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Abstract: Building Electrical Systems

The design, construction, and operation of electrical systems in educational facilities call for careful consideration of fire safety, energy efficiency, and electrician safety. The Life Safety  Code  (NFPA  101)  and ICC documents are adopted as a whole or used  as resource documents in state codes. Both NFPA and ICC reference the NEC wiring protocol.   ASHRAE is also one   of the leaders in creating national standards to meet federal energy efficiency  objectives  proposed  by  EPA and the Department of Energy. OSHA is the leading advocate for electrician safety and worked with NFPA to produce NFPA 70E, which details operational and maintenance rules for protecting electricians working on live electrical equipment. Guidelines issued by these organizations do not always agree and often are  hampered by the cost of older system retrofits. This chapter points to the effectiveness and cost savings generated by advance planning  during  the  building design phase.

Understanding Electric Power

Most higher education institutions are on a bulk electrical distribution grid (e.g., 13.2kV, 4160V, 2400V) three-phase alternating current (AC) and are subject to NESC safety rules developed by IEEE. This chapter uses  a sample building system with low-voltage electrical distribution systems (less than 600V). Most building electrical systems have one transformer with 480V three-phase and 277V single-phase windings for motor control and fluorescent lights and another transformer with 208V three-phase and 120V single-phase windows for 208V loads and general purpose power outlets (see Figure 2.14).

From the transformer, power flows to the main distribution panels (circuit breakers) and on to secondary panels and eventually to the actual power consumer (load). A good system design  factors  in  25  percent future growth.

Electrical  Safety  and  Flash  Hazard  Analysis.   NEC  (NFPC 70) requires placing a flash hazard mark and incident energy level on all equipment that might be serviced while energized. Computing energy levels is confusing because  most  systems  use  both  AC  and direct current (DC)  power.  For  DC  power,  engineers must compute power (watts) as a function of amperes (amps) and volts and also consider the power factor (inductance). The three-phase  AC  power  formula consists of the same variables, but the formula differs; power factor is a measure of actual available energy as a proportion of energy drawn. Institutional electrical systems are known for lagging power factors (power drawn exceeds available power) that decrease energy efficiency and incur power company penalties.

Increasing system efficiency by using capacitors to minimize power loss and using external sources for needed system magnetization can best be achieved during the design and construction phase of the buildings.

Power System Modeling

Power system modeling is complicated by design and operations professionals who define key terms (e.g., low, medium, high voltage) differently, but a per-unit system for analyzing power networks is universally accepted. The per-unit technique is used primarily in distribution and transmission studies but also is applied to building power systems (e.g., transformer and machine parameters). Short circuit analysis is essential to design an electrical safety program and requires maintenance of accurate circuit diagrams (with details about the size and path of the electrical distribution system). With this information, the experienced electrical power engineer can make solid judgments about system design and safety.

Electrical Wiring

A conductor (commonly copper or aluminum) is the term for a material that conducts electricity. Size and temperature determine the current-carrying capacity of a conductor.   Heat dissipation is crucial to the functioning  of  a  conductor  because  excess  heat  increases resistance to current-carrying capacity. Larger wire size increases heat dissipation, but cost-efficiency calls for minimizing wire size.

Wire Sizes. Wire size is noted in circular mils (one- thousandth of an inch). AWG is the generally accepted U.S. standard of measure (with sizes of 14, 12, 10, 8, 6, 4, and 2 the most common). A smaller AWG number means that the wire diameter is larger and the resistance is lower.

Wire Insulation. Indoor building wire is appropriate for voltages up to 600V and is normally insulated with moisture-resistant thermoplastic (type TW) and heat- resistant thermoplastic (type THW).

Wire Size Selection. Wire size selection is determined by the need to prevent and minimize voltage drop.

Wires in Parallel. Running wires in parallel (paralleling) entails running two lines as one to minimize the requirement for large-sized conduits in small spaces.

Wire Splices and Terminations. Wire splices and terminations are generally the weak link in building electrical systems. Soldering is rarely used in building wiring, and connections must be made very precisely and carefully to ensure building safety.

Conduit Systems

A conduit wiring system provides a channel for running electrical wires, is easy to install and service, and protects components.

Rigid Galvanized Conduit. Rigid Galvanized Conduit (RGC) provides the best protection but is costly, heavy, and difficult to install.

Electric Metal Tubing. This tubing is normally used in drop ceilings because it is lighter than RGC.

Rigid PVC Conduit. PVC conduit has no voltage limitation, resists corrosive ozone and sun effects, and is lightweight but does not conduct electrical current, so a grounding conductor is needed.

Flexible Conduit. Flexible conduit is effective when moving parts (e.g., motors) are involved. Wire selection takes varying installed conduit diameters into account.

Electric Power Quality

Electric power quality is directly related to system noise. Transients and harmonics clutter the current electrical distribution system and cause  approximately  $750 million of damage annually to sensitive electronic equipment. Two types of noise are relevant: normal-mode noise, which only occurs in line-to-line or line-to- neutral connections, and common-mode noise, which occurs in line-to-ground or neutral-to-ground connections.

Transients. Transients are high-amplitude short-duration noises that normally are the result of lighting and power surges caused by switching.  Surge  suppressors  and active power line conditioners mitigate it.

Harmonics. Harmonics are the result of introducing nonlinear equipment (e.g., switching devices, ferromagnetic devices, arcing devices) into a linear system.

De-Rating Distribution Transformers. The presence of harmonics requires de-rating the transformer load to prevent the transformer from overloading.

