Sizing of Telecommunication Bonding Backbone (TBB) TBB length FT Grounding Conductor Size (AWG) DC Resistance Per 100 Ft (Copper Conductor) ShortTime Rating (A) 8 0.0778 391 < 13 Ft 6 0.0491 621 14 - 20 Ft 4 0.0308 988 21 -26 Ft 3 0.0245 1245 27 - 33 Ft 2 0.194 1571 34 - 41 Ft 1 0.0154 1981 42 - 52 Ft 1/O 0.0122 2499 53 - 66 Ft 2/O 0.00967 3150 > 66 FT 3/O 0.00766 3972 4/O 0.00608 5008 Kcmil 250 0.00515 5917 300 0.00429 7101 350 0.00367 8284