What blood alcohol concentration (BAC) while operating a commercial motor vehicle (CMV) triggers mandatory CDL disqualification under federal regulations?

49 CFR 383.51, Table 1 to Section 383.51 (alcohol concentration disqualification); AAMVA CDL Manual Sec. 1.3.2, p.1-8

Commercial drivers are held to a stricter alcohol standard than ordinary drivers because a CMV crash has far greater potential for mass casualties and property damage. A first offense costs a full year without a CDL, and a second offense is a lifetime disqualification, so the exact .04% number (not .08%) matters in real consequences, not just on the test.