Special electrical systems requirements

Aboveground conductors over 600 volts must be installed in approved raceways or as approved open runs listed in the rule. Allowed methods include rigid metal conduit, intermediate metal conduit, electrical metallic tubing, rigid nonmetallic conduit, cable trays, busways, cablebus, other identified raceways, or open runs of metal-clad cable; in locations accessible only to qualified persons, open runs of Type MV cable, bare conductors, and bare busbars are also permitted. See the specific list in 1910.308(a)(1)(i).

Conductors that emerge from the ground must be enclosed in an approved raceway. This means you cannot leave them exposed where they leave the earth; install them in a conduit or other approved enclosure as required by 1910.308(a)(1)(ii).

The braid on open runs of braid-covered insulated conductors must be flame retardant or treated with a flame-retardant saturant after installation. The treated braid must then be stripped back a safe distance at conductor terminals depending on operating voltage. See 1910.308(a)(2).

Yes. Metallic shielding components such as tapes, wires, or braids, and their associated conducting and semiconducting components, must be grounded. The rule requires grounding of metallic shielding to prevent hazardous potentials on shield members; see 1910.308(a)(3)(ii).

A cable sheath terminating device must be used where cable conductors emerge from a metal sheath and protection against moisture or physical damage is necessary, so the conductors’ insulation is protected at the termination. See 1910.308(a)(4).

Indoor circuit breaker installations must be metal-enclosed units or fire‑resistant cell‑mounted units; open mounting is allowed only in locations accessible to qualified employees, and breakers must have an indicator showing open/closed position. See 1910.308(a)(5)(i).

When fuses protect conductors and equipment, a fuse must be placed in each ungrounded conductor. If two power fuses are used in parallel to protect the same load, they must be identical, mounted together in a common identified mounting, and wired so the current divides equally. Also note restrictions on vented power fuses indoors or in metal enclosures unless listed for that use. See 1910.308(a)(5)(ii).

Fused cutouts installed indoors or in transformer vaults must be of a type identified for that purpose and must be readily accessible for fuse replacement; distribution cutouts may not be used indoors, underground, or in metal enclosures. If fused cutouts cannot manually interrupt the circuit while carrying full load, an approved means must be installed to interrupt the entire load, and unless interlocked, a conspicuous sign reading "WARNING – DO NOT OPERATE UNDER LOAD" must be posted. See 1910.308(a)(5)(iii) and 1910.308(a)(5)(iv).

When load interrupter switches are used with other devices, they must be electrically coordinated to safely withstand closing, carrying, or interrupting currents up to the assigned maximum short‑circuit rating; and where multiple switches provide alternate supply connections, each switch must have a conspicuous sign stating "WARNING – SWITCH MAY BE ENERGIZED BY BACKFEED." See 1910.308(a)(5)(vi)(A) and 1910.308(a)(5)(vi)(B).

Employers must provide a means (for example a fuseholder and fuse designed for that purpose) to completely isolate equipment for inspection and repairs; isolating means not designed to interrupt load current must be interlocked with an approved interrupter or be posted with a warning against opening under load. See 1910.308(a)(5)(vii).

Mobile machines must have a metallic enclosure that houses cable terminals and provides a solid grounding connection to the machine frame; cable terminations must prevent strain. All energized switching and control parts must be enclosed in effectively grounded metal cabinets, with operating means projecting through the cabinet so breakers/protective devices can be reset without opening locked doors. Enclosures must be locked and marked to restrict access to authorized qualified persons. See 1910.308(a)(6)(i) and 1910.308(a)(6)(ii).

Conductors in tunnels must be installed in metal conduit or other metal raceway, Type MC cable, or other approved multiconductor cable; they must also be located or guarded to protect them from physical damage, and an equipment grounding conductor must run with circuit conductors inside the raceway or cable jacket. See 1910.308(a)(7)(i) and 1910.308(a)(7)(iii).

All nonenergized metal parts of electrical equipment, metal raceways, and cable sheaths in tunnels must be effectively grounded and bonded to metal pipes and rails at the portal and at intervals not exceeding 305 meters (1000 feet) throughout the tunnel. See 1910.308(a)(7)(viii).

