How equipment ratings are used to determine capacity

A number on a hosting capacity map is a distance from whichever of several ceilings is lowest. Most maps won’t tell you which one that is.

There are many different types of ceilings. Every wire or conductor has a thermal rating, which is the current it can carry before heat becomes a safety problem, independent of anything else happening on the circuit. This is a fixed physical property of the equipment rather than a planning choice. To raise the rating, the utility would need to replace the conductor with a heavier one, which utilities call reconductoring. It's a disruptive process that requires new wire, new poles or trenching in some cases, which is an outage to cut it in.

Upstream lies the transformer, and its ceiling is a separate number from the wire's. A transformer's nameplate rating is usually given in kVA or MVA rather than MW. Converting between the two requires a power factor assumption, commonly somewhere around 0.9, so a transformer's true MW ceiling is an estimate. They’re independent constraints, so a feeder can be nowhere near its own thermal limit while the transformer feeding it is nearly full, or the reverse.

Another ceiling, which isn’t a physical restraint, is the margin utilities deliberately hold in reserve. Protective relays are set to trip below the equipment's physical maximum, set by a planning choice. Sometimes it’s a product of contingency planning, where the utility is sizing a substation for what it needs to carry today plus what it needs to carry if a piece of upstream equipment fails and its load has to be picked up elsewhere. Orange & Rockland's regulatory filings describe this kind of tiered approach: normal, long-term emergency, and short-term emergency ratings. N-1 and N-2 contingencies A substation can look full while still having headroom according to its physical maximum depending on how much contingency was planned.