Specifying electrical distribution systems

It is useful for both electrical and nonelectrical engineers to understand basic features when selecting, specifying, and applying electrical distribution systems.
By Brian Rener, PE, LEED AP, SmithGroupJJR, Chicago March 17, 2017

This article is peer-reviewed.Learning objectives

  • Understand basic features when selecting and specifying electrical distribution equipment.
  • Know how much current to apply to various equipment, such as circuit breakers.
  • Glean a basic understanding of panelboards, transformers, and other electrical equipment.

To narrow the broad scope of electrical distribution, this discussion will focus on practical considerations for specifying electrical distribution systems. The discussion will be limited to more common low-voltage 480/120 V electrical distribution equipment encountered in most facilities.

Figure 1: Typical switchboard. Courtesy: Eaton

Primary service equipment

In some smaller facilities, panelboards may be used as a primary service; but for larger facilities, the primary service equipment will be based on a switchboard or switchgear. Engineers, architects, contractors, and facility owners often use the terms “switchboard” and “switchgear” interchangeably when referring to 480 V circuit breaker distribution equipment. But there are notable differences in configurations, components, standards, applications, reliability, and selection criteria between these two types of power distribution equipment.

There are several notable differences between switchboards and switchgear including physical size, front or rear access, and how breakers are mounted and removed. The type of breakers used also is a major difference between switchboards and switchgear. The basic types that we are concerned with, here, are sealed, semi-open, and open types. Specifically, these are called molded-case, insulated-case, and power circuit breakers.

Molded-case circuit breakers. MCCBs are the most commonly used in all types of low-voltage switchboards and panel boards. One will find these breakers in ratings from 15 to 3,000 amps. The breaker mechanism is totally sealed within an external molded case. If the breaker has a failure or problem, it must be replaced. These breakers typically are bolted onto the bus, or may have plug-in designs. The removal or addition of MCCBs to a switchboard should take place only with the switchboard power turned off (see Figure 1).

Figure 2: Typical switchgear. Courtesy: Eaton

Power circuit breakers. Typical ratings range from 800 to 5,000 amps. PCBs are designed and tested under much different standards than MCCBs or ICCBs. PCBs are connected to the bus in a drawout design, allowing the breakers to be withdrawn partially or fully while the entire switchgear is powered on (see Figure 2). PCBs have numerous components that can be inspected and replaced, such as contacts, pole assemblies, and arc chutes.

Insulated-case breakers. ICCBs are a type of MCCB designed to provide features typically available in PCBs. The internal parts are mostly, but not completely, sealed like those in an MCCB. Typical ratings range from 400 to 5,000 amps. These breakers are available as options in switchboards and can be fixed or drawout. Designed to the same standards as MCCBs, they provide access to replaceable parts, such as contacts.

Application considerations

The amount of continuous current that can be put on a 400-amp circuit breaker depends on the breaker. With MCCBs and ICCBs, the breaker typically is rated for only 80% of its capacity within a switchboard or panelboard. In this case, you could put no more than 320 amps continuously on that breaker. This is a limitation that not everyone is aware of. It is possible to specify optional 100%-rated MCCBs and ICCBs in some frame sizes with some cost premium. PCBs are 100%-rated as standard. Refer to NFPA 70-2017: National Electrical Code (NEC), Article 220.10, for more on this topic.

Beyond continuous current, there are important differences when considering short circuits and faults. While beyond the scope of this article, is important to note that PCBs are tested and rated to higher levels of initial (or asymmetrical) fault than MCCBs or ICCBs. Depending on the engineer’s detailed calculations, the MCCB’s or ICCB’s listed fault rating may need to be derated.

