NEC 90.13: plain language explanation

What NEC 90.13 Actually Covers

NEC 90.13 is the section that tells you how to read the Code itself. It covers the metric and inch-pound measurement system used throughout the NEC, and clarifies which value governs when a conflict appears. For working electricians, this section rarely gets flipped to, but it matters when specs, plans, or inspector questions hinge on exact dimensions.

The rule is short. The NEC lists trade sizes and measurements in both SI (metric) and inch-pound units. The inch-pound value is the one you install to. The metric figure is a soft conversion, included for international reference and harmonization, not for enforcement.

If a plan sheet or submittal calls out a metric value and it does not match the inch-pound equivalent in the Code, the inch-pound value wins on the jobsite in the United States.

Trade Sizes vs. Actual Dimensions

Trade size is a label, not a measurement. A 1/2 inch EMT is not 0.500 inches inside or outside. A 3/4 inch rigid is not 0.750 inches in any real dimension. NEC 90.9(C)(1) and Chapter 9 tables make this explicit: trade sizes are designators, and the dimensional data in Chapter 9 is what governs for fill, bending, and clearance calculations.

When you pull Chapter 9, Table 4, you will see both metric designator and trade size columns. Metric designator 16 equals trade size 1/2. Metric designator 21 equals trade size 3/4. These are not math conversions. They are parallel labels for the same pipe.

• Metric designator 12 = trade size 3/8
• Metric designator 16 = trade size 1/2
• Metric designator 21 = trade size 3/4
• Metric designator 27 = trade size 1
• Metric designator 35 = trade size 1 1/4
• Metric designator 41 = trade size 1 1/2
• Metric designator 53 = trade size 2

When the Metric and Inch-Pound Values Disagree

You will see rules like "900 mm (3 ft)" in working clearance requirements under NEC 110.26(A)(1). The metric number 900 mm is actually 35.43 inches, not 36 inches. The Code rounds for practicality. Per 90.13, the 3 ft value is what you install and what the inspector measures against.

Same story with 1.8 m versus 6 ft in 110.26(E) dedicated equipment space rules. 1.8 m is 70.87 inches, not 72. Build to 6 ft. Measure in feet and inches. If a foreign-trained hand or an imported spec sheet flags the discrepancy, 90.13 is your citation.

Field tip: when an inspector shows up with a metric tape, that is not a failure point for you. Pull up 90.13 on your phone and the inch-pound value is the enforceable one. Keep your 25 ft Stanley and measure in feet and inches.

Why This Matters on Submittals and RFIs

Engineers sometimes spec conduit or clearances in metric on drawings, especially on federal work, pharma, or projects designed by international firms. If a plan shows a 27 mm conduit run and the schedule calls for trade size 1, those are the same pipe. Do not order based on the metric number as if it were a measurement. Order trade size 1 EMT, RMC, or whatever the spec demands.

When you respond to an RFI where the metric and inch-pound values create ambiguity, cite 90.13 and the relevant Chapter 9 table. That closes the loop fast and keeps the submittal moving. Inspectors and engineers know the rule, but they appreciate seeing it pinned to a specific citation rather than a vague "the Code says so."

Quick Reference for the Field

Commit these to memory so you are not thumbing through the Code every time a metric value shows up on a print:

1. Inch-pound value governs installation and inspection in the U.S.
2. Metric values in the NEC are soft conversions, rounded for clarity.
3. Trade size is a designator, not a dimension. Use Chapter 9 tables for real numbers.
4. Metric designators and trade sizes are parallel labels. 21 mm equals 3/4 inch, not 21 mm of actual pipe.
5. On federal or international-spec jobs, confirm which system the contract documents require before pricing.

Chapter 9 tables do the heavy lifting for fill calculations, bending radius, and conductor dimensions. 90.13 just tells you which column to read when you are on a U.S. jobsite.

Common Mistakes and How to Avoid Them

The most frequent slip is treating the metric number as a precise measurement instead of a label. Someone reads "metric designator 35" and thinks it means 35 mm of actual pipe, then tries to calculate fill from that. Fill calculations come from the cross-sectional area values in Chapter 9, Table 4, not from the designator.

Another mistake is rounding the other direction on clearance. If a plan shows 900 mm for working space and you install to 35.43 inches because that is the exact metric conversion, you are short by more than half an inch of the 36 inch requirement. Build to 36 inches. Always round up to the inch-pound value, never down to the metric equivalent.

Field tip: if you are ever unsure whether a dimension on a print is a hard metric spec or an NEC soft conversion, check whether the same number appears in parentheses after an inch-pound value in the Code. If yes, it is a soft conversion and the inch-pound value rules.

NEC 90.13 is one of those sections you read once, understand, and then apply without thinking about it. Knowing it exists is what matters when a dispute comes up over a measurement or a metric spec. Pin it, cite it, and move on.