Panelboard installation and circuit directory field guide

A panelboard, and the parts that make it one

A panelboard is the assembly that takes one supply, the feeder or service conductors, and divides it into branch circuits, with an overcurrent device protecting each one. It is the point where the big conductor becomes many small ones. Everything else in the box exists to do that job safely and to let someone work on it later without guessing.

Learn the parts by name, because the code and the inspector use those names. The enclosure, or can, is the steel box and the deadfront cover that keeps fingers off live parts. The bus is the set of copper or aluminum bars the breakers clip onto, and the bus has an ampere rating that is the real ceiling on the panel. The main is either a main breaker that protects the whole panel or, on a main-lug-only panel (MLO), a set of lugs with the protection sitting upstream. The branch breakers are the overcurrent devices for each circuit. The neutral bar lands the grounded conductors, and the ground bar lands the equipment grounding conductors. The directory is the schedule that says what each breaker feeds.

Two parts get treated as afterthoughts and should not be. The bus rating, not the main breaker, is what a panel is. A 100 A main in a 225 A bus is a 225 A panelboard with a 100 A main. And the directory is code, not courtesy. A panel with no legible schedule fails inspection as surely as one with a loose lug.

Bus

The copper or aluminum bars the breakers connect to; its ampere rating is the panel's true capacity

MLO

Main lugs only; a panel with no main breaker, protected by an overcurrent device upstream

Deadfront

The cover that exposes only the breaker handles and keeps live parts out of reach

What is the difference between a panelboard, a switchboard, and a load center?

A panelboard is a dead-front assembly built into a cabinet and placed in or against a wall, accessible only from the front. A switchboard is a larger free-standing structure, often accessible from front and rear, carrying heavier bus and a service or main distribution role. A load center is the residential and light-commercial flavor of a panelboard, the term the manufacturers and the home-center shelf use for the same thing the NEC calls a panelboard.

The practical line between a panelboard and a switchboard is size, access, and where it sits in the system. Switchboards take the service or the main distribution feed and sit free-standing on the floor; panelboards are the branch and sub-distribution gear hung on the wall. Switchgear is a separate, heavier class again, with drawout breakers and a higher short-circuit withstand for industrial and utility-grade work.

Do not get hung up on the marketing word. A load center and a panelboard install to the same rules. What changes the requirements is the role, service versus sub-panel, and the ratings, not the name on the carton.

Sizing and selecting the panel

Pick the panel from the load calculation, not from what fit last time. The minimum bus ampacity and the main size come out of the NEC Article 220 calculation for the service or feeder that feeds it. Our load calculation guide walks that math; the output of it is the input here. Size the bus to the calculated demand with room left, because a panel run at its rating with no spare spaces is a panel someone re-pulls in two years.

After the ampere rating, four things select the specific panel. The voltage and phase have to match the system: 120/240 V single-phase, 120/208 V or 277/480 V three-phase wye, or a delta arrangement, and the panel has to be listed for it. The number of spaces and circuits has to cover the branch circuits plus realistic spares; count poles, because two-pole and three-pole breakers eat more than one space. The interrupting rating has to meet the available fault current, which is its own section below and is the rating people forget. And the enclosure type (NEMA 1 indoors, NEMA 3R outdoors, and so on) has to suit where it lands.

Spare capacity is cheap at order time and expensive later. A common move is to specify a bus a size up from the present main and leave a quarter of the spaces open. The added cost is small against the labor of swapping a full panel when the building adds load.

Does the panel AIC or SCCR have to match the available fault current?

Yes, and this is the rating that gets skipped because it does not show up until something faults. The panel and its breakers have to be rated for at least the available fault current at the point where they are installed. The NEC handles it in two places: the interrupting rating of the overcurrent device, commonly cited at 110.9, has to equal or exceed the current it has to interrupt, and the short-circuit current rating of the equipment, at 110.10 with the panelboard SCCR marking under 408, has to withstand the let-through without coming apart.

