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Section 90.9 explains that the NEC prefers measurements using the International System of Units (abbreviated SI), sometimes called the metric system, or scientific units.
Most NEC rules originally were in English units (such as pounds, inches, feet or gallons) and have been converted into SI units (such as grams, meters and liters). The NEC includes values for both systems of units, with the SI units appearing first and the English units following in parenthesis.
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