Electrical
Distribution equipment field guide: switchgear, switchboard, panelboard, MCC
Match the gear to the level: switchgear, switchboard, panelboard, and MCC, with the fault rating, the drawout question, the clearance, and the record each one demands.
Direct answer
Electrical distribution equipment is the family of gear that steps utility power down from the service to branch circuits: switchgear and switchboards distribute feeders, panelboards split feeders into branch circuits, and motor control centers feed motors. Each is built and listed to its own standard, and its interrupting rating must exceed the available fault current.
Key takeaways
- Gear interrupting and withstand ratings (AIC, SCCR) must equal or exceed the available fault current at its location, set by a fault study, not a guess.
- Switchgear (UL 1558) uses drawout power breakers in isolated compartments and rides through a fault, commonly 30 cycles; switchboards (UL 891) use fixed group-mounted breakers that trip instantly.
- Listing standards sort the gear: UL 1558 switchgear, UL 891 switchboards, UL 67 panelboards, UL 845 motor control centers.
- Low voltage is 1000 V and below; medium voltage runs roughly 1 kV to 38 kV with vacuum interrupters and grounded barriers, built to IEEE C37.20.2.
- Working clearance in front of low-voltage gear is commonly 3 ft (36 in) deep, per NEC 110.26, and must stay clear of stored material.
Distribution equipment, and the job it does
Electrical distribution equipment is the gear that takes the power coming in from the service and splits it, in stages, into the circuits that feed the building. Power does not go straight from the utility to a receptacle. It steps down through progressively smaller gear, and each piece in that chain has a name, a rating, and a listing standard that says what it is allowed to do.
The four names you work with are switchgear, switchboard, panelboard, and the motor control center. They are not interchangeable, and calling one by another's name on a submittal gets it kicked back. A switchboard is not switchgear with a different label. A panelboard is not a small switchboard. The differences are in the construction, the fault rating, and the standard each is built and tested to, and those differences decide which one belongs at which point in the system.
Get the match wrong and you find out the expensive way. Gear that cannot withstand the fault current available where it sits, or a panel doing a switchboard's job without a switchboard's bus. This guide walks the hierarchy from the top down and lays the four kinds of gear side by side, so the right one lands at the right level.
The distribution hierarchy, service to branch
Power steps down in stages, and the gear matches the stage. At the top is the service, where the utility lands and the building's electrical system begins, covered in the service entrance guide. From there, large gear, switchgear or a switchboard, takes the service or a main feeder and divides it into feeders that run to the next level down. Those feeders land in smaller distribution: more switchboards, panelboards, or motor control centers. Panelboards take a feeder and split it into branch circuits, the conductors that actually reach the loads. That is the bottom of the chain.
The rule that keeps this straight is simple. Bigger conductors and higher fault current at the top, smaller and lower as you go down. The gear at each level has to carry the current that passes through it and survive the fault current available at its location, which is highest near the service and drops as you move downstream through impedance.
Match the gear to the level. A 4000 A service does not land in a panelboard. A 20 A branch circuit does not need switchgear. The skill is sizing each step so it carries the load, holds the fault, and leaves room to work on it later.
The switchboard
A switchboard is a free-standing or wall-mounted assembly that takes the service or a feeder and distributes it to feeders and branch circuits through breakers or fused switches, all behind a dead front. It is the workhorse of medium-sized commercial and light-industrial buildings: a school, a strip of retail, a mid-rise office. Built and listed to UL 891, which now covers systems up to 1000 V, with main bus ratings commonly up to about 4000 to 5000 A and short-circuit ratings up to roughly 100 kA depending on the construction.
Inside, the overcurrent devices are usually molded-case or insulated-case breakers, group-mounted on a common bus rather than each in its own sealed compartment. Access can be front-only against a wall or front-and-rear when it sits in a room with space behind it, and the rear access is where the incoming and bus connections live on the larger units. A switchboard is frequently service-entrance rated, meaning it is built and labeled to serve as the service disconnect where the utility power lands.
The switchboard is fixed gear. To work on a breaker, the section it feeds generally comes down. That is the trade-off against switchgear, and it is the right trade for buildings that can take a planned outage.
The panelboard
A panelboard is the load center, the enclosure and bus and branch breakers mounted in or on a wall that takes one feeder and splits it into branch circuits. It is the smallest piece of distribution and the one most people picture when they hear panel. Built and listed to UL 67, it sits at the end of the chain, fed from a switchboard, switchgear, or a feeder, and hands power off to the circuits that reach the loads.