Keep the two terms straight, because a pro keeps them straight. AIC, the ampere interrupting capacity, is the breaker's ability to open a fault without exploding. SCCR, the short-circuit current rating, is the assembly's ability to survive the same fault without failing. Both have to be at or above the available fault current where they sit. A 10 kA breaker on a system that can deliver 22 kA is a bomb with a date on it.

Get the available fault current from the utility or the engineer's short-circuit study, then compare. Standard residential gear is often 10 kA, commercial panels commonly 22 kA or 65 kA, but the available current at your point in the system decides. Series-rated combinations can satisfy the requirement at a lower device rating, but only the listed, marked combination, applied exactly as listed, qualifies. Verify the available fault current against the study, not against habit.

Is there still a 42-circuit limit in a panel?

Not as a flat NEC number. The old rule capped a lighting and appliance branch-circuit panelboard at 42 overcurrent devices, which is why so many older jobs ran two cans where one would have fit. The 2008 NEC removed that limit by deleting the lighting and appliance panelboard definition and the sections that carried it, and the limit on circuit count now comes from the panel's own listing instead of a blanket code figure.

Here is the part that trips people who heard the limit went away. The listing still controls. If the panel is listed for a maximum of 30, 42, or 60 overcurrent devices, that listed maximum is the limit, and exceeding it violates the listing and the code rule that says you install equipment per its listing. The change did not abolish the number; it moved the number from the code book to the label on the panel.

What it did do is let manufacturers build and list panels that take more than 42 poles, so a job that needs 48 circuits can now use one panel where it used to need two. Read the panel label for its maximum, count your poles, and stay inside it. Confirm against the adopted edition, since a jurisdiction can be on an older code that still carries the old rule.

What working clearance does a panel need?

A panel needs clear working space in front of it so someone can work on it live without being trapped or crowded into energized parts. The NEC sets this in 110.26. The depth in front is at least 3 ft for the common case, measured from the live parts or the enclosure face, and it increases with voltage and with what is on the opposite wall, per the conditions table. The width is the greater of 30 in or the width of the equipment. The headroom is at least 6 ft 6 in.

The depth is not the only dimension that fails inspection. The 30 in width does not have to be centered on the panel, but it has to be there, and the door has to open at least 90 degrees. Above the panel sits the dedicated equipment space, in 110.26(E): a zone from the floor up to 6 ft above the equipment, or to the structural ceiling if lower, that foreign systems are kept out of. Plumbing, ducts, and sprinkler piping that do not serve the electrical gear cannot run through that space.

The clearance is the first thing the inspector checks, before a single wire. It is also the first thing the building violates after you leave, because the space in front of a panel is exactly the space the tenant wants for storage. No storage in the working space, ever. The day a worker has to clear a stack of boxes to reach a panel in a fault is the day the rule earns its keep. Confirm the exact dimensions for the voltage and condition against the adopted edition.

Mounting height and where a panel can and cannot go

Mount the panel so the highest breaker handle is reachable and so the panel is not where the code forbids. The reach rule is in 240.24: the center of the grip of the operating handle, in its highest position, sits no more than 6 ft 7 in above the floor or working platform. That is what fixes the top of the panel. On a tall panel you set the can so the top breaker lands under that line, which usually puts the bottom around 12 to 16 in off the floor.

Location matters as much as height. In dwelling units, dormitories, and guest rooms or suites, overcurrent devices are not allowed in bathrooms, and in any occupancy they are not allowed in clothes closets or directly over the steps of a stairway. The clothes closet ban is about stored fabric next to a device that can spark on a fault; the bathroom ban, scoped to those residential occupancies, is about wet hands and limited egress, and commercial bathrooms are not covered by it. The panel also has to be readily accessible, which means you reach it without a ladder and without moving anything.

