This document discusses cable sizing and voltage drop calculation. It covers factors that affect cable sizing like current capacity, correction factors for temperature and grouping, and how to determine cable size based on factors like load current and protection device rating. Voltage drop calculation and limits are also explained.
This document discusses cable sizing and voltage drop calculation. It covers factors that affect cable sizing like current capacity, correction factors for temperature and grouping, and how to determine cable size based on factors like load current and protection device rating. Voltage drop calculation and limits are also explained.
This document discusses cable sizing and voltage drop calculation. It covers factors that affect cable sizing like current capacity, correction factors for temperature and grouping, and how to determine cable size based on factors like load current and protection device rating. Voltage drop calculation and limits are also explained.
This document discusses cable sizing and voltage drop calculation. It covers factors that affect cable sizing like current capacity, correction factors for temperature and grouping, and how to determine cable size based on factors like load current and protection device rating. Voltage drop calculation and limits are also explained.
Introduction The tabulated current-carrying capacity relates to a single circuit in the installation methods shown in Table 4A (Refer to IEE Wiring Regulation Sixteenth Edition page 181, 182, 183, 184 & 185), in an ambient air temperature of 30C. The current-carrying capacities given in the tables for a.c operation apply only to frequencies in the range 49 to 61 Hz. The tabulated current-carrying capacity relates to continuous loading and is also known as the full thermal current rating of the cable, corresponding to the conductor operating temperature indicated in the headings to the tables concerned. It is intended to provide for a satisfactory life of conductor and insulation subject to the thermal effects of carrying current for sustained periods in normal service. A cable may be seriously damaged, leading to early failure, or its service life may be significantly reduced, if it is operated for any prolonged period at a temperature higher than the indicated value. In addition, there are other consideration affecting the choice of the crosssectional area of a conductor, such as the requirements for protection against electric shock, protection against thermal effects, over current protection, voltage drop and the limiting temperatures for terminals of equipment to which the conductors are connected. Other than that, the conductors will need to be sized in accordance with the relevant circuit criteria. This sizing will also have to be taken into account the type of cable and its insulation. Other requirements that will also affect the size indirectly may include: availability, type of building, environment, security, life expectancy, adaptability, aesthetics and cost.
2.0
Correction factors for current-carrying capacity
In order to determine the current-carrying capacity of the cable, it may be necessary to apply one or more correction factors to the tabulated value given in the appropriate table for the cable. a)
For ambient temperature
Tables 4C1 and 4C2 (Refer to IEE Wiring Regulation Sixteenth Edition page 188) give the correction factor to be applied to the tabulated currentcarrying capacity depending upon the actual ambient temperature of the location in which the cable is to be installed.
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b)
For grouping Tables 4B1, 4B2 and 4B3 (Refer to IEE Wiring Regulation Sixteenth Edition page 186 & 187) give the correction factor to be applied to the tabulated current-carrying capacity where cables or circuits are grouped. c) For thermal insulation For a cable installed in a thermally insulating wall or above a thermally insulated ceiling the cable being in contact with a thermally conductive surface on one side, the rating factor to applied may, in the absence of more precise information, be taken as 0.75 times the current carrying capacity for that a cable likely to be totally surrounded by thermally insulation material. The applicable rating factor may be as low as 0.5
3.0
Relationship of current-carrying capacity to other circuit parameters
The relevant symbols used in the Regulations are as follows: Iz
the current-carrying capacity of a cable for continuous service, under the
particular installation condition concerned
It
the value of current tabulated in the Table with referring to the IEE Wiring Regulation Sixteenth Edition for the type of cable and installation method concerned, for a single circuit in an ambient temperature of 30C
Ib
the design current of the circuit, i.e. the current intended to be carried by the circuit in normal service
In
the nominal current or current setting of the device protecting the circuit against over current
a correction factor to be applied where the installation conditions differ
from those for which values of current-carrying capacity are tabulated in IEE Wiring Regulation Sixteenth Edition. The various correction factors are identified as follows: Ca for ambient temperature Cg for grouping Ci for thermal insulation Ct for operating temperature of conductor
