Transformer loading with secondary protection @ 125%

[winslowfam]

Hi...our client has calculated his expected load on a group of new equipment and, based on this, the distribution transformer was sized...and, it includes a safety factor based on his company's load history. The calculated load is about 10% below the KVA of the transformer. His question to us is: since the code allows the secondary protection to be set at 125% of the transformer FLA, does this mean he would have 25% more capacity on that transformer for additional loads than just the basic KVA rating, should he ever needed it? He is just wondering how much of a safety factor he really has. I didn't have a quick answer for him. What does it mean, practically, to the potential loading on a transformer when the secondary protection is allowed to be set at 125% of FLA? Thanks.

 

[ron]

The secondary is typically sized to 125% because of the 80% continuous rating on molded case breakers.

The transformer shouldn't be used above it's kVA nameplate

 

[jim dungar]

The secondary is typically sized to 125% because of the 80% continuous rating on molded case breakers.

The 80% rating is applicable to enclosed fuses as well as molded case breakers. 100% rated protective devices require 'special' enclosures.

The secondary 'main' protective device is sized (rated) in a manner similar to the conductors, just like for any other feeder or branch circuit.