The panelboard has its own full guide, covering sizing, the bus, AIC and SCCR, working clearance, neutral bonding, and the circuit directory, so this stays short. The point for the hierarchy is placement. The panelboard is downstream gear. It carries less current and sees lower fault current than the switchboard or switchgear ahead of it, and its bus rating, commonly up to about 1200 A, reflects that. When a job needs more than a panelboard can carry, the answer is a switchboard, not a bigger panel.
Switchgear
Switchgear is heavy metal-enclosed gear built around drawout power circuit breakers, each in its own isolated compartment, rated and tested to hold a fault rather than just clear it. It sits where the stakes and the fault current are highest: the service of a large facility, a hospital, a data center, an industrial plant, or anywhere on the medium-voltage system. Low-voltage power circuit breaker switchgear is listed to UL 1558, with the breakers themselves tested to UL 1066. Main bus ratings reach about 10,000 A and short-time withstand ratings run far higher than a switchboard's.
The defining features are compartmentalization and drawout construction. Each breaker lives in its own grounded metal compartment, separated from the bus and from the other breakers, so a failure in one cell is contained. The power breakers rack out to a test position and fully out for service without de-energizing the bus, which is the whole reason a facility that cannot go dark buys switchgear in the first place.
The other defining trait is the short-time withstand. Power circuit breakers are built to carry fault current for a set interval, commonly 30 cycles, instead of tripping instantly. That delay is what lets a downstream device clear its own fault first while the main holds, so a fault on one feeder does not take down the whole bus. A molded-case breaker in a switchboard does the opposite: it trips as fast as it can, with no intentional delay.
What is the difference between switchgear and a switchboard?
The short answer: switchgear has drawout power breakers in individual compartments and a higher fault-withstand rating, built to UL 1558; a switchboard has fixed, group-mounted breakers on a common bus and a lower withstand, built to UL 891. Both distribute power at low voltage, but they are different classes of equipment with different prices and different reasons to exist.
Five differences matter on a job. Construction standard: UL 1558 for switchgear, UL 891 for switchboards. Breaker type: drawout power circuit breakers tested to UL 1066 versus fixed molded-case or insulated-case breakers. Compartmentalization: each switchgear breaker in its own isolated cell versus group-mounted devices sharing a common bus space. Withstand: switchgear is tested to ride through a fault for an interval, commonly 30 cycles, while a switchboard's breakers trip with no intentional delay and the structure is tested to a shorter withstand. Maintainability: a switchgear breaker racks out for service with the bus live, while a switchboard usually needs an outage.
Cost follows. Switchgear runs several times the price of a switchboard for the same ampacity, so you buy it where the fault current, the redundancy, or the need to maintain without an outage justifies it. For an ordinary commercial building that can take a planned shutdown, a switchboard is the right call. Verify the ratings and the listing against the project documents.
| Trait | Switchboard (UL 891) | Switchgear (UL 1558) |
|---|---|---|
| Breakers | Fixed, group-mounted MCCB/ICCB | Drawout power circuit breakers (UL 1066) |
| Compartments | Common bus, group-mounted | Each breaker in its own grounded cell |
| Fault withstand | Short, breakers trip instantly | Rides through, commonly 30 cycles |
| Typical max bus | About 4000 to 5000 A | About 10,000 A |
| Maintenance | Usually needs an outage | Rack out the breaker, bus stays live |
| Relative cost | Lower | Several times higher |
The motor control center
A motor control center is a lineup of vertical sections filled with plug-in or bolt-in bucket compartments, each holding the starter, contactor, overload relay, or drive for one motor. Where a switchboard or panelboard mainly holds branch-circuit protection, the MCC mainly holds motor control units, and that is what separates it from the other gear and puts it under its own listing standard, UL 845. It feeds and protects a group of motors, with a common horizontal bus running the lineup and vertical buses tapping each section.
On the hierarchy, the MCC is downstream distribution like any other: fed from a switchboard, switchgear, or a feeder, and placed wherever a plant or building has a cluster of motors to run. The buckets are often drawout or removable, so a starter or drive can be pulled and replaced without killing the whole lineup. That is the same maintainability logic that drives switchgear, applied to motor control. The motor circuits inside it draw on NEC Article 430, which sets the overload and short-circuit protection rules for motors.
