Wiring methods and grounding

Metal raceways and other metal noncurrent-carrying parts that are to serve as grounding conductors must be effectively bonded to ensure electrical continuity and to safely carry any fault current likely to be imposed on them. See 1910.305(a)(1)(i).

• Remove any nonconductive paint, enamel, or similar coating at threads, contact points, and contact surfaces so the bonding connection is metal-to-metal, or use fittings specifically designed so removal is unnecessary. See 1910.305(a)(1)(i).

Under 1910.305(a)(1)(ii), an equipment enclosure may be isolated from a raceway at the point of attachment by using listed nonmetallic raceway fittings when necessary to reduce electrical noise (EMI), provided an internal insulated equipment grounding conductor is installed to ground the enclosure. See 1910.305(a)(1)(ii).

• The metal raceway must still be supplemented by an internal insulated equipment grounding conductor run to the enclosure so the enclosure remains grounded even though the raceway is electrically isolated. See 1910.305(a)(1)(ii).

No—wiring systems of any type may not be installed in ducts used to transport dust, loose stock, flammable vapors, or in ducts used for vapor removal or commercial-type cooking ventilation, nor in shafts containing only such ducts. See 1910.305(a)(1)(iii).

• This prohibition applies to all wiring system types to avoid fire, contamination, or ignition hazards inside ducts. See 1910.305(a)(1)(iii).

Temporary electrical power and lighting of 600 volts nominal or less may be used during remodeling, maintenance, or repair activities; for up to 90 days for Christmas decorative lighting, carnivals and similar purposes; or for experimental/development work and emergencies. See 1910.305(a)(2)(i).

• The 90‑day limit specifically covers seasonal or special-event uses like decorative lighting and similar short-term needs under 1910.305(a)(2)(i)(B).
• All other permanent wiring requirements still apply to temporary installations unless specifically modified by paragraph (a)(2). See 1910.305(a)(2).

Temporary wiring must be removed immediately upon completion of the project or purpose for which it was installed. See 1910.305(a)(2)(ii).

• Employers should plan for prompt removal so temporary installations do not remain as unsafe or noncompliant permanent wiring. See 1910.305(a)(2)(ii).

Temporary electrical installations of more than 600 volts may be used only during periods of tests, experiments, emergencies, or construction-like activities. See 1910.305(a)(2)(iii).

• This exception is narrowly drawn for short-term, specialized work and does not permit general long-term high-voltage temporary installations. See 1910.305(a)(2)(iii).

Temporary feeders must originate in an approved distribution center, and conductors should be run as multiconductor cord or cable assemblies; however, if used for experimental work or accessible only to qualified persons, feeders may be run as single insulated conductors. See 1910.305(a)(2)(iv)(A) and 1910.305(a)(2)(iv)(B).

• Originating in an approved distribution center helps ensure overcurrent and disconnect protection for temporary feeders. See 1910.305(a)(2)(iv)(A).
• Single insulated conductors are permitted only under the limited conditions of 1910.305(a)(2)(iv)(B).

Temporary branch circuits must originate in an approved power outlet or panelboard; if run as open conductors they must be fastened at ceiling height every 3.05 m (10 ft); and no branch-circuit conductor may be laid on the floor. See 1910.305(a)(2)(v)(A), 1910.305(a)(2)(v)(B), and 1910.305(a)(2)(v)(C).

• These requirements reduce mechanical damage and trip hazards for temporary branch circuits. See 1910.305(a)(2)(v).

Each temporary branch circuit that supplies receptacles or fixed equipment must contain a separate equipment grounding conductor if run as open conductors, and all receptacles must be of the grounding type and electrically connected to the grounding conductor unless installed in a continuous grounded metallic raceway or metallic covered cable. See 1910.305(a)(2)(v)(D) and 1910.305(a)(2)(vi).

• If you use a continuous metallic raceway or metallic-covered cable, that system can serve as the grounding path; otherwise provide a separate grounded conductor and grounding-type receptacles. See 1910.305(a)(2)(vi).

