NEC 2023 Code Guide

The NEC is a minimum safety standard. It is not a design manual, and it does not guarantee an efficient or expandable installation (90.1). Good design usually goes beyond the code minimum.

Recommended review sequence for a project

1. Confirm the adopted edition and local amendments. Many jurisdictions lag one cycle behind, or modify rules such as 210.8 and 210.12.
2. Identify the occupancy and any Chapter 5–7 articles that apply: 517 (health care), 518 (assembly), 500–516 (hazardous locations), 680 (pools), 690/705 (PV), 625 (EV), 700–702 (emergency/standby).
3. Apply Chapters 1–4 for the general design: loads, conductors, OCPDs, grounding, wiring methods.
4. Check product listings and installation instructions (110.3(B)). They are enforceable.

Since the 2023 edition, all definitions are consolidated in Article 100. Using the correct term matters: many rules depend on whether a conductor is a service, feeder, or branch circuit.

Article 110 sets the ground rules that apply to all equipment. Several of these items are frequent plan-review and inspection comments on commercial projects.

3.1 Termination temperature limits: 110.14(C)

A conductor may be insulated for 90°C (e.g., THHN), but the terminal it lands on may be rated only 60°C or 75°C. The lowest-rated component in the circuit (conductor, terminal, or device) sets the usable ampacity column.

Torque (110.14(D)): where the manufacturer gives a tightening torque, use a calibrated torque tool. Loose connections are a leading cause of electrical fires.

3.2 Interrupting rating and short-circuit current rating: 110.9, 110.10

• Interrupting rating (AIC): every breaker and fuse must be able to safely interrupt the available fault current at its line terminals. Residential breakers are commonly 10 kA. Commercial services near large utility transformers often need 22 kA, 42 kA, 65 kA, or more.
• SCCR: equipment such as panelboards, motor controllers, HVAC units (440.10), and industrial control panels (409.22) must have a short-circuit current rating that is not less than the available fault current.
• Series-rated combinations (240.86) must be tested and marked, and in some cases engineered by a licensed professional.

3.3 Field markings for commercial equipment

110.26 protects the person working on energized equipment. It requires a clear area to stand, room to move back, enough light, and a way out. It has two separate parts that are often confused:

• Working space (110.26(A)): the clear area in front of the equipment, for people.
• Dedicated equipment space (110.26(E)): the zone directly above and below switchboards, switchgear, panelboards, and MCCs, kept free of foreign systems (pipes, ducts).

The working-space dimensions apply to equipment operating at 1000 V or less to ground that is likely to require examination, adjustment, servicing, or maintenance while energized. Examples are panelboards, switches, circuit breakers, controllers, VFDs, and controls on HVAC equipment. “Examination” includes checking for the presence of voltage with a meter.

4.1 Depth: 110.26(A)(1)

Depth is measured from the exposed live parts, or from the front of the enclosure (or its opening) if live parts are enclosed. The required depth depends on the voltage to ground and on what is on the opposite side of the space (the “condition”).

Depth: special provisions

• Dead-front assemblies, 110.26(A)(1)(a): working space is not required at the back or sides where all connections and renewable or adjustable parts are accessible from the front. Where rear access is needed for work on de-energized parts, provide at least 30 in horizontally.
• Low voltage, 110.26(A)(1)(b): smaller spaces are permitted by special permission where exposed live parts operate at no more than 30 V rms, 42 V peak, or 60 V dc.
• Existing buildings, 110.26(A)(1)(c): where equipment is replaced, Condition 2 spacing is permitted between dead-front switchboards, panelboards, or MCCs facing each other across an aisle, if written procedures prevent both sides from being open at the same time and only authorized, qualified persons service the installation.

4.2 Width: 110.26(A)(2)

The working space must be at least 30 in (762 mm) wide or the width of the equipment, whichever is greater. Equipment doors and hinged panels must be able to open at least 90°.

4.3 Height: 110.26(A)(3)

4.4 Limited access, separation, and floor surface: 110.26(A)(4)–(A)(6)

4.5 Clear spaces: 110.26(B)

• The working space may not be used for storage, at any time. This is one of the most common citations in occupied buildings.
• If normally enclosed live parts are exposed for inspection or servicing in a passageway or general open area, the working space must be suitably guarded.

4.6 Entrance to and egress from working space: 110.26(C)

A worker injured by an arc flash must be able to get out quickly, without being trapped behind large equipment. The rules depend on the size and rating of the equipment.