Beyond a circuit breaker’s ability to withstand and interrupt a maximum short circuit, there are trip levels or regions to consider. Circuit breakers will open based on various magnitudes and durations of current. These trip levels are expressed as a curve on a graph of current versus time. There are three regions to consider: long-time (continuous-current range) faults, short-time faults, and instantaneous fault. The area of difference between MCCBs, ICCBs, and PCBs is in the short-time regions. Essentially, PCBs have higher short-time ratings, which along with the ability to eliminate the instantaneous range, allows PCBs to wait for breakers further downstream in the distribution system to trip and isolate faults. This is of particular use in large distribution systems where one doesn’t want main circuit breakers to trip when a fault occurs on a smaller downstream breaker. This is referred to as a selective or fully coordinated system. This type of coordination is more readily achieved with the use of PCBs at main service points.

Space is another consideration. Switchgear is larger than switchboards and requires front and rear access. In addition, the clearance in front must take into account the space needed to draw out a breaker. While not covered in the code when withdrawing a drawout breaker, it may occupy the NEC-required clearances—making egress and access difficult. Rear-connected switchboards, depending on specified options, also will require similar careful space considerations. Front-accessible switchboards have the least space requirements and may be located against a wall.

Both switchboards and switchgear are code-compliant and proven in the industry. But there are some advantages to switchgear and rear-connected switchboards that can reduce downtime and failures. First, there is the idea of individual compartments for breakers. In the event of a short circuit on a breaker, the resulting energy will be contained and isolated from other breakers and from the bus and cable compartment. Second, the ability to have drawout breakers also permits repair, inspection, and replacement of a breaker while the rest of the switchboard or switchgear continues to operate. Third, PCBs, and to a lesser extent ICCBs, have exposed and accessible parts that can be regularly inspected and replaced without having to buy an entirely new breaker. Lastly, PCBs have a more rugged construction and are able to handle more closing and opening operations, including faults, and provide for automatic remote control for transfer schemes.

So how does one make a selection? Initial costs often play a major role in the selection. The cost differences between a low-end switchboard and high-end switchgear can be substantial—as high as two or three times—and must be weighed against the long-term issues of maintainability, reliability, and downtime. Project type and complexity often determine the choice. A simple office facility with no maintenance staff is much different from a manufacturing facility. Recommended applications for switchgear include manufacturing or process facilities with round-the-clock operations, data centers, telecommunication switching sites, airports, convention centers, or skyscrapers. Hybrid or high-end rear-access switchboards are good choices for medical facilities, laboratories, light manufacturing, large institutional, or commercial facilities. Front-accessible switchboards are recommended for basic office and commercial buildings, K-12 schools, warehouses, or retail facilities.

Figure 3: Typical lighting panelboard. Courtesy: Eaton

Basic configurations

In its most basic form, a low-voltage switchboard is a common grouping of fixed molded-case “sealed-type” circuit breakers in a common enclosure. The breakers are directly connected to the bus and may be group-mounted or individually mounted in their own compartment within the entire enclosure. Cable connections are made by an electrician standing in the front of the board. They typically require only front access and may be mounted against a wall. These are often seen in small- and medium-size commercial or institutional facilities.

Switchgear consists of individually mounted and compartmentalized drawout power, open-type circuit breakers. There are physical barriers between the breaker and between the breakers and the bus. Cable connections are made in the rear compartment. They are larger and require front and rear access. These typically have been used in industrial and large commercial and institutional facilities.

Historically, these two simple explanations have helped to highlight the differences between a low-voltage circuit breaker switchboard and switchgear. Recently, however, the lines have been blurred with the availably of rear-connected switchboards that can provide hybrid options of individual compartments and drawout circuit insulated-case semi-open breakers with semi-open or open-type power circuit breaker construction.

Distribution panelboards

In the 2005 and earlier versions of the NEC, panelboards were classified as either lighting and appliance branch-circuit panelboards or power panelboards, based on their content. Specifically, NEC 2005 Article 408.34: Classification of Panelboards contained the following two definitions of panelboard types:

  • (A) Lighting and Appliance Branch-Circuit Panelboard. A lighting and appliance branch-circuit panelboard is one having more than 10% of its overcurrent devices protecting lighting and appliance branch circuits (see Figure 3).
  • (B) Power Panelboard. A power panelboard is one having 10% or fewer of its overcurrent devices protecting lighting and appliance branch circuits.

