At this point, we can either develop our own limits or we can adopt limits of others, like the IEC. We are fortunate in that a number of studies in the U.S. that have already been undertaken. The reports on these efforts cover the spectrum of low power devices like compact fluorescent lamps [Pileggi & Emanuel], larger loads like variable speed heat pumps and central air conditioning [Grady] and large electrical loads like battery chargers for electric vehicles [Grady]. These studies determine how much of a given nonlinear load a distribution system can tolerate before one exceeds 5% voltage THD. This is probably a more meaningful starting point since these studies involve system models conforming to the distribution systems found in the U.S. and the harmonic spectrum of modern single-phase converters.
The other option is to adopt the proposed IEC limits (i.e., 61000-3-2 and 61000-3-4). If the distribution systems were similar enough around the world, and if the use of such a standard was validated by field experience, this would make sense. However, there are two basic distribution systems found around the world and the two are different enough – in terms of harmonic implications.