Outdoors or in a wet location, the enclosure rating carries the load, NEMA 3R or better, with the right fittings and a means of disconnect where required. The mistake here is rough-in. Set the panel location at layout against the clearance and the prohibited spots, because relocating a panel after the drywall is up is a change order nobody wants to write.

Do you bond the neutral and ground in a sub-panel?

No. The neutral and ground are bonded together in exactly one place, at the service, through the main bonding jumper. Everywhere downstream, the neutral and the equipment ground stay separate. Bonding the neutral at a sub-panel is the classic, dangerous, and extremely common violation, and it is the first thing a sharp inspector looks for in a sub-panel.

At the service, the main bonding jumper, the green screw or strap the panel ships with, ties the neutral bar to the enclosure and to the grounding electrode system. That bond gives fault current a low-impedance path back to the source so the breaker trips. At a sub-panel, you remove that bonding screw. The neutral lands on an insulated, isolated neutral bar, and the equipment grounding conductors land on a separate ground bar that is bonded to the can. The feeder to the sub-panel is a 4-wire run: two or three ungrounded conductors, a neutral, and a separate equipment grounding conductor.

Get this wrong and the harm is not abstract. Bond the neutral at a sub-panel and the equipment grounding conductor and the metal it connects to start carrying normal neutral return current, in parallel with the neutral. Now the conduit, the ground bar, and anything bonded to them are live with return current under normal load, and a person who touches two grounded surfaces can complete a path. Pull the bonding screw in every sub-panel.

The enclosure itself has to be bonded so a fault to the can finds a path back to the source and trips the breaker. The sub-panel ground bar is often a separate piece you add and bond to the can with the kit screw, and each equipment grounding conductor lands under its own hole, one conductor per terminal unless the terminal is listed for more. Doubling grounds under one screw is a common find and a real problem: pull one circuit and you loosen the ground on another. Every metal raceway and fitting in the path has to be made up tight, since a loose coupling is an open ground hiding in plain sight.

Where you upsize phase conductors, for voltage drop or any reason, the equipment grounding conductor grows in proportion, commonly cited at 250.122(B). The ground exists to carry fault current long enough to open the breaker, so it has to keep pace with the conductors it protects. Size it to what you actually pulled, and verify the ground path is continuous and bonded end to end, with the neutral and ground proven as separate systems downstream, before the panel goes live.

Terminating the feeder and torquing the lugs

The feeder or service conductors land on the main lugs or main breaker, and the connection is only as good as the torque. Every lug has a torque value, stamped on the lug, printed on a label inside the can, or in the manufacturer's instructions. Use a calibrated torque wrench or torque screwdriver and hit that value. Do not guess by feel, and do not run it down with an impact and call it tight.

Under-torqued lugs are where panels burn. A loose connection has resistance, resistance makes heat under load, heat loosens the connection further, and the cycle ends at the discoloration on the insulation or the smell before anyone reads it with a meter. Over-torqued is its own failure: you crush stranded aluminum, fracture the lug, or strip the threads, and the joint fails later anyway. The number on the label is the number for a reason. Hitting torque verification right is the same habit that keeps breaker terminations and bus connections from failing, and it is worth treating as its own checklist step on every job.

Conductor prep is part of the termination. Strip to the right length so no bare conductor shows past the lug and no insulation gets clamped under it. On aluminum, wire-brush the conductor and apply the antioxidant the listing calls for if required. Match the conductor to the lug's listed range and temperature column. A lug rated for a range you have exceeded is not a termination; it is a future fault.

Installing the branch breakers

Use breakers listed for the panel. A panelboard is tested and listed as a system with a specific breaker line, and the listing is what the AIC and SCCR depend on. The temptation is the classified breaker, a third-party breaker marketed as a fit for another maker's panel. Inspectors and many manufacturers reject classified breakers because the panel was never tested with them, and a mismatch can defeat the interrupting rating you are counting on. Stay with the listed breaker for that panel unless the panel's own listing accepts the substitute.