Oversizing the Neutral Conductor. Neutral circuits do not normally have an overload device, so neutral wire should not be shared (or wire diameter should be doubled if sharing is necessary).

Loading Circuit Breakers. Circuit panel breakers should only be loaded to 80 percent capacity when serving nonlinear loads to protect against the heat and vibration caused by harmonics.

Power Factor Considerations. Installation of a harmonic trap and use of low-impedance distribution transformers in a delta-wye configuration mitigates system power factor degradation from harmonics.

Power Quality Considerations. Determining whether a power problem is on the line side or the load side is the first factor in crafting a solution to power quality considerations. Tracking the change in current and isolating the problem area are crucial.

Electric Motors

Electric motors convert electrical energy into mechanical energy. They consist of stationary and rotating elements and can be classified as synchronous or induction-type. Almost all buildings use induction motors. The standard measure of motor power is horsepower (1 horsepower equals 746W, or 0.746kW).

Electric Motors and Energy Management. Improvements in electrical efficiency in commercial and industrial applications could produce economic rewards; they account for approximately 40 percent of annual electrical energy consumption.

Efficiency. Efficiency measures how effectively a motor converts electrical energy into mechanical energy. Large motors are generally more efficient than small ones, and proper care and maintenance can increase the efficient life of a motor.

Variable-Frequency Drives. Industrial processes generally require the use of variable-frequency drives. The three common types are adjustable voltage input (AVI), Current Source Invertors (CSI), and pulse width modulation (PWM). AVI is simplest; CSI is used in DC systems; and PWM is complicated but efficient.

Lighting

Lighting can be divided into three types: functional, safety and security, and ornamental or architectural.

Elements of Lighting. Lighting is the result of running an electrical current through resistance filament or a gas. Lighting quantity is a function of luminous flux. The accuracy of color generated by a specific light is measured with a color rendering index (CRI); incandescent lamps measure 100 (best score) and fluorescent lamps measure 65. Lighting efficiency is measured in lumens per watt. (1) Incandescent lamps use a tungsten alloy filament that glows at a high temperature and have high CRI ratings and low efficiency ratings. (2) Fluorescent lamps, the oldest and simplest type, are least expensive and have high CRI ratings. (3) Metal halide lamps have a CRI of 65 and good efficiency ratings but are fragile. (4) High-pressure sodium lamps have a CRI of only 22 but offer the best efficiency. (5) Low-pressure sodium lamps are highly efficient, but with a CRI of 0, are rarely used. (6) Inductance lamps are efficient and have a CRI of 80 but are very costly. (7) Light-emitting diode (LED) technology offers excellent color rendering, but the cost of supporting components is high. Over time, LED technology is likely to gain significant market share, and increased use should lower costs.

Lighting, Part 2

Lighting Application. Well-planned lighting placement creates a higher-quality environment. Lighting levels should be as uniform as possible to prevent glare and shadows. Indoor commercial spaces have historically used direct light because it is more efficient. Because indirect lighting is easier on the eyes and more desirable than direct lighting, many institutions are switching to indirect lighting in classroom settings. Instituting a regular schedule of lamp replacement and fixture cleaning increases lighting levels by 50 to 100 percent, making indirect lighting more cost-efficient.

Lighting  Design  Calculations.   Lumens  (measuring light quantity) and foot-candles (measuring intensity of illumination) are fundamental to understanding lighting design calculations. IESNA  recommends  sample lighting levels (foot-candles) for typical campus spaces; the foot-candle range varies (e.g., by quality of material viewed, accuracy of task, person’s age). This section summarizes the lumen method (one of several) for calculating number of required fixtures, including relevant formulas and factors such as lighting quality and geometry. Lighting design software is available. Lighting should be one voltage throughout the building (usually 227V in larger post-1960 facilities). (See Figure 2.15.)

Figure 2.15. Example of Recommended Lighting Levels
Space Type Footcandles
Office Space 20 – 50
Classrooms 50 – 100
Conference Rooms 20 – 50
Laboratories 50 – 100
Libraries 20 – 50
Lobbies 10 – 20
Dining Rooms 5 – 10
Outdoors 1 – 3

Lighting Control. A variety of lighting control tools are available. A pole switch (on-off switch) is the simplest form of lighting control. Dimmer switches enable illumination control (dimming). Three-way and four-way switches allow lighting to be controlled from multiple locations. A lighting contactor is used for remotely controlling large banks of light. Lighting accounts for more than a third of U.S. energy use, so improving lighting control systems is a major part of the national energy efficiency policy.

Grounding

Grounding minimizes the risk of shocks or lightning damage and is a required safety mechanism for   secondary electrical distribution. A system  ground consists of a grounded current-carrying wire; equipment grounding involves placing a non-current-carrying grounding wire between a metal equipment frame and a ground rod or armor. The effect of an electrical shock depends entirely on current that flows through the  human body. A current between 1mA and 8mA merely causes perceptible sensations,  but  15mA  causes muscular freeze, and more than 75mA is fatal.

Ground  Fault  Protection.   The  NEC  requires  ground fault equipment protection. Ground Fault Interrupters (GFIs) shield equipment from low-grade faults by using a current transformer. If leakage occurs, the GFI trips the circuit breaker (or blows a fuse).   Code requires GFIs if   an electrical system is between 150V and 600V, and they are a good idea even for smaller systems. Routine system cleaning and maintenance are the first line of defense in preventing ground faults, but GFIs offer another level of protection.