Transformer Overcurrent protective device

[Dennis Alwon]

If a 30 kva 480/277 to 208/120 transformer is installed into a gutter and tapped to the service conductors I assume I need protection on the line side but how do we determine whether or not the load side needs overcurrent protective device. Table 450.3(B) has me confused and I haven't worked on a trany in over 25 years.

I have to go but we be back and hopefully there will be 100 posts....:lol:

 

[david luchini]

If a 30 kva 480/277 to 208/120 transformer is installed into a gutter and tapped to the service conductors I assume I need protection on the line side but how do we determine whether or not the load side needs overcurrent protective device. Table 450.3(B) has me confused and I haven't worked on a trany in over 25 years.

I have to go but we be back and hopefully there will be 100 posts....:lol:

If the primary OCPD is greater than 50A (125% of primary current, next standard size up), then you will need secondary transformer protection.

You will still need protection for the secondary conductors, even if the transformer is 'primary only protection,' but the sizing isn't per 450.3(B).

 

[Dennis Alwon]

If the primary OCPD is greater than 50A (125% of primary current, next standard size up), then you will need secondary transformer protection.

You will still need protection for the secondary conductors, even if the transformer is 'primary only protection,' but the sizing isn't per 450.3(B).

Thank you David. What code section calls for the greater than 50 amps

 

[david luchini]

Thank you David. What code section calls for the greater than 50 amps

Table 450.3(B) - "primary only protection" - Note 1 allows the next higher standard rating. If you exceed max allowed primary only protection, then you would be forced into "primary and secondary protection."

 

[Dennis Alwon]

Table 450.3(B) - "primary only protection" - Note 1 allows the next higher standard rating. If you exceed max allowed primary only protection, then you would be forced into "primary and secondary protection."

Okay , I see you did the math. I was looking for a statement that said over 50 amps.... So If the primary is fused at 50 amps then no overcurrent protective device is needed on the secondary. Is that correct? I just assumed the secondary would need overcurrent protective device.

 

[david luchini]

So If the primary is fused at 50 amps then no overcurrent protective device is needed on the secondary. Is that correct? I just assumed the secondary would need overcurrent protective device.

There is no need for a secondary ocpd to protect the transformer, but you still need an ocpd to protect the secondary conductors per 240.21(C).

 

[Dennis Alwon]

There is no need for a secondary ocpd to protect the transformer, but you still need an ocpd to protect the secondary conductors per 240.21(C).

Yes but 240.21 (C) has options for no overcurrent protective device. Correct?

 

[david luchini]

Yes but 240.21 (C) has options for no overcurrent protective device. Correct?

Yes, but only for 2-wire single voltage secondary and 3-wire delta-delta single voltage secondaries. You can protect those secondary conductors with the primary OCPD if you meet the requirements in 240.21(C)(1).

For a 208/120V secondary transformer, you would need secondary conductor protection on the secondary side.

 

[Dennis Alwon]

Yes, but only for 2-wire single voltage secondary and 3-wire delta-delta single voltage secondaries. You can protect those secondary conductors with the primary OCPD if you meet the requirements in 240.21(C)(1).

For a 208/120V secondary transformer, you would need secondary conductor protection on the secondary side.

Thank you-- I read that backwards.