A 3 trade size conduit is sized at

Question

Options

• A78 mm (3 in)
• B91 mm (3 1/2 in)
• C103 mm (4 in)
• D129 mm (5 in)

• A
  73% (36)
• B
  14% (7)
• C
  8% (4)
• D
  4% (2)

Explanation

A 3 trade size conduit corresponds to the metric designator 78 mm, which is the NEC-standardized metric equivalent of the 3-inch trade size designation used throughout electrical code tables. Options B (91 mm), C (103 mm), and D (129 mm) are the correct metric designators for trade sizes 3-1/2, 4, and 5, respectively - each is one step larger than the 3 trade size, making them easy to confuse when working through conduit fill problems under time pressure. The metric designators are not exact inch-to-millimeter conversions; for example, 3 inches × 25.4 = 76.2 mm, but the NEC rounds and standardizes this to 78 mm, which is why memorizing the table values matters more than calculating them.

Memory tip: The four answer choices (78, 91, 103, 129) map directly to trade sizes 3, 3-1/2, 4, and 5 in order. If you remember that 78 mm anchors the 3-inch trade size, you can count up through the sequence for larger sizes.

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