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%	167%	300%	Not required	Not required
Primary and secondary protection	250%	250%	250%	125%	167%

	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%	167%	300%	Not required	Not required
Primary and secondary protection	250%	250%	250%	125%	167%

3Ø, Three Phase 1Ø, Single Phase 3Ø or 1Ø, Phase
Transformer Size, "KVA"
Primary Voltage, "P_L-L"
Secondary Voltage, "S_L-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.