Table of Contents Introduction...............................................................................2 Distribution Systems.................................................................4 Panelboards...............................................................................6 Overcurrent Protection Devices.................................................9 Panelboard Construction.......................................................... 19 Individual Overcurrent Protection......................
Introduction Welcome to another course in the STEP series, Siemens Technical Education Program, designed to help our distributors and customers better understand Siemens Industry, Inc. products. This course covers Basics of Panelboards.
This knowledge will help you better understand panelboard applications. In addition, you will be better prepared to discuss panelboards with others. You should complete Basics of Electricity and Basics Circuit Breakers before attempting Basics of Panelboards. An understanding of many of the concepts covered in Basics of Electricity and Basics of Circuit Breakers is required for Basics of Panelboards.
Distribution Systems Power distribution systems are used in every residential, commercial, and industrial building to safely control the distribution of electrical power throughout the facility. Residential Power Distribution Most of us are familiar with the power distribution system found in the average home. Power, purchased from a utility company, enters the house through a metering device.
Commercial and Industrial Power Distribution Power distribution systems used in multi-family, commercial, and industrial facilities are more complex. A power distribution system consists of metering devices to measure power consumption, main and branch disconnects, protective devices, switching devices to start and stop power flow, conductors, and transformers. Power may be distributed through various switchboards, transformers, and panelboards. Good distribution systems don’t just happen.
Panelboards Electrical distribution systems, whether simple or complex, typically include panelboards, the focus of this course. Even the load center used in a home is a type of panelboard. However, the focus of this course is on panelboards used in commercial and industrial facilities. Panelboard Definition A panelboard is a type of enclosure for overcurrent protection devices and the busses and connections that provide power to these devices and their associated circuits.
Used to Control Light, Heat, or Power Circuits Placed in a Cabinet or Cutout Box Flush Mounted Mounted in or Against a Wall Accessible Only From the Front Surface Mounted 1.75 in. (44 mm) 0.25 in. (6 mm) For additional information, refer to National Electrical Code® Article 408, Switchboards and Panelboards.
Prior to the 2008 National Electrical Code®, the distinction between these two panelboard types was described in NEC® Articles 408.34 and 408.35. These articles placed restrictions on lighting and appliance panelboards and indicated that panelboards that did not comply with these restrictions were defined as power panelboards.
Overcurrent Protection Devices Because current flow in a conductor always generates heat, the greater the current flow, the hotter the conductor. Excess heat is damaging to electrical conductors. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Current beyond the rated capability of a conductor is referred to as overcurrent. Overcurrent can result from a short circuit, an overload, or a ground fault.
Overcurrent protection devices are used to protect conductors from excessive current flow. Some overcurrent protection devices only provide protection in the event of a short circuit, some provide both short circuit and overload protection, and some devices provide protection in the event of any of the three overcurrent types. Circuit protection would be unnecessary if overcurrents could be eliminated.
Fuse Classes Underwriters Laboratories (UL) establishes and standardizes basic performance and physical specifications for products that undergo its safety test procedures. Among the standards developed by UL are standards for classes of low voltage fuses (fuses with voltage ratings of 600 volts or less). The following table shows the fuse classes most commonly found in panelboards.
Fusible Disconnect Switch A fusible disconnect switch is one type of device used on panelboards to provide overcurrent protection. Fuses located in the switch are selected to handle the specified levels of current and voltage and to provide the appropriate interrupting rating. Siemens Vacu-Break fusible switches, through 600 A, feature a Clampmatic action that holds the current carrying contact surfaces in a vise-like grip. Heat build-up due to current is minimized.
High Contact Pressure Fusible Switch Siemens high contact pressure (HCP) fusible switches have continuous current ratings from 400 A to 1200 A. Circuit Breaker Another device used for overcurrent protection is a circuit breaker. Although some circuit breakers do incorporate fuses, most do not, but, like a fusible switch, a circuit breaker provides overcurrent protection and a manual means of controlling power distribution.
When an overcurrent occurs, the circuit breaker trips to remove power from the circuit. The greater the overcurrent, the more rapidly the circuit breaker trips. Once the overcurrent condition has been corrected, a simple flip of the breaker’s operating handle restores the circuit. The ability to restore a circuit without replacing a fuse is one of the key advantages of a circuit breaker. However, circuit breakers have other advantages as well.
Some circuit breakers have what is referred to as a “slash” voltage rating, such as 120/240 VAC or, as shown in the following graphic, 600/347 VAC for 2, 3, or 4-pole NEB breakers and 115/250 VDC for 2-pole NEB breakers. In such cases, the breaker may be applied in a circuit where the nominal voltage between any conductor and ground does not exceed the lower rating and the nominal voltage between conductors does not exceed the higher rating.
