Learning Module 10: Loadcenters 101 Basic Series
What You Will Learn We ll step through each of these topics in detail: What Does a Loadcenter Do? 4 Applications 4 Basic Circuitry and Wiring 5 Residential Service Entrance Panel 5 Residential Distribution Panel 6 Commercial/Industrial Service Entrance Panel 6 Commercial/Industrial Distribution Panel 7 Grounding Issues 7 Grounding the Service Entrance Panel 7 Equipment Grounding Bus 8 Grounding Downstream Loadcenters 8 Ground Fault 9 Review 1 11 Loadcenter Components and Installation 12 Anatomy of a Loadcenter 12 Enclosure 13 Installation and Mounting 14 Circuit Breakers 15 Installing Circuit Breakers 15 How Many Circuit Breakers Fit in a Loadcenter? 16 Ratings 17 Loadcenter Types 17 Main Breaker Loadcenter 18 Main Lug Only Loadcenter 18 Convertible Loadcenter 18 Selecting a Loadcenter 19 Required Application Information 19 Sample Application 20 Review 2 21 Glossary 22 Review 1 Answers 24 Review 2 Answers 24 Page 2
Welcome Welcome to Module 10, which is about Loadcenters. A loadcenter is a device that delivers electricity from a supply source to loads in residential and light commercial/industrial applications. Figure 1. Typical Loadcenter A Note on Font Styles Viewing the Glossary Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, then answer the questions without referring back to what you ve just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module. Key points are in bold. Glossary items are italicized and underlined the first time they appear. Printed versions have the glossary at the end of the module. You may also browse the Glossary by clicking on the Glossary bookmark in the left-hand margin. Page 3
What Does a Loadcenter Do? Every residential and light commercial/industrial building utilizes electricity for lighting, receptacles, and/or appliance loads. Conductors are used to bring power from the power lines to the building. A device is needed to divide this power for branch circuits and to protect these branch circuits from overloads and short circuits. This device is called a loadcenter. Electricity moves from the supply source (example, utility) into the building to the loadcenter and is then distributed through the building s Branch Circuits to the loads. Each branch circuit is connected, or Terminated, at the loadcenter. Each branch circuit is protected by a circuit breaker housed in the loadcenter. In the event of a short circuit or an overload on a branch circuit, the circuit breaker cuts the power before any property damage or personal injury can occur. NOTE: New electrical installations use circuit breakers in the loadcenter, so we will not discuss Fuses in this training module. Figure 2. Electrical Distribution to Loads Applications There are two main applications for a loadcenter: As a Service Entrance Panel. This is the term for a loadcenter used at the point at where the power enters a building. There can only be one Service Entrance panel per building. As a Distribution Panel. A distribution panel is simply a loadcenter used at a point beyond the building s service entrance. This can be useful when adding additional electrical service to an existing building. The power to the distribution panel usually comes from a branch circuit of the service entrance panel. Page 4
Figure 3. A Service Entrance Panel and a Distribution Panel Basic Circuitry and Wiring Residential Service Entrance Panel Figure 4 shows an example of a residential application. Loadcenters are typically rated 225 amps or less, and a maximum of 240 volts. Most homes have 200 amp service and a 120/240-volt, single-phase, three-wire system. Three conductors run from the Distribution Transformer to the service entrance panel. This transformer converts the utility voltage into a voltage and current supply suitable for use in most residential applications. Two of the Main Service Conductors (or Mains) are ungrounded ( hot ), and the third is the neutral. The hots are connected to the Main Circuit Breaker in the loadcenter. The neutral is attached to the neutral bar. If a Voltmeter reading is taken between the two hot conductors ( A and B ), it will measure 240 volts. If a reading is taken between a hot conductor and the neutral ( N ), it will measure 120 volts. Figure 4. Typical Single-Phase, Three-Wire Loadcenter This is convenient because 240 volts are needed to power central air conditioners, electric clothes dryers, and electric range tops. At the same time, 120 volts are required to service lighting, small appliances, and receptacle loads. Page 5
