9.1 System Grounding, 250.24 and the Main Bonding Jumper
Key Takeaways
- NEC 250.4(A)(5) requires an effective ground-fault current path that is permanent, electrically continuous, capable of safely carrying the maximum fault likely to be imposed, and of sufficiently low impedance to facilitate operation of the overcurrent device.
- NEC 250.4(A)(5) also states that the earth shall not be considered as an effective ground-fault current path.
- NEC 250.24(A)(1) permits the grounding electrode conductor connection at any accessible point from the load end of the service conductors to and including the terminal or bus where the grounded service conductor terminates at the service disconnecting means.
- NEC 250.24(A)(5) prohibits reconnecting the grounded conductor to equipment grounding conductors, enclosures, or ground on the load side of the service disconnecting means.
- NEC 250.28(D)(2) requires the main bonding jumper to be not less than 12.5 percent of the area of the largest phase conductor where the service-entrance conductors exceed 1100 kcmil copper or 1750 kcmil aluminum.
Grounding and Bonding Are Different Jobs
Grounding connects a system or equipment to earth. Bonding connects conductive parts to each other so they are at the same potential and can carry fault current. The Code's own performance statement in 250.4(A) distinguishes five functions:
- Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
- Grounding of Electrical Equipment. Normally non-current-carrying conductive materials enclosing electrical conductors or equipment shall be connected to earth so as to limit the voltage to ground on these materials.
- Bonding of Electrical Equipment. Normally non-current-carrying conductive materials shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
- Bonding of Electrically Conductive Materials and Other Equipment. Metal piping, structural steel, and similar materials likely to become energized shall be connected together and to the supply source.
- Effective Ground-Fault Current Path. Electrical equipment and wiring shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.
That last sentence is the single most important idea in Article 250 and appears constantly on exams. Earth is not a fault path. A ground rod does not clear a fault; it cannot. Soil resistance of 25 ohms on a 120-volt system passes 4.8 amperes — not enough to move a 20-ampere breaker. The metallic bonding path back to the source clears faults; the electrode handles lightning, surges, and voltage stabilization.
250.4(B) gives the parallel performance requirements for ungrounded systems.
Systems Required to Be Grounded — 250.20
250.20(B) Alternating-Current Systems of 50 Volts to 1000 Volts. AC systems of 50 volts to 1000 volts supplying premises wiring shall be grounded under any of the following conditions:
- Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts — this captures 120/240 V and 208Y/120 V;
- Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor — 480Y/277 V and 208Y/120 V;
- Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor — the high-leg delta.
250.21 covers AC systems of 50 to 1000 volts not required to be grounded, and requires ground detectors on ungrounded systems operating at 120 volts or more where the system is 1000 volts or less.
Service Grounding Connections — 250.24
250.24(A)(1) General. The grounding electrode conductor connection shall be made at any accessible point from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.
So the GEC may land in the meter socket, in a wireway ahead of the disconnect, or in the service disconnect enclosure — anywhere in that span.
250.24(A)(2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.
250.24(A)(3) Dual-Fed Services. For services that are dual fed in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single grounding electrode connection to the tie point of the grounded conductors from each power source shall be permitted.
250.24(A)(4) Main Bonding Jumper as Wire or Busbar. Where the main bonding jumper is a wire or busbar and is installed from the grounded conductor terminal bar or bus to the equipment grounding terminal bar or bus in the service equipment, the grounding electrode conductor shall be permitted to be connected to the equipment grounding terminal bar or bus.
250.24(A)(5) Load-Side Grounding Connections. "A grounded conductor shall not be connected to normally non-current-carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the service disconnecting means" except as otherwise permitted in Article 250.
This is the rule behind isolated neutrals in every subpanel.
250.24(B) Main Bonding Jumper. For a grounded system, an unspliced main bonding jumper shall be used to connect the equipment grounding conductor(s) and the service disconnect enclosure to the grounded conductor within the enclosure for each service disconnect.
250.24(C) Grounded Conductor Brought to Service Equipment. Where an AC system is grounded at any point, the grounded conductor shall be routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means grounded conductor terminal or bus. The grounded conductor shall not be smaller than the size specified in Table 250.102(C)(1).
250.24(D) Grounding Electrode Conductor. A grounding electrode conductor shall be used to connect the equipment grounding conductors, the service-equipment enclosures, and, where the system is grounded, the grounded service conductor to the grounding electrode(s).
Why the Bond Exists at Exactly One Point
At the service, the grounded conductor and the equipment grounding system are joined by the main bonding jumper. Downstream of that point they must remain separate. The reason is a matter of current paths.
