Equipment Grounding Using Steel Conduit
Steel conduit as equipment grounding conductor
Steel RMC, IMC and EMT are recognized by the NEC® as equipment grounding conductors. Using a supplemental equipment grounding conductor 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 areas in NEC 517.13. Steel conduit is the main equipment grounding conductor regardless of whether a supplemental equipment grounding conductor 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 path to ground in circuits, equipment and metal enclosures for conductors shall be permanent and continuous. Complying with guidelines in the Fittings section 4.3 and Support section 4.4 is the 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, which would result in the failure to carry a ground fault. Good installation workmanship is critical.
The NEC further requires that the path to ground have the capacity to safely conduct any fault current likely to be imposed and have sufficiently low impedance to limit the voltage to ground to cause operation of the circuit protective device. 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 conduit / EMT
Copper, aluminum and copper-clad aluminum equipment grounding conductors must be sized according to NEC Table 250.122. Just as with these types of ”wire” equipment grounding conductor, conduit runs and couplings must be properly sized. The installed length of any wiring method will impact the operation of the overcurrent device. In the event of a phase-to-neutral or phase-to-conduit ground fault, the length of the particular 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 steel RMC, IMC and EMT.
Threads must be clean 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 rust-resistant coating (see 4.1.3).
Continuity of the raceway system
The NEC does not permit certain circuits to be grounded. However, steel raceways and all metal parts likely to become energized must still have ensured continuity and be bonded together and run to a grounding electrode to prevent electric shock.
Bonding is used to provide electrical continuity so that overcurrent devices will operate and shock hazards will not be present. This is the “finishing touch” for a metallic raceway system, and close attention is to be paid to detail. All fittings, lugs, etc., shall be securely made up.
Bonding around steel raceway joints / couplings is not necessary when EMT, IMC and RMC are properly made up as recommended in this installation guideline. A secure joint provides excellent low impedance continuity. Bonding is not required because this joint already meets the NEC definition of bonding.
Metal raceways for feeder and branch circuits operating at less than 250 volts to ground shall be bonded to the box or cabinet. Do one or more of the following:
- Use listed fittings.
- For steel RMC or IMC, use two locknuts: one inside and one outside of boxes and cabinets.
- Use fittings, such as EMT connectors, with shoulders that seat firmly against the box or cabinet, with one locknut on the inside of boxes and cabinets.
NOTE: Remove paint in locknut areas to ensure a continuous ground path. Repaint or cover any exposed area after installation is completed.
Service raceway system bonding
A service raceway system includes service equipment enclosures, meter fittings, boxes, etc., and requires special consideration for bonding the enclosures to the raceways where the connection relies on locknuts only. Service equipment must be connected with threaded bosses and fittings such as locknuts, wedges, and bushings of the bonding type.
Standard locknuts are not to be used on circuits over 250 volts to ground where 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 systems; or boxes and enclosures listed for bonding are to be used.
Additional bonding considerations
Expansion fittings and telescoping sections of metal raceways shall be listed for grounding or shall be made electrically continuous by the use of equipment bonding jumpers or other suitable means in accordance with NEC 250.98.