No—bare conductors and earth returns are not permitted for the wiring of any temporary circuit. See 1910.305(a)(2)(vii).

• Use insulated conductors and approved grounding methods for all temporary circuits to avoid shock and fire hazards. See 1910.305(a)(2)(vii).

Temporary circuits must have suitable disconnecting switches or plug connectors to disconnect all ungrounded conductors of each temporary circuit, and multiwire branch circuits must provide a means to disconnect simultaneously all ungrounded conductors at the power outlet or panelboard where the branch circuit originated. See 1910.305(a)(2)(viii).

• Circuit breakers with handles tied together are considered a single disconnecting means for this purpose. See 1910.305(a)(2)(viii).

All lamps for general illumination must be protected from accidental contact or breakage by a suitable fixture or lampholder with a guard, and metal-cased sockets (such as brass shell, paper-lined sockets) may not be used unless the shell is grounded. See 1910.305(a)(2)(ix).

• Use guarded fixtures and ensure metal sockets are properly grounded to prevent shock hazards. See 1910.305(a)(2)(ix).

Flexible cords and cables must be protected from accidental damage (for example from sharp corners, projections, doorways, or pinch points) and cable assemblies and flexible cords must be supported at intervals that protect them from physical damage using staples, cable ties, straps, or similar fittings that do not cause damage. See 1910.305(a)(2)(x) and 1910.305(a)(2)(xi).

• Support intervals should be close enough to prevent sagging or abrasion; fittings must be installed so they do not damage the insulation. See 1910.305(a)(2)(xi).

Cable tray systems may contain a range of listed cable types, including armored cable; electrical metallic tubing; electrical nonmetallic tubing; fire alarm cables; flexible metal conduit; instrument tray cable; liquidtight flexible conduits; metal-clad cable; mineral-insulated metal-sheathed cable; multiconductor service-entrance cable; multipurpose and communications cables; nonmetallic-sheathed cable; power and control tray cable; power-limited tray cable; optical fiber cables; and other factory-assembled, listed multiconductor control, signal, or power cables approved for cable trays. See 1910.305(a)(3)(i).

• Only cables specifically approved and listed for cable tray use may be installed there. See 1910.305(a)(3)(i).

In industrial establishments where maintenance and supervision ensure only qualified persons will service the cable tray system, certain additional cable types—such as single-conductor cable No. 1/0 or larger listed for tray use, welding cables in dedicated trays, insulated or bare single conductors used as equipment grounding conductors of No. 4 or larger, and specific multiconductor cables—may be installed in ladder, ventilated-trough, or ventilated-channel trays. See 1910.305(a)(3)(ii) and 1910.305(a)(3)(ii)(A)-(D).

• These allowances depend on an industrial setting where qualified personnel perform service and maintenance. See 1910.305(a)(3)(ii).

Metallic cable trays may serve as equipment grounding conductors only where continuous maintenance and supervision ensure that qualified persons will service the installed cable tray system. See 1910.305(a)(3)(iii).

• If the installation does not have that ongoing qualified maintenance and supervision, provide a separate equipment grounding conductor rather than relying on the tray. See 1910.305(a)(3)(iii).

Cable tray systems may not be used in hoistways or in locations where they are subjected to severe physical damage. See 1910.305(a)(3)(v).

• Avoid installing tray systems where mechanical damage from hoists, heavy traffic, or similar hazards is likely. See 1910.305(a)(3)(v).

Open wiring on insulators is permitted only on systems of 600 volts nominal or less in industrial or agricultural establishments (indoors or outdoors, in wet or dry locations, and where subject to corrosive vapors) and for services; conductors smaller than No. 8 must be rigidly supported on noncombustible, nonabsorbent insulating materials and may not contact other objects. See 1910.305(a)(4)(i) and 1910.305(a)(4)(ii).

