Wiring design and protection

A conductor used as a grounded conductor must be clearly identifiable and distinguishable from all other conductors. Employers should permanently mark or use a distinct color for the grounded conductor so workers can quickly tell it apart from ungrounded and grounding conductors. See the identification requirement in 1910.304(a)(1).

An equipment grounding conductor must be identifiable and distinguishable from all other conductors. Use a unique color, marking, or labeling method so the grounding conductor cannot be confused with grounded or ungrounded conductors. This requirement is stated in 1910.304(a)(1)(ii).

No — a grounded conductor may not be attached to any terminal or lead so as to reverse designated polarity. Terminals and wiring must preserve the correct polarity to prevent hazards from reversed connections. That prohibition is in 1910.304(a)(2).

No — a grounding terminal or grounding-type device on a receptacle, cord connector, or attachment plug may only be used for grounding and not for other purposes. Do not use the grounding contact as a neutral or carrying conductor. See 1910.304(a)(3).

Each ungrounded conductor of a multiwire branch circuit that is accessible must be identified by phase and system, with the identification permanently posted at each branch-circuit panelboard. This helps prevent misidentification that could create shock or equipment damage. See the multiwire identification rule in 1910.304(b)(1).

Receptacles installed on 15- and 20-ampere branch circuits must be of the grounding type except where replacement rules in 1910.304(b)(2)(iv) allow otherwise. Also ensure grounding-type receptacles match the circuit voltage and current rating. See 1910.304(b)(2)(i).

Grounding contacts on receptacles and cord connectors that have grounding contacts must be effectively grounded by connection to the equipment grounding conductor of the supplying circuit. In other words, the grounding contact must be tied to the circuit's equipment grounding conductor to ensure the device is grounded. See 1910.304(b)(2)(ii) and 1910.304(b)(2)(iii).

If a grounding means does not exist in the receptacle enclosure, you may:

• Replace a nongrounding-type receptacle with another nongrounding-type receptacle; or
• Replace it with a GFCI-type receptacle that is marked "No Equipment Ground" and do not connect an equipment grounding conductor to it; or
• Replace it with a grounding-type receptacle supplied through a GFCI, marked "GFCI Protected" and "No Equipment Ground," and do not connect an equipment grounding conductor to that receptacle.

These options are listed in 1910.304(b)(2)(iv)(C). For cases where a grounding means does exist in the enclosure, grounding-type receptacles must be used and connected as required in 1910.304(b)(2)(iv)(A).

Receptacles connected to circuits having different voltages, frequencies, or types of current must be designed so the attachment plugs for those circuits are not interchangeable. This prevents accidental mis-plugging into the wrong system and is required by 1910.304(b)(2)(v).

All 125-volt, single-phase, 15- and 20-ampere receptacles installed in bathrooms or on rooftops must have ground-fault circuit-interrupter protection for personnel. This specific GFCI requirement is contained in 1910.304(b)(3)(i).

All 125-volt, single-phase, 15-, 20-, and 30-ampere receptacle outlets that are not part of the permanent wiring and are in use by personnel during construction-like activities must have GFCI protection for personnel. Note that an extension cord connector on a cord set counts as a receptacle outlet in this context. See 1910.304(b)(3)(ii)(A).

If GFCI protection is not available for receptacles other than 125-volt, single-phase, 15-, 20-, and 30-ampere used temporarily, your employer must implement an assured equipment grounding conductor program covering cord sets, non-permanent receptacles, and cord-and-plug-connected equipment used by employees. This alternative program is described in 1910.304(b)(3)(ii)(C) and the related requirement is in 1910.304(b)(3)(ii)(B).

Your assured equipment grounding conductor program must be written and jobsite-available, designate competent person(s), require daily visual inspections of cord sets and equipment, perform specific electrical tests for grounding conductor continuity and correct attachment, test at the required intervals, prevent use of noncompliant equipment, and keep records of tests and dates. These detailed program elements are listed in 1910.304(b)(3)(ii)(C)(1)-(6).

