ECN Forum Forums General Topics of the Trade General Discussion Area Sizing a Transformer

Here you go:
Amps = 1000 × kVA / (√3 × volts)

Amps = 1000 × 150KVA / (1.73 × 208V)
The 208Y/120V output of a 150kVA transformer is 416.7 Amps.

Keep in mind that this is does not take into account any reduction due to harmonics generated by the solid state switching power supplies in the computer equipment.

John, you hit the nail right on the head man. Everything I was taught about electricity suggests that what you are saying is correct and I have spent the last 20 years of my life believing in that logic.
Bear in mind, I only had 3 months education in electrical as that's all that's needed for hvac.
I've lost recent arguments with people that have degrees that made me question my beliefs.

Will you do me a favor and read through this thread and tell me what you think?
http://www.contractortalk.com/f5/am...t/#/forumsite/21253/topics/254793?page=1

Larry, I appreciate your concern. We've done testing and determined the average computer we're using with monitor pulls 3.3 amps x 100 rigs = 330 amps. We're using 400 amps for good measure.
Networking gear is on its own circuit.
For the record, I originally thought we needed a 150 but someone I work with convinced me of the flawed logic above.
Thanks

Mort;
Always remember that once the undersized 75KVA transformer failed, it would be considered YOUR fault and the person who convinced you to use a smaller transformer (so it could be done cheaper) will suddenly disappear and simply leave you swinging in the wind.

It's the same false logic that tells you that beating the train at the crossing should be possible ...just as long as you have a good reason for going around the gates.

Mort, I'm slow to be critical ... but if idiots could fly, that link would take you to an airport!

Not only is the use of multi-pole breakers to control separate circuits common, the NEC actually requires it in several circumstances.

As for this thread, the math for a transformer is pretty simple. The only 'jokers' in the deck are that you use the 'nominal' voltage in the calculation, and that 1.73 figure in three-phase calculations.

Not enough transformer? Transformers are pretty tolerant of abuse. If you're really overloading one, it will be reflected in a severe voltage drop.

Harmonics are pretty much a non-issue, in that it is the practice today that every circuit have its' own neutral. This has the effect of over-sizing the neutral, which takes care of the higher amp loads caused by the harmonics.

A 'normal' common spec would be 200 amp, 3 phase, 480 volt feeder for primary of a 150 KVA transformer, with the secondary (at 400 amp, 3 phase, 4 wire; 400 amp OCP) feeding an 84 circuit panel setup (If needed).

There seems to be a pretty strong movement in NFPA to not allowing shared neutrals for single phase circuits anymore.
It works fine on the initial installation but when breakers get rearranged to create a 2-pole or 3-pole space so a new breaker can be added to the panelboard the odds are strongly against things ending up on the corect phase.

The recent changes are more about keeping the whole circuit bundled in some way in the panel. As long as the grouping is made apparent on single phase multiwire circuits there is no problem and they do have superior voltage drop characteristics.
The problems come up in 3 phase wye with harmonics. It is fairly well understood and people are doing things to mitigate it these days.

I'm not trying to be difficult... But this formula suggests that at 120 volts a 75KVA would be rated for 625 amps.
Can you help me understand wye (pun intended) we have to use the 208/3 formula?

I'm not sure how you got 625 Amps.
The secondary of a 75kVA 3 phase transformer is 208 Amps at 208Y/120 Volts (kind of a weird coincidence, huh?).

There are 2 possible ways to connect the 3 transformer windings in a 3-phase transformer; one has each winding connected to the next and forms a triangle when drawn (this is the Delta connection) and the other has 1 end of each winding connected together and forms a 3-pointed star when drawn (this is the Wye connection).

The square root of 3 comes about because the ratio of the magnitude of the voltage vector between phases and the magnitude of the voltage vector between any phase and the common point (Neutral) is equal to the square root of 3.

Take a look at this pdf from Jefferson transformer and see if this helps...sometimes the vectorial math involved in electricity gets confusing.

http://www.jeffersonelectric.com/s3/site/catalog/basics.pdf