• Supports for conductors smaller than No. 8 must be within 152 mm (6 in.) of a tap or splice, within 305 mm (12 in.) of a dead-end to a lampholder or receptacle, and at intervals not exceeding 1.37 m (4.5 ft) or closer where disturbing is likely. See 1910.305(a)(4)(ii)(A)-(C).
• Open conductors crossing ceiling joints or exposed to physical damage (for example within 2.13 m (7 ft) of the floor) must be protected. See 1910.305(a)(4)(v).

In dry locations not exposed to severe physical damage, conductors may be separately enclosed in flexible nonmetallic tubing in continuous lengths not exceeding 4.57 m (15 ft) and secured by straps at intervals not exceeding 1.37 m (4.5 ft). Where open conductors pass through walls or partitions, they must be separated from contact by tubes or bushings of noncombustible, nonabsorbent insulating material; if the bushing is shorter than the hole, a waterproof nonconductive sleeve plus insulating bushings must be used to keep conductors out of contact with the sleeve. See 1910.305(a)(4)(iii) and 1910.305(a)(4)(iv).

• Follow the specified tubing lengths and fastening intervals to maintain protection and support. See 1910.305(a)(4)(iii).

Conductors entering cutout boxes, cabinets, or fittings must be protected from abrasion and openings through which conductors enter must be effectively closed; unused openings must be effectively closed as well. See 1910.305(b)(1)(i) and 1910.305(b)(1)(ii).

• Use bushings, fittings, grommets, or other approved means to prevent abrasion and to close conduit or enclosure openings. See 1910.305(b)(1)(i)-(ii).

A nonmetallic-sheathed cable entering the top of a surface-mounted enclosure through a nonflexible raceway 457 mm (18 in.) to 3.05 m (10 ft) long need not be secured to the enclosure provided all of the following conditions are met: the cable is fastened within 305 mm (12 in.) of the outer end of the raceway; the raceway extends directly above the enclosure without penetrating a structural ceiling; fittings protect the cable from abrasion and remain accessible; the raceway is sealed or plugged at the outer end to prevent access to the enclosure; the cable sheath is continuous through the raceway and extends at least 6.35 mm (0.25 in.) beyond the fitting into the enclosure; the raceway is fastened at its outer end and elsewhere as necessary; and the allowable cable fill does not exceed conduit fill limits when installed as conduit or tubing. See 1910.305(b)(1)(iii)(A)-(G).

• Each of the seven conditions in 1910.305(b)(1)(iii)(A)-(G) must be satisfied for the securing exception to apply.

The cable sheath must be continuous through the raceway and must extend into the enclosure not less than 6.35 mm (0.25 in.) beyond the fitting. See 1910.305(b)(1)(iii)(E).

• This ensures the sheath provides protection at the fitting and inside the enclosure. See 1910.305(b)(1)(iii)(E).

All pull boxes, junction boxes, and fittings must be provided with covers identified for the purpose. See 1910.305(b)(2)(i).

• Use covers listed or marked for their intended use to maintain accessibility and protect conductors inside the box. See 1910.305(b)(2)(i).

Yes. Metal covers used on outlet boxes or similar fittings must be grounded so the metal cannot become a shock hazard. See the grounding requirement in 1910.305(b).

You must protect cord pendants where they pass through cover holes by using bushings designed for that purpose or by providing smooth, well-rounded surfaces so the cord won't be damaged. This protection requirement is stated in 1910.305(b).

You must cover any combustible wall or ceiling finish exposed between the edge of a fixture canopy or pan and the outlet box with noncombustible material. That requirement is in 1910.305(b)(2)(ii).

Pull and junction boxes for systems over 600 volts must fully enclose the conductors, be closed with suitable covers securely fastened, and have covers permanently marked "HIGH VOLTAGE" so the marking is readily visible. See 1910.305(b)(3), 1910.305(b)(3)(ii), and 1910.305(b)(3)(iii).

Single-throw knife switches must be mounted so that gravity does not tend to close them (i.e., arranged so the normal position will not let gravity shut the switch). This placement requirement is found in 1910.305(c)(1).

Yes. A single-throw knife switch that is approved for use in the inverted position must have a locking device that ensures the blades remain in the open (off) position when set open. See 1910.305(c)(2).

