What are the standard accuracy tolerances for mechanical meters vs smart meters? AEP Texas keeps telling us their old meters are "within acceptable limits" at +/- 2% but I'm seeing mechanical meters that are consistently reading 4-5% low. Is there a difference between ANSI standards for mechanical vs AMI meters? Also wondering if accuracy tolerances are different for residential vs commercial applications.
Question about meter accuracy tolerance standards
ANSI C12.20 sets accuracy standards for mechanical meters at +/- 2% for standard residential/small commercial. ANSI C12.1 covers electronic meters (including smart meters) with tighter tolerances - typically +/- 0.5% to 1.0% depending on the accuracy class. Commercial and industrial meters generally have stricter tolerances than residential. If your mechanical meters are consistently reading 4-5% low, they're definitely outside acceptable limits and should be replaced.
Indianapolis Power & Light uses +/- 2% as their standard tolerance for both mechanical and smart meters in their tariff, but the smart meters typically perform much better than that in practice. We've had independent testing done on questionable mechanical meters and found some reading as much as 8% low after 30+ years of service. The utility has to replace meters that test outside the published tolerance, but you have to pay for the testing upfront ($600-800 typically) and only get reimbursed if the meter fails.