Residential Distribution Panel Now, let s consider the case of a residential distribution panel. Remember that a distribution panel is a panel used at a point beyond the service entrance and is sometimes called a subpanel. This means that the distribution panel is typically fed by a service entrance panel. You can add on to a loadcenter in two ways. The first way is to use a branch circuit breaker. The second way is to use Feed-Through Lugs. Figure 5. A Service Entrance Panel Feeding a Distribution Panel Commercial/Industrial Service Entrance Panel In the example shown here, the service conductors (the two hots and the neutral) run from the feed-through lugs of the service entrance panel to the Main Lugs of the distribution panel. In this way, the distribution panel itself is protected by the main circuit breaker in the service entrance panel. The neutral conductor is isolated. The importance of this will be discussed later. Usually, a residential distribution panel is used to supply an air-conditioner, hot tub, garage, out-building, and/or home addition. Many commercial/industrial service entrance panels are supplied with a different power system than the residential one. A distribution transformer provides a 120/208 volt, three-phase, four-wire system to the panel. Four conductors go to the service entrance. The three mains are ungrounded ( hot ), and the fourth is the neutral. The hots are connected to the main breaker in the loadcenter. The neutral is attached to the neutral bar and is connected the same way as in the residential application. If a voltmeter reading is taken between any two hot conductors ( A, B and C ), it will measure 208 volts. If a reading between any hot conductor and the neutral ( N ) is taken, it will measure 120 volts. Page 6
Figure 6. Typical Three-Phase, Four-Wire Loadcenter Commercial/Industrial Distribution Panel Grounding Issues Grounding the Service Entrance Panel This system can handle single-phase or three-phase applications. The 120/ 208 volt single-phase power is used the same way as in the residential application. The three-phase, 208-volt power is rarely used in loadcenters. For this reason, we will not discuss three-phase any further in this module. The distribution panel is treated the same as the residential distribution panel. It is typically fed by the service entrance panel and it can be single phase or three phase. Again, it is important to remember that the neutral is isolated. Grounding is an important aspect of any electrical equipment and must be considered carefully. The National Electrical Code (NEC) defines ground as a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. There are two objectives to the intentional grounding of electrical equipment: Keep voltage differentials between different parts of a system at a minimum which reduces shock hazard. Keep impedance of the ground path to a minimum. The lower the impedance the greater the current is in the event of a fault. The greater the current the faster an overcurrent device will open. In the service entrance panel shown in Figure 7, the neutral from the transformer is connected to the neutral bar in the loadcenter. The neutral bar is then grounded to the earth. Page 7
Figure 7. Grounding a Service Entrance Panel Equipment Grounding Bus This is accomplished by running a conductor from the loadcenter s neutral bar to a cold water pipe or copper rod buried in the ground (or other suitable grounding electrode.) Then, a Bonding screw or strap is used to physically bond the neutral bar to the metal enclosure. In this way, the neutral bar, the neutral of the transformer, and the enclosure are grounded. When the neutral is grounded in this manner, the equipment ground from the branch circuits is sometimes allowed to connect to the neutral. The Equipment Grounding Bus is connected directly to the loadcenter enclosure. For safety, all equipment (on both the feeder and branch circuits) is connected to the equipment grounding bus. This requirement keeps the connected equipment at the same potential as the enclosure itself. Figure 8. Equipment Grounding Bus Grounding Downstream Loadcenters One additional point for service entrance applications: this bonded and grounded neutral can also be used as the point of connection for equipment grounding. It is important to remember that the service entrance panel is the only ground connection point for the neutral. The neutral is both insulated and isolated at any downstream panels. As shown in Figure 9, the downstream panel s enclosure is grounded with a grounding conductor running back to the service entrance panel. Page 8
Figure 9. Grounding the Downstream Loadcenter In Figure 10, the table lamp has a short circuit. If you trace the thick line back, you will see how fault current is returned to the source. This is why the downstream loadcenter contains a branch circuit breaker. It trips, disconnecting power from the load. Figure 10. A Short in a Downstream Load Ground Fault In most cases, NEC also requires equipment to have an additional grounding conductor. This conductor connects the metal housing of an appliance to the system ground to reduce the possibility of a shock due to a Ground Fault. A ground fault is simply current leakage from an ungrounded conductor to the Grounding Path in an electrical system. Consider Figure 11. A piece of insulation covering a conductor at an electrical outlet has flaked off. This could allow current to pass from the conductor to the metal outlet box. The box is part of the system ground. If properly grounded, the current should follow the grounding path to ground, instead of an unfortunate person s finger that touched the box. Page 9