If double-throw knife switches are mounted so the throw is vertical, the switch must have a locking device that ensures the blades remain in the open position when set open. See 1910.305(c)(3).

Generally, switch blades and the terminals supplying the load must be deenergized when the switch is open; however, where the circuit or equipment can backfeed the load side, the load-side terminals may remain energized but a permanent warning sign must be installed warning that "LOAD SIDE TERMINALS MAY BE ENERGIZED BY BACKFEED." These requirements are in 1910.305(c)(3)(i) and 1910.305(c)(3)(ii).

Snap switches and dimmer switches must be effectively grounded and provide a means to ground metal faceplates; if no grounding means exists in the enclosure or the wiring method lacks an equipment ground, a non-grounding snap switch is allowed only for replacement and must be fitted with a nonconducting, noncombustible faceplate when within reach of conducting floors or surfaces. See 1910.305(c)(5).

Switchboards with any exposed live parts must be in permanently dry locations and must be accessible only to qualified persons. That requirement appears in 1910.305(d)(1).

Panelboards must be mounted in cabinets, cutout boxes, or enclosures designed for the purpose and shall be dead front; an exception allows non-dead-front externally-operable panelboards only where they are accessible solely to qualified persons. See 1910.305(d)(2).

Enclosures (cabinets, cutout boxes, fittings, boxes, panelboards) in damp or wet locations must be installed to prevent moisture or water from entering and accumulating, mounted with at least 0.25 in airspace from the supporting surface (except some nonmetallic enclosures), and enclosures must be weatherproof in wet locations; switches, circuit breakers, and switchboards in wet locations must be enclosed in weatherproof enclosures. See 1910.305(e)(1) and 1910.305(e)(2).

All conductors used for general wiring must be insulated unless an exception applies; the insulation must be approved for the voltage, operating temperature, and location; and insulated conductors must be distinguishable by color or other means as grounded, ungrounded, or equipment grounding conductors. See 1910.305(f)(1), 1910.305(f)(2), and 1910.305(f)(3).

Flexible cords and cables may be used only for a limited set of purposes (for example pendants, wiring of fixtures, connection of portable lamps or appliances, crane wiring, temporary wiring allowed by 1910.305(a)(2), etc.), and they may not be used as a substitute for fixed wiring, run through holes in walls/ceilings/floors, run through doorways or windows, attached to building surfaces, concealed behind walls/ceilings/floors, or installed in raceways except where specifically permitted. See allowed uses in 1910.305(g)(1)(ii) and prohibited uses in 1910.305(g)(1)(iv).

Yes. When flexible cords are used for connecting portable lamps or appliances or for connections that are permitted in those specific paragraphs (such as (g)(1)(ii)(C), (G), or (I)), the cord must be equipped with an attachment plug and must be energized from an approved receptacle outlet. See 1910.305(g)(1)(iii).

Flexible cords and cables used as grounded or equipment grounding conductors must be distinguishable; many cord types must be durably marked at intervals with type, size, and conductor count; flexible cords must be used in continuous lengths without splice or tap except for certain repairs on hard-service cords; and cords must be connected to devices and fittings so strain relief prevents pull from being transmitted to joints or terminal screws. See 1910.305(g)(2), 1910.305(g)(2)(ii), and 1910.305(g)(2)(iii).

Portable cables over 600 volts nominal must use multiconductor construction with No. 8 or larger flexible-stranded conductors, be shielded when operated over 2,000 volts to confine voltage stresses, have grounding conductors provided and shields grounded, and use fittings and splices that lock firmly and prevent opening while energized; terminations must be marked with a high-voltage warning and be accessible only to authorized qualified employees. See 1910.305(h), 1910.305(h)(2), 1910.305(h)(3), and 1910.305(h)(4).

Fixture wires may be used inside lighting fixtures or similar equipment where they are enclosed and not subject to bending or twisting, and for connecting lighting fixtures to branch-circuit conductors; fixture wires may not be used as branch-circuit conductors except where permitted for certain limited circuits (Class 1 power-limited and fire alarm circuits). See 1910.305(i)(1), 1910.305(i)(2), and 1910.305(i)(3).

