The National Electrical Manufacturers Association (NEMA) has published ANSI C12.20 “American National Standard for Electricity Meters—0.2 and 0.5 Accuracy Classes”. Last published in 2002, the standard was produced by the ANSI C12 Accredited Standards Committee for Electricity Metering, Subcommittee 16.

ANSI C12.20 establishes acceptable performance criteria for electricity
meters, and covers accuracy class designations, current class
designations, voltage and frequency ratings, test current values,
service connection arrangements, pertinent dimensions, form
designations, and environmental tests.

In this revision, the standard was broadened to allow three-phase
current and voltage sources as an optional test method to the existing
single-phase, series, parallel method.

CLICK HERE for ANSI C12.20.

CLICK HERE for NEMA.