X-frmr calculation

[david luchini]

What about this table's note 3, if this criteria is met OCP can be up to 6 times the rated current.

Yes, if the transformer was equipped with "coordinated thermal overload protection" and has an impedance of not more than 6%, then the primary OCPD could be 1000A. The primary conductor ampacity would have to be at least 1000.

But I don't see anything in the OP that suggests the xfmr has "coordinated thermal overload protection."

 

[tryinghard]

Yes, if the transformer was equipped with "coordinated thermal overload protection" and has an impedance of not more than 6%, then the primary OCPD could be 1000A. The primary conductor ampacity would have to be at least 1000.

But I don't see anything in the OP that suggests the xfmr has "coordinated thermal overload protection."

It was asked in Post 13

 

[david luchini]

It was asked in Post 13

Huh -

Post 13 says:

So for a transformer with Primary and Secondary OCPD, The primary OCPD is based upon the 125 % of the Primary Current withe the option to go up to 250 % as permitted in the second half of of table 450.3(B) ? And if this is the case, is this expansion in percentage the cure for inrush? Must I then resize my primary conductors?

There's nothing about "coordinated thermal overload protection" in Post 13.

 

[tryinghard]

Yes, if the transformer was equipped with "coordinated thermal overload protection" and has an impedance of not more than 6%, then the primary OCPD could be 1000A. The primary conductor ampacity would have to be at least 1000.

But I don't see anything in the OP that suggests the xfmr has "coordinated thermal overload protection."

I have a 400 amp buss that is a 3 phase 4 wire system rated for 277/480 volts. This application is suppose to feed a 150kva step down transformer = 480-120/208 volts 3 phase 4 wire. I'm having diffuculties trying to figure out the feeder conductors aswell as over current protection for this particular installation. Where do I start?

You also won’t see “Primary and secondary protection” from T450.3B - I hope you’re not rubbed or bothered; the OP is asking how to feed and protect this XFMR. There are great answers early on revealing excellent methods of how to arrive at this info answering to start with the load.

You pointed out some good stuff and what’s evolved topically is this XFMR’s OCP can be 1000A its primary feeder would have to be sized per OCP, a panelboard is not sized per XFMR but must be sized at least for it’s specific calculated load and fed as such and it will also include OCP (it can be 2000A). The only thing actually related to the XFMR in this method is the primary circuit and XFMR protection, not the secondary conductors or downstream items.

Why stop at assuming only part of T450.3B?

 

[david luchini]

Why stop at assuming only part of T450.3B?

Well why muddy the water with T450.3(B) Note 3? The OP didn't mention that he planned on using a transformer equipped coordinated thermal overload protection.

A standard transformer would have a 400A max primary c/b. The overload protected could have a 1000A max primary c/b. The transformer is being supplied from a 400A bus. Why bring the 1000A max c/b for the overload protected transformer into the mix?

 

[tryinghard]

Well why muddy the water with T450.3(B) Note 3? The OP didn't mention that he planned on using a transformer equipped coordinated thermal overload protection.

A standard transformer would have a 400A max primary c/b. The overload protected could have a 1000A max primary c/b. The transformer is being supplied from a 400A bus. Why bring the 1000A max c/b for the overload protected transformer into the mix?

I understood some (maybe only me) were sizing from the XFMR info only, and hypothetically, not only from the OP's 400A supply point. We got it though good info here - thanks for your energy!