The rated current for a circuit breaker is often represented as In, as shown in the following chart for an NEB circuit breaker. This should not be confused with the current setting (Ir) which applies to those circuit breakers that have a continuous current adjustment. Ir is the maximum continuous current that circuit breaker can carry without tripping for the given continuous current setting. Ir may be specified in amps or as a percentage of In.
Circuit Breaker Frame Size The circuit breaker frame includes all the various components that make up a circuit breaker except for the trip unit. For any given frame, circuit breakers with a range of current ratings can be manufactured by installing a different trip unit for each rating. The breaker frame size is the highest continuous current rating for a breaker with a given frame.
The interrupting ratings for a circuit breaker are typically specified in symmetrical RMS amperes for specific rated voltages. As discussed in Basics of Electricity, RMS stands for root-mean-square and refers to the effective value of an alternating current or voltage. The term symmetrical indicates that the alternating current value specified is centered around zero and has equal positive and negative half cycles. Siemens circuit breakers have interrupting ratings from 10,000 to 200,000 amps.
Panelboard Construction Panelboards are available in different sizes with variations in construction. The components that make up a panelboard, however, are similar. Panelboards contain a can, interior, circuit protection devices, label, and trim. Can The can is typically constructed of galvanized steel and houses the other components. The can is also referred to as a box or enclosure. It is designed to provide component and personnel protection.
Interior The interior consists of several components, including overcurrent protection devices, bus bars and insulated neutral bus bars. A lighting panel interior is mounted to the four mounting studs in the can. Jacking screws (not shown) allow adjustment of the interior within the enclosure. Bus Bars A bus bar is a conductor that serves as a common connection for two or more circuits. Standard bus bars on Siemens panelboards are made of aluminum, but copper bus bars are available as an option.
High Leg Some power supply systems use a transformer with a threephase, four-wire (3Ø4W), delta-connected secondary with grounded, center-tap connection on one phase. The following illustration shows an example of such a system with 240 volts phase-to-phase. The midpoint of one phase winding is grounded to provide 120 volts between phase A and neutral and 120 volts between phase C and neutral. Between phase B and neutral, however, the voltage is 208 volts. This is referred to as the high leg.
Split Neutral Siemens panelboards feature a split neutral design which means that neutral connections are available on both sides of the panelboard. Split neutrals are connected by means of an insulated neutral bus bar. A Service Neutral Lug B C Neutral Bus Bar Branch Neutral Lugs Branch Neutral Lugs Insulation Supply Bus Bars 200% Neutral Some loads can cause harmonics and non-linear loading on a distribution system. This requires special consideration when ordering a panelboard.
Circuit Identification Specifications typically require panelboard circuit terminals to be labeled or for a wiring diagram to be provided. One approach for numbering terminals is to use odd numbers for poles on the panelboard’s right (your left as you face the panelboard) and even numbers on the panelboard’s left. This is sometimes referred to as NEMA numbering. For some specifications, vertical numbering is required.
Dead Front and Trim The dead front and trim are the front surfaces of the panelboard that cover the interior. The trim includes an access door. These components provide access to the overcurrent devices while sealing off the bus bars and internal wiring from contact. Access Door Trim Dead Front Filler Plates QF3 filler plates are used to cover any unused pole spaces not filled by a circuit breaker.
Enclosures The National Electrical Manufacturers Association (NEMA) has established guidelines for electrical equipment enclosures. Siemens panelboards are supplied as standard in a NEMA Type 1 enclosure intended for general purpose indoor use. The following enclosures are available as an option: Type 3R Enclosures are intended for outdoor use primarily to provide a degree of protection against rain, sleet and damage from external ice formation.
Installation Panelboard installation requires careful planning to ensure a safe environment for personnel and equipment. Article 110.26 of the NEC® covers spaces about electrical equipment, such as panelboards. The intent of Article 110.26 is to provide enough working space for personnel to examine, adjust, service, and maintain energized equipment. Article 110.26 sets requirements for depth, width, and height of a working space. In addition, Article 110.
Individual Overcurrent Protection NEC® Article 408.36 requires panelboards to be protected by an overcurrent protection device with a rating that does not exceed the panelboard’s rating. The following illustration shows two ways individual panelboard overcurrent protection can be accomplished. A main overcurrent protection device, such as a circuit breaker, can be located as an integral part of the panelboard or located on the supply side of the panelboard.