15- and 20-amp attachment plugs and connectors must have no exposed current-carrying parts except the prongs/blades/pins and must be dead-front in construction; attachment plugs must not be energized unless inserted into an energized receptacle, and receptacles/cord connectors must not accept a plug with a different voltage or current rating than intended (with a limited exception for a 20‑amp T-slot receptacle accepting a 15‑amp plug of the same voltage). See 1910.305(j)(2)(i) and 1910.305(j)(2)(ii).

No. Nongrounding-type receptacles and connectors may not be used for grounding-type attachment plugs; grounding-type plugs require compatible grounding-type receptacles. That prohibition is in 1910.305(j)(2)(iii).

A receptacle in wet or damp locations must be suitable for the location, and the required weatherproof enclosure depends on whether the receptacle is exposed to weather and whether the plugged-in equipment will be attended while in use.

• A receptacle installed in a wet or damp location must be suitable for that location (1910.305(j)(2)(iv).
• A receptacle outdoors but protected from the weather (for example, under a roofed porch or canopy) or in other damp locations must have an enclosure that is weatherproof when the receptacle is covered (plug not inserted and cover closed) (1910.305(j)(2)(v)). The standard’s note explains that “protected from the weather” includes locations like roofed porches where the receptacle will not be exposed to driving rain or runoff.
• If the receptacle is in a wet location and the product plugged into it is not attended while in use (examples: sprinkler controllers, landscape or holiday lighting), the enclosure must be weatherproof even with the attachment plug cap inserted or removed (1910.305(j)(2)(vi)).
• If the receptacle is in a wet location but the product plugged into it will be attended while in use (examples: portable power tools), the enclosure must be weatherproof when the attachment plug cap is removed (i.e., cover closed when not plugged in) (1910.305(j)(2)(vii)).

Practical tip: choose receptacle and cover assemblies rated for the exact exposure and expected use (attended vs. unattended) and follow the examples in the standard to ensure you select the correct weatherproof enclosure.

Each appliance must prevent exposure to live parts (except necessary heating elements), have a means to disconnect it from all ungrounded conductors, and be marked with an accessible nameplate showing identifying name and electrical rating.

• Live parts of appliances must not be normally exposed to contact, except for parts necessarily exposed for open-resistance heating (for example, a toaster element) (1910.305(j)(3)(i)).
• Each appliance must have a means to disconnect it from all ungrounded conductors; if the appliance is supplied by more than one source, the disconnecting means must be grouped and identified (1910.305(j)(3)(ii)).
• Appliances must have a nameplate showing the identifying name and the rating in volts and amperes (or volts and watts); frequency and external motor-overload protection information must be marked where applicable, and the marking must be visible or easily accessible after installation (1910.305(j)(3)(iii)–(iv)).

Practical tip: ensure disconnects are clearly labeled and accessible and that nameplates remain visible after installation to help maintenance and emergency responders quickly evaluate and isolate appliances.

An individual disconnecting means required to be “within sight of” the controller must be visible and not more than 15.24 m (50 ft) from the controller.

• The standard defines the phrase: equipment required to be “within sight of” another piece of equipment must be visible and located not more than 15.24 m (50.0 ft) away (1910.305(j)(4)(i)).
• Paragraph (j)(4) also requires an individual disconnecting means for each controller and that the disconnecting means be located within sight of the controller (1910.305(j)(4)(ii)).

Note on temporary energized work: if work requires temporary re-energization for testing or positioning, follow the lockout/tagout sequencing and employee-protection steps in the Control of Hazardous Energy standard; OSHA’s interpretation of 1910.147 explains that temporary re-energization is limited and must be done with protections in place (OSHA letter on minor servicing exception).

Practical tip: place and label disconnects so operators can see them from the controller location and follow lockout/tagout procedures whenever isolating/re-energizing motors for service.

A single disconnecting means may be used for a group of motors only when one of three specific conditions in the standard is met.