Tests must be performed before first use, after any repairs, after any incident that may have caused damage, and at intervals not exceeding 3 months for cord sets and receptacles exposed to damage; cord sets and receptacles that are fixed and not exposed to damage may be tested at intervals not exceeding 6 months. See the testing frequency rules in 1910.304(b)(3)(ii)(C)(4)(iii).

You must keep test records that identify each receptacle, cord set, and cord- and plug-connected equipment that passed the test and show the last test date or the test interval; records must be kept on the jobsite until replaced and made available to OSHA and affected employees. This recording requirement is in 1910.304(b)(3)(ii)(C)(6).

A single receptacle on an individual branch circuit must have an ampere rating not less than the branch-circuit rating; where a branch circuit supplies two or more receptacles, the total cord-and-plug load must not exceed the maximums in Table S-4 and receptacle ratings must conform to Table S-5 (or for circuits over 50 A, be at least the branch-circuit rating). See 1910.304(b)(4)(ii)(A)-(C).

A receptacle outlet must be installed wherever flexible cords with attachment plugs are used, unless the flexible cords are permitted to be permanently connected under the standard. This requirement is in 1910.304(b)(5).

For power conductors alone or above communication conductors at 300 volts or less on poles, the required horizontal climbing space is 610 mm (24 inches). See the pole conductor spacing rules in 1910.304(c)(1)(ii)(A).

Power conductors below communication conductors must provide 762 mm (30 in.) of horizontal climbing space; power conductors alone or above communication conductors require 610 mm (24 in.) if 300 volts or less and 762 mm (30 in.) if over 300 volts; communication conductors below power conductors must meet the same spacing as power conductors. These requirements are in 1910.304(c)(1)(i)-(iii).

Open conductors and service-drop conductors of not over 600 volts must meet the minimum clearances from ground specified in Table S-6; for example, older installations typically require about 3.05 m (10 ft) clearance in pedestrian-accessible areas and greater clearances where vehicular or truck traffic is present. Review the specific condition and voltage columns in Table S-6 for exact clearance values. See 1910.304(c)(2) which contains Table S-6.

For a 15-amp receptacle on a 15- or 20-amp circuit, the maximum cord-and-plug-connected load allowed is 12 amperes per Table S-4. See the load limits in 1910.304(b)(4)(ii) and Table S-4 located in that section.

On a 20-ampere branch circuit supplying two or more receptacles, you may use receptacles rated 15 or 20 amperes in accordance with Table S-5; the total cord-and-plug load must not exceed the maximum specified in Table S-4. See 1910.304(b)(4)(ii)(B)-(C).

If you replace a nongrounding-type receptacle with a GFCI-type receptacle without a grounding conductor, the device must be marked "No Equipment Ground." If you replace with a grounding-type receptacle fed through a GFCI, it must be marked "GFCI Protected" and "No Equipment Ground." In both cases, do not connect an equipment grounding conductor to such receptacles. These marking and connection rules are in 1910.304(b)(2)(iv)(C)(2)-(3).

Yes — cord sets and devices incorporating the required ground-fault circuit-interrupter that are connected to the receptacle closest to the source of power are acceptable forms of GFCI protection for temporary power, as noted in 1910.304(b)(3)(ii)(A) Note 2.

Where connected to a branch circuit having a rating in excess of 20 amperes, lampholders must be of the heavy-duty type; heavy-duty lampholders must be rated not less than 660 watts if of the admedium type and not less than 750 watts if of any other type. See 1910.304(b)(4)(i).

Flexible cords may be permanently connected in limited situations permitted by the standard, and when they are permanently connected receptacle outlets may be omitted; however, permanent connections must still meet the wiring and safety requirements for the installation. See 1910.304(b)(5) for the receptacle requirement and 1910.304(c)(1) for related outdoor conductor provisions.

