Sizing conductors for transformers

[rjenkins]

When sizing conductors for primary and secondary do you have to increase the size by 125% when there is nothing but the transformer involved? The secondary side of the transformer has branch circuits that go to lights, fans, and defrost circuits in refridgerated cases?
The transformer sizes are 75KVA,45KVA,30KVA,15KVA,& 9KVA. If you take the KVA and divide by the voltage x square root of 3 you would have amps. Then size the wire correct?

 

[charlie b]

If you have a 750 KVA transformer, and install a 20 amp breaker immediately at its secondary side, you can then install #12 wires from there to a couple of lights. The 125% value that appears in Table 450.3(B) is a maximum. There is no minimum.

 

[rjenkins]

When sizing conductors for primary and secondary do you have to increase the size by 125% when there is nothing but the transformer involved? The secondary side of the transformer has branch circuits that go to lights, fans, and defrost circuits in refridgerated cases?

The size of transformer is 75KVA,45KVA,30KVA,15KVA and 9KVA. The primary side is protected, but what I am asking is conductors not OCP.

 

[RUWired]

The size of transformer is 75KVA,45KVA,30KVA,15KVA and 9KVA. The primary side is protected, but what I am asking is conductors not OCP.

RJ, Charlie is saying that you base the conductor size for each transformer on the ocpd.The conductor size must meet the ocpd for each of the transformers you have listed above.Article 240.21(C) has some of the rules for the minimum secondary conductors.
Rick

 

[RUWired]

. There is no minimum.

Reading article 240.21(C) promted me to point out that there are a few minimum requirements.

10' rule
(1) The ampacity of the secondary conductors is
c. Not less than one-tenth of the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary to secondary transformer voltage ratio

25' rule
(1) The secondary conductors shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the primary of the transformer.
Rick