These definitions were removed from the NEC beginning in 2008, but the general concept of separating panelboards into different application types still persists in the industry. One of the reasons is that major electrical-equipment manufacturers typically have several types of panelboards with different features, capabilities, and space needs. A further benefit to using these terms is when designating panelboards on one-line diagrams, risers, and plans, one can link different types to different specifications.

Figure 4: One-line diagrams show details associated with switchgear, switchboards, panelboards, and connected equipment. Courtesy: Jacob Clatanoff, SmithGroupJJR

A common application approach might be to use the terms distribution or power panels for panels that feed other panels or large loads, lighting panels for lighting loads and lighting controls, and receptacle or branch panels for panels that feed outlets or smaller loads. Given the benefits of using these panel designations, consider the common application issues for these applications (see Figure 4).

Most panelboards are available in ampere ratings from 100 to 1,200 amps. Common ratings are 100, 200, 225, 400, 600, 800, and 1,200 amps. If an amperage rating of more than 1,200 amps is needed, switchboards must be specified, as panelboards are not available above 1,200 amps. Commercial and industrial facilities use voltage ratings of 120/208 V, 277/480 V, and 480 V. Residential often will use 120/240 V.

A common point of discussion is whether panels should have a main breaker (main disconnect) or main lugs only. Certain requirements in the code may mandate a main breaker (or disconnect) nearby, such as when a panelboard is supplied from a transformer per NEC Article 240.21(C). Additionally, if the panel is fed from a breaker in an upstream panel, switchboard, or switchgear, it is technically protected under code. However, providing a main breaker in some panelboards that are not within sight of their upstream protection can provide enhanced safety and maintainability.

Panelboards also offer the choice of bolt-on or plug-on (stab) breaker-to-bus connections. A bolt-on breaker connection secures to the panel bus by screws. Stab-on uses a spring-clip-type conductor. Generally speaking, bolt-on connections are considered more reliable and secure and are common in distribution panels. Plug-on breakers are more common on lighting or branch panels.

In nearly all panelboards, the circuit breakers used are fixed-rated, molded-case thermal magnetic type. However, numerous optional breaker types are available. Adjustable magnetic trips are available on larger-frame breakers. In addition, adjustable electronic trip breakers are available with various trip settings on most breakers, providing enhanced selective coordination. Also, it is possible to specify 100%-rated breakers in larger frames to use the full ampacity of the rating, rather than the 80% rating standard with most MCCBs. When a main breaker is used in larger panels, or as a larger branch breaker on a distribution panel, it may be desirable to select 100%-rated and electronic trip features to permit the full rating of the panel to be used and improve coordination with upstream panels, switchboards, or switchgear.

Lastly, numerous optional features can be specified for molded-case breakers in special applications, such as motor circuit protectors (magnetic only), ground-fault circuit interrupters for wet areas, ground-fault equipment protectors for heat trace, shunt trips for remote control, key interlocks and padlocked operators for safety, arc fault circuit interrupters typically for residential, and more.

Like switchgear and switchboards, it’s also important to consider whether the main feeder and branch feeders will be top- or bottom-feed. This can affect the panel enclosure and may also necessitate the addition of vertical gutter ways to accommodate all the branch feeders.

When laying out floor plans, it’s important to understand that distribution panelboards will require significantly more space than lighting or branch-circuit panelboards. In addition, distribution panelboards are usually surface-mounted (not recessed), and some may actually require floor mounting or equipment pads.

Transformers

The most common use of general-purpose, dry-type transformers in commercial facilities is to provide secondary 120/208 V power from 480 or 480/277 V primary power. While fairly straightforward, there are some practical considerations for their use.

Figure 5: A typical general-purpose transformer showing the 3-phase cores, associated windings, and insulation around each core. Courtesy: EatonEnergy efficiency is a key consideration for transformers. The basic components of a transformer are the core and the windings. Each part plays a different role in transformer efficiency (see Figure 5). In every transformer, the efficiency depends on core losses and winding losses. Core losses generally are constant over the full loading range. Winding losses are proportional to the square of the current, thus the power delivered by the transformer.

