Rising Edge Services, LLC

Table 450.3(B) Calculator: Maximum Overcurrent Protection for Transformers 1000 Volts and Less

Primary Protection Secondary Protection
Protection Method Currents of 9 Amperes or More Currents Less Than 9 Amperes Currents Less Than 2 Amperes Currents of 9 Amperes or More Currents Less Than 9 Amperes
Primary only protection 125%
(See Note 1)
167% 300% Not required Not required
Primary and secondary protection 250%
(See Note 3)
250%
(See Note 3)
250%
(See Note 3)
125%
(See Note 1)
167%

3Ø or 1Ø, Phase
Transformer Size, "KVA"
Primary Voltage, "PL-L"
Secondary Voltage, "SL-L"

Note: NEC 240.4(C) Where the over current device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6.