Rising Edge Services, LLC

Table 450.3(A) Calculator: Maximum Overcurrent Protection for Transformers Over 1000 Volts

Secondary Protection
Primary Protection over 1000 Volts Over 1000 Volts 1000 Volts or Less
Location Limitations Transformer Rated Impedance Circuit Breaker Fuse Rating Circuit Breaker Fuse Rating Circuit Breaker or Fuse Rating
Any Location Not more than 6% 600%
(See Note 1)
300%
(See Note 1)
300%
(See Note 1)
250%
(See Note 1)
125%
(See Note 1)
More than 6% and not more than 10% 400%
(See Note 1)
300%
(See Note 1)
250%
(See Note 1)
225%
(See Note 1)
125%
(See Note 1)
Supervised locations only Any 300%
(See Note 1)
250%
(See Note 1)
Not required Not required Not required
Not more than 6% 600% 300% 300% 250%
(See Note 5)
250%
(See Note 5)
More than 6% and not more than 10% 400% 300% 250% 225%
(See Note 5)
250%
(See Note 5)

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.