Service conductors installed as open conductors or multiconductor cable without an overall jacket must be at least 914 mm (3.0 ft) away from windows that are designed to be opened, doors, porches, balconies, ladders, stairs, fire escapes, and similar locations. See 1910.304(c)(3)(i).

• Conductors that run above the top level of a window may be closer than 914 mm (3.0 ft).
• Final spans that are above or within 914 mm (3.0 ft) horizontally of platforms or projections must still meet the vertical-clearance requirements in 1910.304(c)(2).

(Authority: 1910.304(c)(3)).

No. Overhead service conductors may not be installed beneath openings through which materials may be moved, and they may not be installed where they will obstruct entrance to those openings. This is explicitly prohibited by 1910.304(c)(3)(ii).

• If materials are handled through an opening (e.g., farm or commercial building openings), plan the routing so conductors do not pass over that opening or create an access obstruction.

(Authority: 1910.304(c)(3)(ii)).

Open conductors and open multiconductor cables must have at least 2.44 m (8.0 ft) vertical clearance above the roof surface, maintained for a distance of at least 914 mm (3.0 ft) in all horizontal directions from the roof edge. See 1910.304(c)(4).

Reductions and special cases:

• If the area above the roof is subject to pedestrian or vehicular traffic, then the clearance must meet the values in 1910.304(c)(2). (1910.304(c)(4)(i)).
• A 914 mm (3.0 ft) clearance may be reduced where the voltage between conductors does not exceed 300 V and the roof slope is at least 102 mm in 305 mm (4 in. in 12 in.). (1910.304(c)(4)(ii)).
• The clearance above an overhanging portion of a roof may be reduced to not less than 457 mm (18 in.) when voltage does not exceed 300 V provided the conductors do not pass above the overhang for more than 1.83 m (6.0 ft) vertically or 1.22 m (4.0 ft) horizontally and the conductors terminate at a through-the-roof raceway or approved support. (1910.304(c)(4)(iii) and its subparts).
• The 914 mm (3.0 ft) requirement from the edge of the roof does not apply to the final conductor span when conductors are attached to the side of a building. (1910.304(c)(4)(iv)).

(Authority: 1910.304(c)(4)).

Outdoor lamps must be located below any energized conductors, transformers, or other energized electrical equipment unless that equipment can be locked in the open (de-energized) position or other safeguards provide adequate protection for relamping operations. See 1910.304(d).

• If you cannot lock out the electrical equipment, provide adequate clearances or protective measures (for example, insulated platforms, barriers, or remote relamping methods) so workers are not exposed to live parts during relamping.

(Authority: 1910.304(d)).

You must provide a service disconnecting means that will disconnect all conductors in a building or structure from the service-entrance conductors, and it must clearly show whether it is open or closed, be installed at a readily accessible location, and be nearest the point where the service-entrance conductors enter the building. See 1910.304(e)(1)(i).

Additional requirements:

• The service disconnect must simultaneously disconnect all ungrounded conductors (1910.304(e)(1)(ii)).
• The disconnecting means must be suitable for the prevailing conditions (for example, rated properly for voltage and environment) (1910.304(e)(1)(iii)).

(Authority: 1910.304(e)(1)).

For services over 600 volts nominal, open-wire service-entrance conductors must be guarded so they are accessible only to qualified persons, and warning signs about high voltage must be posted where unqualified employees might come into contact with live parts. See 1910.304(e)(2)(i) and 1910.304(e)(2)(ii).

• Guarding can include barriers, enclosures, or other means that prevent access except by trained/qualified personnel.
• Post clear, visible signs where non‑qualified people could possibly encounter energized parts.

(Authority: 1910.304(e)(2)).

No. Except for motor running overload protection, overcurrent devices may not interrupt the continuity of the grounded conductor alone; all conductors of the circuit must be opened simultaneously. See 1910.304(f)(1)(ii).

