2.2. Electrical installation design
Contents
2.2.1. CABLE SIZE SELECTION
Design Current
-Rating and type of protective device
-Correction factors
-Grouping factor
-Thermal insulation
-Type of protection device
2. 2.2. Voltage Drop Calculation
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2.2.1. CABLE SIZE SELECTION
DESIGN PROCEDURE
The selection of size of cable to carry a load current
involves the consideration of the following factors:
- Design Current
-Rating and type of protective device
-Correction factors
-Grouping factor
-Thermal insulation
-Type of protection device
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1. Design current (Ib): It is the rating of the
apparatuses to be operated . Manufacturers may
give it or if not it should be calculated as follows:
For single phase Ib=
𝑃∗100
𝑉∗𝑃𝐹∗𝑒𝑓𝑓
P= power consumed by the apparatuses
V= single phase voltage
For three phase Ib =
𝑃∗100
3∗𝑉∗𝑃𝐹∗𝑒𝑓𝑓
V – voltage between two lines
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But ac circuits may be capacitive or inductive
can produce poor power factor.
Ib =
Ib=
𝑃∗100
𝑉∗𝑃𝐹∗𝑒𝑓𝑓
for single phase
𝑃∗100
3∗𝑉1∗𝑃𝐹∗𝑒𝑓𝑓
for three phase
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. Rating of protective Device (In)
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After having established the design current of the circuit,
the next stage is to determine current rating or setting
protective device. Allowance should be made for possible
additional future load, the rating of the protective device.
It should not be less than design current of the circuit
(In>=Ib)
The reason for this is self-evident since the protective
device must be capable of passing sufficient current for the
circuit function at full load without the device operating5
3.Correction Factors
There may be a situation in which cables can find
themselves being overheated. The more obvious are the
conditions set up when over currents are carried due to:
- Overloading
- Short circuit occurred
So to choose cable size or to determine current
carrying capacity of conductor the following factors
should be considered.
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3.1.Ambiant Temperature (ca)
It is the temperature of
the immediate surroundings of the
equipment and cables before the temperature of the equipment or
cables contribute to the temperature rise .
 The cable ratings in the IEE regulations are of the time based
upon 300c , and hence it is only above this temperature that
adverse correction is needed.
3.2.Groupng Factor (cg)
When cables are grouped together they impart heat to each other.
Therefore, the more the cables there will be more heat they will
generate, thus increasing the temperature of each cable.
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3.3.Thermal Insulation (ci)
When cables are in contact with thermal insulation,
thermal insulation reduces rate of flow of heat from the
conductors, thus rising the conductors temperature.
3.4.Protection by fuse (cf)
Because of the high fusing factor of BS 3036 fuses, the
rating of the fuse In, should be less than or equal to
0.725Iz. Hence, 0.725 is the correction factor to be
used.
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Application of Correction Factors
 Some or all of the conditions outlined may affect a
cable along its whole length or part of it, but not all
may affect it at the same time. So consider the
following:
 If the cable run for the whole length, grouped with
others of the same size in a high ambient temperature
and was totally surrounded with thermal insulation, it
would seem logical to apply all correction factors( cfs),
as they all affect the whole cable run.
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Certainly the factors for the BS 3036 fuse, grouping
and thermal insulation should be used hence:
Iz
=
In
Cg x Cf x Ci
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Choice of cable size
Having established the tabulated current carrying
capacity Iz of the cable to be used, it is now remains to
choose cable to suit this value
( for full information refer Ethiopian Building code
standard, Electrical Installation of Building of EBCS10, Section 4, page 66)
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2. 2.2. Voltage Drop Calculation
The resistance of a conductor increases as the length
increases and / or the cross sectional area decreases.
Associated with an increased resistance is drop in
voltage, which means that a load at the end of along
thin cable will not have the full supply voltage
available.
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The IEE regulation require that the voltage drop Vc shouldn’t
be so excessive, that equipment doesn’t function safely. The
drop of not more than 5% (IEE)
 Therefore for single phase 230v , the drop should not
exceed 5% of 230v = 11.5v
 For three phase 400v, the voltage drop should not exceed 5%
of 400v = 20v
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