What goes into a substation capacity figure

A utility-published capacity value for a substation and a sum of feeder values grouped under that substation describe different things. The first needs to be interpreted using the utility's assessment method. The second is a derived total whose usefulness depends on the underlying records, their grouping and any constraints they share.

A feeder sum is not proof of additional load that the substation can deliver. Several feeders may depend on the same transformer or upstream supply. Adding their individual values can obscure a shared constraint and overstate what can be connected simultaneously.

Start with the asset and the metric

Before comparing capacity figures, identify whether each describes a feeder, transformer bank or entire substation. Then establish whether it represents an equipment rating, remaining load-serving capacity or generation hosting capacity. Converting units does not resolve differences in equipment coverage or assessment conditions.

For example, a published feeder limit can reflect conditions on that feeder without establishing the unused capacity of its supplying transformer. Conversely, a substation value does not establish whether the equipment between it and a particular property can serve a new load.

Aggregation introduces its own assumptions

When grouping feeder records, retain the source identifier and the evidence for each feeder's substation assignment. Check for duplicate records, missing feeders and changes in network configuration. An unchanged capacity value does not establish that the feeder still belongs to the same group.

Records also need compatible definitions and units. A total assembled from different seasons, load scenarios or assessment methods can look precise while answering no consistent engineering question. Where a derived total is shown, label the aggregation method and its limits explicitly.

Keep three dates separate

• Retrieval date: when the data was downloaded.
• Publication or update date: when the utility says the record or dataset was released or changed.
• Assessment date: when the underlying capacity analysis was performed, where disclosed.

A new download can contain old assessments. A republished record may reflect a label change rather than a new engineering calculation. Where an assessment date is unavailable, say so; a retrieval timestamp cannot fill that gap.

An unchanged value is not evidence of either freshness or staleness. Preserve the observation and its provenance, then use the utility's documented dates and methodology to interpret it. Do not remove a record merely because its number did not change between downloads.

What a client should receive with the number

A useful site comparison identifies the source, asset level, metric, unit, dates and any derivation. It also flags missing information and differences that prevent a direct comparison. This lets a reader distinguish a published engineering estimate from a convenient aggregation.

Retaining source files and calculation history makes the result reproducible when records or methods change. The commercial value is traceability: a project team can understand why a site screened favorably and take a specific, qualified question to the utility. The number alone cannot establish the connection that the utility will offer.