• This prevents leaving parts of a circuit energized while the neutral is opened, which could create unexpected voltages and hazards.

(Authority: 1910.304(f)(1)(ii)).

A disconnecting means must be provided on the supply side of all fuses in circuits over 150 volts to ground and for cartridge fuses in any accessible circuit so each individual fused circuit can be disconnected independently, unless an allowed exception applies. See 1910.304(f)(1)(iii).

Key exceptions and notes:

• A current-limiting device without a disconnecting means is permitted on the supply side of the service disconnecting means.
• A single disconnect may serve more than one set of fuses as permitted by the exception in 1910.305(j)(4)(vi) for group operation of motors, and a single disconnect is permitted for fixed electric space‑heating equipment.

(Authority: 1910.304(f)(1)(iii) and 1910.305(j)(4)(vi)).

Overcurrent devices must be readily accessible to employees or authorized building management personnel. They may not be located where they can be exposed to physical damage or placed near easily ignitable materials. See 1910.304(f)(1)(iv) and the related limitations in 1910.304(f)(1)(v).

• Fuses and breakers should be shielded or positioned so that employees are not burned or otherwise injured when they operate.
• Moving parts (handles or levers) that could strike nearby workers must be guarded or isolated.

(Authority: 1910.304(f)(1)(iv) and 1910.304(f)(1)(v)).

Circuit breakers must clearly indicate whether they are in the open (off) or closed (on) position. Where breaker handles on switchboards operate vertically, the up position must be the closed (on) position. See 1910.304(f)(1)(vi) and 1910.304(f)(1)(vii).

• Ensure breakers are labeled or designed so personnel can instantly tell their status.
• For vertical-handle switchboards, adopt the up = on convention to avoid confusion during operations or emergencies.

(Authority: 1910.304(f)(1)(vi)-(vii)).

A circuit breaker may only be installed in a circuit where the nominal voltage between any two conductors does not exceed the breaker's straight voltage rating (for example, a 240 V breaker cannot be used where conductor-to-conductor voltage exceeds 240 V). Additionally, a two-pole breaker may not be used to protect a three-phase corner-grounded delta circuit unless the breaker is specifically marked "1Φ -- 3Φ" to indicate suitability. See 1910.304(f)(1)(ix).

• Always verify the breaker’s voltage rating and marking before installation.

(Authority: 1910.304(f)(1)(ix)).

A slash-rated breaker may be installed only where the nominal voltage of any conductor to ground does not exceed the lower of the two values and the nominal voltage between any two conductors does not exceed the higher value on the breaker's rating (for example, a 120/240 V slash-rated breaker can be used when conductor-to-ground voltage ≤ 120 V and conductor-to-conductor voltage ≤ 240 V). See the slash-rating guidance and 1910.304(f)(2)(i)(A).

• Verify both conductor-to-ground and conductor-to-conductor voltages before applying a slash-rated breaker.

(Authority: 1910.304(f)(2)(i)(A)).

Feeder and branch-circuit conductors energized over 600 volts nominal must have overcurrent protection in each ungrounded conductor located where the conductor receives its supply or at a location determined under engineering supervision. See 1910.304(f)(2)(i).

• For three-phase circuits, circuit breakers shall have a minimum of three overcurrent relays operated from three current transformers, with specific exceptions for residual relays as described in the standard. (1910.304(f)(2)(i)(A)).

(Authority: 1910.304(f)(2)(i)).

Protective devices on feeders and branch circuits must detect and interrupt any current in excess of their trip setting or melting point, and their operating time, available short-circuit current, and conductor selection must be coordinated to prevent damaging temperatures under short-circuit conditions. See 1910.304(f)(2)(ii) and 1910.304(f)(2)(iii).

