Transformer Secondary Protection

[Rich Elec.]

Table 430.3 (B)
Secondary Protection
Currents of 9 amps or more
(second row)125% (see Note 1)
Note 1 allows "next higher standard rating shall be permitted." for fuse or breaker.

240.21(C) "The provisions of 240.4(B) shall not be permitted for transformer secondary conductors."
240.4(B) being next higher device shall be permitted.

I am confused at how 430.3(B) allows next size up for secondary OCPD, and 240.21(C) does not allow next size up OCPD.

 

[raider1]

Remember that 450.3(B) is dealing with the transformer secondary winding protection only and does not address secondary conductor protection.

Secondary conductor protection is regulated by 240.21(C).

Chris

 

[Rich Elec.]

I am with you there Chris.
Maybe I want to ask:
1. When is secondary transformer protection required?
2. Where would it be located?

Using "Currents of 9 amps or more"
Is Transformer secondary protection required only if the primary OCPD is sized over 125%?
Then where is this OCPD located. It would be on the secondary conductors, then the next size up question comes up again.

 

[Rich Elec.]

I found some wording that is helping me understand.
"if you provide overcurrent protection on the secondary side and properly size the secondary conductors, you can often use this one device to protect the ttransformer and the supplied conductors."

 

[augie47]

IMO, the first thing you need to do, in most situations, is to keep in mind, as Chris states, Art 450 is for transformer protection and Art 240 for conductor protection.
Determine how you plan to protect the transformer (125% secondary/250% primary, or 125% primary). and select your primary conductor and OCP accordingly.
You then look at your secondary conductors. In a limited situation, explained in 240.21(C)(1), the secondary conductors are protected by the primary OCP device. In a vast majority of transformer installs you are required to have secondary OCP regardless of the primary protection. You determine which of the 240.21(C) rules fits your application and select the conductor and OCP based on that conductor.