What is the difference between low voltage and medium voltage switchgear?
Low-voltage gear operates at 1000 V and below; medium-voltage gear covers roughly 1 kV up to 38 kV. The split is not just a number on a nameplate. It changes the construction, the breaker technology, the clearances, the standards, and who is allowed to work on it.
Low-voltage switchgear, UL 1558, uses air power circuit breakers and is what lands at the secondary of a building's main transformer, distributing 480 V or 208 V. Medium-voltage switchgear is metal-clad gear built to IEEE C37.20.2, with vacuum interrupters in drawout breakers, grounded metal barriers between every compartment, and much larger electrical clearances because the voltage will arc across distances that mean nothing at 480 V. Maximum voltage ratings for metal-clad gear run from about 4.76 kV to 38 kV, with bus ratings commonly 1200, 2000, 3000, and 4000 A.
Where does MV sit? At the service of a large campus or plant, where the utility delivers at medium voltage and the building owns the step-down. The MV switchgear feeds transformers, the transformers drop to low voltage, and the low-voltage switchgear or switchboard takes it from there. Medium-voltage work carries its own hazard class and qualification requirements, so it is not a place to improvise.
The bus and its rating
The bus is the set of conductor bars inside the gear that carries current from the incoming connection across to the breakers, and its ampere rating is the real ceiling on what the gear can pass. A switchboard labeled 2000 A has a 2000 A bus, and no arrangement of breakers changes that limit. The main rating, the section rating, and the through-bus rating can differ, so read the nameplate. The through-bus that carries current to the next section down may be rated lower than the main if the design tapers it.
Bus is copper or aluminum. Copper carries more current per cross-section, takes a cleaner termination, and costs more. Aluminum is lighter and cheaper and is common on larger gear, but it needs more cross-section for the same ampacity and the connections demand the right hardware and torque, because aluminum cold-flows and loosens if it is terminated like copper. Plating, tin or silver, matters at the joints where heat concentrates.
The bus bracing is a separate rating from the ampacity. Bracing is how hard the bus can be hit by a fault without the bars deforming or tearing loose, and it has to match the available fault current, which is the next section.
Why does the AIC rating have to exceed the available fault current?
Because the gear has to survive and clear the worst fault that can happen where it sits, and if it cannot, the failure is violent. The available fault current is the current that would flow if a bolted short circuit happened at the gear, set by the utility transformer, the conductors, and the impedance upstream. The interrupting rating, the AIC (ampere interrupting capacity) of the breakers, and the short-circuit withstand or bracing of the bus and structure are what the gear can take. The first must be at least as large as the second. This is firm: the equipment's interrupting and withstand ratings must equal or exceed the available fault current at its location, every time.
Under-rate it and a fault does not trip the breaker cleanly. It blows the breaker apart, vaporizes bus, and throws an arc-flash blast at whoever is standing in front. The NEC requires equipment to have an interrupting rating sufficient for the available fault current, and it requires the assembly's short-circuit current rating (SCCR) to be adequate too. These are not design targets you can shade. They are pass-or-fail.
Available fault current is highest near the service and drops downstream as impedance adds up. That is why switchgear and service-entrance switchboards carry the highest AIC and bracing, and why a fault-current study, not a guess, sets the number. The fault-current calculation and the arc-flash study that follows from it are covered as their own topic.
Main, main-lug, and feeder breakers
Distribution gear is either main-breaker or main-lug. A main breaker is a single overcurrent device that protects and disconnects the whole assembly at once. Main-lug-only (MLO) gear has lugs where the incoming feeder lands and relies on an upstream device for protection. Which one you need depends on where the gear sits and what the code requires for a disconnect at that point, so check the application against the adopted code.
Below the main, the feeder breakers send power to downstream gear: another switchboard, a panelboard, an MCC. Each feeder breaker is sized to its conductor and its load, and it is the point where selective coordination is won or lost. Coordination means the breaker closest to a fault trips first, so a fault on one feeder does not open the main and drop the whole building. On critical systems that coordination is engineered and sometimes required, and it is one reason switchgear with its short-time-rated main exists. The main can hold while the feeder breaker clears.
Service-entrance gear has its own rule on the number of disconnects, addressed in the service entrance guide. For downstream gear, the main-versus-lug decision is mostly about coordination, maintenance, and whether one handle should kill the whole assembly.
