It is common in the sub-metering market to use simple terms to express the accuracy of electric energy meter (such as 0.2%), but the reality is very complicated. This article summarizes the common meter and current transformer (CT) accuracy standards, as well as overall system accuracy. It explains how the system accuracy (meter with CTs) can be much worse than just the meter accuracy or even the naïve addition of the meter and CT accuracies (a 0.2% meter with 0.3% CTs may not result in 0.5% accuracy).
- Download the full application here: AN-136: Metering System Accuracy (PDF, 5 pages)
- For information on the accuracy of CTs, see the CT Accuracy Standards page.
Certain WattNode® meter models meet the accuracy requirements of two commonly used U.S. metering standards:
- ANSI C12.1-2014
- ANSI C12.20-2010
Revenue WNC series meters (the model number starts with RWNC) meet ANSI C12.1-2008 accuracy. All WND series
meters meet ANSI C12.1-2008 accuracy. Models RWNC-3Y-208-MB, RWNC-3D-240-MB and RWNC-3Y-480-MB are
certified by MET Laboratories to meet ANSI C12.1. MET Labs is a nationally recognized testing laboratory (NRTL).
To achieve C12.1 system accuracy, the WattNode meters must be used with sufficiently accurate CTs, such as the Accu-CT® with Opt C0.6 (IEEE C57.13 class 0.6) or Opt C0.3 (IEEE C57.13 class 0.3).
The C12.1 standard is nominally accuracy class 1, corresponding to a 1% system accuracy under the most typical conditions, with added errors allowed at high and low current, varying power factor, varying temperature, and other varying conditions.
All WND series meters (model numbers starting with WND) meet ANSI C12.20-2010 class 0.5 accuracy.
To achieve C12.20 class 0.5 system accuracy, the WattNode WND series meters must be used with sufficiently accurate CTs, such as the Accu-CT with Opt C0.3 (IEEE C57.13 class 0.3).