Figure 11. Ground Fault Because a distribution panel takes its power from the service entrance panel (Figure 9), the neutral needs to be isolated from the ground. It is considered a branch circuit of the main service panel, and needs to have its own ground to prevent a ground fault. If all the loads in a panel are connected evenly between A-N and B-N, no current will flow in the neutral. However, loads are rarely balanced. As a result, some current usually flows through the neutral. The amount of current flowing in the neutral conductor at any one time is the difference between the current flowing through leg A and leg B. This is why a path to ground is needed. Page 10
Review 1 Answer the following questions without referring to the material just presented. Begin the next section when you are confident that you understand what you ve already read. 1. Loadcenters have two main applications. These are: 2. In which two market segments are loadcenters typically used? / 3. For safety reasons, the National Electrical Code (NEC) requires the service entrance panel s to be grounded. Beyond the service entrance equipment, it is always and. 4. In your own words, explain under what conditions no current will flow through the neutral. Then explain how to figure the amount of current flowing through the neutral. 5. In your own words, explain the main purpose of a loadcenter. Page 11
Loadcenter Components and Installation Anatomy of a Loadcenter Before we go any further in our exploration of the loadcenter, let s take a moment to consider the main components and features of a typical loadcenter. 1. Front Cover Provides a neat, protective face for the product. Usually includes a hinged door to allow access to the circuit breaker handles. 2. Inner Cover or Dead Front 3/4. Inner Cover Adjustment Screws Serves as a protective barrier to prevent contact with energized ( hot ) parts within the unit. Used to secure the inner cover tightly against the tops of the circuit breakers. This eliminates any space that might allow access to hot parts inside. 5. Twistouts Removable perforated metal spaces that can be twisted out, to allow the protrusion of the circuit breaker handles. 6. Pub (or Publication) Lists the specifics of the loadcenter, including: circuit breaker application, UL information, short circuit ratings, series ratings, wire connection torque values, unit accessories, and wiring diagram. 7. Sliding Latch Provided on indoor loadcenters to allow easy opening of the loadcenter door. 8. Door The hinged part of the front cover which allows access to the circuit breaker handles. 9/19. Backpan Top and Bottom Mounting Screws Allows for easy removal of the loadcenter interior assembly. Page 12
10/18. Enclosure Mounting Keyholes 11. Drywall Scribe and Markings 12. Surface Mounting Holes (4) 13. Main Lugs or Main Breaker Located at top center and bottom center. Whether surface-mounted right-side up or inverted, the unit can be easily centered on the wall by hanging the enclosure with the uppermost keyhole. Serve as a guide for mounting the loadcenter flush with the drywall between the wall studs. Once centered for surface mounting, the loadcenter is affixed with screws through these four holes. Provide the means for the termination of the power cables coming from the electric meter. Main lugs simply provide a safe mechanical connection to the Bus Bars. A main breaker provides thermal magnetic protection for the loadcenter. 14. Backpan The solid steel piece that supports the loadcenter bus bar assembly. It also provides the means to affix branch circuit breakers. 15. Bonding Strap Used to bond the neutral to the enclosure (when required by code). 16. Bus Bars (2) An extension of the incoming power cables. Provide a mechanical means of affixing branch circuit breakers. Notches are added to (or omitted from) the bus bars to limit the number of branch circuits allowed. Bus bars may be aluminum or copper. 17. Knockouts Circular indentations in the top, bottom, sides, and back of the enclosure. When removed, the electrician can bring wire into and out of the loadcenter without compromising safety. 