The core efficiency typically is associated with no-load losses, i.e., a magnetizing field must be established in the core upon energizing regardless of load. Studies of transformer applications in the field indicate that many transformers are lightly loaded; therefore, it was determined that improving core efficiencies would have the greatest impact on overall transformer efficiency.

As mentioned, the windings also have a role in transformer efficiency. The role of the windings in energy efficiency is tied to load. As current rises, the resistance in the windings produces power losses and temperature increases. Dry-type transformers are available in three standard temperature rises: 80°C, 115°C, and 150°C. The transfer temperature rise classification approximates winding efficiencies. These values are based on a maximum ambient temperature of 40°C. Therefore, an 80°C-rise dry transformer will operate at a winding temperature of 120°C at full-rated load in a 40°C ambient environment. The most common rating is 150°C due to lower cost, but it also is the least efficient. most efficient temperature rise is 80°C, which can be up to 20% more efficient than a 150°C-rise transformer. A secondary benefit of the lower-rise (80°C and 115°C) ratings is that the transformer will have higher temporary overload capabilities and longer life.

The U.S. Department of Energy (DOE) has issued standards for transformer efficiency based on core designs. On Jan. 1, 2016, the DOE required transformers manufactured and sold in the U.S. and U.S. territories to meet higher energy efficiency requirements. Transformers manufactured and/or imported into the U.S. prior to that date may still be sold and installed until stock is depleted. The DOE standards for transformer efficiency are optimized at 35% load.

A downside to the more stringent DOE requirements is that transformer impedances will be decreasing, which will increase available inrush and fault currents. In practical terms, this will require careful selection of both upstream and downstream breaker, panelboard, and switchboard short-circuit and ratings.

K-factor transformers also are available to accommodate harmonics from electronic loads. Understand that for the most troublesome harmonic currents, most transformers keep the harmonic currents created by downstream electronic loads to the secondary side, but this results in overheating. The K-factor classifications indicate the ability to withstand the nonlinear (harmonic) load current’s impact on transformer overheating as specified in ANSI/IEEE C57.110-2008: IEEE Recommended Practice for Establishing Liquid-Filled and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents. Typical K-factor ratings include 4, 13, and 20, with K4 being the least robust and K20 the most robust in handling harmonics.

Additional application considerations

Electrical distribution equipment requires proper space and environmental conditions in which to operate. It’s a challenge to coordinate with architects to allocate sufficient space in electrical equipment rooms to provide both required clearances and adequate space for maintenance activities. The key is to work with the architect and owner very early in programming to estimate good working spaces and to show that going beyond code minimums can enhance safety, reliability, and longevity of the equipment.

It’s also important to review ventilation and cooling requirements with the HVAC engineer. Electrical equipment may experience issues in rooms that exceed 104°F. When a room exceeds 104°F, issues include the false opening of circuit breakers, premature failures, and reduced life span. While limiting cooling improves energy conservation, it is desirable from an owner-maintenance standpoint to keep comfortable temperatures in electrical rooms housing important electrical equipment. Smaller electrical closets with less-critical equipment could be allowed to operate at a higher temperature than might be comfortable. Transformers are the main source of heat in electrical rooms. The electrical engineer should coordinate heat-rejection values with the HVAC engineer.

Focus on future issues

Primary service equipment, panelboards, and transformers are the most common parts of electrical distribution in most facilities. A basic understanding of electrical distribution equipment types and features is beneficial to their practical application. These practical application considerations include ratings, circuit-protection capabilities, maintenance needs, reliability, safety, space and access requirements, temperatures, and energy efficiency. In the future, we will continue to see increased focus on these issues.


Brian Rener is an associate and electrical discipline leader at SmithGroupJJR. He is a member of the Consulting-Specifying Engineer editorial advisory board.