When designing an underfloor duct system, which of the following would be the cross-sectional area of the duct serving each 100 square feet of floor space with 3 cables to each of the work areas.

Question

Options

• A3 square inches.
• B4 square inches.
• C1 square inches.
• D5 square inches.

• A
  6% (3)
• B
  4% (2)
• C
  78% (42)
• D
  13% (7)

Explanation

Option C (1 square inch) is correct based on NEC Article 390 standards for underfloor duct systems, which specify that 1 square inch of cross-sectional duct area is required per 100 square feet of floor space when serving work areas with 3 cables each. Options A (3 sq in) and B (4 sq in) are common traps because test-takers may confuse the number of cables (3) with the duct area, or conflate different raceway sizing rules from other NEC articles. Option D (5 sq in) is simply too large and does not correspond to any standard underfloor duct sizing formula.

Memory tip: Think "1 square inch for 100 square feet" - the duct size is 1, not the cable count. The 3-cable detail is a distractor designed to make you choose 3; anchor on the floor area (100 sq ft → 1 sq in) instead.

No community discussion yet for this question.