In all circumstances Iz must be not less than Ib and In also must be not less than Ib. However CKE practice is Iz > I n > Ib
4.0
Determination of the size of cable
As a preliminary step it is useful to identify the length of the cable run and the permissible voltage drop for the equipment being supplied. The permissible
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voltage drop in mV, divided by Ib and by the length of run, will give the value of voltage drop in mV/A/m which can be tolerated. A voltage drop not exceeding that value is identified in the appropriate table and the corresponding cross-sectional area of conductor needed on this account can be read off directly before any other calculation are made. The conductor size necessary from consideration of the conditions of normal load and overload is then determined. All correction factors affecting Iz (i.e. the factors for ambient temperature, grouping and thermal insulation) can, if desired, be applied to the values of It as multipliers. This involves a process of trial and error until a cross-sectional area is reached which ensures that I z is not less than I b and not less than In of any protective device it is intended to select. In any event, if a correction factor for protection by a semi-enclosed fuse is necessary, this has to be applied to In as a divisor. It is therefore more convenient to apply all the correction factors to In as divisors. Once the cable type and other characteristics have been considered, the conductor size can be determined. This size will be dependent upon the current required by the circuit load The following procedure enables the designer to determine the size of cable it will be necessary to use in order to comply with the requirement for overland protection. 1. For single circuits Divide the nominal current of the protective device (In ) by any applicable correction factor for ambient temperature (Ca) given in Table 4C1 and then further divide by any applicable correction factor for thermal insulation (Ci). The size of cable to be used is to be such that its tabulated currentcarrying capacity (It) is not less than the value of nominal current of the protective device adjusted as below: It In CaCi 2. For groups Divide the nominal current of the protective device (In) by the correction factor for grouping (Cg) given in Tables 4B1, 4B2 or 4B3: It In Cg Where a rewire able fuse to BS036 is to be used, an additional factor (0.725) must be included, hence It In 0.725Ca CiCg Thus, the factors to determining current carrying capacity of conductors will be such as: a. b. c. d.
cross-sectional area type of cable or conductor method of installation number of conductors grouped together
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e. Environmental conditions, e.g. high ambient temperature, enclosure in
thermally insulating material. Once the factors that apply to a particular conductor have been ascertained the value of It can be found by calculation and reference then made to the relevant table i.e. 4D1A (single core PVC insulated), to obtain the cable size.
5.0
Voltage Drop Values of voltage drop are tabulated for a current of one ampere for a meter run i.e. for a distance of 1 m along the route taken by the cables, and then present the result of the voltage drops in the entire circuit conductor. For any given run the values need to be multiplied by length of the run in meters and by the current the cables are to carry, in amperes. The voltage drop for any particular cable run must be such that the voltage drop in the circuit of which the cable forms a part does not exceed 4% of the nominal voltage of the supply. As a guide, refer to the following diagram. Volt drop from DB to final point need not be calculated. It is assumed to be negligible since we are limiting our final circuit to not more than 80 meters from the DB. DB DB 1% DB
1.5%
SSB
1.5%
SSB
1%
4%
4% 4% 2.5%
SSB 2%
MSB / SOURCE Diagram 1
However for external lighting the volt drop from DB to final point must be calculated. The total must not exceed 4%.
MSB
2%
SSB
DB CL
1.5%
0.5%
Final Point
4%
Diagram 2
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The voltage drop calculated from the formula:Voltage Drop VD
Where Vd In VD L
= Vd L In 1000
= permissible voltage drop (mV/A/m)
= the device current of the circuit (A) = volt drop (V) = length of cable (m)
It is pertinent to point out that the cable rating must always be higher than (or equal to) that of the fuse or circuit breaker that is supposed to protect that part of the installation.
6.0
Type of cables commonly used
It used to be a standard practice in CKE to use PVC/PVC cables for non conduit wiring (surface and concealed). However these two (2) types of wiring are not used anymore in CKE. All wiring must now be in conduit using PVC insulated cables. Underground cables that are widely used: i)
PILCDSTAS Cable (Paper Insulated, Lead Covered, Double Steel Tape
Armoured and Served cable). This is use conjunction with cable-box termination and is available only in the 3 phase 4 core types, normally used when the 3 phase line current exceeds 40A.
