Grounding Design Calculations – Part Six


I indicated that the Earthing Systems Design Steps process has (3) main steps:




Earthing Systems Design Steps

A grounding system design process has (3) main steps:


  1. Data Collection,
  2. Data Analysis,
  3. Grounding Design Calculations.






And I explained the first step: Data Collection in the following Articles:





I explained the second step: Data Analysis in the following Articles:




And I explained What we are going to design for grounding system in any building in the following Articles:





And, in Article 
 Grounding Design Calculations – Part One ", I indicated the following:





Grounding System Design Calculations according to type of the building

The procedures for performing the Grounding System Design Calculations can differ slightly according to the type of the building as follows:

  1. Domestic, commercial and industrial premises,
  2. High and medium voltage electricity substations.








First: Domestic, commercial and industrial premises

We mean by domestic, commercial and industrial premises, all installations up to 1,000 V ac and 1,500 V dc - between phases, with some minor exceptions.





And I started discussion for Methods of Grounding Design Calculations of Domestic, commercial and industrial premises as follows:






Methods of Grounding Design Calculations

There are many methods can be used for performing Grounding System Design Calculations But the common methods are:

  1. Equations Method,
  2. Nomographs Method,
  3. Excel Spreadsheets Method,
  4. Tables Method,
  5. Online Earthing Calculators Method,
  6. Software Programs Method.






In this Article and Article 
Grounding Design Calculations – Part Two ", I explained the first method of grounding design calculations: Equations Method and solved examples.


And I explained the second method of grounding design calculations: Nomographs Method in Article 
Grounding Design Calculations – Part Three".


Also, I explained third method of grounding design calculations: Excel Spreadsheets Method in Article 
Grounding Design Calculations – Part Four ".


Today, I will explain NEC Article 250 tables for sizing of earth conductors.


You can preview the following Articles for more info:


  







Third Method: Tables Method - Continued






Important

The Tables Method is used for sizing earthing conductors only, not for calculating the earth resistance value.






Methods Of Sizing Earthing Conductors

There are two methods for sizing protective conductors including earthing conductors as follows:

  1. By using the adiabatic equation,
  2. By using Tables.






In Article " Grounding Design Calculations – Part Five ", I explained method#1: By using the adiabatic equation, also I explained the first case: BS 7671 Requirements for Electrical Installations (IEE Wiring Regulations) from method#2: By using Tables which include two cases as follows:




Method#2: By Using Tables

The easier method is to determine the earthing conductor (protective conductor) size from Tables but this may produce a larger size than is strictly necessary, since it employs a simple relationship to the cross-sectional area of the phase conductor(s).

The used tables for determining of earth conductor size are existing in two standards/codes as follows:

  1. In BS 7671 Requirements for Electrical Installations (IEE Wiring Regulations),
  2. In NEC, Article 250.







Second: NEC Article 250 tables for sizing of earth conductors

NEC Code provide (2) tables for sizing grounding conductors as follows:

  1. Table 250.66: Sizing Grounding Electrode Conductor (GEC),
  2. Table 250.122: Sizing Equipment Grounding Conductor (EGC).








First: Table 250.66
Sizing GEC according to the largest service entrance conductor Size



As per NEC 250.66, the grounding electrode conductor must be sized based on the largest service-entrance conductor or equivalent area for parallel conductors in accordance with Table 250.66, Except for:

  1. A ground rod electrode,
  2. A concrete-encased electrode,
  3. A ground ring electrode.




Table 250.66


The above (3) exceptions will be sized according to the following table:


Exceptions to Table 250.66 (see Fig.1)
grounding electrode conductor
connected to
grounding electrode conductor
 largest size
Rod, Pipe, or Plate Electrodes
6 AWG copper or 4 AWG aluminum wire.

Concrete-Encased Electrodes
4 AWG copper wire
Ground Rings
the size of conductor used for the ground ring
Note: the above (3) exceptions are applied only when that portion of the grounding electrode conductor is the sole connection to the grounding electrode.



Fig.1









Example#1:

Determine the size of the grounding electrode conductor used for a service equipment with service-entrance conductor of 3/0 AWG.

Solution:

Use Table 250.66 to size the grounding electrode conductor. According to the third row, “2/0 or 3/0” the size should be 4 AWG copper or 2 AWG aluminum.



Example#2:

If a second service equipment with service-entrance conductor of 3 AWG is added to the case in example#1, Determine:
  • The size of the grounding electrode conductor used for each service equipment.
  • The size of the main grounding electrode conductor for both service equipments.



Solution:

Step#1: Use Table 250.66 for each service conductor
The size of the grounding electrode conductor used for first service equipment ( with service conductor 3/0 AWG)= 4 AWG copper or 2 AWG aluminum (as in example#1)
The size of the grounding electrode conductor used for second service equipment (with service conductor 3 AWG)= 8 AWG copper or 6 AWG aluminum

Step#2: calculating equivalent area for these parallel conductors
In this case we have (2) parallel service conductors (3/0 AWG & 3 AWG), so we need to calculate the equivalent area for these parallel conductors as follows:




Using Table 8 in Chapter 9, find the total circular mil area for each service conductor:
3 AWG = 52,620 circular mils
3/0 AWG = 167,800 mils
Then, equivalent Total area = 220,420 circular mils or 220.42 kcmil


Step#3: Use Table 250.66 for equivalent area for these parallel conductors
Using Table 250.66 and According to the fourth row, “Over 3/0 through 350,” the size should be 2 AWG copper or 1/0 AWG aluminum.







Table-8, Chapter 9






Second: Table 250.122
Sizing EGC according to the size of the over-current device ahead of the conductor

As per NEC 250.122, the Equipment grounding conductor must be sized according to the size of the over-current Protective device (OPCD) ahead of the conductor.