When two main circuit breakers are used in a panelboard, a split bus is used. Half of the branch circuits are protected by one main circuit breaker, and the other half are protected by the other main circuit breaker. Keep in mind that the combined ratings for these circuit breakers must be no greater than the panelboard rating. 200 Amp Service 100 Amp Main Breakers Panelboard Supplied by a Transformer Frequently a panelboard is supplied by the secondary of a transformer. According to NEC® Article 408.
Panelboards Main Configurations There are three types of panelboard main configurations: main lug only, main breaker, and main switch. In this context, the term switch refers to a fusible switch. All three of these configurations are available for power panels. Lighting panels are available with either a main lug only or a main breaker configuration.
Siemens main breakers are bus connected to the main bus bars. This means there are no cable connections required from the main circuit breaker to the lugs on the main bus bars. Bus connecting provides a higher degree of circuit integrity because there is less chance for loose connections which lead to overheating. Bus Connected Depending on the panelboard, the main breaker can either be mounted horizontally or vertically.
Feed-thu Lugs There are a variety of ways a main breaker or main lug only panelboard might be used in the same application. For example, feed-thru lugs, mounted on the opposite end of the main bus from the main breaker, could be used to connect a main breaker panelboard to a main lug only panelboard. The feed-thru lugs mounted on the main bus of the main breaker panelboard are connected to the main lug only panelboard. The main breaker protects both panelboards from overcurrent.
Sub-feed Lugs Sub-feed lugs are mounted directly beside the main incoming lugs on a panelboard and are used to connect one or more additional panelboards to the same feeder. In the example shown below, two adjacent main lug only panelboards are connected to the feeder through a fusible switch or circuit breaker. Power supplied by the overcurrent protection device is routed to the panelboard on the left and through sub-feed lugs to the panelboard on the right.
Unit Space and Number of Circuits Circuit breaker or fusible switch mounting height is sometimes referred to as unit space. The unit space available for mounting branch devices depends on the panelboard type, enclosure dimensions, and main device configuration. Because the unit space required by circuit breakers and fusible switches varies, the number branch devices that can be mounted in a panel also varies.
Sub-feed Breaker When an application requires a circuit breaker that is a larger frame size than the branch circuit breakers available and will not fit in a branch circuit location, a sub-feed breaker can be used. One possible application is to supply a second panelboard located some distance from the first panelboard. However this is not the only application. A sub-feed breaker can supply any load that a branch circuit breaker can supply.
Power Supply Systems Panelboards receive power from a variety of sources. For example, a downstream panelboard typically receives power from an upstream panelboard or switchboard. However, power for the distribution system originates from a utility power company. Power from the power company is stepped down through a transformer for distribution to a residential, commercial, or industrial facility. There are a number of ways that the transformer secondary windings providing service may be configured.
3Ø4W Wye-connected Transformer The following illustration shows the secondary of a 480 Y/277 V three-phase, four-wire (3Ø4W), wye-connected transformer. The “480 Y” indicates the transformer is wye-connected and has 480 volts between any two phases. The “277 V” indicates the voltage between any phase and neutral (N) is 277 V. If you know the phase voltage for a system like this, you can calculate the phase-to-phase voltage by multiplying 1.732 times the phase-to-neutral voltage (277 V x 1.732 = 480 V).
You may remember that NEC® Article 110.15 requires that the high leg bus bar or conductor be permanently marked with an orange finish “or by other effective means.” This will help prevent someone from connecting a 120 V single-phase load to the 208 V high leg.
Service Entrance Panelboards Sometimes panelboards are used as service entrance equipment for a building. This is the equipment located near where the power enters the building. The incoming power is connected to this equipment which provides a means to control and cut off the supply. According to NEC® Article 408, panelboards used as service entrance equipment must be approved and labeled as such.
In the example on the right, a main lug only panelboard is equipped with up to six circuit breakers to disconnect power to all equipment being supplied by the service. Regardless of which of these examples is used, each circuit breaker must be clearly labeled to show the load it supplies.
Panelboard Grounding Grounding is an important aspect of any electrical equipment and must be considered carefully. A ground connection is a connection to earth or to a conductive object that is connected to earth. The accompanying illustration, for example, shows the neutral (N) conductor of a wye-connected transformer connected to ground. The intentional grounding of electrical equipment is done to limit voltage differences between parts of a system.
The neutral and the panelboard enclosure are bonded together at the service entrance so that the enclosure is also connected to ground through the grounding electrode connector. Bonding permanently joins metal parts to form a low-resistance path for electrical current.