Drawout or fixed breakers: which do I need?
Drawout breakers rack in and out of the gear without disturbing the bus or the connections; fixed breakers are bolted in place and wired permanently. The choice is a maintenance and cost decision. Drawout costs more up front and pays back on any system that cannot tolerate an outage to service a breaker.
With a drawout breaker, you rack it to a test position to exercise and check it, or all the way out to replace it, while the bus stays energized and the rest of the gear keeps running. That is standard on switchgear and on better MCC buckets. With a fixed molded-case breaker in a switchboard or panelboard, servicing or replacing it generally means de-energizing the section, and sometimes the whole assembly, because the line-side terminals stay live until something upstream opens.
The honest version: most commercial buildings run fine on fixed breakers because a planned Saturday outage is acceptable. Hospitals, data centers, water treatment, continuous-process plants, anything where downtime is measured in dollars per minute or in risk to life, that is where drawout earns its premium. Decide it on the cost of an outage, not on the cost of the gear.
Metering and monitoring
Distribution gear is where you meter the building, and modern gear carries far more than a utility kilowatt-hour meter. Built-in metering on a switchboard or switchgear reads voltage, current, power, power factor, and energy at the main and often at each feeder, and it feeds a power monitoring system that trends the building's load over time.
On larger and critical facilities this becomes an electrical power monitoring system (EPMS), a network of meters and relays reporting to a head-end so operators see load, alarms, and power quality across every piece of gear from one screen. The value is catching the problem before it is a failure: a feeder creeping toward its rating, a power-factor penalty on the utility bill, a harmonic problem cooking a transformer, a phase imbalance loading one leg of the bus. Metering also captures the event record after a trip, which is how you reconstruct what actually happened instead of guessing.
Specify the metering with the gear, not after. Adding revenue-grade meters or EPMS connectivity to existing switchboard sections later is far more work than ordering it built in, because the CTs and the wiring have to go somewhere. The metering and monitoring scope is its own topic to coordinate with the controls and the owner.
Service-entrance rated equipment
Service-entrance rated gear is built and labeled to serve as the point where the utility power lands and the building's system begins, which carries extra requirements the rest of the distribution does not. A switchboard or switchgear used as service equipment has to provide the service disconnecting means, it has to make the main bonding jumper connection that bonds the neutral to ground at the service, and it has to be marked as suitable for use as service equipment.
That neutral-to-ground bond happens once, at the service, and nowhere downstream. Bond it again in a downstream panel and you put neutral current onto the ground system, which is both a code violation and a shock and noise problem. The service entrance and metering guide covers the bonding and the disconnect rules in depth, so the point here is just placement. The service-entrance rated gear is the one piece in the lineup that gets the bond and the service label.
The number of service disconnects and how they are grouped is set by the code, and recent editions have tightened it, so confirm the rule against the adopted edition. The gear has to be ordered to match, because the service-entrance label and the bonding provisions are factory features, not field add-ons.
Arc flash and arc-resistant gear
The fault current that the gear is rated to interrupt is the same energy that, in an arc, can kill the person standing in front of it. Arc flash is the explosion that follows an arcing fault: heat that vaporizes copper, a pressure blast, and shrapnel. The gear with the highest fault current, the switchgear and service-entrance switchboards, carries the highest arc-flash hazard, which is exactly the gear people most want to work on live.
Two defenses matter. First, the labeling and PPE. The equipment has to carry an arc-flash warning label with the incident energy or PPE category, derived from an arc-flash study, and the worker has to wear the rated gear and follow NFPA 70E. Second, arc-resistant switchgear, built and tested to IEEE C37.20.7, is constructed to contain and vent an internal arc away from the operator instead of blowing the doors off into the room. It does not prevent the arc. It redirects the blast.
The arc-flash study and the boundary calculations are their own topic. The point for the gear is this: specify arc-resistant construction where the hazard and the need to work near energized gear justify it, and never treat a label as optional. Verify the gear is dead and verified dead before the door comes off, every time. If you cannot establish an electrically safe work condition, the work waits.
How much working clearance does the gear need?
