Download permissible voltage drop

ElemeNts of electrical design
TOPIC
•LOAD ASSESSMENT
•PERMISSIBLE VOLTAGE DROP
•CONDUCTOR SIZE CALCULATION
GUIDED BY : M.R.VASAVdA
SR NO NAME
1.
PAVAGADHI MEGHANA
ENROLLMENT
NO.
140233109007
2.
SABALPARA NIKUNJ
140233109008
3.
CHOPDE NILESH
140233109010
LOAD ASSESSMENT
Basic Terms
• Active power (useful or real power)
Time average of instantaneous power when the
average is taken over a complete cycle of an A/C
waveform, expressed in Watt (W)
For single phase, P = V I cos ø
For balanced three-phase, P = √3 Vph-ph I cos ø
• Apparent power
For single phase, AP = V I
For three phase, AP = √3 Vph-ph I
• Reactive power
In a direct current circuit, or in an alternating
current circuit whose impedance is a
pure resistance, the voltage and current are in
phase, and the following formula holds:
P= ErmsIrms
where P is the power in watts, Erms is the rootmean-square (rms) voltage in volts, and Irms is the
rms current in amperes.
• Connected load
 Sum of all the loads connected to the electrical
system, usually expressed in watts
• Demand factor
= Maximum demand / Connection load
• Load factor
 ratio of the average demand to
the maximum demand over a defined intervaI
• In commercial buildings, between the hours 9:00
to 18:00 the load factor is usually close to unity
• Power factor
 The ratio of the apparent power in a circuit (V.A)to the
useful power (W) if the voltage and current are sinusoidal
 Power factor = kW/ Kv.A
PERMISSIBLE VOLTAGE DROP
• Maximum voltage drop from supply terminals to any point on
the installation is not exceed the permissible limit viz.2% of
the supply voltage plus one volt for light load wiring and 5%
percent of declared supply voltage for power load wiring.
• How Much Voltage Drop Is Acceptable?
• The National Electrical Code (NEC) recommends that the
combined voltage drop of the electrical system (branch circuit
and feeders) not exceed 5% for optimum efficiency. This
recommendation not only can improve safety but can insure
proper equipment operation and power efficiency.
 the table shows below gives rating for single, twin or multi-core
cable with VIR, PVC, polythene insulation including TRS, PVC lead
or aluminium sheathed, run singly or in conduit, toughing, casing
or in free air. The ratings are based on an ambient temperature of
30⁰C.
 Use multiplying factor of 1.13 for 25⁰C, 0.86 for 35⁰⁰C, 0.69 for
40⁰C and 0.47 for 45⁰C.
 If more than one cable is used, multiply the given current ratings
by a factors of 0.8 for 2 cables, 0.7 for 3 cables and 0.65 for 4
cables.
 Voltage drop on ac cable is greater than dc due to inductance and
capacitance but difference is vary small and need only be taken
into account when we are dealing with heavy loads and long run.
Current rating of cu and equivalent aluminium
conductor/cable
The maximum voltage drop values taken
from Table
 For 230 VOLTS
For 400 VOLTS
CONDUCTOR SIZE CALCULATION
• The following points must be considered while
calculating the size of the conductors.
1. Current carrying capacity
2. Voltage drop
3. Minimum permissible size
• Current carrying capacity
 the conductor or wire should be of size sufficient to carry the
maximum circuit current continuously without overheating.
• Voltage drop
 the voltage drop in a conductor or wire depends upon the
current flowing through the conductor and resistance of the
conductor.
 The resistance is inversely proportional to area, therefore the
voltage drop will be less if the area of conductor is more.
 The permissible voltage drop in a lighting circuit is 2% of the
supply voltage plus one volt. In case of industrial loads, the
maximum voltage drop at the extreme end equipment should not
be more than 5% of the declared supply voltage.
• Minimum permissible size
 the minimum permissible size of conductor or wire should be
as follow:
 The conductor used in domestic wiring should not be size less
than 1/1.12 mm in copper or 1/1.40 mm in aluminum wire.
 The various sizes of copper and aluminium conductor with
current ratings and voltage drops are given in table bellow
1,2&3.
Current rating of copper conductor single core cable (VIR, PVC or
polythene insulated including tough rubber, PVC or lead sheathed).
Current rating of aluminium conductor single core cable (VIR, PVC or
polythene insulated including tough rubber, PVC or lead sheathed).
Current rating of aluminium conductor twin, three core or four core
cables. (VIR, PVC or polythene insulated and sheathed with tough rubber,
PVC or lead)
Thank you