2021 IRC Chapter 37
Every branch circuit in a dwelling ends up at a panelboard, and the panelboard is where an inspector can read the whole electrical system at a glance: whether the overcurrent devices match the wire they protect, whether the panel is somewhere a person can actually get to safely, and whether the circuit directory tells the truth. This lesson covers where a panelboard may and may not be located, the working space and access rules that make it usable in an emergency, how breakers are sized against conductors, and the common panel defects GCHS finds in older rehab houses: double-tapped breakers, missing directories, and mixed neutrals in a subpanel.
A panelboard is the assembly of overcurrent protective devices, typically circuit breakers, mounted in a cabinet that distributes power to the branch circuits and feeders of a dwelling. Two ratings define it: the panel's bus rating, the maximum current the internal bus bar can carry, and its main breaker or main lug rating, which is often but not always the same number. A 200-ampere service panel commonly has a 200-ampere main breaker sized to the bus, though E3706.3 only sets a ceiling, so a panelboard may be protected by an overcurrent device rated below its bus rating. Every panelboard also carries a short-circuit current rating, the maximum fault current the panel and its overcurrent devices are evaluated to interrupt safely, and E3404.2 sets a floor of 10,000 amperes for equipment meant to interrupt fault current; that number never changes without a fault-current study and is not something a retrofit breaker of a different brand should be assumed to preserve.
E3705.5 and its parent NEC Article 240 set the basic rule: an overcurrent protective device, a circuit breaker or fuse, must be rated at or below the allowable ampacity of the conductor it serves, using the standard ampere ratings in Table E3705.6 (15, 20, 25, 30, 35, 40, 45, 50, 60, 70 amperes and up). Where the conductor's ampacity falls between two standard ratings, the next higher standard rating may be used (E3705.5.2), but not on a branch circuit that supplies more than one receptacle for cord-and-plug portable loads; those circuits are protected at the standard rating at or below the conductor's ampacity. Table E3705.5.3 also caps small copper conductors at 15 amperes for 14 AWG, 20 for 12 AWG, and 30 for 10 AWG. That is the same 15/20/30-ampere framework covered in Lesson 4, applied here at the panel where the breaker is actually selected and installed.
E3705.7 (with note d to Figure E3405.1) prohibits installing a panelboard, or overcurrent devices generally, in a bathroom, in a clothes closet, or over the steps of a stairway. Both prohibitions rest on the same two concerns: bathrooms put a person in a wet, often bare-skinned environment right next to an enclosure of energized parts, and closets stack combustible material against an enclosure that runs warm and occasionally arcs. In a rehab house where a panel was historically shoehorned into a small bathroom or a bedroom closet to save space, relocating it is one of the more disruptive but non-negotiable corrections GCHS scopes into the rewire.
Lesson 1 introduced the working space and dedicated equipment space rules from E3405; this lesson applies them specifically to the panelboard. The clear working space in front of the panel must be at least 30 inches wide, or the width of the equipment if wider, 36 inches deep measured out from the panel face, and 6 feet 6 inches high, and nothing else, no water heater, no shelving, no stored materials, may occupy that space. Above and below the panel, the dedicated equipment space extends the full width and depth of the panel from the floor to a point 6 feet above the panel or to the structural ceiling, whichever is lower, and is reserved for the panel's own conduits, raceways, and cables; foreign systems such as plumbing or ductwork are not allowed to run through it. E3705.7 adds a specific access rule for the breakers themselves: the center of the grip of the highest circuit breaker handle, when in its highest position, must generally be not more than 6 feet 7 inches above the floor or working platform, so a panel is not mounted so high that a person needs a stool to reach the top row in an emergency. In an existing dwelling unit, a panelboard rated 200 amperes or less may stay where the working space is lower than 6 feet 6 inches (E3405.2, Exception 1), which matters on rehab jobs.
A breaker's rating protects the conductor connected to it, not the appliance or load at the far end. A 20-ampere breaker on a run of 12 AWG copper is correctly matched; the same 20-ampere breaker feeding 14 AWG copper is not, no matter how light the actual connected load is, because the breaker will not trip fast enough to protect the smaller wire from a developing fault before the insulation is damaged. This is why field re-terminations, extending a 14 AWG lighting circuit with scrap 12 AWG wire found in a truck, or the reverse, splicing 14 AWG onto a 12 AWG circuit, both create a mismatch that an inspector traces breaker size against conductor size to catch. Multi-wire branch circuits, covered in Lesson 4, add a further requirement at the panel: each multiwire circuit needs a means that simultaneously disconnects all of its ungrounded conductors where the circuit originates (E3701.5.1), in practice a two-pole breaker or single-pole breakers with an identified handle tie, so a person working on one half cannot be surprised by the other half still being live.