20. Neutral Bar Provides the termination point for the neutral wires from both the incoming service and the load circuits. Most loadcenters have a twin neutral design, positioning terminations on both sides of the unit. This is a wiring convenience for the electrician. Enclosure The loadcenter enclosure is typically constructed of cold rolled or galvanized steel. It serves to house the other loadcenter components. It is designed to provide component and personnel protection. The National Electrical Manufacturers Association (NEMA) and UL have established guidelines for electrical enclosures. They are as follows: NEMA Type 1 General Purpose NEMA Type 3R Raintight This enclosure type is for general purpose, indoor use. It is suitable for most applications where unusual service conditions do not exist. It provides protection from accidental contact with enclosed equipment This enclosure type is intended for outdoor use. It provides protection against falling rain, sleet, and snow. Page 13
Installation and Mounting A loadcenter can be Flush-Mounted or Surface-Mounted against a wall. A flushmounting is recessed into a hole between the studs in the wall. A surface-mounting is attached to (and projects out from) the wall. Flush mounting offers some benefits such as: Space savings Because the box is behind the wall, the room does not sacrifice too much space. Appearance This option is more attractive, as the wiring and box are concealed. Safety Except for the trim, the panel does not stick out to possibly catch clothing or other objects. Figure 12. Flush-Mount Vs. Surface-Mount Surface-mounted loadcenters are generally used in industrial buildings, and in basements or garages. Because most of these areas have walls made of poured concrete, flush-mounting is impractical. The NEC specifies clearances around loadcenters. This is to provide access and working space. There are three basic rules: Headroom in the location must be a minimum of 6-1/2 feet. For systems up to 150 volts, the minimum distance from the loadcenter to the ground shall be 3 feet. For access, there must be 30 inches minimum space in front of the loadcenter, and sufficient space to allow the hinged door to open and rotate 90 degrees. Figure 13. Loadcenter Clearance Requirements Page 14
Circuit Breakers Installing Circuit Breakers Now, let s take some time to look at one of the main components of a loadcenter: the circuit breaker. Each branch circuit in the loadcenter is protected by a Branch Circuit Breaker (or Miniature Circuit Breaker). Miniature circuit breakers are covered in much more detail in Module 9, Miniature Circuit Breakers. Miniature circuit breakers are also called Plug-In breakers because they are connected by plugging them into the bus bar Stabs. The bus bars provide a convenient means of providing electricity to the various loads in a building. The circuit breakers attach to the bus bars on vertical stabs that alternate from each hot source. Single Pole and Double Pole circuit breakers are the most commonly used breakers in a loadcenter. If a double pole breaker is plugged onto two adjacent stabs, 240 volts are drawn. A single pole breaker is plugged onto a single bus stab and feeds a 120-volt circuit. To understand how this is possible, consider the physical shape of the bus bars. Figure 14. Getting 120V and 240V Power from the Bus Bars Now, to reinforce the point, consider Figure 15. Figure 15. Typical Single-Phase, Three-Wire Loadcenter Page 15
How Many Circuit Breakers Fit in a Loadcenter? On Figure 15, trace the 240-volt M circuit. Current comes in on conductor A, flows through the left bus bar and branch circuit breaker, and enters the motor load s branch circuit. From there, current passes through another branch breaker and out on conductor B. The circuit is made across both ungrounded legs of the transformer. Trace the 120-volt L circuit. Current comes in on conductor A, flows through the left bus bar and branch circuit breaker, and enters the lighting load s branch circuit. From there, current passes out on the neutral N. The circuit is made from an ungrounded leg of the transformer to the transformer s grounded leg. A 240-volt circuit typically consists of four wires: two hots, one neutral, and one ground. A 120-volt circuit consists of three wires: one hot, one neutral, and one ground. Usually, hot wires are both color-coded black, although the second hot could be red. Neutral wires are always white. Ground wires are either green or bare (uninsulated) copper. The number of single-pole circuit breakers that can be installed in a loadcenter is limited by the number of stabs on the bus bars. A breaker can be applied to each side (right and left) of a stab. Therefore, a loadcenter with six stabs would accommodate twelve single-pole circuit breakers. To balance the loads, the breakers should be evenly applied to each side of the stab. Figure 16. Breaker Installation (Viewed from Above) If the stab is notched, then Half-Size Branch Circuit Breakers can be used. Halfsized branch circuit breakers allow for two single-pole circuit breakers to be installed in one space. Each unit is typically only 1/2 wide. These breakers are every bit as functional as the full-size units and allow the added convenience of a more compact installation. Some local inspectors will not allow the use of half-size branch circuit breakers. This is because there is a possibility of overloading the loadcenter and/ or unbalancing the system. Figure 17. Solid and Notched and Stabs The number of notched and unnotched stabs in a particular loadcenter and therefore the number of breakers that may be installed is limited by UL requirements. This is to prevent a panel from being overloaded. Page 16
