay at 120/208v 3ph 4w. What I have done in the past for factories that required 230V, or whatever... is calculate the amount of load required at the non standard voltage... and have a separate transformer and sub electrical distribution and panels, etc., inside the building serving that specific equipment. The rest of the electrical equipment... like fans and lighting and receptacles, etc., etc., I would feed off of distributions at standard voltages... which in my area are 120/208v 3ph 4w and 347/600v 3ph 4w.

That way you could have standard type pole mounted transformers for your area... and just specify one, or maybe two relatively smaller sized transformers at 120/240v 3ph.

You also could being saving yourself the bother of trying to get 240v 3ph HVAC equipment and pumps, etc.

Margaret

 

[Powerlineguy]

I wish to thank you for all of your answers! The Electric foreman decided that he wanted the opnion of our city contracted engineers "Stewart Electrical Consulants" and they have decided that the use of (7200 volts hi side)(3) 100 kva 120-240volt (secondary side) transfomers would handle the load requirements. I disagree, but seeing as I'm the #2 man the foreman has the last say and I guess we will be going with the 100kva transfomers even though I believe that they will might be at times overloaded. But then again if they are overloaded the foreman can place the blame on the city contracted engineers!
I guess sometimes I just worry too much about our system! Anyway again, thanks!

 

[jghrist]

The 100 kVA transformers will take 167 kVA for short peak periods without loss of life if the normal load is less than 100 kVA. Chances are, the actual peak load for a 1200A service will be a lot less than 500 kVA anyway because of load diversity.