Article 90.9 of the National Electrical Code (NEC) outlines the units of measurement used in the Code, ensuring clarity and consistency in electrical installations.
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Article 90.9: Units of Measurement
(A) Measurement System of Preference
- The NEC uses the International System of Units (SI) as the primary measurement system.
- SI units are part of the modernized metric system.
(B) Dual System of Units
- Primary Units: SI units are listed first.
- Secondary Units: Inch-pound units follow in parentheses.
- Conversion: Hard conversion is used unless otherwise specified in 90.9(C).
- Exception: Informative Annex C tables may list trade sizes before SI units.
(C) Permitted Uses of Soft Conversion
Soft conversion is allowed in the following cases:
- Trade Sizes:
- Where the actual measured size differs from the nominal size, trade size designators are used.
- Trade practices must be followed.
- Extracted Material:
- When material is extracted from another standard, the original context must not be compromised.
- Editing is limited to making the style consistent with the NEC.
- Industry Practice:
- If industry practice uses inch-pound units, SI units are not required.
- Safety:
- Soft conversion is used if hard conversion would have a negative impact on safety.
(D) Compliance
- Approximate conversions from inch-pound units to SI units are permitted.
- Compliance with either system (SI or inch-pound) constitutes compliance with the Code.
Informational Notes:
- Informational Note No. 1:
- Hard Conversion: Changes dimensions or properties into new sizes, which may or may not be interchangeable.
- Soft Conversion: Direct mathematical conversion without changing actual dimensions.
- Informational Note No. 2:
- SI conversions are based on IEEE/ASTM SI 10-1997, the standard for using the International System of Units.
Summary of NEC Article 90.9
| Section | Description |
|---|---|
| (A) Measurement System | SI units are the primary system of measurement. |
| (B) Dual System | SI units listed first, followed by inch-pound units in parentheses. |
| (C) Soft Conversion | Allowed for trade sizes, extracted material, industry practice, and safety. |
| (D) Compliance | Approximate conversions permitted; compliance with either system is acceptable. |
Why This Matters
- Consistency: Ensures uniformity in measurements across the NEC.
- Flexibility: Allows for both SI and inch-pound units, accommodating industry practices and safety needs.
- Clarity: Provides clear guidelines for conversions and compliance.