Emergency circuits must be kept entirely independent of other wiring and equipment and may not share raceways, boxes, or cabinets with other circuits except where common elements are required or for transferring power from normal to emergency sources. Emergency illumination must cover means of egress, exit signs, and other required lights, and must be arranged so that failure of an individual lighting element cannot leave a space completely dark. See 1910.308(b)(1) and 1910.308(b)(2).

A sign must be posted at the service entrance equipment identifying the type and location of on-site emergency power sources (except for individual unit equipment), and if the grounded circuit conductor from an emergency source is connected to a grounding electrode conductor at a remote location, a sign must identify all emergency and normal sources connected there. See 1910.308(b)(3)(i) and 1910.308(b)(3)(ii).

Class 1, Class 2, and Class 3 circuits are classified by voltage and power limits: a Class 1 power‑limited circuit is supplied by a source not exceeding 30 volts and 1000 VA; Class 1 remote control or signaling circuits may have voltages up to 600 volts (with no power limit); and Class 2/3 sources must be listed equipment marked for the class (with limited exceptions). See 1910.308(c)(1)(i), 1910.308(c)(1)(ii), and 1910.308(c)(1)(iii).

A Class 2 or Class 3 power supply unit must be durably marked to show its class and rating where plainly visible, and Class 2/3 circuit conductors may not share enclosures or raceways with light, power, Class 1, nonpower-limited fire alarm, or certain broadband communications conductors unless a barrier or equivalent protection is provided. See 1910.308(c)(2) and 1910.308(c)(3).

Fire alarm circuits must be classified as nonpower‑limited (NPLFA) or power‑limited (PLFA); NPLFA power sources shall have output voltages not exceeding 600 volts nominal, and PLFA power sources must be listed and marked as PLFA. Power‑limited fire alarm conductors may not share raceways with other power or certain communications circuits and must be separated by at least 50.8 mm (2 in.) from power, Class 1, nonpower-limited fire alarm, or medium‑power broadband conductors unless an equally protective method is used. See 1910.308(d)(2)(i), 1910.308(d)(2)(ii), and 1910.308(d)(3)(iii).

Fire alarm circuits must be identified at terminal and junction locations in a way that prevents accidental interference during testing and servicing; power‑limited fire alarm circuits must be durably marked where plainly visible at terminations. See 1910.308(d)(4).

Communications systems need not comply with many other electrical provisions except they must meet the protective device rule that requires a listed primary protector on each aerial circuit run and on circuits exposed to accidental contact with power conductors over 300 volts to ground; aerial-drop cables must be kept away from power conductors and maintain at least 1.83 m (6 ft) from lightning conductors; indoor communications cables must be separated 50.8 mm (2 in.) from certain power conductors; and metal sheaths of aerial cables entering buildings exposed to power conductors must be grounded or interrupted by an insulating joint. See 1910.308(e)(1)(i), 1910.308(e)(2)(i), 1910.308(e)(2)(ii), and 1910.308(e)(4)(i).

Photovoltaic source and output circuits may not be contained in the same raceway, cable tray, cable, outlet box, junction box, or similar fitting as feeders or branch circuits of other systems unless separated by a partition or connected together. A disconnecting means must be provided to disconnect all current‑carrying PV power source conductors from all other conductors in a building or structure; and if the grounding connection is not automatically interrupted, a disconnecting switch or breaker must not have a pole in the grounded conductor. See 1910.308(f)(1) and 1910.308(f)(2).

An integrated electrical system may have overcurrent devices that are not readily accessible to employees if the system is unitized, orderly shutdown minimizes hazards, only qualified persons service it, effective safeguards are maintained, and mounting heights secure devices from operation by nonqualified persons; see the definition and 1910.308(g)(1)-(2). Overcurrent devices critical to such integrated systems can be located at heights ensuring security from nonqualified persons (see 1910.304(f)(1)(iv) as referenced in 1910.308(g)(2)).