Prevailing wage vs. union wage

A negotiated rate and a legal minimum - and where they meet

Prevailing wage and union scale are related but not the same thing. Union scale is the wage-and-benefit package negotiated in a collective-bargaining agreement (CBA) between a union local and signatory employers. A prevailing wage is a legal minimum set by government for covered public work - which may or may not equal the union rate, depending on what prevails locally.

When they're the same number

In areas where union agreements cover most workers in a trade, DOL's determination process adopts the CBA rate as the prevailing rate - determination lines whose identifier begins with a union abbreviation (e.g. ELEC0001) are exactly that (see how to read a determination). Those rates then bind union and non-union contractors alike on covered projects.

When they differ

Where union coverage is thin, the prevailing rate comes from DOL's wage surveys (SU lines) and is typically below union scale. And on private work, union scale applies only to signatory employers - the prevailing wage doesn't apply at all unless a prevailing-wage law covers the project.