The NEC allows copper busbars to carry 1000 amps per square inch. If a bus bar measures 1/4 inch by 2 inches, how many amps is this bar allowed to carry?

• A. 250
• B. 750
• C. 500
• D. 1000

Answer: (C)

Current capacity for copper busbars is determined by current density per unit area. Multiply the cross‑sectional area by the allowable current per square inch. The bar is 1/4 inch by 2 inches, giving an area of 0.25 × 2 = 0.5 square inches. At 1000 amps per square inch, the bar can carry 0.5 × 1000 = 500 amps. So the bar is allowed to carry 500 amps. (If the area were smaller, the allowable current would be lower; if larger, higher.)