Lesson 5

GROUNDING AND BONDING

 

 

          The previous lesson discussed grounding, now lets look at bonding.  Bonding is achieved by creating a permanently low impedance to ground in which all non-current carrying metal parts of electrical equipment will have a voltage and current potential of zero.  Its purpose is to divert any excess current flow resulting from lightning, stray currents or shorts as fast and easy as possible to ground or earth without frying the whole system.  Installation of equipment bonding jumpers (panel to ground bar, bonding bushing to panel) and main bonding jumper (neutral to ground bar) connections as well as grounding electrode conductors are accomplished within the first disconnecting means.  This disconnecting means can either be in the panel where the first main breaker is located or in the first disconnect switch which houses the service entrance conductors and feeder conductors.

 

          No bonding of the neutral to ground bar connections should be made within the second piece of equipment where the feeder conductors terminate.  Current takes the path of least resistance.  If there are stray currents coming back through the equipment grounding conductors (through the armor, metallic conduit, green, or bare conductor) toward the ground bar in the second piece of equipment and there is a bonding jumper from the ground bar to neutral bar, the excess current will divert through the larger conductor (path of least resistance) which is the grounded conductor thus frying the neutral.

 

          Current takes the path of least resistance.  The larger the CMA (circular mils area) the quarter the amount of resistance (chapter 9 Table 8) thus allowing the ease of which current will flow (conductance).  Main bonding jumpers should only be installed within the first disconnecting means with the grounding electrode conductors.  Bonding jumpers should also be sized according to Table 250.66 since they must be capable of the total systems load stray current.  Remember like a tree, the base represents the grounding electrode conductor which must support its branches or load distribution. 

 

 

 

GENERAL WIRING METHOD REQUIREMENTS

 

          Chapter 3 of the National Electrical Code addresses different types of wiring methods used and the requirements of those applications.  Article 300 defines the general requirements of all the later wiring methods addressed including ampacity, wiring protection, as well as box fill.

 

          One type of wiring method used is the use of conduit.  Conduit comes in different types.  Rigid Metal Conduit (RMC) can be made of steel (ferrous) or aluminum (non-ferrous).  IMC or Intermediate Metal Conduit is a thinner walled Rigid Metal Conduit and can be used in most cases in place of RMC.  Both of the above mentioned are of the threadable type, offer conductor protection and can be used as an equipment grounding conductor.  EMT or Electrical Metallic Tubing as well can protect conductors and be used as an equipment grounding conductor (250.118). 

 

          Other types of conduit might be of the nonmetallic type.  Rigid nonmetallic conduit also known as PVC comes in two different types, Schedule 40 and Schedule 80.  Rigid Polyvinyl Chloride or PVC is much cheaper and easier to use than the metallic type.  Schedule 40 offers little protection with the raceway not suitable to protect conductors and not to be used as an equipment grounding conductor.  Schedule 80 on the other hand may be used to protect conductors from physical damage but not be used as an equipment conductor.  Therefore Table 250.122 must be used to size the equipment grounding conductor.

 

          FMC or Flexible Metal Conduit also known as Greenfield may be used in exposed and concealed locations.  However the armor is not suitable to protect conductors against physical damage and must meet certain requirements to be used as an equipment grounding conductor.  Horizontal runs shall be supported at intervals not to exceed that permitted in the code.  Certain restrictions do apply to the lengths of FMC depending on its use and size.  For 3/8” or less FMC, there are certain requirements.  Lighting taps for example are only allowed to be 6’ and must contain an equipment grounding conductor.  FMC is different than FMT (Flexible Metallic Tubing) as well as EMT is different than Electrical nonmetallic tubing.  EMT cannot be bent by hand where ENT can.  ENT is also flame retardant, resistant to moisture and chemical atmospheres but however cannot be used as an equipment grounding conductor.

 

          Nonmetallic Sheathed Cable is broken down into 3 major types containing a nonmetallic jacket and an equipment grounding conductor.  UF cable or Underground Feeder and Branch Circuit Cable fall under a different section in the code and like NM cable contain a nonmetallic jacket and an equipment grounding conductor.  Romex or Nonmetallic Sheathed Cable is designated as NM.  The C in the NMC means corrosion resistant.  The S in the NMS designates the cables use for signaling.  Most confusion occurs when a sample test question states NMC and the student thinks it means nonmetallic conduit.  Remember RNC is Rigid Nonmetallic Conduit or PVC. 

 

          Two common cable armors of interlocking metal are MC or AC.  Armored Cable may not contain an equipment grounding conductor and the armor may be used as an equipment grounding conductor.  Metal Clad Cable will contain a green or equipment grounding conductor and must be used as an equipment grounding conductor.  Both types of cables as well as Romex or NM cable do not require us to size the equipment grounding conductor.  We only have to understand the ampacity chart of table 310.15(B)16 (2011 NEC) which we will cover in the next lesson.

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