What is the typical slab thickness requirement for a two level underfloor duct system where feeder and distribution ducts intersect?

Question

Options

• A4 inch.
• B5 inch.
• C6 inch.
• D7 inch.

• A
  13% (2)
• B
  6% (1)
• D
  81% (13)

Explanation

Option D (7 inches) is correct because a two-level underfloor duct system requires additional concrete depth at intersection points where larger feeder ducts cross over distribution ducts - the stacked duct layers plus minimum required concrete coverage above and below the assembly necessitate the full 7-inch slab thickness to meet NEC Article 390 requirements.

Options A (4") and B (5") are insufficient - these thicknesses may apply to single-level systems or shallow installations, but they cannot accommodate the vertical stacking of two duct levels plus the mandatory concrete coverage above the upper duct. Option C (6") is close but still falls short; it's a common distractor because it seems reasonable for a "double" system, but the actual intersection geometry and cover requirements push the minimum to 7 inches.

Memory tip: Think "2 levels = 7 inches" - the number 7 has two distinct parts just like a two-level duct system has two distinct layers. Alternatively, remember that going from a single-level to a two-level system adds roughly 3 inches of depth (from ~4" to 7") to account for the second duct tier crossing underneath.

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