Introduction:
Cable (or conductor) sizing is the process of selecting appropriate sizes for electrical power cable conductors.
The proper sizing of cables is important to ensure that the cable can:
- Operate continuously under full load without being damaged
- Provide the load with a suitable voltage (and avoid excessive voltage drops)
- Withstand the worst short circuits currents flowing through the cable
Cable sizing methods do differ across international standards and some standards emphasis certain things over others. However the general principles that underpin all cable sizing calculation do not change. When sizing a cable, the following general process is typically followed:
- Sizing of the cable, its installation conditions, the load that it will carry, etc
- Determine the minimum cable size based on Ampacity (continuous current carrying capacity)
- Determine the minimum cable size based on voltage drop considerations
- Determine the minimum cable size based on short circuit temperature rise
- Select the cable based on the highest of the sizes calculated in the steps above.
Types of Cables:
Type of cable used is a matter of common knowledge, such as when more mechanical strength is required or for outdoor above ground or underground installation, armored cables are used. They are also used for hazardous areas.
- For high temperature areas like furnaces, boilers etc, HRPVC or mineral insulated cables are used
- FRLS i.e. fire retardant and low smoke cables are used in area prone to fire hazard.
- Oil filled and gas insulated cables are used for extra high voltage systems but cost becomes an important factor when compared to transmission line.(Cost of such cables is around 8 to 15 times, that of transmission lines for same voltage and capacity).
- For LT voltage up to 1000V, for general purpose use, PVC insulated and sheathed cables are good enough.
- Whereas for voltages of 3.3KV to 33KV, XLPE cables serve the purpose. XLPE cables are also used for voltages up to 1000 volts since they have various advantages over PVC cables especially with reference to higher conductor temperature and higher short circuit ratings.
- Rubber cables are used as flexible cables for cranes lifts etc.
Cable Sizing calculation Procedure
Having selected the type of cable, next task is to choose correct size of cable, which is most important and depends on variety of factors.
- Number of Cores
- Selection of Conductor
- Current carrying capacity
- De-rating factors
- Voltage drop consideration
- Short circuit withstand capacity
Selection of Conductor:
Selection of conductor either aluminum or copper should be paid attention depending on whether cable to be used as power or control purpose. Conductivity of aluminum is around 60% of that of copper. Control cables shall use copper conductor.
Current Carrying Capacity:
Each power cable is designed to operate under certain temperature conditions.
Current carrying capacity of power cable is also dependent on conductor material (copper/Aluminum) and insulation type.
Thus, Copper conductor cable has greater current carrying capacity than aluminum.
XLPE insulation is better than PVC, hence the current carrying capacity of XLPE cable is more than that of PVC insulated cable.
Operating a cable continuously beyond its rated current carrying capacity shortens the lifespan of the cable, as the insulation becomes prone to failure.
The current carrying capacity is also dependent on operating temperature. Higher the temperature, lower is the current carrying capacity of the cable and vice versa.
Consumed Load = Total Load × Demand Factor
Consumed Load in KVA = KW/P.F
Full Load Current = (KVA × 1000)/ (1.732 * Voltage)
De-rating Factors:
The factor due to which electrical cable lose its current carrying capacity means it cannot carry the amount of current for which it is designed.
Following are the reason of de-rating factor comes into action:
- Ambient temperature: If temperature of the environment increases correspondingly cable starts getting de-rated due to resistance changes.
- Air laying/Underground laying: Cable get de-rated if it laid underground rather than upper ground.
- Soil Details: Thermal resistivity of soil is not known. Nature of soil is damp soil.
- Multi-core cables laid in horizontal formation.
Actual Current carrying capacity = De-rating Factor × cable current carrying capacity under std. conditions.
 Voltage drop consideration:
A cable’s conductor can be seen as an impedance and as a result, whenever current flows through a cable, there will be a voltage drop across it, derived by Ohm’s Law (i.e. V = IZ). The voltage drop will depend on two things:
- Current flow through the cable – the higher the current flow, the higher the voltage drop.
- Impedance of the conductor – the larger the impedance, the higher the voltage drop.
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The impedance of the cable is a function of the cable size (cross-sectional area) and the length of the cable. Most cable manufacturers will quote a cable’s resistance and reactance in Ohms/km or Ohms/ft.
For AC systems, the method of calculating voltage drops based on load power factor is commonly used. Full load currents are normally used, but if the load has high startup currents (e.g. motors), then voltage drops based on starting current (and power factor if applicable) should also be calculated.
For a three phase system:
For a single phase system:
Where,
V is the three phases or single phase voltage drop (V)
I is the nominal full load or starting current as applicable (A)
is the ac resistance of the cable (Ohms/km or Ohms/ft)
is the ac reactance of the cable (Ohms/km or Ohms/ft)
cos 
 is the load power factor (p.u)
L is the length of the cable (m or ft)
When sizing cables for voltage drop, a maximum voltage drop is specified, and then the smallest cable size that meets the voltage drop constraint is selected.
Maximum voltage drops are typically specified because load consumers (e.g. appliances) will have an input voltage tolerance range. This means that if the voltage at the appliance is lower than its rated minimum voltage, then the appliance may not operate correctly.
In general, most electrical equipment will operate normally at a voltage as low as 80% nominal voltage. For example, if the nominal voltage is 230VAC, then most appliances will run at >184VAC. Cables are typically sized for a more conservative maximum voltage drop, in the range of 5 to 10% at full load.
