A newbie question about 75KVA transformer

[unsaint33]

So, it sounds like there are two common wiring methods for 75KVA (480V/Y208V) transformers as far as the OCPD sizing.


1) Primary OCPD - 90A Secondary OCPD - 200A
2) Primary OCPD - 125A Secondary OCPD - 300A.


If inrush current was not a factor, what factors make one choose between the two? And the factors change the primary or secondary OCPDs? And What is the rule to size the secondary OCPD once the primary OCPD size is determined?

 

[don_resqcapt19]

The primary conductor size is just based on the rating of the OCPD that feeds those conductors. The secondary conductors are based on the rules in 240.21(C). Remember that the rules for the overcurrent protection of the transformer itself are in Article 450, the rules for the protection of the primary conductors are found in Article 240, and if you have a panel on the secondary of the transformer, it has to be protected per 408.36.

All of the requirements must be complied with, but a single OCPD can statistic more than one requirement. For example a breaker in a panel can protect the transformer secondary, the secondary conductors, and the panel itself.

 

[augie47]

Ignoring inrush, the actual and proposed load might effect your decision along with, of course, dollars.
Both sizes wpiuld be Code compliant but the 125 would allow you to get the full capacity from the transformer but, since the primary conductors are based on the OCP, the cost of installing the circuit could influence you8r decision.
As don notes, if your primary is protected at 125% per 450.3, the secondary rules are covered in 240.21(C) & Note 2 of 450.3(B)

 

[unsaint33]

For the 90A primary example, using 240.21 C rule, the secondary conductor minimum ampacity shall be (480/208) x (1/3) x 90 = 69 A. I was wondering where the 200A came from?

 

[david luchini]

For the 90A primary example, using 240.21 C rule, the secondary conductor minimum ampacity shall be (480/208) x (1/3) x 90 = 69 A. I was wondering where the 200A came from?

The rated secondary current of a 75kVA, 208/120V transformer is 208A.

 

[kwired]

For the 90A primary example, using 240.21 C rule, the secondary conductor minimum ampacity shall be (480/208) x (1/3) x 90 = 69 A. I was wondering where the 200A came from?

You must be referring to 240.21(C)(6), the 25 foot tap rule. It is the only one I am seeing with that 1/3 factor involved. Here that just means minimum size tap conductor in that situation would be a 69 amp conductor.

A 75 kVA 208 volt secondary putting out full rating would deliver 208 amps, so there is more than 69 amps available without overloading from the transformer itself, and nothing says you must use all 208 amps or design a circuit that can handle all 208 amps.

 

[unsaint33]

You must be referring to 240.21(C)(6), the 25 foot tap rule. It is the only one I am seeing with that 1/3 factor involved. Here that just means minimum size tap conductor in that situation would be a 69 amp conductor.

A 75 kVA 208 volt secondary putting out full rating would deliver 208 amps, so there is more than 69 amps available without overloading from the transformer itself, and nothing says you must use all 208 amps or design a circuit that can handle all 208 amps.

So, I take that to mean this transformer produces 208A max on the secondary. Then, why some setups have 300A OCPD on the secondary side, which requires bigger wires?

 

[augie47]

You are allowed to go 125% + ext size up so the 300 amp breaker would meet Code.
Really pushing the transformer but some folks like that.

 

[unsaint33]

You are allowed to go 125% + ext size up so the 300 amp breaker would meet Code.
Really pushing the transformer but some folks like that.

That is not that uncommon so I'm trying to understand why? What advantage does that give over using 200A OCPD?
Or 225A or 250A for that matter.

 

[kwired]

That is not that uncommon so I'm trying to understand why? What advantage does that give over using 200A OCPD?
Or 225A or 250A for that matter.

The transformer isn't exactly limited to 208 amps, that is what current will be when 75kVA is being supplied. It can and will deliver more than that, but at cost of more heating in the transformer.

Short term overloading may be deemed acceptable in some cases, continuous overloading generally is not without at least dealing with ways to manage excess heat.