The following earthing terms are defined according to NEC standard (National Electrical Code) which is a foundational pillar of NFPA 70 (National Fire Protection Authority).
Having a good understanding of the terms used within Article 250 of NEC is critical to understanding the grounding and bonding requirements within the NEC. The following definitions are commonly used throughout Article 250, and knowing them is essential to installing an effectively grounded and bonded electrical system.
Ground - The earth
Grounded (Grounding) - Connected (connecting) to the ground or to a conductive body that extends the ground connection.
An example of a conductive body that extends the ground connection is the first 5 feet of a water pipe entering a building. It is permitted to connect at this point even though it is not a grounding electrode, as the first 5 feet of the pipe are not physically in contact with the earth.
Grounded Conductor - A system or circuit conductor that is intentionally grounded.
These are often regarded as "neutral" conductors to be identified as continuous white, continuous gray, or having three continuous white or gray stripes along the conductor's entire length on the other than green insulation. Be mindful that grounded conductors can also be phase conductors in certain applications.
Grounding Conductor, Equipment (EGC) - The conductive path(s) that provides a ground fault current path and connect normally non-current-carrying metal parts of equipment together and o the system grounded conductor or to the grounding electrode conductor, or both.
These are often called "ground" conductors in the trade. Section 250.119 requires these conductors to be bare, covered, or insulated. Covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes.
Grounding Electrode - A conducting object through which a direct connection to the earth is established.
Grounding Electrode Conductor - A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.
This is the conductor that connects the grounding electrodes to the electrical system.
Grounding - Fault Current Path - An electrically conductive path from the point of a ground fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source.
Effective Ground Fault Current Path - An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors.
An effectively designed ground-fault current path will allow for circuit breakers, fuses, and ground-fault detectors to open properly when ground-fault conditions arise within the electrical system.
Bonded (Bonding) - Connected to establish electrical continuity and conductivity
Bonding Conductor or Jumper - A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.
Bonding Jumper, Equipment - The connection between two or more portions of the equipment grounding conductor.
Bonding Jumper, Main - The connection between the grounded circuit conductor and the equipment grounding conductor at the service.
Bonding Jumper, System - The connection between the grounded circuit conductor and the supply-side bonding jumper, or the equipment grounding conductor, or both, at a separately derived system.
 |
| The main bonding jumper was installed at the service between the grounded service conductor and the equipment grounding conductor |
Sizing Grounding Electrode Conductors Using Table 250.66
Table 250.66 of the NEC is used to size grounding electrode conductors for alternating-current systems located at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at a separately derived system, such as on the load side of transformers.
This table shows you how to use the size of the largest ungrounded (hot) conductor to determine the necessary size of your grounding electrode conductor. Where you have two or more sets of service-entrance conductors run in parallel, the grounding electrode conductor must be sized according to the largest sum of the corresponding conductors of each set.
Notes:
service drop, set of overhead service conductors, set of underground
service conductors, or service lateral, the equivalent size of the largest
service-entrance conductor shall be determined by the largest sum of
the areas of the corresponding conductors of each set.
2. Where there are no service-entrance conductors, the grounding
electrode conductor size shall be determined by the equivalent size of
the largest service-entrance conductor required for the load to be
served.
3. See installation restrictions in 250.64.
Example: Parallel Conductors
A 3-phase, 800A service is supplied using two 500kcmil (250mm2) THWN copper conductors per phase. The parallel conductors per phase. The parallel conductors are installed in two separate runs of rigid metal conduit. Using Table 250.66, determine the maximum size grounding electrode conductor required for this service.
Step 1 - Determine the size of the largest ungrounded conductor in each raceway.
Step 2 - Determine the maximum size grounding electrode conductor (GEC) required for this service.
SOLUTION: Two parallel 500kcmil copper conductors: Total equivalent area = 2 x 500kcmil = 1000kcmil (500mm2)
Then refer to Table 250.66 for the ungrounded conductor and copper column values:
1000 kcmil falls into the "Over 600 through 1100" row = size 2/0 (70mm2) copper GEC
To get a hold of the full article from NFPA/NEC. You can download the below PDF file
Comments
Post a Comment