Section 8: Grounding and Bonding

8.1 Effective Ground-Fault Current Path
8.2 Separately Derived Systems
8.3 Grounding Separately Derived Alternating-Current Systems
8.4 Buildings or structures supplied by Feeder(s) or Branch Circuit(s)
8.5 Grounding Electrode System
8.6 Grounding Electrodes
8.7 Grounding Electrode Conductor Installation
8.8 Size of Alternating-Current Grounding Electrode Conductor
8.9 Types of Equipment Grounding Conductors
8.10 Size of Equipment Grounding Conductors

8.1 Effective Ground-Fault Current Path
Proper materials & techniques are required for an effective ground fault path.

When a ground-fault occurs and metal equipment is energized, the circuit must be de-energized as quickly as possible.  If metal equipment remains energized, it is a deadly trap waiting for someone to come in contact with it.

The safest way to deal with a ground-fault is to trip the overcurrent device that supplies the faulted circuit.

In order to trip the circuit breaker, the gound-fault current path must be a low-impedance circuit.  An effective ground fault current path must have lower impedance than any other available path that fault current might take such as building steel or water pipes.  A low-impedance (AC resistance) path means the fault current will be high enough to trip the overcurrent device.  If the fault-current path is not low-impedance, the fault-current will be too low to trip the circuit breaker, but high enough to kill a person.

An effective ground-fault current path must be capable of carrying the maximum ground-fault current that it is likely to see.  When equipment grounding conductors are sized according to NEC Table 250.122 and grounding electrode conductors are sized according to Table 250.66, they are large enough to carry likely levels of fault current.

NEC Table 250.122

NEC Table 250.122


NEC Table 250.66

NEC Table 250.66

Review Question Section 8.1

45. Which of the following statements best describes an effective ground-fault current path?

8.2 Separately Derived Systems

Separately derived systems include solar photovoltaic systems, wind turbines, and fuel cells, but by far the most common separately derived systems are supplied by generators and transformers.

Just like services, separately derived systems are grounded at the source of the separately derived system, but not after the first disconnecting means.  For both transformers and generators, the neutral on the secondary (derived) side of the separately derived system is connected to the grounding electrode, equipment grounding conductors, and the metal enclosure.  A system bonding jumper connects the grounded conductor to the supply-side bonding jumper and the normally non-current-carrying metal enclosure.  If the first disconnecting means and the source are in separate enclosures, a supply-side bonding jumper is installed with the circuit conductors from the source enclosure to the first disconnecting means.

NEC article 250.30(A)(4) requires the grounding electrode to be as near as practicable to the location where the grounding electrode conductor is connected to the system.  The grounding electrode conductor for a separately derived system is selected from NEC Table 250.66.

Informational Note No. 1 of NEC article 250.30 says that an on-site generator is not a separately derived system if the grounded conductor from the generator is solidly connected to the service-supplied grounded conductor.  If the transfer switch for the generator does not switch the grounded conductor from the generator then the generator is not connected as a separately derived system.

Review Question Section 8.2

46. What is the minimum size copper grounding electrode conductor required for the connection to a structural steel electrode, when the secondary conductors are 4/0 copper?

A transformer is the most common separately derived system.
8.3 Grounding Separately Derived Alternating-Current Systems
Transformers are permitted to be grounded at the transformer or at the first disconnect but NOT both.

A separately derived system is an electrical source, other than at a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.

The most common separately derived system is a transformer.  The requirements for grounding separately derived systems are specified in NEC article 250.30.

Transformers may be grounded either at the transformer or at the first disconnecting means, but not at both.  However, most installations are bonded and grounded using a system bonding jumper to connect the grounding electrode conductor, the transformer’s derived neutral conductor, and the equipment grounding conductor at the transformer rather than at the first disconnect.

A system bonding jumper is used to connect the grounded circuit conductor to the equipment grounding conductor of a separately derived system.

System bonding jumpers are permitted to be a bus, screw, or wire terminated by listed pressure connectors, listed clamps, exothermic welding, or other listed means.  If the system bonding jumper is a wire its size is based on the size of the largest secondary phase conductor and is selected from NEC Table 250.102(C)(1).

For example, using NEC Table 250.102(C)(1), a transformer with 250 kcmil CU secondary phase conductors requires a No. 2 AWG CU system bonding jumper.