4.7 Illumination: 110.26(D)

Lighting is required for all working spaces about service equipment, switchboards, switchgear, panelboards, and motor control centers installed indoors.

• The lighting may not be controlled by automatic means only. Timers or occupancy sensors are allowed if a manual means can turn the lights on.
• Additional lighting outlets are not required where the working space is illuminated by an adjacent light source, or as permitted by 210.70(A)(1) Exception 1 for switched receptacles.

4.8 Dedicated equipment space: 110.26(E)

Switchboards, switchgear, panelboards, and motor control centers must be located in dedicated spaces and protected from damage. Exception: control equipment that, by its nature or because of other code rules, must be adjacent to or within sight of its machinery may be installed in those locations.

4.9 Locked electrical rooms: 110.26(F)

Electrical equipment rooms or enclosures housing equipment that are controlled by a lock are considered accessible to qualified persons.

For dwelling units, the NEC prescribes specific circuits and receptacle locations so occupants do not need extension cords. Commercial buildings have fewer prescriptive outlet rules, but several targeted requirements apply.

5.1 Required dwelling branch circuits: 210.11(C)

5.2 Dwelling receptacle locations: 210.52

5.3 Receptacle types

• Tamper-resistant (406.12): required for 15 A and 20 A, 125 V and 250 V receptacles in dwelling units, hotel/motel guest rooms, child-care facilities, schools, business-office corridors and waiting rooms, clinics, places of awaiting transportation, gyms, auditoriums, dormitories, and assisted-living facilities. Exceptions include receptacles more than 5½ ft above the floor.
• Weather-resistant with in-use covers (406.9(B)): required for 15 A and 20 A receptacles in wet locations. The cover must be an “extra-duty” in-use type.

5.4 Commercial receptacle requirements

210.25 applies to two-family dwellings, multifamily dwellings, and multi-occupancy buildings such as retail centers and offices with tenants. It keeps one occupant from controlling, switching off, or paying for loads that everyone relies on.

5A.1 The rule: 210.25(A) and (B)

5A.2 What counts as a common area

5A.3 Feeding from a house panel in another building: 225.30

A house panel in an adjacent building on the same property satisfies 210.25(B). However, where the light is mounted matters:

5A.4 Frequently asked questions

A GFCI compares the current on the hot conductor with the current returning on the neutral. If about 4–6 mA leaks to ground, for example through a person, it opens the circuit within a fraction of a second. GFCI protection is required where water, damp floors, or contact with earth increase the risk of shock.

Other GFCI requirements

An AFCI analyzes the current waveform for the signature of arcing, caused for example by a damaged cord, a loose terminal, or a nail through a cable, and trips before a fire can start. A GFCI protects people from shock; an AFCI protects the building from fire.

Methods of protection: 210.12(A)(1)–(6)

• A listed combination-type AFCI breaker protecting the entire circuit. This is the most common method.
• An outlet branch-circuit (OBC) AFCI at the first outlet, where the home run is in metal raceway (RMC, IMC, EMT), steel-armored cable (Type AC or MC), or is encased in 2 in of concrete.
• Other listed system combinations, as permitted.

8.1 Required lighting outlets: 210.70

8.2 Switches

• Neutral at switch locations, 404.2(C): a grounded (neutral) conductor must be provided at most switch locations that serve habitable rooms, bathrooms, hallways, and stairways, so electronic controls and occupancy sensors can be installed later. There are limited exceptions (e.g., the raceway allows the neutral to be added later, or the switch serves a receptacle).
• Mounting height, 404.8(A): the center of the operating handle in its highest position may be no more than 6 ft 7 in (2.0 m) above the floor or working platform.
• Wet locations: switches must be in weatherproof enclosures (404.4). In bathtub or shower spaces, a switch is permitted only as part of a listed tub or shower assembly.

8.3 Luminaires in clothes closets: 410.16

The load calculation sets the minimum size of feeders and services. Article 220 was reorganized in 2023, so section numbers from the 2017 and 2020 editions have moved. Use the 2023 numbers on new drawings.

9.1 Article 220 map (2023 numbering)

Article 220 has five parts. Pick the part that matches what you are sizing: a single branch circuit (Part II), or a feeder or service (Part III standard method, or Part IV optional method).

9.2 Dwelling unit: standard method

9.3 Non-dwelling (commercial) occupancies

9.4 Other sections you will need

9.5 Optional methods (Part IV) and farm loads (Part V)

See the worked examples for a complete dwelling and a small retail service calculation.

