NEC Calculation Requirements, Part 12

NEC Calculation Requirements, Part 12In the previous issue, we started to look at the few ways Article 215 affects your calculations. We left off with the 125% rule presented in 215(A)(1). This same rule is presented in 215.3 for sizing overcurrent protection devices.

The other Article 215 requirements that could affect your calculations are in Subsection 215, which is divided into Point A (Under 600V) and Point B (Over 600V). Let’s look at what those are.

  • Size the grounded conductor of the feeder circuit per 250.122 (or larger).
  • If the feeder conductors carry the total load of the service, their ampacity must be at least that of the service conductors (if 55A or less).
  • The ampacity of feeders over 600V supplying transformers must be at least the sum of the transformer nameplate ratings (if the feeders supply only those transformers).
  • The ampacity of feeders over 600V supplying transformers must be at least the sum of the transformer nameplate ratings and 125% of the designed potential load (if the feeders supply those transformers and utilization equipment).

That second bullet point above is confusing to many people. If the feeder conductors and the service conductors are carrying the same load, wouldn’t they be sized at the same ampacity anyhow?

It might seem so at first glance. But you size feeder conductors per the requirements of 215.2 and service conductors per the requirements of Article 230 (different subsections depending on the type). These requirements can produce different results.

« Part 11 | Part 13 » | Source: Mark Lamendola | Mindconnection

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