Rockwell Automation 1492 APPL.NOTE/PROTECT TRANS.CRCTS. - Manual (Page 3)

Using Circuit Breakers
To Protect Transformer Circuits
Application Note
3
Circuit breaker protection for the primary side of the transformer can be
sized per NEC (National Electrical Code) Article 450-3(b)(2) which states:
”All transformers of 600V, nominal, or less, shall be protected by an
individual overcurrent device on the primary side . . .”
Article 450-3(b)(2) defines guidelines for sizing an overcurrent protection
device. The correct sizing of a transformer primary winding depends upon
whether or not the secondary windings have an overcurrent device.
Overcurrent Device on Primary Winding Only
Load Source
Device
Up to 125%
of Load
Overcurrent
Primary
A primary overcurrent device can be sized at, but not more than 125% of the
rated transformer load if the secondary side is not protected.
In this application, the primary overcurrent device protects the load as well
as the transformer primary windings.
Overcurrent Devices on Both Primary and Secondary Windings
Load Source
Device
Up to 250%
of Load
Overcurrent
Device
Up to 125%
of Load
Overcurrent
Primary Secondary
An overcurrent device on the primary side can be rated at, but not more than
250% of the rated transformer load, if the secondary side of the transformer
has an overcurrent device rated at but not more than 125% of the rated
transformer load.
In this application, the primary overcurrent device is protecting the
transformer primary windings only.
Selecting a Circuit Breaker
For Transformer Protection