E3706.2 and its parent NEC 408.4 require every circuit and circuit modification at a panelboard to be legibly identified on a circuit directory located on the face of the panel, inside it, or adjacent to the panel door, describing the purpose or use of the circuit clearly enough that someone unfamiliar with the installation can figure out what a given breaker controls. The directory has to reflect the current condition of the circuits, not the condition when the panel was first wired; a house that has been added onto, rewired, or had rooms repurposed over the years commonly has a directory that is years out of date, and updating it is a routine but necessary part of any GCHS electrical scope. "Outlets," "misc," or a blank label does not meet the requirement; the directory should read like "kitchen counter receptacles," "hall and bedroom lights," or "water heater."
A standard circuit breaker is evaluated and listed for one conductor under its terminal screw. Landing two conductors under a single-conductor terminal, commonly called double-tapping, is a frequent finding in older or amateur-modified panels and is not permitted unless the breaker is specifically listed and marked for two conductors, information printed on the breaker itself or in the panel's listing documentation. The concern is a loose, overheating connection: two conductors under a screw meant for one rarely land with even pressure on both, and the one that is not fully seated arcs and heats until it fails. Unused openings in a panel cover, left after a breaker is removed, must be closed with an identified closure such as a filler plate (E3907.5), not left open to expose the bus and energized terminals inside; this is one of the fastest and cheapest corrections available at final inspection, and one of the most commonly missed.
A main service panel bonds the grounded (neutral) conductor to the equipment grounding system at one point, the main bonding jumper, as covered in Lesson 3. A subpanel fed from that main panel does not repeat that bond: the subpanel's neutral bus and its equipment grounding bus must be kept separate, with the neutral conductors isolated from the enclosure and from the equipment grounding conductors, and the enclosure itself grounded back to the main panel through an equipment grounding conductor run with the feeder. Bonding the neutral bus to the subpanel enclosure, a mistake carried over from main-panel wiring habits, creates a parallel path for neutral current over the equipment grounding system and metal building parts, a defect an inspector checks for whenever a detached garage, workshop, or basement subpanel is on the job. E3908.7 bars connecting the grounded conductor to enclosures or equipment grounding conductors on the load side of the service disconnect, and for a detached building E3607.3.1 calls for an equipment grounding conductor run with the feeder in new work; a neutral bond at the outbuilding survives only on existing premises under E3607.3.2.
A worked example on breaker-to-conductor matching: a rehab job has an existing 15-ampere breaker feeding a run of 12 AWG copper NM cable to a new baseboard heater circuit with no receptacles, drawing a calculated continuous load of 12.5 amperes. E3701.2.1 and E3702.10 require the conductor to carry at least 125 percent of that load: 12.5 times 1.25 is 15.6 amperes. Twelve AWG copper is rated 20 amperes, so the wire is adequate (14 AWG at 15 amperes would not be). The breaker is a separate question: the NEC continuous-load rules (210.20(A), and 424.3(B) for baseboard heaters) set its minimum at the same 15.6 amperes, so the 15-ampere breaker is undersized for the load. The correct fix is a 20-ampere breaker, the next standard rating in Table E3705.6, which is also the most 12 AWG copper may be protected at under Table E3705.5.3.
In the field
A common finding in older Gulf Coast houses GCHS inspects for rehab is a panel with two or three double-tapped breakers, usually where a homeowner or handyman added a circuit years ago and reused an existing breaker rather than adding a new one or replacing it with a twin-rated unit. GCHS photographs each instance, notes the breaker make and whether it is listed for two conductors, and scopes a licensed electrician to correct any that are not. The same walk-through checks for a current circuit directory, closed unused openings, and a subpanel with its neutral and ground properly separated, all quick checks that catch a disproportionate share of a panel's real problems.
1. Under E3705.7, a dwelling panelboard may not be installed in:
2. The required clear working space in front of a dwelling panelboard is at least:
3. Under E3705.7, the center of the grip of the highest circuit breaker handle, in its highest position, must generally be mounted no higher than:
4. A breaker's ampere rating is chosen to protect:
5. An inspector finds a 20-ampere breaker feeding a run of 14 AWG copper NM cable with no other changes to the circuit. This is a concern because:
6. Under E3706.2, every circuit at a panelboard must be:
7. Landing two conductors under a single circuit breaker terminal screw, commonly called double-tapping, is permitted only when:
8. An inspector finds an open, unused breaker slot on a panel cover with the bus bar visible behind it. The correct correction is:
9. In a subpanel fed from the main service panel, the neutral (grounded conductor) bus must be:
10. A multiwire branch circuit sharing one neutral must have its two ungrounded conductors:
11. A fixed baseboard heater on a circuit with no receptacles has a calculated continuous load of 12.5 amperes. Under the 125 percent continuous-load rule (E3701.2.1 and E3702.10), the minimum conductor ampacity needed is closest to:
12. A panelboard's bus rating and its main breaker rating describe:
13. A branch circuit supplies several receptacle outlets for portable cord-and-plug loads, and its conductor ampacity falls between two standard overcurrent device ratings. The overcurrent device is generally:
14. An inspector finds a new detached garage subpanel with its neutral bus bonded to the enclosure, the same as a main panel. This is: