Exploring basic components of a low-voltage electrical system

Low-voltage electrical system insights

• Low-voltage electrical systems can be used in commercial and residential locations and is classified as either less than 48 volts or less than 1,000 volts, depending on the application.
• The codes and many other considerations will determine electrical equipment, such as switchgear and switchboards, transfer switches and generators.

Voltage class can be defined in several different ways. For electrical design engineers, low-voltage is below 1,000 volts and representative of what is typically seen in commercial and residential applications. For telecommunications engineers, however, low-voltage is usually defined as 48 V and lower.

The most common application for commercial projects is bringing 480 V/277 V into the building and then stepping down to 208 V/120 V. When moving forward in specifying and sizing components, design teams must carefully consider basic components of the low-voltage system, main codes, standards and common practices.

Figure 1: This switchboard image shows feeders and branch circuits. Courtesy: Page Southerland Page Inc.

Switchboard and low-voltage switchgear

These two terms are often used interchangeably, but in actuality, they are technically different. Both are required to be service entrance rated when placed after the medium-voltage utility transformer. The biggest differences are: testing standards, ampere ratings, short-circuit testing, cost, reliability and physical size.

There are also many other differences such as breaker types used in each. Switchgear is used more often than switchboards in critical facilities such as hospitals and data centers, where increased power continuity is a priority.

Design teams should have a discussion with the clients to identify the priorities for system selection. It is crucial to establish whether the project is primarily driven by budget, schedule, reliability, sustainability, ease of maintenance and/or any other factors the electrical engineer needs to account for during design. Another factor to consider is space availability, especially because electrical equipment such as panelboards, switchboards/switchgears, etc., requires working clearances (refer to NFPA 70: National Electrical Code section 110.26 for all working clearances requirement).

• The width of working spaces in front of the equipment should be the width of the equipment or 30 inches, whichever is greater.
• The workspace shall be clear and extend from the floor/grade/platform to a height of 6.5 feet or the height of the equipment, whichever is greater.
• Follow NFPA 70 Table 110.26(A)(1) for the depth of working spaces.
• Most of the owner’s project requirements indicate spare wall space for future expansion when it comes to sizing new electrical rooms.

Feeders and branch circuits

Just like switchboard and switchgear, feeder and branch circuits are sometimes used interchangeably. Per NFPA 70, feeders include all circuit conductors between the service equipment, the source of separately derived systems or another power supply source and the final branch circuit overcurrent device.

Branch circuit, on the other hand, is the circuit conductor between the final overcurrent device protecting the circuit and the outlet(s) or load. They follow different code sections.

Branch circuits:

• General requirements: NFPA 70 Article 210.
• Sizing: NFPA 70 Article 210.19(A)(1) and Article 310.

Exception: If the overcurrent protective device is listed for operation at 100% of its rating, the allowable ampacity of the branch-circuit conductor is allowed to be a sum of continuous and noncontinuous loads.

Feeders:

• General requirements: NFPA 70 Article 215.
• Sizing: NFPA 70 Article 215.2(A)(1) and Article 310.
• Size of the feeder’s circuit grounded conductor: NFPA 70 Article 250.122.

Figure 2: An automatic transfer switch is shown. Courtesy: Page Southerland Page Inc.

For sizing conductors in low-voltage system, designers often use table 310.16 (NEC 2020). Weight, electrical capacity and cost are major considerations when selecting aluminum or copper for an electrical application. Copper offers a better electrical capacity per volume. However, aluminum has better capacity per weight. Aluminum conductors frequently are used from the secondary of the utility transformer to a building’s switchboard/switchgear and feeders between panels. Contractor needs to be careful with wire termination with aluminum conductors to avoid damage during installation. However, some critical applications, such as for hospitals, government projects and data centers, may not allow aluminum on the project.

Selecting transformers

Typically, transformers in low-voltage systems refer to dry-type 480 V/208 V (delta-wye), three-phase, stepdown transformers. Electrical engineers should refer to NFPA 70 Article 450 for transformers, its feeders and overcurrent protective device sizing. Some important considerations for transformer design include:

• Size based on the calculated demand load.
• Energy code efficiency requirement.
• Location/noise.
• Winding material/temperature rise/insulation temperature.

Table 1: ANSI C84.1: Electric Power Systems and Equipment – Voltage Ratings and IEEE 141: Recommended Practice for Electric Power Distribution for Industrial Plants divide system voltage into voltage classes. Courtesy: Page Southerland Page Inc.

