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Electrical Load Diversity

Tuesday, June 22nd, 2010

The National Electric Code (NEC) has a number of rules for determine the wire size required to feed a load. Design Master Electrical incorporates most of these rules automatically. Branch circuits and panel feeders are sized differently.

Branch Circuits

The NEC requires that branch circuits be sized to serve 125% of the continuous load and 100% on 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” option in “DM Electrical->Customization->Options.”

Here are how the load types are broken down between continuous and noncontinuous:

Continuous

  • Lighting
  • Motor (NEC 430.24)
  • Continuous
  • Heating (NEC 424.3B)

Noncontinuous

  • Noncontinuous
  • Receptacle (NEC Handbook Exhibit 220.4)
  • Kitchen
  • Diverse

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” option in “DM Electrical->Customization->Options.”

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.

Panel Feeders

Design Master Electrical does not automatically size feeders. The software calculates the total load on a panel, but you have to select the final size for the feeder.

Lighting

The calculated lighting load is equal to 125% of the connected lighting load.

Motors

The calculated load on the largest motor on the panel is equal to 125% of the motor load. All other motors on the panel are sized at 100% of their connected load. The largest motor is based upon all the motors connected to the panel and any panels it feeds.

More information about how motor loads are sized can be found here.

Receptacles

The receptacle load is calculated at 100% up to 10 kVA and 50% above that.

Continuous

Continuous loads are calculated at 125% of the connected load.

Heating

Heating loads are calculated at 100% of the connected load.

Noncontinuous

Noncontinuous loads are calculated at 100% of the connected load.

Kitchen

Kitchen loads are calculated based upon the number of pieces of kitchen equipment. Each receptacle or equipment connection is counted as a separate piece. The diversity factor is based upon NEC 220.56.

Diverse

Diverse loads do not have a default calculated load. You specify the diversity factor for the load when you create it that is used for the calculated load.

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