• A single disconnect is allowed if several motors drive parts of a single machine or piece of apparatus (for example, multiple motors on a single metalworking machine, crane, or hoist) (1910.305(j)(4)(vi)(A)).
• A single disconnect is allowed if the group of motors is protected by one set of branch-circuit protective devices (they share the same branch-circuit protection) (1910.305(j)(4)(vi)(B)).
• A single disconnect is allowed if the group of motors is located in a single room that is within sight of the disconnecting means (1910.305(j)(4)(vi)(C)).

Practical tip: do not assume a grouped disconnect is allowed — confirm one of the three conditions applies and document the rationale so maintenance staff know how to isolate the motors safely.

Indoor transformers with exposed live parts must have operating voltage indicated by markings, and certain transformers must be in vaults; vaults must contain fires and liquids, prevent unauthorized access, and not allow foreign pipe or duct systems to pass through. Combustible materials must be safeguarded from oil-insulated transformer fires and transformer vaults must not be used for storage.

• The operating voltage of exposed live parts of transformer installations must be indicated by signs or visible markings on the equipment or structure (1910.305(j)(5)(ii)).
• Dry-type, high fire point liquid-insulated, and askarel-insulated transformers installed indoors and rated over 35 kV must be installed in a vault (1910.305(j)(5)(iii)). Oil-insulated transformers installed indoors must be in a vault as well (1910.305(j)(5)(iv)).
• Combustible material and nearby building parts (including fire escapes and openings) must be safeguarded from fires that may originate in oil-insulated transformers (1910.305(j)(5)(v)).
• Transformer vaults must be constructed to contain fire and combustible liquids within the vault and to prevent unauthorized access; vault doors must be arranged to open readily from the inside (1910.305(j)(5)(vi)).
• No foreign pipe or duct system (other than those for vault fire protection or cooling) may enter or pass through a transformer vault (1910.305(j)(5)(vii)).
• Material may not be stored in transformer vaults (1910.305(j)(5)(viii)).

Practical tip: keep transformer rooms and vaults free of stored materials, post voltage markings clearly, and ensure vault construction and access meet the standard to protect personnel and property.

Capacitors on circuits over 600 volts must use group-operated switches that meet high current and inrush requirements, have means to isolate units from all voltage sources with a visible gap, and use sequenced switching, interlocks, or posted procedures for series capacitor installations.

• Switching for capacitor banks must be group-operated and capable of carrying at least 135% of the rated current, interrupting the maximum continuous load current, withstanding maximum inrush currents (including contributions from adjacent capacitors), and carrying fault currents on the capacitor side of the switch (1910.305(j)(6)(ii)(A)(1)–(4)).
• There must be a means to isolate each capacitor, capacitor bank, or capacitor installation that will be removed from service as a unit from all sources of voltage (1910.305(j)(6)(ii)(B)). The isolating means must provide a visible gap adequate for the operating voltage (1910.305(j)(6)(ii)(C)).
• Isolating or non–interrupting-rated disconnects must be interlocked with load-interrupting devices or have prominent caution signs to prevent switching under load (1910.305(j)(6)(ii)(C)).
• For series capacitor arrangements, proper switching must be assured by at least one of these methods: mechanically sequenced isolating and bypass switches, interlocks, or a prominently displayed switching procedure at the switching location (1910.305(j)(6)(ii)(D)(1)–(3)).

Practical tip: when working on high-voltage capacitors, confirm the group-operated switch ratings, verify visible isolation, follow interlocks or written switching procedures, and treat the equipment as energized until capacitors have been properly discharged and isolated.

Storage battery installations must be arranged so gases generated can diffuse and ventilate sufficiently to prevent the accumulation of explosive mixtures.

• The standard requires provisions for sufficient diffusion and ventilation of gases from storage batteries to prevent the accumulation of explosive mixtures (1910.305(j)(7)).

Practical tip: design battery rooms with forced or natural ventilation sized to remove hydrogen and other gases, follow battery manufacturer guidance for ventilation rates, and avoid storing combustible materials near battery installations.