The code requires clear working space in front of electrical gear so a person can work on it and get away from it, and the dimension depends on the voltage and what is across from you. For most low-voltage gear the depth in front is commonly 3 ft (36 in) and grows with voltage and with what faces the working space, but the exact dimensions, the width, and the headroom come from the NEC working-space requirements, commonly cited at 110.26, and the adopted edition controls.
The clearance is not negotiable and it is the first thing an inspector checks, before a single wire. The width has to cover the equipment and let the doors open, the headroom has to clear, and the space stays clear. You cannot store conduit, ladders, or stock in front of a switchboard. Larger service gear also requires a second way out of the working space under some conditions.
Medium-voltage and big service gear get a dedicated electrical room, and the code reserves the space above and around dedicated equipment so plumbing and ductwork cannot encroach. Plan the room early. The most common version of this failure is architectural. The electrical room shrinks on a late drawing revision, the gear no longer fits with its clearance, and now something moves at the worst possible time. Hold the space on the plan and confirm the dimensions for the voltage and condition against the adopted code.
Grounding and bonding the gear
Every piece of distribution gear has a ground bus, and bonding the gear to it is what gives fault current a low-impedance path back to the source so the breaker trips. An open or high-resistance ground means a fault energizes the enclosure instead of clearing, and the gear sits there live and waiting. The ground bus ties the enclosure, the equipment grounding conductors, and the raceway system together.
The one bond that gets its own rule is the neutral-to-ground bond at the service, made once, at the service-entrance gear, through the main bonding jumper. Downstream gear keeps neutral and ground separate, on isolated bars, so neutral current stays on the neutral and the ground carries only fault current. Tie them together in a downstream switchboard or panel and you have created a parallel neutral path through the ground system.
On medium-voltage and larger installations the grounding extends to the building grounding electrode system and sometimes a ground grid, and the connections are inspected and often tested for resistance. Grounding and bonding is a deep topic with its own rules. The point at the gear is that the ground bus is sized, the bonds are tight to torque, and the neutral bond happens in exactly one place.
NEMA enclosure: indoor or outdoor
The enclosure rating has to match where the gear lives. Indoor gear in a clean dry electrical room is typically a NEMA 1 enclosure, ventilated and not sealed against water. Put that same gear outside or in a wash-down area and it fails fast, because rain, dust, and condensation get in and corrode the bus and the connections. Outdoor gear is commonly NEMA 3R, built to shed rain and sleet, and wetter or harsher locations call for higher ratings.
The trap is condensation, not just rain. An outdoor NEMA 3R switchboard still breathes, and the daily temperature swing pulls moist air in and condenses it on the cold bus at night. That is why outdoor gear carries space heaters, and why leaving the heaters disconnected is a quiet way to grow corrosion and eventual flashover inside a sealed-looking enclosure. Coastal and industrial sites add salt and chemicals that eat standard finishes, so the enclosure material and coating get specified to the environment.
Enclosure ratings are their own topic. The point at the gear: order the enclosure for the actual location, account for temperature derating in hot outdoor sun, and keep the heaters working. The wrong enclosure for the location is a failure you build in on day one.
Maintenance and NETA testing
Gear does not maintain itself, and the failures that take out a switchboard or switchgear are almost always the connection and the contamination, not the breaker mechanism. Acceptance testing when the gear is new and periodic maintenance testing after follow the InterNational Electrical Testing Association (NETA) specifications, which spell out what to test and the values to compare against.
The work that earns its keep: infrared scans of the gear under load to find the hot connection before it fails, because a loose or corroded joint runs hot long before it lets go and a thermal camera sees it through the cover ports. Torque checks on the bus and lug connections, because thermal cycling backs them out. Insulation-resistance testing on the bus. Exercising the drawout breakers, racking them in and out and trip-testing them, because a breaker that has sat closed for ten years may not open when it finally has to. On power breakers, primary-injection testing confirms the trip unit actually trips at its settings.
The blunt version: most catastrophic gear failures are maintenance that nobody did. The breaker that did not trip, the lug that ran hot for a year, the bus that flashed over on contamination, all of it shows up on a NETA scan and a maintenance schedule, and none of it shows up if the gear is installed and forgotten. Tie the testing scope to the criticality of what the gear feeds.
Selecting the gear for the level
Picking the gear is a decision with five inputs, and they pull against each other. Service size and load set the ampacity, which sets the bus and the main. Available fault current sets the minimum AIC and bracing, and near a big utility transformer that number alone can push you from a switchboard to switchgear. Maintainability, whether the facility can take an outage to service a breaker, decides fixed versus drawout. Space and the electrical room decide what physically fits with clearance. Budget is the constraint the other four argue with.
