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Branch Circuit Load Calculations

Tuesday, June 22nd, 2010

The NEC requires that branch circuits be sized to serve 125% of the continuous load and 100% of the noncontinuous load (NEC 210.20(A)). Design Master Electrical includes an option to calculate the branch circuit load at 125% of both the continuous and noncontinuous. This is a more conservative approach than what the NEC requires. You can change this setting using the Size breakers to 100% of noncontinuous load setting in the Circuits, Load Types, and Wire Sizes section of the Options menu.

Ribbon: DME: Customization->Project Customization->  Options

Pulldown Menu: DM Electrical->Customization->Options->Edit Project List

The load types are broken down between continuous and noncontinuous as follows:

Continuous

  • Lighting
  • Motors (NEC 430.24)
    • Largest Motor
    • Other Motors
    • Cooling, Largest Motor
    • Cooling, Other Motors
    • Heating and Cooling Motor
    • Appliance Motor
  • Continuous
  • Heating (NEC 424.3B)
  • Appliance, Continuous

Noncontinuous

  • Receptacles (NEC Handbook Exhibit 220.4)
  • Noncontinuous
  • Kitchen
  • Diverse
  • Small Appliance
  • Laundry
  • Appliance, Noncontinuous
  • Electric Dryer
  • Electric Cooking
  • Marina / Mobile Home / RV

When you create a custom load type, you specify whether it should be treated as a continuous or noncontinuous load for branch circuit sizing. Do this using the Custom Load Type: Load Type setting in the Circuits, Load Types, and Wire Sizes section of the Options menu.

All motors are considered continuous at the branch circuit level. NEC 424.19 allows you to treat the largest as continuous and the rest as noncontinuous. If that is required for your design, you will need to manually downsize the motor breaker and wires.

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