A feeder is already carrying 520 A on conductor rated for 640 A. A developer wants to add a load that would draw another 90 A at the feeder’s coincident peak. Does it fit? Answer before you read on.
It fits, barely. The test is total coincident load against the conductor’s ampacity: 520 + 90 = 610 A against a 640 A rating, leaving 30 A of headroom. Ampacity is the current a conductor can carry continuously before it overheats. It is a thermal limit, set by how much heat the conductor can shed, and it is a hard ceiling that has nothing to do with voltage quality. A feeder can run out of thermal room or voltage room first, and you check the two separately.
The number that trips people up is the 90 A. A new subdivision might have 240 A of connected nameplate load, but its homes and appliances never all peak in the same hour. That is diversity: the coincidence factor is well below 1, so the group’s contribution to the feeder’s peak hour is far smaller than the sum of its parts. You add the diversified, coincident contribution to the feeder’s peak, not the raw connected nameplate. Add nameplate and you would reject a load that actually fits; ignore diversity the other way and you would approve one that does not.
That 30 A margin is thinner than it looks. If the diversity assumption is wrong and the new load’s coincident draw is 120 A instead of 90, the feeder lands at 640 A, dead on the rating with no room left. This is why planners do not load a feeder to 100 percent of ampacity in the first place. Headroom is what lets an operator transfer load onto this feeder when a neighboring one is out, and ampacity itself shrinks as ambient temperature climbs, so a summer afternoon rating is lower than a nameplate value. The thermal limit is specific to the conductor and its conditions, not a single fixed number.
A load fits only when its coincident contribution, not its nameplate, still leaves the feeder below its ampacity with room to spare. A buyer sizing a service, an operator weighing a transfer, and an engineer approving an interconnection are all running the same check: diversified load in, thermal ceiling as the wall, and deliberate headroom in between.
SAIDI and SAIFI targets drive capital planning, and capital planning drives what gets ordered and when. The Feeder tracks the reliability investment and regulatory signals behind those orders, free, every week.