Grounding Panelboards Downstream The neutral conductor is only connected to ground at the service entrance. As shown in the following illustration, when a downstream panel is used, the neutral is isolated from ground in that panel and connected to the neutral bus in the service entrance panel. In addition, the enclosure of the downstream panel is connected to ground through a grounding conductor which connects to the ground bus in the service entrance panel.
Review 4 1. If the secondary of a four-wire, wye-connected transformer is 480 V phase-to-phase, the phase to neutral voltage is _____ V. 2. If the secondary of a four-wire, delta-connected, BØ high leg transformer is 240 volts phase-to-phase, determine the following phase to neutral voltages. _____ V from A-N _____ V from B-N _____ V from C-N 3. According to NEC® Article 230.
Ground Fault Protection A ground fault is a condition in which electrical current unintentionally flows to ground. Because ground faults can cause damage to equipment and can endanger lives, ground fault protection is required in some situations. For example, NEC® Article 230.95 requires ground fault protection of equipment for service disconnects rated 1000 amps or more on solidly-grounded wye services exceeding 150 volts-to-ground but not exceeding 600 volts phase-to-phase.
Ground Fault Sensor Around all Conductors Another way a ground fault protector works is with a sensor around all the circuit conductors. When current is flowing normally, the sum of all the currents is zero. However, a ground fault causes an imbalance of the currents flowing in the individual conductors. When the imbalance reaches a set level, the shunt trip opens the circuit breaker, removing the load from the line.
Panelboard Interrupting Ratings Interrupting Rating When selecting panelboards and overcurrent protection devices, it is essential to know the available fault current for an application and the interrupting rating for the protective devices under consideration for use in the panelboard. NEC® Article 110.9 requires circuit protection equipment to have an interrupting rating sufficient for the circuit voltage and available current.
Series Rating Method An alterative to the full rating method is the series rating method, which requires that the main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system, but subsequent downstream circuit protection devices connected in series can be rated at lower values.
Siemens P Series Panelboards Siemens P series of panelboards offers a stepped approach to power distribution. The P1 panel fits the majority of lighting panel needs in a cost effective package. P1 offers a flexible design that virtually eliminates the impact of common mistakes in feed direction or main lug versus main breaker selection. The next step in the series is the P2 panel, which offers maximum flexibility and options to fit demanding specifications. P3 is also a flexible and innovative panel.
General Specifications P series panelboard interiors are designed to accommodate top or bottom feed. Regardless of which is specified for three-phase panels, branch device poles are arranged with the uppermost pole always on “A” phase, the second pole down always on “B” phase, and the third pole down always on “C” phase. As a standard configuration, branch breakers are mounted at the top of the panel with “spaces” at the bottom, regardless of the direction the panel is fed.
P1, P2, and P3 Panelboards P1, P2, and P3 panelboards are grouped together in this section because they are similar in construction and function. Like all P series panelboards, these panels have symmetrical interior mounting studs to eliminate the problem of upside down mounting. P1, P2, and P3 panelboards feature concealed fasteners and hinges with a flush door lock. P1, P2, and P3 panelboards are designed to be wall mounted.
P1 Panelboards P1 panelboards are pre-engineered to accept the most common modifications without increasing box height. P1 Features P1 panelboards have the following features: • Symmetrical interiors - No top or bottom. To change from top to bottom or vice versa, simply invert the interior. The deadfront labeling is always right-side up. • Field convertible from main lug to main breaker and vice versa with no increase in enclosure height.
P2 Panelboards P2 panelboards offer a wide variety of factory-assembled options to meet most lighting panel application requirements. The P2 design also offers the ability to mix breaker frames in unit space up to 250 A to meet many power distribution panel requirements in a much smaller package. In addition to the standard bussing, P2 panelboard bussing options include temperature rated copper, 750 A/sq. in. aluminum, or 1000 A/sq. in. copper.
P3 Panelboards P3 panelboards are small footprint power distribution panelboards designed for use in applications that require more or larger branch devices than a lighting panelboard typically includes. P3 panelboards can include a wide variety of factory assembled options and have the ability to mix and match breaker frames in unit space up to 250 A. P3 panels are available with enclosure heights of 56, 62, 68, 74, or 80 inches.
P4 and P5 Panelboards P4 and P5 power panelboards are similar in design and features, but vary in the ratings available. P4 panelboards have a medium footprint to fit applications that require more or larger branch devices and higher current ratings than lighting and appliance panelboards can accommodate. P4 panelboards can incorporate circuit breaker frames in unit space up to 800 A and fusible switches up to 200 A.