Polyvinyl Chloride Cable). This cable is used with cable gland terminations and is available both in the 3 phase 4 core as well as in the single phase 2 core. It is generally used when the current demand is less than 60A.
Calculate cable size used and voltage drop from SSB G to DB GAL a) Consider copper conductor used e.g. single core PVC insulated cable and installation method is enclosed in conduit on a wall where L2 = 60 metre. Iz > In > Ib Refer to table 4D1A from IEE Wiring Regulation Sixteenth Edition, I z = 50A Iz > 40 > 22.28 50 > 40 > 22.28 From value of Iz we get cable size of 10mm. From table 4D1B voltage drop (per ampere per metre), V d = 4.4 mV/A/m Voltage drop (VD)
= Vd x L 2 x I n / 1000 = 4.4 x 60 x 40 / 1000
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= 10.56V Therefore cable to be used is 4 x 10mm PVC in conduit. b) Say L 2 is longer i.e. L2 = 80 metre Voltage drop (VD)
= Vd x L 2 x I n / 1000 = 4.4 x 80 x 40 / 1000 = 7.84V (exceeding allowable voltage drop of 6.225V)
Thus, 4 x 10mm PVC in conduit cannot be used.
Check again the Iz value. Refer to table 4D1A from IEE Wiring Regulation Sixteenth Edition, Iz = 68A Iz > 40 > 22.28 68 > 40 > 22.28 From value of Iz we get cable size of 16mm. From table 4D1B voltage drop (per ampere per metre), V d = 2.8 mV/A/m Voltage drop (VD)
= Vd x L 2 x I n / 1000 = 2.8 x 80 x 22.28 / 1000 = 4.99V
Therefore cable to be used is 4 x 16mm PVC in conduit.
ii)
Calculate cable size used and voltage drop from MSB NE to SSB G a) Consider copper conductor XLPE/SWA/PVC cable, installed underground where L1 = 110 metre Iz > In > Ib Refer to table 4E4A from IEE Wiring Regulation Sixteenth Edition, Iz = 251A Iz > 250 > 137.09 251 > 250 > 137.09 From value of Iz we get cable size of 70mm. From table 4E4B voltage drop (per ampere per metre), Vd = 0.60 mV/A/m Voltage drop (VD)
= Vd x L 1x I n / 1000 = 0.60 x 110 x 137.09 / 1000 = 9.05V
Therefore cable to be used is 4 core 70mm XLPE/SWA/PVC laid in the
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b) Say L1 is longer i.e. L1 = 155 metre
Voltage drop (VD)
= Vd x L 1 x I n / 1000 = 0.60 x 155 x 137.09 / 1000 = 12.75V (exceeding allowable voltage drop of 10.375V)
Thus, 4 core 70mm XLPE/SWA/PVC cannot be used.
Check again the Iz value. Refer to table 4E4A from IEE Wiring Regulation Sixteenth Edition, I z = 304A Iz > 250 > 137.09 304 > 250 > 137.09 From value of Iz we get cable size of 95mm. From table 4E4B voltage drop (per ampere per metre), Vd = 0.45 mV/A/m Voltage drop (VD)
= Vd x L 1 x I n / 1000 = 0.45 x 155 x 137.09 / 1000 = 9.56V
Therefore cable to be used is 4 core 95mm XLPE/SWA/PVC laid in the
ground. c) Say L1 is longer i.e. L1 = 250 metre Voltage drop (VD)
= Vd x L 1 x I n / 1000 = 0.60 x 250 x 137.09 / 1000 = 20.56V (exceeding allowable voltage drop of 10.375V)
Thus, 4 core 70mm XLPE/SWA/PVC cannot be used.
Check again the Iz value. Refer to table 4E4A from IEE Wiring Regulation Sixteenth Edition. Refer next value of Iz and calculate the volt drop until the acceptable volt drop is achieved. In this case the Iz that result in an acceptable volt drop is Iz = 406A Iz > 250 > 137.09 406 > 250 > 137.09 Therefore cable to be used is 4 core 150mm XLPE/SWA/PVC laid in the ground.
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