What is the actual vertical hole spacing (center-to-center) based on the EIA-310 standard for one mounting unit or U?

Question

Options

• A25.40 mm (1 inch)
• B50.80 mm (2 inches)
• C38.10 mm (1.5 inches)
• D44.45 mm (1.75 inches)

• A
  2% (1)
• B
  10% (6)
• C
  5% (3)
• D
  83% (49)

Explanation

Option D is correct because the EIA-310 standard defines one rack unit (1U) as 1.75 inches (44.45 mm) in total height, and the vertical hole spacing - measured center-to-center between the two holes within a single U - follows this same 1.75-inch pitch. Option A (25.40 mm / 1 inch) is a common unit of measurement but has no direct role in defining U spacing. Option B (50.80 mm / 2 inches) would be closer to a 2U measurement, not 1U. Option C (38.10 mm / 1.5 inches) is simply not a value defined in the EIA-310 standard and does not correspond to any standard rack interval.

Memory tip: Think of "1U = 1.75 inches" as the foundational rack fact - the 0.75 extra beyond 1 inch accounts for the panel border tolerances built into the standard. If you remember 1.75 inches, the millimeter conversion (44.45 mm) follows directly.

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