Ratings Regardless of ampere ratings, a lighting and appliance type of loadcenter is limited to 42 circuits in one enclosure. If the specifications require more than 42 circuits, two or more loadcenters will be required. Every circuit breaker has a specific ampere, voltage, and fault current interruption rating. Ampere rating The ampere rating defines the maximum current a circuit breaker can carry without tripping. The typical for miniature circuit breakers is ratings from 15-150 amps. Voltage rating In residential applications, single pole breakers protect 120 volt branch circuits and two-pole breakers protect 240 volt branch circuits. The rating of a circuit breaker can be higher than the circuit voltage, but never lower. Short circuit interrupting rating This is the maximum available fault current that a breaker is designed to interrupt. Typical ratings are from 10,000 to 65,000 amps. When selecting loadcenters and overcurrent protection devices, it is extremely important to know both the maximum continuous amperes and available fault current. There are two ways to meet this requirement: the Full Rating Method and the Series-Rated Method. The full rating method selects circuit protection devices with ratings equal to or greater than the available fault current. Consider a building service entrance with 22,000 amps of fault current available. All downstream circuit protection devices must be rated at 22,000 amps. The requirements of the series-rated method are somewhat looser. The building s main circuit protection device must have an interrupting rating which is at least equal to the system s available fault current. However, the all downstream, series-connected panels can have lower ratings. Consider the same building, still with 22,000 amps of available fault current. While the breaker at the service entrance is rated at 22,000 amps, additional downstream breakers could be rated at only 10,000 amps. Figure 18. Full Rating Method (on top) Vs. Series-Rated Method (on bottom) Loadcenter Types In the typical single family home, available fault current is normally 10,000 amps. There are three different types of loadcenters commonly used today. These are: Main Breaker Main Lug Only Convertible Page 17
Figure 19. Main Configurations Main Breaker Loadcenter The incoming supply cables of a main breaker loadcenter are connected to the main breaker, which in turn feeds power to the loadcenter and its branch circuits. Power from the main is fed through the main breaker to the bus bars, which are an extension of the utility power cables. With the main circuit breaker in the off position, no power will flow to the bus bars for the branch circuit breakers. This serves as a quick and simple way to disconnect power to the entire building. Figure 20. Simplified Main Breaker Type Loadcenter Main Lug Only Loadcenter Convertible Loadcenter With the main breaker in the on position, power is allowed to flow from the bus bars to the branch circuits. Branch circuits run throughout the building to supply power for lighting and appliances. Because of these features, the main breaker is normally used as a service entrance panel. The Main Lug Only (MLO) Loadcenter does not contain a main circuit breaker to protect the loadcenter itself. Instead, it is protected by a breaker back in an upstream panel. The supply cables from the upstream panel are connected to the main lugs and bus bars. This type of loadcenter is used primarily used as a distribution panel. The MLO is often called an add-on, secondary, or downstream panel. It is often used when the main breaker loadcenter s circuit slots are full, or to provide power at a remote point. Also, main lug loadcenters are used in the southeastern and western parts of the country because, in these areas, local codes mandate that the main circuit breaker be located outside the home so that emergency personnel can turn off the power. The Convertible Loadcenter is supplied without lugs or breakers. Once the customer decides to use main lugs or main breakers, the appropriate kit is installed by an electrician. Page 18
