Equipment Grounding Using Steel Conduit
Steel conduit as equipment grounding conductor
Steel and stainless steel RMC, IMC, and EMT are recognized by the National Electrical Code® as equipment grounding conductors (EGCs). Using a supplemental EGC in the form of a copper, aluminum, or copper-clad aluminum conductor in addition to the raceway is a design decision, except where the NEC® requires it in some specific installations such as patient care spaces (517.13). The conduit is the primary fault current path regardless of whether a supplemental wire-type EGC is installed. In the event of a fault, the raceway will carry most of the current and therefore must be continuous. For this reason, each raceway must be installed securely and with tight joints to provide mechanical and electrical continuity.
Continuity of grounding path
The NEC states that the fault current path to for circuits, equipment, and metal enclosures for conductors must be continuous and have low impedance. Complying with the guidelines in the Fittings section 4.3 and Support section 4.4 is a major factor in maintaining electrical continuity. Using less than the NEC-required supports or failing to properly tighten joints can cause discontinuity in a raceway system. This would cause an increase in the impedance of the equipment grounding circuit and result in energized metal parts of the installation. Good installation workmanship is critical.
The NEC further requires that the ground fault current path have the capacity to safely conduct any fault current likely to be imposed and have sufficiently low impedance to initiate operation of the circuit protective device, such as a fuse or circuit breaker. Steel RMC, IMC, and EMT are conductors permitted to carry current in the event of a ground fault. All three have been tested, and they all meet the NEC requirements when properly designed and installed (see Annex B).
Maximum length of steel raceways
Copper, aluminum, and copper-clad aluminum equipment grounding conductors of the wire type must be sized according to NEC 250.122. Just as with these types of wire-type EGCs, conduit installations must be properly “sized.” The installed length of any wiring method will impact the operation of the overcurrent protective device. In the event of a phase-to-neutral or phase-to-conduit ground fault, the length of the conduit run determines safe operation, assuming proper overcurrent protection has been provided. For a phase-to-phase fault, it is the conductor length that determines safe operation. See Annex B for tables that show examples of the maximum run lengths for RMC, IMC, and EMT.
Clean threads
Threads must be clean and undamaged to ensure electrical continuity of the assembled raceway system. Leave the thread protectors on the conduit until ready to use. Wipe field-cut threads with a clean cloth to remove excess oil, and apply an electrically conductive corrosion-resistant coating (see 4.1.3).
Bonding
Bonding is used to provide electrical continuity so that overcurrent protective devices will operate and remove shock hazards. This is the “finishing touch” for a metal raceway system, and close attention is to be paid to detail. All fittings, locknuts, terminations, lugs, and similar, must be made secure.
Bonding around steel raceway joints / couplings is not necessary when EMT, IMC, or RMC are properly made up as recommended in this installation guideline. A secure joint provides excellent low impedance continuity. Bonding is not required around fittings because fittings are tested for grounding and bonding performance.
Metal raceways for feeder and branch circuits operating at less than 250 volts to ground must be bonded to the enclosure. Do one or more of the following:
- Use listed fittings.
- For RMC or IMC, use two locknuts: one inside and one outside the enclosure.
- Use connectors with shoulders that seat firmly against the enclosure, with a locknut on the inside of the enclosure.
NOTE: Remove paint at connections to enclosures, unless installing the locknut removes the paint.
Service raceway bonding
A raceway containing service conductors may not rely on locknuts alone. Service raceways must be connected to enclosures with threaded bosses or with fittings such as bonding-type locknuts, wedges, and bonding bushings.
For circuits over 250 volts to ground, additional bonding is required if the raceway is terminated at concentric or eccentric knockouts. The raceway must be bonded to the enclosure using the same methods as noted above for service raceway; or enclosures listed for bonding are to be used.
Additional bonding considerations
Expansion fittings and telescoping sections of metal raceways must be listed for grounding or be made electrically continuous by the use of equipment bonding jumpers or other suitable means in accordance with NEC 250.98.