Article 750 sets the safety rules for the EMS itself. 220.70 lets you use its setpoint in the service or feeder calculation. 625.42(A) and 705.13 apply the same idea to EV chargers and to solar and battery systems.

9A.1 How it works

9A.2 Article 750 requirements

9A.3 Using an EMS in calculations

10.1 The 125% rule

Standard circuit breakers are not rated to carry 100% of their rating continuously inside an enclosure. For loads that run 3 hours or more, both the conductor and the OCPD are sized at 125% of the continuous load plus 100% of the non-continuous load.

• Typical continuous loads: commercial lighting, EV chargers (625.42), electric water heaters (422.13), fixed space heating (424.4), process loads, and data equipment.
• Exception: where the OCPD assembly is listed for 100% operation (common for large switchboard breakers), the continuous load may be taken at 100%.
• The conductor must satisfy two separate checks: (1) ≥ 125% of the continuous load before any correction or adjustment; and (2) ≥ the load after correction and adjustment factors.

10.2 Feeder tap rules: 240.21(B)

Normally, conductors must be protected at the point where they receive their supply. The tap rules allow a smaller conductor to be connected to a larger feeder without its own OCPD at the tap point, if strict limits are met.

Dwelling service and feeder conductors: the 83% rule (310.12)

For single-phase 120/240 V dwelling services and main feeders rated 100–400 A that carry the entire dwelling load, conductors may be sized at 83% of the service rating. This reflects the diversity of residential loads.

The NEC recommends, in informational notes, a maximum drop of 3% for a branch circuit or feeder and 5% total for feeder plus branch circuit. These limits are not enforceable NEC rules, but energy codes (e.g., ASHRAE 90.1: 2% feeder / 3% branch) and project specifications often make them mandatory. A few NEC articles do set enforceable limits, such as 647.4(D) for sensitive electronic equipment and 695.7 for fire pumps.

The basic rule (240.4) is that conductors are protected at their ampacity. Several important modifications follow.

14.1 Small conductor rule: 240.4(D)

14.2 Next size up: 240.4(B)

Where the conductor ampacity does not match a standard OCPD rating, the next higher standard rating is permitted only if all of these apply:

• The conductors do not supply a multioutlet branch circuit serving cord-and-plug-connected portable loads.
• The ampacity does not match a standard rating.
• The next higher standard rating does not exceed 800 A. Above 800 A, the conductor ampacity must be equal to or greater than the OCPD rating (240.4(C)).

14.3 Standard ratings: 240.6(A)

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 A. (Fuses also: 1, 3, 6, 10, 601 A.)

14.4 Location: 240.24

• Readily accessible, with the handle center no more than 6 ft 7 in (2.0 m) above the floor.
• Occupants must have ready access to the OCPDs that protect their own circuits. In multi-tenant buildings, this is usually done by placing a panel in each tenant space.
• Not in clothes closets, bathrooms of dwelling units and hotel guest rooms, or over stairway steps.

Table 310.16 gives ampacities for insulated conductors rated up to 2000 V: no more than three current-carrying conductors in a raceway, cable, or earth, at a 30°C (86°F) ambient. For other conditions, apply correction and adjustment factors.

* OCPD limited by 240.4(D). Insulation types: 60°C = TW, UF; 75°C = THW, THWN, XHHW, USE; 90°C = THHN, THWN-2, XHHW-2, RHW-2.

16.1 Protection against physical damage: 300.4

16.2 Underground installations: 300.5

• Streets, roads, driveways, parking lots: 24 in for all methods. One- and two-family dwelling driveways allow 18 in for some methods; check the table.
• Under a building: cables must be in a raceway. Under a 4 in concrete slab, the cover may be reduced.
• Where conductors emerge from grade: protect them from the required depth (max 18 in) up to 8 ft above grade (300.5(D)(1)).
• Warning ribbon: at least 12 in above underground service conductors buried 18 in or more that are not encased in concrete (300.5(D)(3)).
• Wet location: the interior of underground raceways is a wet location, so use wet-rated conductors such as THWN-2 or XHHW-2 (300.5(B)). Seal raceways entering buildings (300.5(G)).

16.3 Common wiring methods compared

Grounding (connecting to earth) limits voltages from lightning, line surges, and contact with higher-voltage lines. Bonding (connecting metal parts together and back to the source) creates the low-impedance fault path that makes breakers trip. The earth is not an effective fault-current path (250.4(A)(5)).