Transformers are sized based on the calculated demand load. Typically, designers try to leave 25% spare capacity for future expansion.

The most efficient transformers tend to have lower temperature rise and the higher the insulation temperature rating, the longer the expected lifespan. Additionally, transformers can be built with copper or aluminum windings. Aluminum transformers are cheaper but are physically larger and should be selected based on project priorities and available space.

How to select panelboards

There are many considerations when specifying panelboards, including:

• Location.
• Interrupting rating.
• Accessories.
• Rating.
• Mounting.
• Enclosure.
• Height, branch circuit quantities.
• Working clearances.
• Arc flash/personal protective equipment.

Electrical engineers should consider where to place the panel to assure the voltage drop for the branch circuits meets NFPA 70 requirements. If space programming allows, a small electrical closet is typically placed in every 10,000 square feet of the floor plan.

Personnel safety is the most important aspect to keep in mind when specifying electrical systems. Occupational Health and Safety Administration and NFPA 70E: Standard for Electrical Safety in the Workplace require arc flash warning hazard label indicating:

• Nominal system voltage.
• Available fault current at the service overcurrent protective devices.
• Clearing time of service overcurrent protective devices based on the available fault current at the service equipment.
• Date the label was applied (see NEC 110.16(B)).

Standby generators for low-voltage power

Standby generators can be installed on a project to provide electrical power if the primary power gets interrupted. A prime rated generator is used as a primary power source. For typical applications, designers use a standby rated generator to comply with requirements of NFPA 70 Articles 700, 701, 702 and 708. The generators are classified by class, type and level.

Class is the minimum time in hours for which the emergency power supply system is designed to operate at its rated load without refueling/recharging. Most common is two hours for all life safety equipment and Class 8 (eight hours) when utility power is not considered to be a reliable source and the fire pump is connected to generator. For data centers and health care facilities, the run time can vary between 48 and 96 hours.

Type is maximum transfer time in seconds; most common is Type 10 for Life safety and Type 60 for legally required. NFPA 110: Standard for Emergency and Standby Power Systems recognizes two levels for equipment installation, performance and maintenance requirements:

• Level 1 applications: Life safety illumination, public safety communication systems, fire pumps, ventilation equipment (NEC Article 700) where the loss of power results in loss of human life.
• Level 2 applications: Heating and refrigeration systems, sewage disposal, some industrial processes (NEC Article 701) less critical to human life and safety.

The most commonly used codes and standards include NFPA 110; NFPA 70 (Article 445, 700, 701, 702, 708); NFPA 1: Fire Code; NFPA 30: Flammable and Combustible Liquids Code; and NFPA 99: Health Care Facilities Code.

There are many aspects to consider when specifying a generator and a best practice is to contact the generator representative to confirm the clearances meet or exceed NFPA 37: Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines Subsection 4.1.4 (as they vary by manufacturer and size), fuel storage options and noise requirements.

When sizing generators, designers often use manufacturer genset sizing software to assign load and steps and provide the most cost-efficient design. Additional items to keep in mind are load banks and generator tap box as required per code.

Considering the automatic transfer switch

There are several considerations and options for choosing an automatic transfer switch, or ATS, such as:

• Open or close transitions.
• Bypass switch.
• 3-pole versus 4-pole.
• Service entrance rated.
• 3-cycle versus 30-cycle.
• Ampere interrupting capacity (AIC) rating versus withstand closing current rating (WCR).
• Each branch (emergency, legally required, optional standby, equipment, critical and life safety) typically has its own ATS.

Table 2: There are several differences between switchboards and switchgear. Switchgear is used more often than switchboards in mission critical facilities. Courtesy: Page Southerland Page Inc.

Per UL 1008, transfer switches have WCR, which is either based on a specific device or ability to withstand and close into a fault current until a protective devices opens.

Most branch breakers in low-voltage systems are UL 489 listed. For this, a 3-cycle ATS is sufficient. Similar to switchgear, 30-cycle ATS makes it easier to coordinate the system to assure power reliability, but it is more expensive. A four-pole ATS is required when ground fault sensing is required per NFPA 70 or per the owner’s project requirements.

To ensure a project’s success within any building type, designers are recommended reviewing and thoughtfully considering appropriate code sections, while also coordinating and collaborating with other disciplines. Importantly, when specifying low-voltage systems, it is key to coordinate with the client to determine whether the project is driven by budget, schedule, reliability, sustainability, ease of maintenance or space availability and go from there to best meet or exceed the client’s needs and requirements.

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