OCP For 7.5KVA Transformer

[sparkyrick]

Working on a new restaurant and the plan calls for a 42 circuit, single phase panel dedicated solely to POS equipment. A 7.5KVA, single phase transformer is to feed this panel. The plan calls for a 2 pole, 60A breaker protecting the transformer primary, but another 2 pole, 60A protecting the secondary. Isn't that secondary OCPD way oversized? I think a 40A OCPD should be installed instead. Am I missing something?

 

[raider1]

What voltage is the transformer primary?

Also what size are the secondary conductors?

Chris

 

[bob]

Working on a new restaurant and the plan calls for a 42 circuit, single phase panel dedicated solely to POS equipment. A 7.5KVA, single phase transformer is to feed this panel. The plan calls for a 2 pole, 60A breaker protecting the transformer primary, but another 2 pole, 60A protecting the secondary. Isn't that secondary OCPD way oversized? I think a 40A OCPD should be installed instead. Am I missing something?

If anything the primary seems over sized. I say that without knowing what the primary voltage is. What is the primary voltage? If the secondary voltage is 240 colts, the secondary FLA is 31 amps. 60 amp seems high, but there may be a reason for the 60.

 

[sparkyrick]

If anything the primary seems over sized. I say that without knowing what the primary voltage is. What is the primary voltage? If the secondary voltage is 240 colts, the secondary FLA is 31 amps. 60 amp seems high, but there may be a reason for the 60.

Sorry, My bad. 208V primary, 240V secondary. Primary OCPD can be up to 250% with both sides being protected, but only 125% on the secondary, as I understand it.

 

[sparkyrick]

What voltage is the transformer primary?

Also what size are the secondary conductors?

Chris

Plan called for #6's on both primary and secondary. And as I stated above....208V primary, 240V secondary.

 

[augie47]

I would agree with a 40 on the secondary being more in line with Code.

 

[DARUSA]

Take a look 430.3 B table

 

[raider1]

Sorry, My bad. 208V primary, 240V secondary. Primary OCPD can be up to 250% with both sides being protected, but only 125% on the secondary, as I understand it.

The primary overcurrent device can be 250% if the secondary windings of the transformer have overcurrent protection. Does your transformer have secondary winding protection?

Most likely there is not secondary winding protection so the primary overcurrent protection must be sized at 125%. This would require that you use a 50 amp circuit breaker to protect the primary.

Chris

 

[sparkyrick]

The primary overcurrent device can be 250% if the secondary windings of the transformer have overcurrent protection. Does your transformer have secondary winding protection?

Most likely there is not secondary winding protection so the primary overcurrent protection must be sized at 125%. This would require that you use a 50 amp circuit breaker to protect the primary.

Chris

Yup, the secondaries feed into a 2/60A breaker. I think it should be downsized to a 2/40A.

 

[sparkyrick]

Take a look 430.3 B table

Transformer, not motor. And there is no 430.3 B table in the 2008 NEC :roll:

 

[Pierre C Belarge]

Transformer, not motor. And there is no 430.3 B table in the 2008 NEC :roll:

I believe he meant 450.3

 

[sparkyrick]

I believe he meant 450.3

Yup, thats where I went first. 250% max on primary and 125% on secondary.

 

[DARUSA]

450.3 b Table Sorry my bad! :grin: