Isolation transformer for UPS system

[philly]

We are installing a new isolation transformer upstream of an existing UPS on a 480V system which is being converted to an HRG system. The current configuration on a solidly grounded system has an ATS fed from two upstream feeders with the load side of the ATS feeding into the existing UPS. The existing UPS does not have input OCPD's but rather has a 1200A input switch as well as a 1200A bypass switch with a jumper between these two switches at the input of the UPS.

We are now installing a new 112.5kVA 480D-480Y transformer between the existing ATS and UPS. We plan on replacing the two upstream feeder circuits which feed the ATS with 150A breakers which are within 125% of primary FLA.

Because the primary protection is within 125% of transformer FLA there is no secondary protection required for the transformer itself. However to my knowledge there will still be OCPD required for the for the transformers secondary #1/0 cables between the transformer and UPS. Because the UPS does not have input OCPD's do we need to add some sort of OCPD between transformer and UPS or is there a provision in code that allows for no OCPD required at UPS for protection of these secondary cables?

 

[Andy Delle]

Wouldn't the maintenance bypass breakers act as your transformer secondary OCPD.

 

[drktmplr12]

We are installing a new isolation transformer upstream of an existing UPS on a 480V system which is being converted to an HRG system. The current configuration on a solidly grounded system has an ATS fed from two upstream feeders with the load side of the ATS feeding into the existing UPS. The existing UPS does not have input OCPD's but rather has a 1200A input switch as well as a 1200A bypass switch with a jumper between these two switches at the input of the UPS.

We are now installing a new 112.5kVA 480D-480Y transformer between the existing ATS and UPS. We plan on replacing the two upstream feeder circuits which feed the ATS with 150A breakers which are within 125% of primary FLA.

Because the primary protection is within 125% of transformer FLA there is no secondary protection required for the transformer itself. However to my knowledge there will still be OCPD required for the for the transformers secondary #1/0 cables between the transformer and UPS. Because the UPS does not have input OCPD's do we need to add some sort of OCPD between transformer and UPS or is there a provision in code that allows for no OCPD required at UPS for protection of these secondary cables?

how far is the UPS from the transformer secondaries? look at the tap rule in 240.21. the secondaries will fall within one of the sub-categories there.

 

[Dale001289]

We are installing a new isolation transformer upstream of an existing UPS on a 480V system which is being converted to an HRG system. The current configuration on a solidly grounded system has an ATS fed from two upstream feeders with the load side of the ATS feeding into the existing UPS. The existing UPS does not have input OCPD's but rather has a 1200A input switch as well as a 1200A bypass switch with a jumper between these two switches at the input of the UPS.

We are now installing a new 112.5kVA 480D-480Y transformer between the existing ATS and UPS. We plan on replacing the two upstream feeder circuits which feed the ATS with 150A breakers which are within 125% of primary FLA.

Because the primary protection is within 125% of transformer FLA there is no secondary protection required for the transformer itself. However to my knowledge there will still be OCPD required for the for the transformers secondary #1/0 cables between the transformer and UPS. Because the UPS does not have input OCPD's do we need to add some sort of OCPD between transformer and UPS or is there a provision in code that allows for no OCPD required at UPS for protection of these secondary cables?

Since you don't have overcurrent protection in the UPS you'll need to add protection for the secondary conductors, terminating in a single 3-Pole device within 25 feet, unless your transformer is located outdoors.

 

[philly]

how far is the UPS from the transformer secondaries? look at the tap rule in 240.21. the secondaries will fall within one of the sub-categories there.

My situation falls under the distance restrictions of 240.21(C)(2) and 240.21(C)(3) however both of these sections require the secondary conductors to terminate into an overcurrent device(s) which does not currently exist in the UPS (There is only bypass switch and switch on input to UPS module). To me it sounds like I will need to add a protective device inside of the UPS in order to terminate my secondary conductors into before it splits off to the UPS input and bypass branches.

These UPS input switches and bypass switches are 1200A and 800A respectively but I sure these are just standard frame sizes in order to accommodate various UPS sizes. Is it typical for a UPS not to have an input breaker?

Since you don't have overcurrent protection in the UPS you'll need to add protection for the secondary conductors, terminating in a single 3-Pole device within 25 feet, unless your transformer is located outdoors.

Even if they are outdoors 240.21(C)(4) still requires the secondary conductors to terminate into a single overcurrent protective device.

 

[drktmplr12]

My situation falls under the distance restrictions of 240.21(C)(2) and 240.21(C)(3) however both of these sections require the secondary conductors to terminate into an overcurrent device(s) which does not currently exist in the UPS (There is only bypass switch and switch on input to UPS module). To me it sounds like I will need to add a protective device inside of the UPS in order to terminate my secondary conductors into before it splits off to the UPS input and bypass branches.

These UPS input switches and bypass switches are 1200A and 800A respectively but I sure these are just standard frame sizes in order to accommodate various UPS sizes. Is it typical for a UPS not to have an input breaker?



Even if they are outdoors 240.21(C)(4) still requires the secondary conductors to terminate into a single overcurrent protective device.

The OCPD need not be located inside the UPS. It can be a fused disconnect adjacent to the transformer. Personally that's how I would comply.

The intent of the restriction is to protect secondary unprotected conductors which are too long. Once they are protected there are no resitrictions outside of typical wiring methods.

Sent from my Pixel 2 using Tapatalk

 

[Russs57]

Often the UPS has fuses after the switches. Maybe they are sized so they meet code for your conductors.

Just a thought.