Table 250.122




Note to Table 250.122:

Where there is a long distance between the power source and utilization equipment, larger sizes of EGC can be provided to lower the overall impedance of the ground-fault current return path in order to facilitate quick operation of the overcurrent protective device in the event of a line-to-ground fault.


Example#3:

What size aluminum equipment grounding conductor is needed for a 400 A ciruit breaker protecting the feeder circuit.

Solution:

Using NEC table 250.122, for 400 A Overcurrent Device in Circuit Ahead of conductor, GEC = 1 AWG Aluminum wire.









Special Cases for application of Table 250.122

The following (6) Special cases must be taken into consideration when sizing EGC:

  1. Increasing size of ungrounded conductors,
  2. Ungrounded conductor with Multiple Circuits,
  3. The equipment grounding conductor for Motor Circuits,
  4. Equipment grounding conductor in a Flexible Cord and Fixture Wire,
  5. Equipment grounding conductor for ungrounded Conductors in Parallel,
  6. Equipment Grounding Conductors Run With Feeder Taps.









Case#1: Increasing size of ungrounded conductors

If there is a need to increase the size of the ungrounded conductors to compensate for voltage drop or for any other reason related to proper circuit operation, EGC must be increased proportionately.


Example#4:

A 240-volt, single-phase, 250-ampere load is supplied from a 300-ampere breaker located in a panelboard 500 ft away. The conductors are 250-kcmil copper, installed in rigid nonmetallic conduit, with a 4 AWG copper equipment grounding conductor. If the conductors are increased to 350 kcmil, what is the minimum size for the equipment grounding conductor based on the proportional-increase requirement?

Solution

Step#1: Calculate the size ratio of the new conductors to the existing conductors:
Size ratio = 350,000 circular mils / 250,000 circular mils = 1.4

Step#2: Calculate the cross-sectional area of the new equipment grounding conductor:
By using table 8 in Chapter 9, the cross-sectional area of old equipment grounding conductor = 4 AWG = 41,740 circular mils
So, the cross-sectional area of the new equipment grounding conductor = 41,740 circular mils x 1.4 = 58,436 circular mils





Step#3: Determine the size of the new equipment grounding conductor
By using table 8 in Chapter 9, we find that 58,436 circular mils is larger than 3 AWG. The next larger size is 66,360 circular mils, which converts to a 2 AWG copper equipment grounding conductor.









Case#2: Ungrounded conductor with Multiple Circuits

When a single EGC is run within a raceway with multiple circuits, it should be sized for the largest over-current protective device present.


Example#5:

Find the EGC for three 3-phase circuits in the same raceway, protected by overcurrent devices rated 30, 60, and 100 amperes.

Solution:

The three circuits would require only one equipment grounding conductor, sized according to the largest overcurrent device (in this case, 100 amperes).

By using table 250.122, an 8 AWG copper or 6 AWG aluminum conductor or copper-clad aluminum conductor is required.









Case#3: The equipment grounding conductor for Motor Circuits

The equipment grounding conductor size shall be sized from Table 250.122 using the rating of the branch-circuit short-circuit and ground fault protective device.

Note:

Where the overcurrent device is an instantaneous-trip circuit breaker or a motor short circuit protector, the equipment grounding conductor (EGC) shall be sized from Table 250.122 using the maximum permitted rating of a dual element time-delay fuse selected for branch-circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1), Exception No. 1.








Case#4: Equipment grounding conductor in a Flexible Cord and Fixture Wire

Equipment grounding conductor in a Flexible Cord and Fixture Wire will be sized as per the following table:

Flexible Cord Ungrounded Conductor Size
Equipment Grounding Conductor (EGC) Size
10 AWG or smaller
Shall not be smaller than 18 AWG copper and shall not be smaller than the circuit conductors
larger than 10 AWG
in accordance with Table 250.122









Case#5: Equipment grounding conductor for ungrounded Conductors in Parallel

Where conductors are installed in parallel in multiple raceways or cables as permitted in, the equipment grounding conductors, where used, shall be installed in parallel in each raceway or cable. Where conductors are installed in parallel in the same raceway, cable, or cable tray, a single equipment grounding conductor shall be permitted.

In this case, Each equipment grounding conductor shall be sized in accordance with Table 250.122 based on the rating or setting of the overcurrent device protecting the paralleled circuit conductors.


Example#6:

A 2000 A feeder is installed in parallel using 6 metal clad (MC) type cables, each set contains four 600 Kcmil aluminum alloy conductors. What is the minimum size EGC that can be used?

Solution:

Bu using table 250.122, for 2000 A OPCD, EGC minmum size = 400 Kcmil, and one EGC must be installed in each MC Cable assembly.








Case#6: Equipment Grounding Conductors Run With Feeder Taps

Equipment grounding conductors run with feeder taps shall not be smaller than shown in Table 250.122 based on the rating of the overcurrent device ahead of the feeder but shall not be required to be larger than the tap conductors.


Example#7:

A 600-kcmil copper conductor is tapped to a 1200-ampere feeder and supplies a fusible switch with 400-ampere fuses. What is the minimum size EGCs that can be used?

Solution:

For the 400-ampere overcurrent protection, the equipment grounding conductor from Table 250.122 is a 3 AWG copper or 1 AWG aluminum conductor.
for the 1200-ampere device that is on the line side of the 600-kcmil tap conductors, the equipment grounding conductor from Table 250.122 is a 3/0 AWG copper or 250-kcmil aluminum conductor.





In the next Article, I will explain grounding design calculations by using Online Earthing Calculators Method. Please, keep following.




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