Feeder-specific protection limits (1910.304(f)(2)(iv)):

• A fuse’s continuous ampere rating may not exceed three times the ampacity of the conductor. (1910.304(f)(2)(iv)(A)).
• For breakers, the long-time trip element setting or minimum trip setting of an electronically actuated fuse may not exceed six times the ampacity of the conductor.
• Conductors tapped to a feeder may be protected by the feeder overcurrent device only where that feeder device also adequately protects the tap conductor. (1910.304(f)(2)(iv)(B)).

(Authority: 1910.304(f)(2)(ii)–(iv)).

AC systems rated 50 to 1000 volts must be grounded when any of these conditions apply: the system can be grounded so the maximum voltage to ground on ungrounded conductors does not exceed 150 volts; the system is a nominal three-phase, four-wire wye where the neutral is a circuit conductor; the system is a nominal three-phase, four-wire delta where a phase midpoint is used as a circuit conductor; or a service conductor is uninsulated. See 1910.304(g)(1)(iv).

Common exceptions where grounding is not required include (but are not limited to): systems used exclusively to supply industrial electric furnaces, separately derived systems used exclusively for rectifiers supplying adjustable speed industrial drives, certain separately derived control systems under strict conditions, isolated power systems in health care facilities, and high-impedance grounded neutral systems meeting specific safeguards. See 1910.304(g)(1)(v).

• When you claim an exception (for example, for control circuits), ensure conditions such as qualified-person servicing, ground detectors, and continuity-of-power requirements in the standard are met.

(Authority: 1910.304(g)(1)(iv)–(v)).

For systems required to be grounded by the standard, the conductor to be grounded is: one conductor of a single-phase, two-wire system; the neutral conductor of a single-phase, three-wire system; the common conductor of a multiphase system having one wire common to all phases; one phase conductor where one phase is grounded; and the neutral conductor of a multiphase system in which one phase is used as a neutral conductor. See 1910.304(g)(2).

• Match your grounding conductor choice to the system configuration to comply with the standard and ensure safe operation.

(Authority: 1910.304(g)(2)).

Yes. The frame of a portable generator may serve as the grounding electrode for the system it supplies if the generator supplies only equipment mounted on the generator or cord-and-plug connected equipment through receptacles on the generator, and the noncurrent-carrying metal parts of equipment and the equipment grounding terminals of the receptacles are bonded to the generator frame. See 1910.304(g)(3)(i)(A)–(B).

For vehicle-mounted generators, similar conditions apply: the generator frame must be bonded to the vehicle frame, the generator must supply only equipment on the vehicle or cord‑and‑plug equipment through vehicle-mounted receptacles, receptacle grounding terminals and noncurrent-carrying metal parts must be bonded to the generator frame, and the system must comply with other paragraph (g) provisions. See 1910.304(g)(3)(ii).

• If the generator is part of a separately derived system, required system conductors must be bonded to the generator frame. (1910.304(g)(3)(iii)).

(Authority: 1910.304(g)(3)(i)–(iii)).

For a grounded system, the grounding electrode conductor must connect both the equipment grounding conductor and the grounded circuit conductor (neutral) to the grounding electrode. See 1910.304(g)(4)(i).

• This ensures a common, continuous connection to the grounding electrode so that equipment grounding and neutral conductors share the same grounding reference required by the standard.

(Authority: 1910.304(g)(4)(i)).

The equipment grounding conductor must be tied to the system grounding conductor at the service equipment for ungrounded service-supplied systems and tied at or ahead of the system disconnecting means or overcurrent devices for ungrounded separately derived systems. See 1910.304(g)(4)(ii).

• For an ungrounded service-supplied system: connect the equipment grounding conductor to the grounding electrode conductor at the service equipment.
• For an ungrounded separately derived system: connect the equipment grounding conductor to the grounding electrode conductor at, or ahead of, the system disconnecting means or overcurrent devices.

These connections ensure a common grounding point so equipment enclosures and the grounding electrode are bonded correctly and provide a reliable path for fault currents.

