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Electrical DistributionSystem

General Overview

An electrical distribution system is made up of switchgear and panelboards connected by wires. Switchgear is used to disaggregate the power coming from the utility grid to serve various aggregate loads in a facility, such as lights, motors, receptacle circuits, or miscellaneous equipment. A panelboard is used as the final disconnect and protection point for individual circuits in a space. A lighting panelboard is connected to the light fixture system in one or more spaces, typically on a single floor of a facility, or the outdoor light fixture system. Panelboards may be dedicated to the lighting system or “mixed” serving both lighting loads and other circuits.

Components of an Electrical Distribution System

Figure 1 shows the main components associated with the electrical distribution system: switchgear and panelboard.

Figure 1. Diagram of an electrial distribution system.
Figure 1. Diagram of an electrial distribution system.

Switchgear

Switchgear is an assembly that contains circuit breakers, fuses and other accessories to interrupt current flow and protect electrical equipment and occupants. The electricity delivered by a utility company is passed through a step-down transformer and then to a switchgear as the first layer of protection in a facility. Then electricity is distributed among other assemblies such as a secondary switchgear or panelboards. The configuration of primary and secondary switchgears is dependent on the facility. Not all facilities will have secondary switchgear.

Dedicated Panelboard

A dedicated electrical panelboard supplies electricity only to the light fixture system. Figure 2 shows a dedicated panelboard without a panel cover to show how the electrical wiring is distributed. The electricity from the primary or secondary switchgear enters from the top (or bottom) of the panelboard then splits across all the branch circuits.

Figure 2. Dedicated panelboard system.
Figure 2. Dedicated panelboard system.

Figure 3 is an example of a schedule of circuits on a dedicated panelboard. The schedule typically describes the type of load (e.g., lights) and the space that is served by the breaker. There is no standard naming convention for circuits.

Figure 3. Example of a dedicated panelboard schedule.
Figure 3. Example of a dedicated panelboard schedule.

Mixed Panelboard

A Mixed electrical panelboard supplies electricity to the lighting fixture system and other loads, typically receptacle circuits. Mixed panelboards typically exist in small or older facilities. The electricity from the primary or secondary switchgear enters from the top (or bottom) of the panelboard then splits across all of the branch circuits. The interior of a mixed panelboard will look similar to Figure 2 but the panel schedule will highlight a collection of different loads as shown in Figure 4.

Figure 4: Example of a mixed panelboard schedule. Blue represents lighting fixtures, red represents others electrical loads

Figure 4: Example of a mixed panelboard schedule.1

Evaluation of Energy Consumption

The electrical distribution system does not consume electricity, but it can be a useful point at which to measure the electrical energy associated with the Lighting Plant.

Table 1. Key values, components and measurements

System Quantification

Values to be Quantified

Energy Consuming Component

Measurements

Lighting fixture system electricity consumption (kWh)
  • Average hourly panelboard or switchgear kWh
  • Total operating time for the light fixtures

Light fixtures and automatic controls (if applicable).
  • Average hourly electricity supplied to the light fixture system from the switchgear or panelboard
  • Total operating hours of the light fixtures

Measurement Locations

Figures 5 and 6 show typical measurement points in an electrical distribution system.

Figure 5: Three-phase power measurement using a Dent EliteProXC Power Logger to capture to total power draw of all fixtures served by the panelboard.
Figure 5: Three-phase power measurement using a Dent EliteProXC Power Logger to capture to total power draw of all fixtures served by the panelboard.
Figure 6: Current measurement using current transformers and a data logger to collect measurement data. In this figure measurements are taken at the circuit breaker level to capture specific fixtures. Voltage and power factor measurements are taken with a power meter.
Figure 6: Current measurement using current transformers and a data logger to collect measurement data. In this figure measurements are taken at the circuit breaker level to capture specific fixtures. Voltage and power factor measurements are taken with a power meter.

Measurement points in the electrical distribution system are at the conductors in the panelboard or switchgear. To measure a lighting plant, the best practice is to capture as much of the lighting plant in a single measurement as possible. Measuring at switchgear may be appropriate if all lighting panelboards are dedicated and connected to it, but only the distribution line that supplies electricity to lighting should be measured. Alternatively, a sample of panelboards and/or circuits can be measured but a sampling plan must be considered. For more information on sampling please refer to Bonneville Power Administration’s guide on sampling.

Calculation Methodology

Lighting Plant and Systems Energy Consumption Calculation Methodology and CalculatorCalculation

Further Reading

  • Consulting-Specifying Engineer (20). “Back to Basics: Switchgear, Transformers and UPSs.”. https://www.csemag.com/articles/back-to-basics-switchgear-transformers-and-upss/; accessed May 24, 2022.

  • Bonneville Power Administration (July 2018). “Sampling for M&V: Reference Guide.”

  • EE Richman (February 2016). PNNL-SA-25222. “Measurement and Verification of Energy Savings and Performance from Advantage Lighting Controls” Richland, WA: Pacific Northwest National Laboratory.

Footnotes

  1. Blue represents lighting fixtures, red represents others electrical loads. ↩︎

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