Transformer Calulation

[vortac555]

Hello All
First time poster. Hoping to get some supporting information on a project I am looking at.

Here goes
Deer Park , NY

Existing on Job

45kva step down transformer. 277/480v #2 wire Protected with 100 breaker. Which is approx 250 feet away from main service
Feeding 120/208 100 amp main breaker panel

The landlord is requiring 150 amps 120/208 volts

I am on the fence

I want to install a 75kva transformer and install a 150 amp 120/208 main breaker panel on existing feed

#2 wire 75c @ 480 volts up to 300 plus feet will support the 75kva transformer @ 90 amps Wire rated up to 115 amps

Do I need to run a new feeder for transformer

Thank you for any feed back

 

[iwire]

We often install 100 amp feeders to 75 kVA transfomers much to the chagrin of EEs who would prefer larger OCPD for inrush issues. (I am not saying they are wrong, only that we do it and have no issues)

 

[vortac555]

Transformers

Transformers

I do believe the 100 amp feeder is adequate.

However do I need to factor 125% to the 90 amps bring me to 112 amp.?? In turn increasing breaker size to 125amp. Which I cant do on 100 amp feeder or is this not a factor and I am being to overconcern

Or if the next guy who comes in for another tenant and sees 75kva and says great I can go to 200 amps with out checking the feeder to the transformer

Thanks

 

[Smart $]

I do believe the 100 amp feeder is adequate.

However do I need to factor 125% to the 90 amps bring me to 112 amp.?? In turn increasing breaker size to 125amp. Which I cant do on 100 amp feeder or is this not a factor and I am being to overconcern

Or if the next guy who comes in for another tenant and sees 75kva and says great I can go to 200 amps with out checking the feeder to the transformer

Thanks

Per 240.4(B) you can protect #2 copper at with up to a 125A breaker.

The cap for transformer primary protection is 125% for the primary only protection schema or 250% for the primary and secondary protection schema. Given the existing feeder ampacity being set (i.e. short of replacing it), you may as well opt for the primary only protection schema. With 125% corresponding to 112A, and Table 450.3(B) Note 1 permitting next higher standard rating, you can go with a up to a 125A breaker.

 

[iwire]

Per 240.4(B) you can protect #2 copper at with up to a 125A breaker.

The cap for transformer primary protection is 125% for the primary only protection schema or 250% for the primary and secondary protection schema. Given the existing feeder ampacity being set (i.e. short of replacing it), you may as well opt for the primary only protection schema. With 125% corresponding to 112A, and Table 450.3(B) Note 1 permitting next higher standard rating, you can go with a up to a 125A breaker.

I agree that is allowed but he only gains 12 amps of capcity over the the 100 amp breaker and typically movng to 125 breaker requires more expensive breaker and harware.

YMMV.

 

[vortac555]

Transformer Protection

Transformer Protection

I agree the breaker is expensive to upgrade to 125amp. But upgrading to 125. I can comfortable to at 200amp load side. For future upgrade. However I only need 150 amp main breaker panel on the load side. So Leaving 100 on the primary. I can be comfortable putting the 150 amp panel 120/208v and be in compliance.

 

[Smart $]

I agree that is allowed but he only gains 12 amps of capcity over the the 100 amp breaker and typically movng to 125 breaker requires more expensive breaker and harware.

YMMV.

With the rated primary current at 90A, he doesn't gain any capacity.

What is gained with a 125A primary breaker is inrush headroom.

Justification of hardware and greater cost fall to him and his client, but the least expensive option given the conditions noted is to leave the main as is and change the necessary equipment (transformer and load panel). Can always change out the main later, if and when it becomes necessary.