The honest order: never compromise the fault rating to save money, because that is the one that gets someone hurt. After that, trade maintainability against budget based on the cost of an outage. A building that can go dark on a Sunday does not need switchgear. A data center or a hospital does, and the premium is cheap against the downtime.
Match the gear to the level and the level to the risk. Switchgear at the service of a critical facility, a switchboard for mid-size commercial distribution, panelboards at the branch level, an MCC where the motors cluster. Size each for the current it carries and the fault it must hold, confirm the ratings against a fault study and the project documents, and leave room to work.
The critical-facility distribution lineup
On a data center or other critical facility, the distribution is not one chain but two or more, run in parallel so any single piece of gear can fail or be maintained without dropping the load. The switchgear at the top is doubled, the feeders are doubled, and the load is fed from both through transfer schemes, so a breaker can rack out for service while the redundant path carries everything.
This is where switchgear with drawout breakers and short-time withstand stops being a luxury. Concurrent maintainability, the ability to service any component without an outage, is a design requirement on higher-tier facilities, and it forces drawout gear, paralleling switchgear, and the controls to transfer load without a blink. Generators and UPS tie into the same lineup, and the gear has to coordinate so the right source feeds the load through utility loss, generator start, and back.
The full power chain, from utility through switchgear, transfer, UPS, and the distribution to the racks, is its own topic. The point for gear selection: on a critical facility the question is not just what carries the load, but what carries it while another piece is out. That requirement, more than the load itself, drives the gear up to switchgear and drives the redundancy into the design.
What to document
The gear is only as good as the record that says what it is rated for and how it was set up. The record is what the next person checks before they open a door or add a load, and it is what proves the gear matched the fault current at its location.
Capture the equipment type and listing standard, the location and enclosure rating, the voltage and bus ampacity, the AIC and SCCR against the available fault current from the study, the main and feeder breaker ratings and settings, the service-entrance status and where the neutral bond is, and the acceptance test results. If the gear is part of a coordination study, file the settings with it, because the next person who changes a breaker has to keep the coordination intact.
| Item to record | Why it matters |
|---|---|
| Equipment type and listing | UL 1558, 891, 67, or 845 says what it is |
| Location and enclosure rating | Confirms the NEMA type matches the environment |
| Bus ampacity and main rating | The real ceiling on what it can carry |
| AIC/SCCR vs available fault current | The pass-or-fail safety number |
| Breaker settings and coordination | Lets the next fault clear at the right device |
| Service-entrance status and bond point | Where the neutral-ground bond lives |
| NETA acceptance test results | Baseline for every future maintenance test |
Common mistakes
- Specifying a switchboard where the available fault current needed switchgear's withstand, so the gear cannot ride a fault.
- Ordering gear with an AIC or SCCR below the available fault current at its location.
- Losing the working clearance when the electrical room shrinks on a late drawing revision.
- Undersizing the bus or the through-bus for the load that actually passes through it.
- Leaving an assembly main-lug where a main disconnect was required, or adding a main where coordination wanted lugs.
- Skipping NETA acceptance testing and never scheduling maintenance, so the first fault finds the loose lug.
- Ordering an indoor NEMA 1 enclosure for an outdoor or wash-down location, or disconnecting the space heaters.
- Making a second neutral-to-ground bond in downstream gear instead of only at the service.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The listing standards sort the gear: UL 1558 for low-voltage power circuit breaker switchgear, UL 891 for switchboards, UL 67 for panelboards, and UL 845 for motor control centers, with low-voltage power breakers tested to UL 1066 and molded-case breakers to UL 489. Knowing which standard a piece is built to tells you what it is and is not allowed to do.
The NEC, NFPA 70, governs the installation. Switchboards, switchgear, and panelboards are covered in Article 408, motor circuits and MCCs draw on Article 430, service equipment on Article 230, and the working-space and dedicated-space requirements are commonly cited at 110.26. The interrupting-rating and SCCR requirements that make the AIC firm live in Article 110 as well. Recent editions have moved and tightened several of these, including the service-disconnect rules, so confirm the article and section numbers against the edition the jurisdiction has actually adopted and any local amendments before citing them.