P Series Panelboard Catalog Numbers The following P series panelboard catalog number description provides summary information. For more detail including information on circuit breaker selection, refer to the SPEEDFAX catalog. The catalog number provides a description of the panelboard. There are eight parts to the standard P series panelboard catalog number as the example below shows. 1 2 3 4 5 6 7 8 Part 1 Part 1 identifies the type of panel, P1, P2, P3, P4, or P5.
The panelboard identified in the example is configured for a 208Y/120V, 3Ø4W power system. This indicates it is rated for a 208 volt wye-connected secondary. There are 208 volts phaseto-phase and 120 volts phase-to-neutral. It is a 3-phase (3Ø) 4-wire (4W) system. A 208 Volts 120 Volts B 120 Volts 208 Volts N 120 Volts 208 Volts C Part 3 Part 3 indicates the number of circuits in a P1 or P2 type panelboard. If the panelboard is a P3, P4, or P5 type, this number represents the enclosure height in inches.
You can minimize the potential for error when ordering panelboards by making sure that you have the correct answers to the following questions.
Quick-Spec Coordination Panelboards Siemens Quick-Spec coordination panelboards provide fusible solutions that make it simple and cost effective to selectively coordinate a fused electrical distribution system. These panelboards are designed to address the NEC® selective coordination requirements.
Additional Types of Panels and Cabinets In addition to the P series and coordination panelboards described in this course, Siemens also offers C1 and C2 column type panelboards, lighting control panels, and telephone and equipment cabinets. Siemens C1 and C2 panelboards have a narrow width suitable for column mounting. C1 panelboards are designed for a 250 VAC maximum supply and C2 panelboards are designed for a 480Y/277 VAC maximum supply.
Accessories Accessories add to the performance of a panelboard or adapt the panelboard for specific application requirements. Various accessories are available for Siemens panelboards. The shunt trip accessory described on this page is just one example of a circuit breaker accessory. Refer to the SPEEDFAX catalog for a complete listing. Shunt Trip Some accessories modify the circuit breaker. For example, it is sometimes necessary to trip a breaker from a remote location.
Time Clocks Tork, Sangamo or Paragon time clocks are available as an external accessory for P1 panelboards and can be mounted internally in P2, P3, P4, or P5 panelboards. Time clocks are available in 1 or 2-pole, single or double throw devices, or 3pole, single throw. They are rated for a maximum of 277 volts. A time clock can be used to turn a branch circuit or an entire panelboard on and off at predetermined times.
TPS3 Transient Protection Systems Many electrical devices and systems are susceptible to damage from the high energy levels associated with electrical surges caused by lightning or other electrical equipment. Any component between the source of the surge and ground can be damaged. Therefore, electrical systems aren’t complete unless they incorporate surge protection.
Power Meters In today’s complex business climate, power monitoring systems often require meters with a range of capabilities from basic energy and power meters to advanced power quality meters that can accumulate data from a variety of sources. In addition, while small systems often need low-cost, stand-alone meters, the growing demands for energy management and overall monitoring of system performance mean that communication with a variety of devices is increasingly important.
Embedded Sub-Metering Solutions In a world where tenant square footage is a premium in commercial building designs, the area for electrical metering is being drastically reduced. At the same time, contractor labor costs for the installation of sub-metering systems continues to increase. To meet these sub-metering challenges, Siemens offers proven cost-effective solutions for embedded metering and monitoring.
Review 7 1. Siemens ________ panelboards provide fusible solutions that make it simple and cost effective to selectively coordinate a fused electrical distribution system. 2. Siemens ________ and ________ panelboards have a narrow width suitable for column mounting. 3. A ________ is a circuit breaker accessory designed to trip a breaker from a remote location. 4.
Review Answers Review 1 1) power distribution; 2) b; 3) 2008. Review 2 1) can; 2) interior; 3) bus bar; 4) split; 5) dead front, trim. Review 3 1) man breaker, main lug only, main switch; 2) horizontally, vertically; 3) lug only; 4) Feed-thru; 5) Sub-feed; 6) sub-feed. Review 4 1) 277; 2) 120 A-N, 208 B-N, 120 C-N; 3) 6; 4) Bonding; 5) neutral. Review 5 1) ground fault; 2) interrupting; 3) full; 4) UL. Review 6 1) P1; 2) aluminum, tin; 3) 66, 78, 90; 4) P4, P5; 5) Vacu-Break; 6) top, flush.
Final Exam Before taking the final exam, it is recommended that you delete the temporary internet files from your computer’s web browser. For most versions of Internet Explorer, you can do this by selecting Internet Options from the Tools menu and then clicking on the Delete Files button. If you do not perform this step, you may see a score of 0% after you submit your exam for grading. The final exam for this course is available online at http://www.usa.siemens.com/step.
