Pulled interval data from a warehouse client on ComEd and some of the 15-minute readings are negative. Like -42 kWh in a single interval. The warehouse doesn't have solar or any generation equipment. What causes negative interval readings?
Negative interval values — what does this mean?
Negative values usually mean one of three things: a meter error, a data transmission glitch from the smart meter, or the meter was recently replaced and the new meter's register started at a lower number than where the old one left off. On ComEd specifically, I've seen their AMI meters produce occasional negative spikes during firmware updates. The utility usually catches these and adjusts them but sometimes they flow through to the billing. How many negative intervals are we talking about — a handful or hundreds?
This is worth investigating carefully. A few scattered negative values are likely data glitches and the utility may have already adjusted the billing. But if you see a pattern of negative values — especially during the same time of day or on the same day of the week — it could indicate a CT wiring issue where one of the CTs is installed backwards. That would cause the meter to read negative when current flows in one direction. I've seen this on three-phase installations where a contractor reversed one phase during maintenance. The net effect is that the meter underreads, which means the client is actually being underbilled. Something to be aware of.
Only about 12 negative intervals over a 6-month period, scattered randomly. Sounds like data glitches rather than a wiring issue. Good to know about the CT reversal possibility though — I'll file that away for future reference. Thanks.