Reading negative capacity values

536 out of 2,163 circuits in a PSE&G New Jersey snapshot I pulled came back negative. That's 24.8% of the dataset. The greatest in magnitude was -62.7 MW.

That can happen because PSE&G defines its available capacity estimate as a circuit's maximum load value minus peak load. The utility says outright that the load inputs are raw exports, not cleaned up for weather, meter error, or distributed generation.

So a negative value just means the peak in that subtraction was higher than the reference maximum. It doesn’t tell you if the equipment is overloaded.

Equation definitions

A reference maximum sounds like a hard ceiling, but it isn't measured each time the source updates. It's an engineering assumption about the feeder head rating, and PSE&G has revised it before. In February 2022 the utility raised its hosting capacity figures by about 2 MW per feeder, not because any wires got upgraded, but because the underlying engineering assumption changed. A negative number today doesn't necessarily mean it's negative next year.

Peak load doesn’t have a clear definition either, and how it's measured changes what a negative value represents. Utility distribution planning departments typically don't design around last year's peak. In fact, some utilities focus on a 1-in-10-year peak, a statistically extreme, weather-normalized figure meant to represent the worst plausible year, not the one that happened to occur. PSE&G's map uses the peak the feeder hit in the past year, an observed number, but one that's only as representative as that particular year's weather happened to be. A mild summer produces a low peak and, all else equal, a rosier capacity number than the utility's own internal planning would consider safe.

The hour and season a feeder peaks depends heavily on what's on it. Circuits dominated by air conditioning peak on hot summer afternoons, while circuits with a lot of electric heat pumps tend to peak in cold-weather mornings and evenings instead. Circuits with EV charging depots push peaks toward overnight hours. Two feeders showing the same peak load number can be seeing that peak for different reasons and different times of day or times of year. This matters a lot for developers, since a summer daytime load added to a feeder that's only ever tight in winter mornings might not stress it.

Magnitude interpretation

A feeder that's barely negative in a year with an unusually hot summer might look fine again the moment a normal summer returns. The constraint was the weather, not the equipment. On the other hand, a feeder that's deeply negative regardless of the specific year is a different kind of finding.

Utilities are often treating these negative capacity constraints as solvable, through what they call non-wires alternatives. These include demand response, storage, or shifting load off the peak window specifically. ConEd's Brooklyn Queens Demand Management program is a nice example, as it achieved greater than 50 MW of peak demand reduction for about $200 million, compared to a traditional substation-and-feeder construction that would have cost over $1 billion. A negative number on a screening tool should answer when does this feeder peak, and can the load in question avoid that window?, rather than a definitive dead end.