I indicated that the Earthing Systems Design Steps process has (3) main steps:
Earthing Systems Design
Steps
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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:
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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.
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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:
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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
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Important
The
Tables Method is used for sizing earthing conductors only, not for
calculating the earth resistance value.
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Methods Of Sizing
Earthing Conductors
There are two methods for sizing
protective conductors including earthing conductors as follows:
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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:
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Second: NEC Article 250 tables for sizing
of earth conductors
NEC Code provide (2) tables for
sizing grounding
conductors as follows:
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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:
The above (3) exceptions will be sized
according to the following table:
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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:
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.
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Table-8, Chapter 9
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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.
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.
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Special Cases for application of Table 250.122
The following (6)
Special cases must be taken into consideration when sizing EGC:
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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.
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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.
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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.
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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:
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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.
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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.
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In the next Article, I will explain grounding design calculations by using Online Earthing Calculators Method. Please, keep following.
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