17.1 Grounding electrode system: 250.50, 250.52

All of the following electrodes that are present at the building must be bonded together to form the grounding electrode system:

• Single rod (250.53(A)(2)): a single rod, pipe, or plate electrode must be supplemented by a second electrode at least 6 ft away, unless it has a resistance to earth of 25 Ω or less.
• Bonding of piping (250.104): metal water piping must be bonded (jumper sized per Table 250.102(C)(1)). Metal gas piping, including CSST, must be bonded per 250.104(B) and the manufacturer's instructions.
• Intersystem bonding termination (250.94): provide a terminal with at least three connections, at the service, for communications, CATV, and similar systems.
• Detached buildings (250.32): install a grounding electrode at the second building, run an EGC with the feeder, and do not bond the neutral to ground at the second building.

In most commercial buildings, a 480 V distribution system feeds step-down transformers that create local 208Y/120 V systems. Each transformer secondary is a separately derived system (SDS) and needs its own neutral-to-ground bond and grounding connection.

• GEC limits (250.66(A)–(C)): to rods, pipes, or plates only, the GEC never needs to be larger than 6 AWG Cu. To a concrete-encased electrode only: 4 AWG Cu. To a ground ring only: the size of the ring.
• Upsized conductors (250.122(B)): if the ungrounded conductors are increased in size (e.g., for voltage drop), the EGC must be increased proportionally by circular-mil area.
• Parallel runs (250.122(F)): each parallel raceway or cable needs a full-size EGC based on the feeder OCPD.
• Raceway as EGC (250.118): EMT, RMC, and IMC qualify as EGCs, but many specifications still require a separate wire-type EGC for reliability.

Overfilled boxes cause overheating, damaged insulation, and loose connections. Count the conductor allowances, multiply by the volume per conductor, and compare the result with the box volume (marked on the box, or from Table 314.16(A)).

Box support for luminaires and fans: 314.27

• Ceiling outlet boxes for luminaires must support at least 50 lb. Heavier luminaires need marked boxes or independent support.
• Wall-mounted luminaires over 50 lb need independent support.
• Ceiling fans: boxes must be listed and marked for fan support (up to 70 lb, or marked for more). In dwellings, ceiling boxes in habitable rooms where a fan could be installed must be listed for fan support (314.27(C)).
• Floor boxes must be listed for floor installation (314.27(B)).

1. Find the area of each conductor (in²) in Chapter 9, Table 5, by insulation type and size. Bare conductors are in Table 8.
2. Add all conductor areas, including EGCs.
3. Select the raceway from Chapter 9, Table 4 for the specific raceway type (EMT, PVC 40, PVC 80, RMC, etc.), using the 40% column.

Motors draw 6–8 times their running current while starting. For this reason, Article 430 separates protection into short-circuit and ground-fault protection (the breaker or fuse, sized large enough to allow starting) and overload protection (sized close to the running current).

• Overloads (430.32): 125% of nameplate FLA for motors with a service factor of 1.15 or more, or a temperature rise of 40°C or less; 115% for all others.
• Next size up: where the 430.52 value is not a standard rating, the next higher standard size is permitted (430.52(C)(1) Exc. 1). If the motor still cannot start, higher limits are allowed (Exc. 2).
• Several motors on a feeder (430.24): 125% of the largest motor FLC + 100% of the other motors + other loads.
• Disconnecting means (430.102): must be in sight of the controller and of the motor (in sight = visible and within 50 ft). A lockable disconnect at the controller may serve the motor in some cases. Disconnect rating ≥ 115% of FLC (430.110).
• VFDs (430.122): conductors to the drive are sized at 125% of the drive's rated input current.

For packaged HVAC equipment (condensers, heat pumps, rooftop units), the manufacturer has already done the motor calculations. Use the two values on the nameplate.

• Conductors: ampacity ≥ MCA (minimum circuit ampacity).
• OCPD: not greater than MOCP (maximum overcurrent protection). If the nameplate says “HACR breaker” or “fuse”, that type must be used.
• Disconnect (440.14): within sight of and readily accessible from the equipment. It may be mounted on the unit, but not on panels that give access to the equipment or obscure the nameplate.
• Service receptacle (210.63): within 25 ft, on the same level, GFCI protected outdoors and on rooftops.
• Outdoor dwelling units: 210.8(F) GFCI for outdoor outlets. Coordinate with the equipment's listed compatibility (variable-speed compressors).
• SCCR (440.10): the equipment's short-circuit current rating must be at least the available fault current. This is critical for commercial rooftop units close to the service.

