Switchgear, Panelboards & Electrical Distribution worked example

Breaker Loading Margin with breaker or bus continuous rating of 63 A: a worked example

This worked example runs the breaker loading margin numbers for a tougher week than the baseline: breaker or bus continuous rating of 63 A instead of the typical 125 A. Breaker loading margin is the headroom between what a breaker or bus is rated to carry continuously and what the connected load actually draws.

The inputs for this scenario

  • Breaker or bus continuous rating: 63 A (the input this scenario stresses; the baseline uses 125)
  • Calculated continuous load: 100 A (held at the documented default)
  • Rating reference for percentage: 100 A (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Breaker Loading Margin margin = available value - required value.
  • Margin works out to -37 % at these inputs, and this is the headline figure for the scenario.
  • Absolute margin works out to -37 value at these inputs.
  • Available amount works out to 63 value at these inputs.
  • Required amount works out to 100 value at these inputs.

How this compares with the baseline

  • Against the tool's baseline example, where breaker or bus continuous rating sits at 125 A and the headline result is 25 %, this scenario comes in 248% below the baseline at -37 %.
  • Use it when sizing feeders, adding loads to an existing panel, or verifying a breaker is not overloaded during a coordination study. A result at this level usually justifies acting on the stressed input before touching anything else, because every other figure in the table is downstream of it.

Results at a glance

  • Margin: -37 % (headline result)
  • Absolute margin: -37 value
  • Available amount: 63 value
  • Required amount: 100 value

Run it with your numbers

  • To rerun this with your own numbers, open the live Breaker Loading Margin calculator, set breaker or bus continuous rating to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.