Seat each breaker fully onto the bus stab so the full contact area carries the current. A breaker half-seated arcs at the stab, pits the bus, and you chase a hot spot that moves. Where the branch breaker has its own line and load terminals, torque them to the breaker's spec the same way you torqued the lugs. AFCI and GFCI breakers add a wire: the circuit neutral lands on the breaker, and the breaker pigtail lands on the neutral bar, so the order of operations changes from a standard breaker.

Match the breaker to the conductor and the load, not the other way around. The breaker protects the wire, so a 20 A breaker belongs on a circuit wired for 20 A, and you never upsize a breaker to stop nuisance trips on an undersized conductor. That move turns the overcurrent device into a fire starter waiting for the overload it was supposed to catch.

What goes in a panel directory?

A circuit directory has to identify every circuit with enough specific detail that someone can tell it apart from all the others, legibly and permanently. This is a code requirement, not housekeeping, set in NEC 408.4. The word that matters is specific. A directory that says lights for half the panel does not meet it. Receptacles east wall, room 214 lights, RTU-3 disconnect, those meet it.

The rule has a few sharp edges. Spare positions that hold an unused breaker or switch have to be marked as spares, not left blank, so the next person knows the pole is energized and idle, not unknown. No circuit can be described by something temporary, like the current tenant or a piece of furniture, because that description is wrong the day the occupancy changes. And the directory has to live on the face of the panel, inside the door, or in an approved spot right next to it; the 2020 edition added the adjacent location to give a crowded schedule somewhere to go.

The part of 408.4 that gets ignored is the after-changes part. The directory has to be kept accurate when circuits change. A panel that was right at turnover and never updated through five tenant fit-outs is a panel nobody can safely work on, because the schedule lies. Photograph the finished directory and the open panel as an as-built, and update the schedule every time a circuit is added, moved, or abandoned. The directory the inspector reads at final is the same one a tech reads at 2 a.m. during an outage; make it tell the truth both times.

Balancing the loads across the phases

Spread the single-pole loads across the phases so the legs carry roughly the same current. On a three-phase panel, that means distributing circuits across A, B, and C; on single-phase, across the two legs. The bus is arranged so adjacent breaker positions land on different phases, which lets you balance by where you place breakers, not by rewiring.

Balance is not cosmetic. A panel loaded heavy on one phase pulls that conductor and that part of the bus hotter, wastes the capacity sitting idle on the light phases, and on a wye system loads the neutral with the imbalance current. Push it far enough and you trip a main while two thirds of the panel sits half-used, or you overheat a neutral that the design assumed would carry only the difference.

Do the balancing at design and again as you load circuits, by connected VA per phase, not by counting breakers. Twenty small breakers on one phase and ten big ones on another can be balanced or wildly off; the VA tells you, the count does not. When you finish, the directory and the schedule should let the next person see the per-phase load so they can keep it balanced when they add the next circuit.

Multiwire branch circuits, shared neutrals, and handle ties

A multiwire branch circuit shares one neutral among two or three ungrounded conductors on different phases. It saves a conductor, and it carries a hard requirement: the ungrounded conductors have to be disconnected together. You meet that with an approved handle tie or a multi-pole breaker, so no one can open one leg and leave the shared neutral energized by the others.

The reason is the neutral. On a properly arranged multiwire circuit, the shared neutral carries only the unbalanced current between the phases, which is why it works with one conductor. Put both legs on the same phase by mistake and the neutral currents add instead of subtracting, and the neutral overloads with no breaker watching it. And if someone opens one hot to work on a device while the other is live, the shared neutral can float and put voltage where the worker does not expect it.

Handle ties also show up wherever a circuit needs simultaneous disconnect, like a 240 V load. The point is the same: when conductors share a neutral or feed one piece of equipment, they get switched as a unit. Identify the multiwire sets at the panel and group them so the next person sees the shared neutral before they open one leg of it.

The arc-flash label on the panel

Most panelboards that get worked on energized need an arc-flash warning label, and the label is part of finishing the gear, not an extra. It tells a worker the hazard before they open the cover: the nominal voltage, the arc-flash boundary, and either the incident energy at the working distance or the required PPE, depending on the method the safety program uses. Our arc-flash study and labels guide covers what drives that incident energy and how the label is built.

The label comes from the arc-flash study, which uses the same available fault current and the breaker clearing time you established when you selected the gear. A faster-clearing upstream device lowers the incident energy, which is why the protection settings and the label are connected. NFPA 70E governs the safety program and the label content; the calculation method is commonly IEEE 1584.

Field-wise, the label belongs on the panel before it is energized and worked, and it gets reviewed when the system changes, since a transformer swap or a new service can move the fault current and the energy. A panel with no label is a panel a worker opens blind.

Where are AFCI and GFCI breakers required?

AFCI and GFCI protection is required by location and circuit type, and the list has grown every code cycle, so check the adopted edition rather than what you memorized five years ago. GFCI protects people from shock by opening on current leaking to ground; AFCI protects against arcing faults that start fires. They solve different problems and the code calls for them in different places, with some places now needing both.

GFCI shows up at receptacles in wet and damp areas and near water: bathrooms, kitchens, garages, outdoors, basements, laundry, and similar. AFCI protection covers many of the branch circuits in dwelling living areas, bedrooms, and beyond, with recent editions widening the rooms covered. Some circuits land in both buckets, which is where dual-function breakers earn their place in the panel.

From the panel side, the practical effect is the neutral wiring already mentioned: AFCI and GFCI breakers take the circuit neutral on the breaker and pigtail to the bar. Plan the spaces for them, because they are wider in some lines, and label them in the directory so a future change keeps the protection that the occupancy requires. The adopted edition and local amendments control which circuits need which protection.

Commissioning and the inspection

Commissioning a panel is verifying that what you installed matches what you intended, before the cover goes on for good and before the inspector arrives. The list is short and it is the same list the inspector carries. Confirm the working clearance and the dedicated space are clear. Confirm every lug and breaker termination is torqued to spec, and many crews mark each one so a glance shows the panel is done.

Then the electrical checks. Verify the neutral is bonded only at the service and isolated at every sub-panel, and that the ground path is continuous. On new gear, an insulation-resistance test with a megohmmeter, phase to phase and phase to ground with the breakers off and loads disconnected, confirms there is no short or compromised insulation before energizing. Check the phase balance against the schedule. Confirm the directory is legible, specific, and accurate to the breakers actually installed.

Last, the cover and the close-out. The deadfront goes on with no exposed live parts and no missing knockout fillers, the arc-flash label is in place, and the as-built directory and a photo of the open panel go into the record. The callbacks on panels are rarely the gear. They are the commissioning step nobody finished, the lug nobody torqued, the directory nobody updated.

Data center panelboards, RPPs, and PDUs

In a data center the panelboard shows up as the branch circuit panel inside a power distribution unit (PDU) or as a remote power panel (RPP) out on the floor, and the install rules are the same panelboard rules with the volume turned up. The loads are continuous, the circuit counts are high, and the directory and the per-phase balance stop being good practice and become operational necessities, because the facility tracks branch-circuit load to the pole.

Two things get stricter here. Phase balance is monitored live, often per circuit, because an unbalanced PDU wastes capacity that was paid for at a premium and stresses the upstream UPS and transformer. And the directory has to map to the rack and the receptacle it feeds, since a mislabeled breaker in a live data hall is the difference between dropping the right circuit and dropping a customer. The detailed metering, redundancy, and capacity planning are their own power distribution topic worth treating separately.

Treat the RPP and the PDU branch panel like any panelboard for clearance, AIC, bonding, and termination, then add the discipline the room demands: accurate as-built directories, balanced phases tracked over time, and torque records the facility can audit. The gear is ordinary; the consequences of a sloppy install are not.