Secondary conductors: 240.21(C)

• Conductor protection: transformer protection does not automatically protect the secondary conductors. Only 2-wire (single-voltage) secondaries and 3-wire delta-delta secondaries may be protected by the primary OCPD (240.4(F)). A 208Y/120 V secondary always needs secondary conductor protection.
• 10-ft rule, (C)(2): secondary conductors up to 10 ft long, with ampacity not less than the rating of the device they terminate in.
• 25-ft rule, (C)(6): up to 25 ft, where the secondary ampacity × (secondary-to-primary voltage ratio) is at least 1/3 of the primary OCPD rating, terminating in a single OCPD.
• Outside, (C)(4): unlimited length outdoors, terminating in an OCPD at the building disconnect.

• Ventilation (450.9): maintain the clearances marked on the transformer.
• Accessibility (450.13): transformers must be accessible. Dry-type units 1000 V or less and 50 kVA or less may be installed above suspended ceilings or in hollow spaces under specific conditions.

See Example 28.3 for a complete 75 kVA transformer design.

See Example 28.4 for a residential 48 A charger circuit.

26.1 Load-side connection: the 120% rule (705.12(B)(3)(2))

If the rule fails, the options are to downsize the main breaker, apply the “sum of breakers” method (705.12(B)(3)(3)), use a feed-through or center-fed configuration where permitted, use a power control system (705.13), or make a supply-side connection (705.11).

26.2 Key PV requirements

28.1 Single-family dwelling service: standard method Residential

Given: a 2,400 ft² house at 120/240 V, single-phase. Appliances: 12 kW range, 5 kW dryer, 4.5 kW water heater, 1.2 kW dishwasher, 0.9 kW disposal, 1.5 kW microwave (fastened in place), 10 kW electric heat, and a 5 kVA A/C unit.

28.2 Small retail store service Commercial

Given: a 5,000 ft² retail store at 208Y/120 V, 3-phase, 4-wire. 30 linear ft of show window, 40 general receptacles, one sign circuit, and a rooftop unit with an 18,000 VA nameplate load.

28.3 75 kVA transformer: 480 V Δ to 208Y/120 V Commercial

1. Currents: primary 90.2 A; secondary 208.2 A.
2. Primary OCPD: using primary and secondary protection, the maximum is 250% × 90.2 = 225 A. A practical design uses about 125% × 90.2 = 112.8 A, so select 125 A (check transformer inrush).
3. Primary conductors: 125 A at 75°C → 1 AWG Cu (130 A). EGC per Table 250.122 for 125 A → 6 AWG Cu.
4. Secondary OCPD: maximum 125% × 208.2 = 260 A. Select 250 A (the secondary panel main breaker or a separate disconnect).
5. Secondary conductors: 250 A at 75°C → 250 kcmil Cu (255 A). Keep the length ≤ 10 ft (240.21(C)(2)) or meet the 25-ft rule.
6. Grounding: system bonding jumper and GEC per Tables 250.102(C)(1) and 250.66, based on 250 kcmil Cu → 2 AWG Cu, connected to building steel (250.30(A)(4)).
7. Working space: the 480 V primary terminals are 277 V to ground → 3 to 4 ft depending on the condition (110.26).

28.4 EV charger in a residential garage Residential

1. Load: 48 A EVSE, continuous (625.42) → 48 × 1.25 = 60 A.
2. OCPD: 60 A, 2-pole (standard size, 240.6(A)).
3. Conductors: 75°C terminals → 6 AWG Cu (65 A). With 60°C terminals, 6 AWG is only 55 A, so 4 AWG Cu (70 A) would be needed (110.14(C)).
4. EGC: Table 250.122 for 60 A → 10 AWG Cu.
5. Voltage drop at 40 ft: 2 × 12.9 × 48 × 40 ÷ 26,240 = 1.9 V (0.8%) → acceptable.
6. Raceway: 2 × 0.0507 + 0.0211 = 0.1225 in² → ¾ in EMT (0.213 in² at 40%).
7. Service check: add 7,200 W or the nameplate rating (11,520 W) to the dwelling load calculation (220.57). Use the larger value: 11,520 W.