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Branch circuit sizing playbook

Goal​

Select a branch circuit that meets load needs, conductor limits, and protective device requirements with a clear inspection trail.

Inputs you must know before you start​

  • Load type and continuous or non-continuous classification.
  • Voltage, phase, and environment (ambient, bundling, insulation type).
  • Installation method (raceway, cable, exposed run).
  • Equipment instructions that affect terminals or conductor size.

The path (in order)​

  1. Classify the load and determine the design current.
  2. Select conductors based on ampacity after adjustments.
  3. Choose the OCPD from standard ratings that fit the conductors.
  4. Confirm grounding and bonding paths are complete and compatible.
  5. Document assumptions and NEC references used.

This is​

A practical sequence for sizing a standard branch circuit that ties conductor selection to OCPD choice and grounding continuity.

This is not​

A complete design for specialty equipment or systems with dedicated articles.

Inspection-ready checklist​

  • Load calculation and classification documented.
  • Conductor ampacity after derating recorded.
  • OCPD rating matches conductor limits and standard sizes.
  • Terminations and equipment instructions verified.
  • Grounding and bonding path verified.

Worked example​

A 20A continuous load at 120V requires a branch circuit sized above the design current. Choose a conductor that supports the adjusted ampacity, then pick the nearest standard OCPD rating that stays within conductor limits.

Cross references​

  • Primary: 210
  • Secondary: 110, 300, 310, 240, 250

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