/images/buttons/NEC_button.jpg/images/southwire_100x70.jpg2002 NEC Adds More Metrics - July/August 20022002NECAddsMetrics07and082002.htmThe 2002 NEC continues the process of adding metric units to the code,” says Dave Mercier, technical director for Southwire’s Electrical Division. “Does that mean you have to learn a lot of new numbers? Probably not.”
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2002 NEC Adds More Metrics - July/August 2002

The 2002 NEC continues the process of adding metric units to the code,” says Dave Mercier, technical director for Southwire’s Electrical Division. “Does that mean you have to learn a lot of new numbers? Probably not.”

According to Mercier, the goal of the NEC is to have metrication reflect the needs of end-users, and to support harmonization with international codes and standards. “The strategy isn’t intended to preclude the use of existing products, require redesign of existing products, or require new products,” notes Mercier. “No industry is required to convert sizes or product markings unless the manufacturers support the action.”

How they do it

Here’s what you should know to understand how the 2002 NEC uses metrics.
• For 2002, the NEC now lists metric units first, followed by the equivalent inch-pound units.
• The NEC uses rounded-off inch-metric conversions for trade sizes and approximate dimensions.
• The NEC uses higher-accuracy inch-metric conversions for product dimensions such as conductor areas, and for dimensions that relate to safety.

Approximate measurements and dimensions
An example of rounding for approximate measurements can be found in NEC section 330.30 “Securing and Supporting. Type MC cable shall be supported and secured at intervals not exceeding 1.8 m (6 ft).” In this example 1.8 m is actually 5.9 ft. The difference of 1.2 inches does not compromise safety. An example of approximate dimensions is conduit.

In the 2002 NEC’s Chapter 9, Tables, Table 4 gives dimension of conduit and tubing. For example, when you talk about conduit trade sizes, a nominal 1" conduit has an actual ID of 1.049”. For a conduit trade size of 1 inch, the metric designator is 27. If you ask for 1" conduit, or 27 mm EMT you get the same thing.

Product dimensions are more accurate

Safety-related items and actual product dimensions-- such as conductor area – get more accurate conversions. Wire sizes reflect actual metal areas, so the NEC uses exact numbers. You can also find wire size conversion numbers in Chapter 9. Look in Table 8, Conductor Properties for an example:


 Size (AWG)

 mm2

 Circular Mmils

 6

 13.30

 26240

Will this impact wire sizes in the U.S.?

Unless you’re bidding on an international project, you probably won’t need to worry about metric wire sizes. NAFTA guidelines call for Mexico, Canada and the U. S. to continue to use AWG and kcmil numbers as their primary wire and cable size designations.