Medium-voltage metal-clad switchgear is built to IEEE C37.20.2, arc-resistant construction to IEEE C37.20.7, and acceptance and maintenance testing follow NETA specifications. Electrical safety and the arc-flash study follow NFPA 70E. The equipment listing and the manufacturer's instructions can impose tighter requirements than any of these, and where they do, the listing governs.
Units, terms, and ratings
Distribution gear carries a stack of ratings, and reading the nameplate means knowing which is which. The ampere rating is the continuous current the bus and main can carry. The voltage rating is the system voltage it is built for. The AIC and SCCR are the fault-current numbers, and the short-time withstand is how long the gear holds a fault before it has to clear.
- AIC
- Ampere interrupting capacity, the fault current a breaker can safely interrupt
- SCCR
- Short-circuit current rating, the fault current the whole assembly can withstand
- Withstand rating
- Fault current the bus and structure can take, with short-time gear rated to hold it for an interval such as 30 cycles
- Drawout
- A breaker that racks in and out of the gear without disturbing the bus connections
- Service-entrance rated
- Gear built and labeled to serve as the service disconnect and carry the main bonding jumper
- Metal-clad
- Medium-voltage switchgear with each compartment isolated by grounded metal barriers, per IEEE C37.20.2
- LV / MV
- Low voltage at 1000 V and below; medium voltage roughly 1 kV to 38 kV
FAQ
What is the difference between switchgear and a switchboard?
Switchgear, built to UL 1558, has drawout power breakers in individual compartments and a high fault withstand, so it rides through a fault and is maintained without an outage. A switchboard, built to UL 891, has fixed group-mounted breakers on a common bus, a lower withstand, and costs far less.
What is a panelboard?
A panelboard is the load center, the enclosure, bus, and branch breakers mounted in a wall that takes one feeder and splits it into branch circuits. Built to UL 67, it is the smallest distribution gear and sits at the end of the chain, fed from a switchboard, switchgear, or a feeder.
What is a motor control center?
A motor control center is a lineup of bucket compartments, each holding the starter, overload, or drive for one motor, built to UL 845. Where a switchboard or panelboard mainly holds branch-circuit protection, an MCC mainly holds motor control units, and it feeds and protects a cluster of motors as a group.
What is the difference between low voltage and medium voltage switchgear?
Low-voltage switchgear operates at 1000 V and below with air power breakers, built to UL 1558. Medium-voltage switchgear covers roughly 1 kV to 38 kV with vacuum interrupters and grounded metal barriers between compartments, built to IEEE C37.20.2. Medium voltage carries larger clearances and its own qualification and hazard requirements.
How do I know what AIC rating the gear needs?
The gear's interrupting rating and short-circuit current rating must equal or exceed the available fault current at its location, set by a fault-current study, not a guess. Available fault current is highest near the service and drops downstream. Under-rated gear does not clear a fault, it blows apart.
Is a switchboard service-entrance rated?
A switchboard can be service-entrance rated when it is built and labeled as suitable for use as service equipment, which adds the service disconnecting means and the main bonding jumper that bonds neutral to ground once, at the service. It is a factory feature ordered with the gear, not a field add-on.
What is the difference between drawout and fixed breakers?
Drawout breakers rack in and out of the gear with the bus still energized, so a breaker is tested or replaced without an outage. Fixed breakers are bolted in and usually need the section de-energized to service. Drawout costs more and pays back where downtime is expensive, like hospitals and data centers.
How much working clearance does distribution gear need?
Working space in front of low-voltage gear is commonly 3 ft (36 in) deep and grows with voltage and with what faces it, with width and headroom set alongside it. The exact dimensions come from the NEC working-space rules, commonly cited at 110.26, and the adopted edition controls. The space stays clear of storage.
What is arc-resistant switchgear?
Arc-resistant switchgear is built and tested to IEEE C37.20.7 to contain and vent an internal arcing fault away from the operator instead of blasting the doors into the room. It does not prevent the arc, it redirects the blast. It is specified where the arc-flash hazard and the need to work near energized gear justify the cost.
What do I do if the gear is rated below the available fault current?
Do not energize it. Gear with an AIC or SCCR below the available fault current can fail violently on the first fault. Either reduce the available fault current with upstream impedance such as a current-limiting device or reactor, or replace the gear with a higher-rated assembly. Confirm the corrected rating against the fault study.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.