In a five-wire run of 4/0 ends ending in a 1200A distro box, what is the total amperage capacity?

In a five-wire run of 4/0 ends ending in a 1200A distro box, what is the total amperage capacity?

• A. 690 A
• B. 1200 A
• C. 900 A
• D. 800 A

Answer: (A)

When conductors are run in parallel, their current-carrying capacity adds up. Here we have five parallel 4/0 conductors ending in a distro box rated at 1200 A. Using the typical ampacity for 4/0 aluminum at a 75°C rating, each conductor can carry about 138 A. Five of them together yield 5 × 138 A ≈ 690 A. The distro box rating of 1200 A is higher than this total, so the overall capacity is limited by the conductors themselves, giving about 690 A. If the box rating were lower than the sum, that smaller value would control. Always use the same conductor material and temperature rating when summing ampacities and apply any necessary derating factors.

When conductors are run in parallel, their current-carrying capacity adds up. Here we have five parallel 4/0 conductors ending in a distro box rated at 1200 A. Using the typical ampacity for 4/0 aluminum at a 75°C rating, each conductor can carry about 138 A. Five of them together yield 5 × 138 A ≈ 690 A. The distro box rating of 1200 A is higher than this total, so the overall capacity is limited by the conductors themselves, giving about 690 A. If the box rating were lower than the sum, that smaller value would control. Always use the same conductor material and temperature rating when summing ampacities and apply any necessary derating factors.