Sizing 500 KVA Transformer Conductors

[dasan384]

Hello everybody, I need to be sure that my calculations are OK, so any advises and comments are more than welcome.

The Step-Down Transformer is as follows:
3 Phase, 500KVA
Primary 480V Secondary 120/208V. Transformer is being fed from a Existing 277/480V panel board.

Primary
The Overcurrent Protection for the primary is a 600AMP breaker. The current of a 500 KVA is 601 Amps and according to 215.2 I have to multiply that by 1.25 that is 751 Amps, so the primary feeder is 2 sets of 600KCMIL (75°C)

Secondary
I have to feed a 1200 Amp 120/208 Panel, so the feeder would be 3 sets of 600 KCMIL.

Please let me know if you need any other information to help me.

Thanks in advance.

 

[tkb]

See table 450.3(B)

The primary protection on a 500kVA transformer at 480 volts can be up to 1,200 amps.
The secondary protection can be up to 2,000 amps.

 

[dkidd]

See the transformer chart at http://www.mikeholt.com/freestuff-books.php

 

[charlie b]

I’ll start by asking why you are saying that you have to supply a 1200 amp switchboard. What is the calculated load? If it is closer to 1000 amps, you could get by with a 300 KVA transformer. With a 500 KVA transformer, you can serve a 1600 amp switchboard. Step 1 is always to calculate the load.

If 500 is the right size transformer, then you are right about the rated primary current being 601 amps. But the primary side breaker needs to be higher than 600, or you will likely see trips when you first turn the system on. I would use a 750 amp breaker for the primary.

Two sets of 600 KCMIL will suffice for the primary feeder. But so too would three sets of 250 KCMIL, and this might be cheaper and easier to install. I would have to ask an electrical contractor that question.

The secondary rated current will be 1389 amps. You can protect the secondary at up to 125% of that value, or 1736 amps. That is why I would be thinking about installing a 1600 amp switchboard, and giving it a 1600 amp main breaker.

Three sets of 600 KCMIL would handle a 1200 amp feeder. My design would have five sets of 400 KCMIL, to handle a 1600 amp feeder.

Finally, 215.2 is not relevant to this discussion. The requirements for protecting the primary and secondary sides of the transformer are in 450.3.

 

[Smart $]

... I would use a 750 amp breaker for the primary.
...
Finally, 215.2 is not relevant to this discussion. The requirements for protecting the primary and secondary sides of the transformer are in 450.3.

700 or 800A are standard per the NEC, but an adjustable could be used.

What is relevant is 215.3. The primary conductors must be protected by the primary overcurrent device. Increase it to a 750A rating and the primary conductor ampacity must be greater than the next lower standard rating (700A).

 

[dasan384]

I’ll start by asking why you are saying that you have to supply a 1200 amp switchboard. What is the calculated load? If it is closer to 1000 amps, you could get by with a 300 KVA transformer. With a 500 KVA transformer, you can serve a 1600 amp switchboard. Step 1 is always to calculate the load.

If 500 is the right size transformer, then you are right about the rated primary current being 601 amps. But the primary side breaker needs to be higher than 600, or you will likely see trips when you first turn the system on. I would use a 750 amp breaker for the primary.

Two sets of 600 KCMIL will suffice for the primary feeder. But so too would three sets of 250 KCMIL, and this might be cheaper and easier to install. I would have to ask an electrical contractor that question.

The secondary rated current will be 1389 amps. You can protect the secondary at up to 125% of that value, or 1736 amps. That is why I would be thinking about installing a 1600 amp switchboard, and giving it a 1600 amp main breaker.

Three sets of 600 KCMIL would handle a 1200 amp feeder. My design would have five sets of 400 KCMIL, to handle a 1600 amp feeder.

Finally, 215.2 is not relevant to this discussion. The requirements for protecting the primary and secondary sides of the transformer are in 450.3.

Thanks for the quick response. Let me clarify the scenario. [FONT=&quot] The design is for a new addition for an existing Restaurant, so t[/FONT]he existing service is 600 AMP at 480V, they have an existing 600 AMP panel, from there I have to feed the Transformer and then the 120/208V panel with a total calculated load of 998 AMPS. They had the AC and Hood equipment on the 480V Panel because is existing, and a 150KVA was installed before, but with the new addition and the new equipment I need to supply almost 1000 AMPS total. When you said that a 300 KVA would work for 1000 amps is because you multiply by 125%? Thanks in advance.

 

[don_resqcapt19]

...
Primary
The Overcurrent Protection for the primary is a 600AMP breaker. The current of a 500 KVA is 601 Amps and according to 215.2 I have to multiply that by 1.25 that is 751 Amps, so the primary feeder is 2 sets of 600KCMIL (75°C)
...

I question if the 600 amp primary OCPD will hold the inrush current. I kind of doubt it will.

That being said, I see no reason to use wire with an ampacity greater than 600 amps. Since you are using a 600 amp OCPD, you must have determined that the load is less than 600 amps. Give the general rules, you must have determined that the load is 480 amps or less, and 600 amp conductor comply with the rule in 215.2 based on a 480 amp load.

Note, even with a 480 amp load, if you increase the primary breaker for in rush, the conductor rating cannot be less than the rating of the breaker.