Table 250_102_C_1

Review Question Section 8.3

47. The secondary phase conductors of a separately derived system are 500 kcmil CU. What is the minimum size copper system bonding jumper?

8.4 Buildings or Structures Supplied by Feeder(s) or Branch Circuit(s)

NEC article 250.32 requires that a grounding electrode or grounding electrode system must be installed at separate structures or at a building supplied with a feeder from another building.  An exception permits the grounding electrode to be omitted at a building or structure if it is supplied with a single branch circuit or a single multiwire branch circuit.

A feeder or branch circuit supplying a separate building or structure must include an Equipment Grounding Conductor (EGC) run with the supply conductors and the equipment grounding conductor is required to be connected to the disconnecting means and to the grounding electrode(s) at the separate building or structure.

The size of the EGC is based on the size of the Overcurrent Protective Device protecting the feeder or branch circuit that supplies the structure and is sized from NEC table 250.122.

For example, if a feeder is protected by a 40 amp fuse, the minimum size Equipment Grounding Conductors is a No. 10 AWG CU or a No. 8 AWG AL conductor.  If the OCPD is a 400 amp circuit breaker, the minimum size Equipment Grounding Conductor is a No. 3 CU or a No. 1 AL conductor.

NEC Table 250.122

NEC Table 250.122

Review Question Section 8.4

48. An equipment grounding conductor is run with the feeder conductors to serve an accessory building. The feeder circuit breaker is 70 amperes. What is the minimum size CU equipment grounding conductor?

A feeder to a separate building must include an equipment grounding conductor. A grounding electrode system must be installed at the separate building.
8.5 Grounding Electrode System
All grounding electrodes and metal water pipe are required to be bonded together to form the grounding electrode system.

All grounding electrodes present at a building must be bonded together to form a grounding electrode system.  Individual grounding electrodes are tied together so they are no longer separate electrodes, but part of a grounding electrode system.  A system of grounding electrodes has less resistance than a single electrode and provides a better ground.

Grounding electrodes include metal water pipe, the metal frame of a building, concrete-encased electrodes (such as rebar), ground ring, rod, pipe, and plate electrodes, and other local metal underground systems or structures.

Some state electrical boards have modified this section to say grounding electrodes are required to be bonded to other grounding electrodes if they are available, not if they are present.  For these jurisdictions, if a concrete-encased electrode is buried in the concrete and not available as a grounding electrode, it does not need to be included in the grounding electrode system.

Review Question Section 8.5

49. How is a grounding electrode system created at a building or structure?

8.6 Grounding Electrodes

NEC article 250.52 lists the following eight different types of materials and structures permitted to be used as grounding electrodes:

  1.   Metal Underground Water Pipe in direct contact with the earth for at lease 10-feet.  Except for industrial locations, the connection to the water pipe must be made within 5-feet of where the pipe enters a structure.
  2.   Metal Frame of the Building or Structure that is in direct contact with the earth for 10-feet or more or that is encased in concrete in direct contact with the earth, or hold-down bolts securing the structural steel column that are connected to a concrete-encased electrode and is located in the support footing or foundation.
  3.   Concrete-Encased Electrodes consisting of at least 20-feet of either (1) bare or zinc galvanized or other electrically conductive coated steel reinforcing rods of not less than ½ inch or (2) bare copper conductor not smaller than 4 AWG encased in 2-inches of concrete in direct contact with earth, or within vertical foundations or structural components in direct contact with the earth.
  4. Ground Rings that encircle the building or structure;  the minimum length is 20 ft.; the minimum size is No. 2 AWG bare copper.
  5.   Rod and Pipe Electrodes at least 8-feet long.  The minimum trade size of galvanized pipe or conduit is ¾-inch.  The minimum diameter of zinc coated steel rods is 5/8-inch and, the minimum diameter of listed stainless steel and copper rods is ½-inch.
  6.   Other Listed Electrodes permitted include Chemical Ground Electrodes.
  7.   Plate Electrodes at least 2-feet square; the minimum thickness of steel or iron electrodes is ¼-inch; the minimum thickness for nonferrous electrodes is .06-inch.
  8. Other Local Metal Underground Systems or Structures like underground piping systems, metal tanks, and well casings that are not bonded to a  metal water pipe.