In the Workplace In this home, a main breaker loadcenter supplies power to a main lug only loadcenter in the home s workshop. Using a Main Lug Only Loadcenter Main lug only loadcenters can also be fed from metering equipment when used in apartment installations. Selecting a Loadcenter Required Application Information When assisting a customer in selecting a loadcenter for an application, it is best to start by interviewing the customer to determine the application s requirements. 1. Ask whether this application will involve an indoor or outdoor installation. This will determine the enclosure type for the loadcenter. NEMA Type 1 General Purpose NEMA Type 3R Raintight This enclosure type is for general purpose, indoor use. It is suitable for most applications where unusual service conditions do not exist. It provides protection from accidental contact with enclosed equipment. This enclosure type is intended for outdoor use. It provides protection against falling rain, sleet, and snow. If it is an indoor installation, ask whether it will be flush-mounted or surface mounted. Figure 21. Outdoor, Flush and Surface Mounting Types 2. Ask whether the loadcenter is to serve as a service entrance panel or a distribution panel. 3. Ask whether the application involves a single-phase, three-wire system or a three-phase, four-wire system. 4. Ask what type of main, either a Main Lugs Only or a Main Circuit Breaker. 5. Ask for the main ampere rating. 6. Ask what type and rating main breaker is required, if applicable. 7. Ask for the number of breakers required, and for their ratings. Page 19
Sample Application 8. Ask how many spaces are required. The customer may want to leave extra spaces in the loadcenter for future needs. Once this information is obtained, go to the product catalog. These questions should cover everything needed to recommend products that fit the customer s application needs. To assist you in visualizing the product selection process, let s consider a commonplace customer application. Suppose a customer comes to you and wants to add two branch circuit breakers to an existing loadcenter for a newly remodeled kitchen. How would you go about sizing the breaker? First, interview the customer to find out what loads will be on the branch circuits. Let s say that one will be dedicated to a dishwasher, and the other will be dedicated to a garbage disposal. Next, obtain the amperage requirements of the loads from the customer. Suppose the dishwasher runs on 12.5 amps and the garbage disposal runs on 9.8 amps. Based on these numbers alone, you might think that it would be a good idea to recommend a 15 amp breaker for the dishwasher circuit, and a 10 amp breaker for the garbage disposal circuit. But this would result in a lot of nuisance tripping. So, we need to do a little simple math. Figure in a 25% safety margin by multiplying the amp ratings by 1.25. Dishwasher 12.5 amps x 1.25 = 15.625 amps Garbage Disposal 9.8 amps x 1.25 = 12.25 amps This will be sufficient to eliminate nuisance tripping, but the breakers will still trip in the case of a true overload condition. Based on these calculations, you would recommend a 20 amp breaker for the dishwasher circuit, and a 15 amp breaker for the garbage disposal circuit. Page 20
Review 2 Answer the following questions without referring to the material just presented. 1. List the three main loadcenter types made today. 2. In your own words, explain why a double pole breaker draws twice the voltage of a single pole breaker installed in the same panel. 3. Regardless of ampere ratings, a lighting and appliance type of loadcenter is limited to circuits in one enclosure. 4. List four application questions to ask when working with a customer. Page 21
Glossary Bonding Branch Circuit Branch Circuit Breaker Bus Bars Circuit Breaker Convertible Loadcenter Distribution Panel Distribution Transformer Double Pole Equipment Grounding Bus Feed-Through Lugs Flush-Mounted Full Rating Method Fuse Ground Fault Grounding Path The permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity to ground. A circuit that supplies power to the electrical loads in a building and is terminated at a distribution device (panelboard, loadcenter, etc.). Also Miniature Circuit Breaker. An overcurrent protection device, used to protect a branch circuit. After tripping to break the circuit, it can be reset to protect the branch circuit again. A component of a distribution device (loadcenter, panelboard, etc.) that serves as an extension of the main service conductors. Simplifies the connection of branch circuit breakers to the main service conductors. An overcurrent protection device. After tripping to break the circuit, it can be reset to protect the circuit again. A loadcenter supplied without main lugs or a main breaker. Once in the field, it is up to the electrician to install the appropriate main device. A loadcenter used at a point beyond the building s service entrance. It is not typically supplied with a main breaker. This