Transformer Secondary OCPD

[aramg83]

I was looking for info to double check my work and I found this:

http://ecmweb.com/mag/electric_transformer_installation_made/

Please take a look at Step 4 for secondary OCPD/conductor sizing. The person who wrote this states:

"Where a feeder supplies continuous loads, the rating of the (secondary) overcurrent device may not be less than 125% of the continuous load (215.3) as listed in 240.6(A),"

then goes on to size his secondary ocpd at 125% of the transformer's rated secondary current. Isn't he wrong? 215.3 is referring to all the loads downstream from the transformer, right? Wouldn't the correct method of calculating this be based on the sum all the loads utilizing the transformer? I suppose his calculation would be correct if the transformer were loaded to 100% of its rated current, but I assume that would be very poor practice.

Furthermore, 450.3(B) says the secondary OCPD can be sized at a MAXIMUM of 125% of the transformer's rated secondary current, in this case. If he were correct the NEC would be setting a maximum value equivalent to its minimum value.

From my experience, EC&M has been pretty reputable so I'm wondering what's going on, here. Thanks.

 

[augie47]

I was looking for info to double check my work and I found this:

http://ecmweb.com/mag/electric_transformer_installation_made/

Please take a look at Step 4 for secondary OCPD/conductor sizing. The person who wrote this states:

"Where a feeder supplies continuous loads, the rating of the (secondary) overcurrent device may not be less than 125% of the continuous load (215.3) as listed in 240.6(A),"

then goes on to size his secondary ocpd at 125% of the transformer's rated secondary current. Isn't he wrong? 215.3 is referring to all the loads downstream from the transformer, right? Wouldn't the correct method of calculating this be based on the sum all the loads utilizing the transformer? I suppose his calculation would be correct if the transformer were loaded to 100% of its rated current, but I assume that would be very poor practice..

It does appear the author is looking at "worse case" scenarios. He seems to be looking at a 100% continuous load and also a non-linear load as he is derating his secondary base on 4 ccc.

Furthermore, 450.3(B) says the secondary OCPD can be sized at a MAXIMUM of 125% of the transformer's rated secondary current, in this case. If he were correct the NEC would be setting a maximum value equivalent to its minimum value..

From his examples, it appears his transformers have 125% primary protection so, from a Art 450 standpoint, no secondary protection is required. You would, of course, need to meet Art 240 and 408 requirements, but the 125% max secondary OCP in Art 450 is based on primary OCP > 125%

From my experience, EC&M has been pretty reputable so I'm wondering what's going on, here. Thanks.