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Electrical Fault Current Three Phase Underground Services: Maximum Secondary Fault Currents Transformer Size (kVA) Minimum Impedance Percentage 12/208V 3ø or ᴓ-G Fault 277/480V 3ø or ᴓ-G Fault 45 1.3 9,610 4,170 75 1.5 13,880 6,020 112.5 1.5 20,820 9,020 150 2.0 20,820 9,020 225 2.25 27,760 12,030 300 2.25 37,010 16,040 500 2.25 61,680 26,730 750 3.6 25,060 1000 4.25 28,300 1500 4.25 42,450 2000 5.25 45,800 Three Phase Overhead Services: Maximum Secondary Fault Currents Transformer Size (Total kVA) Minimum Impedance Percentage 120/208V 3ø or ᴓ-G Fault 277/480V 3ø or ø-G Fault 240/120V 4 Wire Delta 3ᴓ Fault 240/120V 4 Wire Delta Ø-G Fault 480V Delta 3 Ø 45 1.5 8,330 3,610 7,220 8,080 3,610 75 1.5 13,880 6,020 12,030 13,470 6,020 112.5 1.5 20,820 9,020 18,040 20,210 9,020 150 1.5 27,760 12,030 24,060 26,950 12,030 225 1.7 36,740 15,920 31,840 35,660 15,920 300 1.7 48,990 21,230 42,450 47,550 21,230 500 1.7 81,640 35,380 70,760 79,250 35,380 750 2.0 104,090 45,110 90,210 101,040 45,110 Single Phase Underground and Overhead Services: Maximum Secondary Fault Currents Transformer Size (kVA) Minimum Impedance Percentage 120/240V L-L Fault 120/240V L-G Fault 120V L-L Fault 480V L-L Fault 10 1.5 2,780 4,360 5,560 1,390 15 1.5 4,170 6,540 8,330 2,080 25 1.5 6,950 10,900 13,890 3,470 37.5 1.5 10,420 16,350 20,830 5,210 50 1.5 13,890 21,810 27,780 6,940 75 1.7 18,380 28,860 36,770 9,190 100 1.7 24,510 38,480 49,020 12,260 167 1.7 40,930 64,260 81,860 20,470 250 2.0 52,080 81,770 104,170 26,040