Distribution vs. transmission interconnection: what changes at the POI

If you're screening candidate sites, you've probably run into the term "point of interconnection" without anyone stopping to define it, and you've definitely had someone confuse a distribution interconnection with a transmission interconnection, as if they're two sizes of the same thing. A distribution interconnection and a transmission interconnection are different queues, different studies, and often a different answer to whether your project pencils out at all.

Where the line actually gets drawn

Distribution interconnection covers facilities tied into the lower-voltage network that serves retail load, generally under 69 kV, sometimes stated as under 100 kV depending on the utility's tariff. Transmission interconnection covers the bulk system, 69 kV and up, running through the regional transmission organization or the utility's transmission planning group instead of its distribution engineering desk.

The practical split shows up fast once you start scoping a site. A 5 MW solar project tapping a 25 kV feeder goes through a distribution interconnection process, often with a simpler application and a shorter review window, because the utility is checking thermal limits, protection coordination, and voltage impact on a local circuit. A 200 MW wind project needs a transmission interconnection, which means a cluster study, a system impact study, a facilities study, and a queue position that can sit for years behind other projects claiming the same network capacity.

Size is the obvious driver, but it's not the only one. A smaller project can still land in a transmission queue if the nearest usable point of interconnection, the actual breaker or bus where your generator ties in, happens to sit on a transmission-class line rather than a distribution feeder. That siting detail decides the process every bit as much as project size does, and it's exactly where a straight-line measurement on a desktop map gets analysts into trouble. The nearest line on a ruler isn't always the nearest usable POI, and it isn't always the voltage class you assumed.

What changes once you pick a process

Once a project is sorted into distribution or transmission, the rest of the timeline follows from that choice. Distribution interconnection studies typically move faster and lean on standardized screens, fast-track criteria, or simplified processes for smaller inverter-based resources. Transmission interconnection studies run on RTO or ISO cluster cycles, stack projects against each other for shared network upgrade costs, and can take a project from initial application to a signed interconnection agreement over several years.

Cost allocation differs too. On the distribution side, upgrade costs are often narrower in scope: a feeder reconductor, a new recloser, a transformer bump. On the transmission side, a single project can trigger network upgrades shared across a whole cluster, and your project's share of that cost depends partly on queue position and partly on which substation or line segment turns out to be the actual constraint. None of that is visible from a map that only shows you the nearest pylon.

Why the point of interconnection decides more than the label

The label, distribution or transmission, is really just shorthand for which rulebook applies. What decides your project's economics is the real point of interconnection: the specific substation or line segment with usable capacity, the corridor your gen-tie would have to follow to reach it, and whether that corridor crosses terrain, parcels, or rights-of-way that add miles to the route a straight line would suggest. Two sites that look identical on a desktop map, same straight-line distance to the nearest visible line, can carry completely different connection costs once you account for the actual route and the actual voltage class at the far end.

That's the gap we built Grid Proximity Mapping to close: a real connection distance and a plausible route to the nearest substation or transmission corridor for each candidate site, instead of a ruler measurement that assumes the land in between is flat and empty. If you're ranking sites before a formal interconnection application, it's worth knowing the real distance before you file.