Chemical Ground Rod installed in service well.

Chemical Ground Rod installed in service well.

Review Question Section 8.6

50. What is the minimum length of a pipe or rod electrode?

Structural metal that may become energized is required to be bonded to the grounding electrode system.
8.7 Grounding Electrode Conductor Installation
In general, grounding electrode conductors are required to be continuous.

Grounding electrode conductors must be securely fastened to the building or structure.  A No. 4 AWG or larger grounding electrode conductor must be protected if it is exposed to physical damage.  A No. 6 AWG grounding electrode conductor is not required to be protected if it is not exposed to physical damage.  They are not required to be buried to the depths in NEC Table 300.5.

8.7 NEC Table 300.5Grounding electrode conductors must be installed in one continuous length without a splice, unless:

  • The splice is made with an irreversible compression-type connector listed for grounding and bonding.
  • The splice is made by exothermic welding.
  • The grounding electrode conductor is made up of busbars which are connected together to form a single conductor.

On new installations there is no reason to splice the grounding electrode conductor.  The permission to splice the grounding electrode conductor is most commonly used on old work or when electrical equipment is being replaced.

For buildings with multiple disconnecting means in separate enclosures the grounding electrode conductor can be installed in any one of three ways:

  1.   A common grounding electrode conductor can be installed based on the size of the largest service entrance conductor and taps can be connected to the common grounding electrode conductor based on the size of the ungrounded conductors in each disconnect.
  2.  Individual grounding electrode conductors can be installed from each disconnecting means to the grounding electrode system.
  3.   A grounding electrode conductor can be installed from a common location, such as a service wireway, directly to the grounding electrode system.

If the grounding electrode conductor is installed in a metallic raceway, the raceway must be bonded at both ends to the grounding electrode conductor so that the metallic raceway is electrically continuous from the point of attachment to the grounding electrode.

Review Question Section 8.7

51. Which of the following installations of a grounding electrode conductor is a violation?

8.8 Size of Alternating-Current Grounding Electrode Conductor

NEC Table 250.66 is used to size the grounding electrode conductor for services and separately derived systems.  It is based on the size of the largest ungrounded service-entrance conductor or the equivalent area for parallel conductors.  The equivalent area for two, 500 kcmil conductors is 1000 kcmil.

The grounding electrode conductor will carry ground-fault current when there is a ground fault, but its purpose is not to be part of the ground-fault current return path back to the electrical source.  The purpose of the grounding electrode and the grounding electrode conductor is to limit the voltage to ground if the building is hit by lightning and to provide a ground reference of zero volts for all the electrical conduit and enclosures.

Since the purpose of the grounding electrode conductor is not to carry fault-current, the largest grounding electrode conductor required by NEC Table 250.66 is 3/0 AWG cu.  If the equivalent size of the service-entrance conductors is 1500 kcmil, the grounding electrode conductor is 3/0 AWG; if the equivalent size of the service-entrance conductors is 2000 kcmil, 2500 kcmil, or 3000 kcmil, the grounding electrode conductor is still only required to be 3/0 AWG cu.

Review Question Section 8.8

52. What is the minimum size copper grounding electrode conductor for a 1250 kcmil cu. ungrounded service-entrance conductor?

The size of the grounding electrode conductor is selected from NEC Table 250.66 based on the size of the service-entrance conductors.
8.9 Types of Equipment Grounding Conductors
Some types of metal raceways are permitted to serve as equipment grounding conductors.

The purpose of an Equipment Grounding Conductor is to provide a low impedance path to ground for fault current in order to facilitate the operation of overcurrent protective devices or the operation of ground fault detection systems in the event of a ground fault.

Section 250.118 gives specifics on what types of items are permitted to be used as an equipment grounding conductor. Many states have modified Section 250.118 of the 2014 NEC by deleting flexible metal conduit (FMC) and liquidtight flexible metal conduit (LFMC) from this list of acceptable grounding paths.