Field checklist

What to document

The record a panel leaves behind is the directory, the as-built schedule, and the commissioning evidence. The directory is the legal minimum; the schedule is what makes the panel maintainable. Build the schedule as a table with the circuit, what it feeds, the breaker, and the phase, so the next person can read the load and keep it balanced.

Capture the panel ratings too: the bus ampacity, the main, the voltage and phase, the AIC and SCCR against the available fault current, the torque values used, and the megger readings if taken. Photograph the open panel and the directory at turnover. When a circuit changes later, update the schedule the same day, because a directory that drifts from reality is worse than none. It tells a worker something false at the moment they are trusting it.

Common mistakes

• Bonding the neutral to the enclosure at a sub-panel instead of isolating it on a separate neutral bar.
• Blocking the working clearance or running foreign piping through the dedicated space above the panel.
• Leaving the directory vague, blank, or out of date after circuits change.
• Installing a panel or breaker whose AIC or SCCR is below the available fault current.
• Running lugs and breaker terminals down by feel instead of to the marked torque value.
• Loading the single-pole circuits heavy on one phase and starving the others.
• Using classified or unlisted breakers the panel was never tested with.
• Doubling equipment grounding conductors under one terminal not listed for more than one.
• Skipping the handle tie or multi-pole device on a multiwire shared-neutral circuit.

Standards and references

The NEC, NFPA 70, is the framework. Panelboards and the circuit directory live in Article 408, with the directory requirement at 408.4 and the panelboard short-circuit current rating addressed in that article. Working space is in 110.26, including the dedicated equipment space. The interrupting and short-circuit requirements are in 110.9 and 110.10. Overcurrent device location and the 6 ft 7 in handle height are in 240.24. Bonding and grounding, including the service main bonding jumper and the equipment grounding conductor sizing, sit in Article 250, with the upsize rule commonly cited at 250.122(B).

The exact article and section numbers move between code cycles, and the 42-circuit history is the clearest example: a rule that was in 408 came out in the 2008 edition. Confirm every number against the edition the jurisdiction has adopted and any local amendments before you cite it on a submittal. UL 67 is the product standard panelboards are built and listed to, and the manufacturer's instructions carry the listed breaker line and the torque values that the NEC then requires you to follow. The AHJ has the final say on the install.

For the safety label, NFPA 70E governs the electrical safety program and the arc-flash label content, with the incident-energy calculation commonly done by IEEE 1584. Cite the standard that controls the point, and let the project specification and the listing override a rule of thumb when they are stricter.

Units, terms, and conversions

The same panel goes by several names across a drawing set, a submittal, and the parts counter, so the vocabulary is worth pinning down.

A panelboard is the NEC term; a load center is the residential and light-commercial name for the same assembly. Bus rating, main rating, and branch ratings are all in amps. AIC (ampere interrupting capacity) and SCCR (short-circuit current rating) are stated in kA, thousands of amps, and both are compared against the available fault current at the point of installation. Torque is in pound-feet (lb-ft) or pound-inches (lb-in) on lug labels, and in newton-meters (N-m) on metric sheets; one lb-ft is about 1.356 N-m. Working clearance dimensions appear in feet and inches on US drawings and in millimeters on metric ones.

Panelboard / load center

The dead-front assembly that splits a feeder into protected branch circuits; load center is the residential name

AIC

Ampere interrupting capacity, the fault current a breaker can open safely, in kA

SCCR

Short-circuit current rating, the fault current an assembly can withstand safely, in kA

Main bonding jumper

The connection tying neutral to enclosure and ground, installed only at the service

MWBC

Multiwire branch circuit, ungrounded conductors sharing one neutral, requiring simultaneous disconnect

EGC

Equipment grounding conductor, the fault-current path that lands on the ground bar

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