Lesson 4

GROUNDING

         

Before we look at the grounding system within a building we must be sure of the grounding electrode in place.  The grounding electrode of the building may be building steel, a ground ring or in most cases a metal underground water system connected to the street main.  This grounding electrode such as a water main is where we will be connecting our grounding electrode conductor.  But how do we size this conductor and what is its purpose?  Let us first discuss the buildings load or effects that it has on sizing our service entrance conductors.  Through this process we can correctly size our grounding electrode conductor as well as our main bonding jumpers while using Table 250.66 and begin to understand the purpose and proper installation methods of ensuring a grounding system.

 

The grounding electrode system consists of primarily

·        Equipment grounding conductors

·        Main bonding jumpers

·        Equipment bonding jumpers

·        Grounded circuits

·        Grounding electrode conductors

         

          The equipment grounding conductors may be either that bare wire in a coil of Romex(NM cable), or that green wire installed in a PVC run.  A metal clad (MC) cable will also contain this green equipment grounding conductor.  Then again the armor of you AC cable or Rigid Metallic Conduit will also be suitable as an equipment grounding conductor.  Either way there should be a path back to ground or to the grounding electrode conductor attached to the grounding electrode to divert any excess current that may stray.

 

          Let’s say you install a series of #12awg conductors fused at 20 amps.  In theory if this circuit should short circuit the highest possible current seen on the equipment grounding conductor should be that in which it is fused at.  This equipment grounding conductor normally carries no current what so ever.  If the ungrounded or hot conductor should energize the grounding electrode conductor, the equipment or raceway then the purpose of the grounding electrode conductor is to divert that excess current until the circuit breaker or fuse should open.  It then serves its purpose and explains why we size this conductor according to that circuits’ fuse in which it is associated with.  If the ungrounded (Hot)  should come in contact with the grounded (Neutral) conductor the excess current should the divert back to the first disconnecting means through the neutral to the neutral bar to the main bonding jumper to the ground bar and on to the grounding electrode conductor and back to earth or ground.

 

          The path in which this excess of current takes is through the following

·        Equipment Grounding and Bonding

·        Equipment Grounding Conductor (Table 250.122)

·        Equipment Bonding Jumper

·         Main Bonding Jumper (Table 250.66)

·        Grounding Electrode Conductor (Table 250.66)

·        Grounding Electrode (ex. Water main)

·        Supplemental Grounding Conductor

 

          Our objective is to divert any excess current, static current, or lightning surges back to earth or ground through the proper coordination of our grounding system.  This importance cannot be overstressed.  If there is a break in the grounding electrode conductor in which this excess current cannot find its way back to earth, then stray currents will build up creating excessive heat on the armor or equipment grounding conductors thus resulting in fire throughout if the breaker should not trip. 

 

          Let us now look at the grounded or neutral conductor.  In a 2 wire circuit supplying a 15amp load the neutral should be capable of carrying 15 amps.  If the neutral should disconnect with no equipment grounding conductor or grounding electrode conductor the load may impose a potential hazard if someone was to come in contact with the grounded metal housings.  In a single phase 3 wire system, the neutral will carry the unbalanced load.  If 2 circuit supplying 15amps and 10amp are on different phases the neutral current will be 5amps.  If they were on the same phase then the neutral current will be 25 amps.  This is what we don’t want.

 

          If there are no equipment grounding conductors in the system or no grounding electrode conductor connected and the service neutral is disconnected, then excessive heat would be seen on every load and grounding conductor through out resulting in fire across every armor or grounding conductor in the walls of the house thus lighting the insulation in the house.  Safety is of utmost importance with the proper installation of grounding connection.  In addition to the grounding electrode conductor installation and within 5 feet of the entrance of the water main, a supplemental ground rod is needed.  The largest size conductor permitted attached to this ground rod is a #6 awg.  The grounding electrode conductor should also be connected to the ground rod or an additional conductor from the ground bar of the first disconnecting mean to the rod should be installed.  But now how do we size our grounding electrode conductor attached to the water main?

 

          After we calculate our load in the house and apply our demand factors we would then be able to size our service entrance conductors.  The service entrance conductors on the line side are not fused on the line side like our grounding electrode conductors.  The main overcurrent protection only protects the panel bus and a chance of overload.  So how then can we size our grounding electrode conductor which is connected to our grounding electrode (water main).  In order to size this conductor(grounding electrode conductor) we had to first calculate our service based on the potential load.  Every load is protected with a breaker and if the service entrance conductors were to come in contact with the equipment, or the grounded wire, the grounding electrode conductor as well as the bonding jumper must be sized according to the possible load on the service conductors.  Therefore the size of the service entrance conductors plays an important role when sizing the grounding electrode conductor.  On the other hand if we were sizing a feeder which by definition is that point or piece of equipment after the first disconnecting means then we would use Table 250.122.  However for grounding electrode conductors and bonding jumpers in the first disconnecting means we size them by the size of the service entrance conductors using Table 250.66 (not the rating of the fuse)

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