Which protective ground size can withstand 21 kA for 15 cycles?

• A. #2 Copper: 14 kA for 15 cycles
• B. 1/0 Copper: 21 kA for 15 cycles
• C. 4/0 Copper: 43 kA for 15 cycles
• D. 2/0 Copper: 18 kA for 15 cycles

Answer: (B)

Protective grounds must carry the fault current long enough for the protective device to clear, so the ground conductor size is chosen to meet the short-time fault current rating for the given duration. Here, the requirement is to withstand 21 kA for 15 cycles (about a quarter of a second). The conductor you pick should have a rating at least equal to that value for that duration.

Evaluating the options, the 1/0 copper option is rated for 21 kA for 15 cycles, meeting the exact requirement. The #2 copper option provides only 14 kA for 15 cycles, which is insufficient. The 2/0 copper handles 18 kA for 15 cycles, also not enough. The 4/0 copper can carry 43 kA for 15 cycles, which exceeds the need but is larger than necessary for this specific requirement.

Therefore, the 1/0 copper ground, rated for 21 kA for 15 cycles, is the appropriate choice because it meets the needed rating without over-design.