Yes, but only for extensions installed before August 13, 2007; in that narrow situation a grounding-type receptacle may be grounded to a grounded cold water pipe near the equipment, but if any element of the branch circuit is later replaced the entire branch circuit must be upgraded to include an equipment grounding conductor that meets paragraph (g). See 1910.304(g)(4)(iii).

• This is a grandfathered allowance limited to extensions made before 8/13/2007.
• If any part of that branch circuit is replaced after that date, the whole branch circuit must be brought into compliance by providing an equipment grounding conductor that satisfies all other provisions of paragraph (g).

It means the path from circuits, equipment, and enclosures to ground must be a reliable bond that is always present and able to carry fault current to clear a fault condition. See 1910.304(g)(5).

Practical points:

• Permanent: the grounding conductor and connections must be part of the fixed wiring or structure (not temporary jumpers) so grounding is not easily removed.
• Continuous: connections must be uninterrupted along the path (no open splices, loose lugs, or removable parts that break the bond under normal use).
• Effective: conductors, fittings, and connections must be sized, installed, and maintained so they can conduct fault current and permit overcurrent devices to operate (i.e., the path must actually clear faults).

Jobsite checks include verifying tight bonding connections, correct conductor routing, and that enclosure-to-ground continuity is measurable with a continuity tester.

Metal enclosures added to such existing installations may be left ungrounded only when all three specific conditions in the standard are met: the run is less than 7.62 meters (25 ft), enclosures are isolated from probable contact with ground or other conductive materials, and the enclosures are guarded against employee contact. See 1910.304(g)(6)(i)(B).

The three required conditions are:

• Runs are under 7.62 m (25.0 ft) (1910.304(g)(6)(i)(B)(1)).
• Enclosures are free from probable contact with ground, grounded metal, metal laths, or other conductive materials (1910.304(g)(6)(i)(B)(2)).
• Enclosures are guarded against employee contact (1910.304(g)(6)(i)(B)(3)).

If any of these conditions cannot be guaranteed, you must ground the metal enclosure.

Exposed noncurrent-carrying metal parts of fixed equipment that may become energized must be grounded when any one of several listed conditions applies, such as being within reach of employees, located in wet locations, electrically connected to metal, in hazardous locations, supplied by certain metal wiring methods, or if equipment terminals operate over 150 volts to ground. See 1910.304(g)(6)(iv).

Key conditions (each is a separate trigger to require grounding):

• Within 2.44 m (8 ft) vertically or 1.52 m (5 ft) horizontally of ground or grounded metal and subject to employee contact (1910.304(g)(6)(iv)(A)).
• Located in a wet or damp location and not isolated (1910.304(g)(6)(iv)(B)).
• In electrical contact with metal (1910.304(g)(6)(iv)(C)).
• In a hazardous (classified) location (1910.304(g)(6)(iv)(D)).
• Supplied by metal-clad, metal-sheathed, or grounded metal raceway wiring methods (1910.304(g)(6)(iv)(E)).
• Equipment operates with any terminal over 150 volts to ground (1910.304(g)(6)(iv)(F)).

Also review the exceptions in paragraph (g)(6)(v) which list equipment types that need not be grounded despite (g)(6)(iv) in certain situations.

Noncurrent-carrying metal parts of fixed equipment required to be grounded must be grounded by an equipment grounding conductor that is contained within the same raceway, cable, or cord, or runs with or encloses the circuit conductors; however, for installations made before April 16, 1981, securing equipment to and metallic contact with the grounded structural metal frame of a building can be considered effective grounding. See 1910.304(g)(8)(i) and 1910.304(g)(8)(iii).

Practical implications:

• New installations: run the equipment grounding conductor with the circuit conductors or include it in the same raceway/cable so the grounding path is continuous and reliable.
• Pre-4/16/1981 installations: bonding the equipment to the building's grounded structural metal frame may be acceptable as the grounding method, but note that if any element of that branch circuit is later replaced, the entire branch circuit must be updated to include an equipment grounding conductor that complies with current paragraph (g) requirements.