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Lighting Calculations

Posted on November 25th, 2013. Last updated on December 12th, 2022.

There are two ways to calculate lighting loads in Design Master Electrical. You can use both in a project.

The General Lighting Circuit setting controls the method that is being used for each circuit in the project. A default value is set for all the circuits on each distribution equipment using the Default General Lighting Circuit Value field in the Feeder and Service Calculation Settings dialog box. The value can be overridden on an individual circuit basis using the General Lighting Circuit field in the Circuiting dialog box.

General Lighting Circuit: No

When the General Lighting Circuit value is set to No, any lights on the circuit are treated as a continuous load. In the service calculation, they are counted at 125% of their connected load. This method is how lighting loads were calculated prior to Design Master 8.3.

General Lighting Circuit: Yes

When the General Lighting Circuit value is set to Yes, all of the loads on the circuit are treated as part of the general lighting load, regardless of what their specific load type is. This restriction is necessary because there are several loads that are considered general lighting but are not actually lighting loads, such as dwelling unit receptacles, small appliance circuits, and laundry circuits.

The minimum general lighting load on the panel is set by the Occupancy Area settings in the Feeder and Service Calculation Settings dialog box. There are multiple unit load values that depend upon the type of building.

Set the occupancy areas in the most downstream panels, as far from the utility as possible. The areas will be carried up the tree to panels upstream, the same way as loads. The area used to calculate the general lighting load on a panel is the sum of all areas on the panels connected to it.

Circuits set to Yes and occupancy areas are further split into two categories, NEC Table 220.45 Demand and 100% Demand. The load calculated for each category using the occupancy area is the minimum general lighting load. If the connected load of the circuits assigned to the category exceeds the minimum general load, the connected load is used instead.

The final calculated value depends upon which category is selected. For the NEC Table 220.45 Demand category, the service load is calculated using that table. The occupancy type for the table is based upon the Load Calculation Method setting for the panel.

For the 100% Demand category, the service load is 100% of the connected load. This category is included for the note at the bottom of the table regarding areas in hospitals and hotels where the entire lighting load is likely to be used at one time.

For multifamily dwellings, the general lighting loads are further split. All of the areas associated with the 3 VA/sf unit load are assumed to be part of the dwelling unit loads. All of the areas associated with any other unit loads are assumed to be part of the house loads.

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