Transformer Sizing OCPD and Conductor Ampacity

[squelch]

Hi Guys ,

just want to ask your second opinion.

We want to put up a 25000TL-30 SMA inverter which as a Max output current of 36.2A. see data sheet SMA 2500TL-30.JPG.

Transformer size: 30kVA

we have a grid voltage of 240V L-L, and putting a step down transformer Wye-Delta 400V-240V LL. my question is how to compute the OCPD and conductor size at my transformer secondary voltage w/c is almost 40 meters total length going to RE meter and to tapping point.



TIA

[inetdog]

You are putting in the transformer because the tripower requires a wye connection while your utility service is 240 delta?

The power formula for three phase matches the 25,000 rated output: 36.2A x 1.732 x 400V = 25,079.
So the current in each of the three phase conductors on the GTI side will be 36.2A.
And the voltage ratio of the transformer will effectively be 400/240 = 1.67. That means that the current on the grid side will be 36.2 x 1.67 = 60.333A.

The NEC provides that since the current from the GTI will be continuous you must multiply each of the currents by 1.25 to get the required conductor ampacity on each side of the transformer.
You need to protect the GTI side of the transformer with the next breaker size above 36.2 x 1.25 = 45.25. So on that side you need a 50A three pole breaker with common trip. In addition, to protect the transformer the breaker should be no larger than the full rated secondary current times 1.25. That would be 43.3 x 1.25 = 54.1. So a 50A breaker meets this condition too. (If it did not, you would then be allowed to go up to the next higher standard size breaker.)
On the grid side, you need to provide for the magnetizing surge of the transformer as well as for the PV current, so the breaker must be no smaller than 74.4A. That would be met by a 75A breaker, but that might be the weaker of the two conditions.

The full load current of the transformer on the grid side is 30,000 divided by 240 x 1.732. That would be 72.2A. But to allow for inrush when power is applied the NEC allows you to protect the transformer with a breaker up to 2.5 times that currrent, since you will have suitable secondary protection in place. It is in your best interests, as long as POCO goes along, to use the full 250% sizing. You do not have to size the wiring in the primary side to that current, just the full load current times 1.25.

The actual wire sizes you use will depend on ambient temperature and number of conductors in the raceway, so I do not have the information to give you those results

Disclaimer: I am not an electrician, I just play one on the internet, so I may have messed this up completely.

[squelch]

tnx man

i think you nailed it