If a subpanel's neutral bar is also bonded to its enclosure, then normal load current returning on the neutral finds two parallel paths back to the service: the neutral conductor and the equipment grounding conductor plus every bonded metal part — raceway, water pipe, structural steel, gas line. Objectionable current flows on equipment that is supposed to be at zero potential. 250.6 addresses this directly as "objectionable current" and requires the arrangement to be altered.
The correct configuration is:
| Location | Neutral bonded to enclosure? |
|---|---|
| Service disconnect | Yes — main bonding jumper, 250.24(B) |
| Separately derived system | Yes — system bonding jumper at one point, 250.30(A)(1) |
| Any subpanel in the same building | No — 250.24(A)(5) and 250.142(B) |
| Panel in a separate structure fed by a feeder | No — 250.32(B)(1) |
Main and System Bonding Jumper Sizing — 250.28
250.28(A) Material. Main bonding jumpers and system bonding jumpers shall be of copper, aluminum, copper-clad aluminum, or other corrosion-resistant material. A bonding jumper shall be a wire, bus, screw, or similar suitable conductor.
250.28(B) Construction. Where a bonding screw is used, it shall be identified with a green finish that shall be visible with the screw installed.
250.28(C) Attachment. Attached in the manner specified by the applicable provisions of 250.8.
250.28(D)(1) General. Main bonding jumpers and system bonding jumpers shall not be smaller than the sizes shown in Table 250.102(C)(1).
250.28(D)(2) Main Bonding Jumper for Service with More Than One Enclosure. Where the service-entrance phase conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper shall have an area not less than 12.5 percent of the area of the largest phase conductor, except that where the phase conductors and the bonding jumper are of different materials, the minimum size of the bonding jumper shall be based on the assumed use of phase conductors of the same material as the bonding jumper and with an ampacity equivalent to that of the installed phase conductors.
250.28(D)(3) Separately Derived System with More Than One Enclosure. Where a separately derived system supplies more than one enclosure, the system bonding jumper for each enclosure shall be sized in accordance with 250.102(C)(1) based on the largest ungrounded feeder conductor serving that enclosure, or a single system bonding jumper shall be installed at the source and sized in accordance with 250.102(C)(1) based on the equivalent size of the largest supply conductor.
Worked Example — the 12.5 Percent Rule
A 480 V service is supplied by three parallel runs of 600 kcmil copper per phase, for 1800 kcmil per phase. What is the minimum copper main bonding jumper?
1800 kcmil exceeds 1100 kcmil, so the 12.5 percent rule applies.
1800 x 0.125 = 225 kcmil
Next standard conductor size above 225 kcmil = 250 kcmil copper
250 kcmil copper. Note that the calculation uses the total circular mil area per phase across all parallel conductors, not the size of one conductor.
Worked Example — Table 250.102(C)(1)
A 200-ampere service uses 3/0 AWG copper service-entrance conductors. What size copper main bonding jumper is required?
From Table 250.102(C)(1), service-entrance conductors of 2/0 or 3/0 copper require a bonding jumper of 4 AWG copper.
Objectionable Current — 250.6
250.6(A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, surge-protective devices, and conductive normally non-current-carrying metal parts of equipment shall be installed and arranged in a manner that will prevent objectionable current.
250.6(B) Alterations to Stop Objectionable Current. Where objectionable current exists, one or more of the following alterations shall be permitted: discontinue one or more but not all of such grounding connections; change the locations of the grounding connections; interrupt the continuity of the conductor or conductive path causing the objectionable current; or take other suitable remedial and approved action.
250.6(C) Temporary Currents Not Classified as Objectionable Currents. Temporary currents resulting from abnormal conditions, such as ground faults, shall not be classified as objectionable current.
250.6(D) Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment being operated on AC systems or branch circuits that are not connected to an equipment grounding conductor as required by Article 250. Currents that introduce noise or data errors in electronic equipment are not considered objectionable currents for the purposes of this section.
That last point is worth remembering: "noise" is never a legal justification for lifting a ground.
According to NEC 250.4(A)(5), what role does the earth play in clearing a ground fault?
A 480-volt service uses two parallel 750 kcmil copper conductors per phase. What minimum copper main bonding jumper is required under NEC 250.28(D)(2)?
Where does NEC 250.24(A)(1) permit the grounding electrode conductor connection to be made at a service?
Under NEC 250.6(D), is electrical noise affecting electronic equipment considered objectionable current that justifies removing a grounding connection?