  • Conductor. A bare or insulated conductor [250.118(1)]
  • Rigid Metal Conduit [250.118(2)]
  • Intermediate Metal Conduit [250.118(3)]
  • Electrical Metallic Tubing [250.118(4)]
  • Listed Flexible Metal Conduit as limited by 250.118(5)
  • Listed Liquidtight Flexible Metal Conduit as limited by 250.118(6)
  • Listed Liquidtight Flexible Tubing as limited by 250.118(7)
  • Armor of Type AC Cable [250.118(8)]
  • Copper metal sheath of Mineral Insulated Cable [250.118(9)]
  • Metal Clad Cable as limited by 250.118(10) [250.118(10)]
  • Metallic cable trays as limited by 250.118(11) and 392.7
  • Cablebus framework as permitted in 370.3 [250.118(12)]
  • Electrically continuous metal raceways listed for grounding [250.118(13)]
  • Surface metal raceways listed for grounding [250.118(14)]

Some States or jurisdictions, as well as electrical engineers, feel that the armor of FMC and LFMC is not adequate to be used as an effective equipment grounding conductor. For this reason, it is quite common for many jurisdictions to have similar amendments to this code section.

Other States have been known to require an equipment grounding conductor to be run inside any kind of raceway that also contains ungrounded circuit conductors. This is usually so that the reliability of the equipment ground path is not dependent on couplings or connectors for the metallic raceway system that might not have been tightened properly when originally installed.  There are also cases where extreme heat or cold can cause expansion even in metal raceway systems.

If the Equipment Grounding Conductor is a conductor or a raceway, it must be listed for the purpose and installed so that it provides a low impedance path to ground.  Fault current travels on the Equipment Grounding Conductor from the point where the fault occurred all the way back to the service or transformer a a high enough value to trip the circuit breaker.

 

Review Question Section 8.9

53. Which of the follo9owing is not permitted to serve as an equipment grounding conductor?

8.10 Size of Equipment Grounding Conductors

In general, conductors used as equipment grounding conductors are sized from NEC Table 250.122.

NEC Table 250.122

NEC Table 250.122 is based on the size of the fuse or circuit breaker that protects the circuit.  As the circuit goes up in rating, a larger equipment grounding conductor is required.

The minimum size equipment grounding conductors for circuits protected by 15 amp fuses or circuit breakers is 14 AWG.  Circuits protected by 20 amp and 30 amp devices must use equipment grounding conductors rated 12 AWG and 10 AWG respectively.

Starting at 40 amps, however, the required size of the equipment grounding conductor does not match the rating of the ungrounded conductors of the circuit.  For example, a No. 10 equipment grounding conductor can be used with a 40 amp or 50 amp circuit.  A no.6 equipment grounding conductor is large enough for a 200 amp circuit, a No. 1 equipment grounding conductor is okay for use on a 600 amp circuit.

The equipment grounding conductors can be smaller than the ungrounded circuit conductors because the equipment grounding conductor carries current only for a short period of time before the fuse or circuit breaker operates to open the circuit.  In fact, the operating time for inverse time circuit breakers is only a fraction of a second.

When the equipment grounding conductor is a raceway all connections must be made up wrench tight, with paint and rust removed to ensure good metal-to-metal connections between raceways and enclosures.  Solid connections along the fault current path means the path will be low resistance.  Loose connections mean a high resistance path that will delay the operation of the fuse or circuit breaker, possibly causing more damage to equipment and property and danger to personnel.

Where conductors are installed in parallel in multiple raceways the equipment grounding conductors must be installed in parallel in each raceway.  Each equipment grounding conductor must be full sized, based on the size of the overcurrent protective device for the parallel run.  For example, if a feeder is protected at 800 amps, but installed in two parallel runs, each equipment grounding conductor in the parallel conduits must be a 1/0 copper, 0r 3/0 aluminum, based on the 800 amp overcurrent device.

If the ungrounded conductors are increased in size because of voltage drop the equipment grounding conductor must be increased in size by the same proportion.  For example if the ungrounded conductors are increased by 15%, the equipment grounding conductor must be increased by 15%.  The required increase in the size of the equipment grounding conductor only applies when the ungrounded conductor is increased in size to carry the load, like when conductors are up-sized because of voltage drop.  If ungrounded conductors are increased in size because of more than 3 current-carrying conductors in conduit, or because of a hot ambient temperature, a larger equipment grounding conductor is not required.

Review Question Section 8.10

54. What is the minimum size equipment grounding Conductor used on a circuit protected by a 150 amp circuit breaker?

The size of the equipment grounding conductor is based on the rating of the circuit’s fuse or circuit breaker.