type of panel can be useful when adding additional electrical service to an existing building. It has an isolated neutral. A device that converts utility voltage into a voltage and current supply suitable for use in most residential applications. Term used to describe a breaker that draws power from two poles of a loadcenter or similar device. Double pole means that it disconnects two wires. A bus connected directly to the enclosure. Used for grounding all feeder and branch circuit equipment. A set of lugs in a service entrance panel where feeder cables for a distribution panel are terminated. A style of mounting a loadcenter in which the loadcenter is recessed into a hole in the wall. A method of selecting circuit protection devices for use in a loadcenter. All devices must have ratings equal to or greater than the available fault current. An overcurrent protection device. After tripping to break the circuit, it must be replaced to restore power to the circuit. Current leakage from an ungrounded conductor to the grounding path in an electrical system. A solid conducting path for electricity to follow to ground. Page 22
Half-Size Branch Circuit Breakers Loadcenter Mains Main Circuit Breaker Main Lugs Main Lug Only (MLO) Loadcenter Main Service Conductors Miniature Circuit Breaker Plug-In Series-Rated Method Service Entrance Service Entrance Panel Single Pole Stab Surface-Mounted Terminated Voltmeter A specialized overcurrent protection device designed to take up only half as much space in a loadcenter as a normal branch circuit breaker. Can only be installed in loadcenters equipped with notched stabs. A wall mounted device that delivers electricity from a supply source to loads in light commercial or residential applications. Also Main Service Conductors. The conductors that bring electricity into a building from the power source. An overcurrent protection device designed to protect an entire loadcenter, panelboard or switchboard. Power from the main service conductors is fed through a main breaker to the bus bars. Component of a loadcenter that is a termination point for a feeder cable from another loadcenter. A loadcenter where power from the mains is fed directly to the bus bars. Also Mains. The conductors that bring electricity into a building from the power source. Also Branch Circuit Breaker. Used to switch and protect the lowest common distribution voltage in an electrical system. Generally used in a loadcenter, panelboard, or similar device. A type of circuit breaker that is literally plugged into the bus bar stabs. Generally found in residential applications. A method of selecting circuit protection devices for use in a loadcenter. The main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system. Downstream devices connected in series can be rated at lower values. The point at which electrical power enters a building. The term used to describe a loadcenter used as a service entrance. Term used to describe a breaker that draws power from one pole of a loadcenter or similar device. Single pole means that it disconnects one wire. A protrusion on the bus bars of a loadcenter which accepts a miniature circuit breaker. A style of mounting a loadcenter in which the loadcenter is attached to and projects out from the wall. The end connection of a circuit. For example, branch circuits are terminated at the service entrance panel. A device used to measure voltage by finding the potential voltage between two points. Page 23
Review 1 Answers 1. Distribution panel Service entrance panel Review 2 Answers 1. Main Breaker Main Lug Only Convertible Loadcenters 2. Residential Commercial / light industrial 3. Neutral, insulated, isolated 4. Answer should basically say If all the loads in a panel are connected exactly evenly between A-N and B-N, no current will flow in the neutral. The amount current flowing in the neutral conductor at any one time is the difference between the current flowing through leg A and leg B. 5. Answer should basically say To supply power to lights, receptacles and loads like dryers, washers, and air conditioners. 2. Answer should basically say A double pole breaker is plugged onto two adjacent bus stabs. The circuit is made across both ungrounded legs of the transformer. A single pole breaker is plugged onto a single bus stab. The circuit is made from an ungrounded leg of the transformer to the transformer s grounded leg. 3. 42 4. Any four of the following: Indoor or outdoor installation? If indoor, flush-mount or surface-mount? Service entrance panel or a distribution panel? Single-phase, three-wire system or a three-phase, four-wire system? What type of main, Main Lugs Only or Main Circuit Breaker? Main ampere rating? What type and rating main breaker is required? (if applicable) How many branch breakers required, and the rating of each? How many spaces are required? Page 24