ELECTRICAL DESIGN AND CALCULATIONS:

VOLTAGE DROP CALCULATION:

BY Victor Robleto
Electrical Engineer EIT and Master electrician.
* National Electric Code NEC, NFPA are registered trademarks of National Fire Protection Association.

WHAT IS VOLTAGE DROP?

To define what is voltage drop, first we need to understand that every electrical equipment is designed to work close to its nominal voltage, depending of the equipment some has more tolerance than other to work at lower voltage than its nominal voltage value. for more details about voltage range and tolerance read the ANSI Standard C84.1L.  We can define as nominal voltage as the commercial available voltage supplied by the utility company, in North America and  most of the America continent countries  120, 208, 240, 277, 480 volts are nominal voltages,  for example  a regular incandescent  light bulb  has a nominal voltage of 120 volts, now if we take  this light bulb and you apply power to it, and for some reason this light bulb just received 105.36 volts at  the light bulb's terminals; this light bulb will be dimmed because of the low voltage applied compared to its nominal voltage.

Now, you will ask where are the 14.64 volts ? ( 120 nominal volt - 105.36 received at the light bulb ), well, this 14.64 volts is the voltage drop of this example.  When we have voltage drop in an electrical circuit, the voltage that really is applied to the electrical load terminals is lower than its nominal voltage applied, in consequence of this, the  electrical equipment may not work properly or in the worse case scenario the equipment  fails because the low voltage applied; As you can see excessive voltage drop is something than we do not  want in an electrical circuit.

There are many factor why and electrical circuit can have voltage drop, some of these  factors described as follow :

1- Electrical circuit was installed with small wire size. Poor planning and calculation of the electrical design and blueprints  or sometimes electrical contractors trying to save some money installing  a small wire size. in this web site, you  will learn how to calculate the right wire size to avoid considerable voltage drop.

2- Improper electrical connections, loose connections, wrong wire connectors etc.   this is associated with the quality of the electrical installation and selection of proper materials for terminations, this part of the job is done usually by an electrician.

3- High resistance in electric contacts  and connections, such as breakers, electrical contacts in relays and contactors  and other electromechanical components of the circuits. This usually happens after the electrical equipments has been in service for long time, and the proper maintenance is not been done, also the environment can affect the electrical connections, high humidity, water intrusion, dust etc can create corrosion on the electrical connections and be sources of high electrical resistance and then having excessive voltage drop in the circuit.

As you can see the voltage drop in a circuit depends of the resistance present  between the power source and the electrical load terminals.  if we apply the Kirchoff''s voltage law to the following circuit:

source_voltage = I*R1_wire + I*R_Load + I*R2_wire

120 volts = I (10+10+144)

I= 120 volts/ 164 =                                        0.7317 Amps

VR1=I*R1_wire = 0.7317* 10 =                    7.32 volts

VLoad= I*R_LOAD = 0.7317* 144 =          105.36 volts

VR2=I*R2_wire = 0.7317*10 =                     7.32 volts

Voltage drop = VR1+ VR2 = 7.32 + 7.32 = 14.64 volts

 Voltage drop is the voltage that drops through the wires of an electrical circuit, wires for an electrical installation are made of copper or aluminum, those materials has  electrical resistance.

Voltage drop has negative effect in electrical loads, in electronic loads the voltage drop may cause erratic response, in motors overheat and lower speed. in light fixtures low illumination output in relays or contactors may not work because the low voltage at the load side, and so on. Voltage drop is always present in an electrical installation circuit, but can be controlled and its negative effect can be reduced using the proper wires size, with the right ampacity for electric current and minimum voltage drop in the circuit.

The National Electric Code NEC does not enforce the voltage drop in an electrical circuit but makes a reference to the maximum voltage drop permissible in an electrical installation.

"The National Electric Code article 210.19 Conductors — Minimum Ampacity and Size. FPN No. 4: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent,provide reasonable efficiency of operation. See FPN No. 2 of 215.2(A)(3) for voltage drop on feeder conductors. "

  voltage drop formula:

VOLTAGE DROP CALCULATOR

WIRE TYPE: COPPER
ALUMINUM
SINGLE PHASE, THREE PHASE OR DC CIRCUIT: SINGLE PHASE
THREE PHASE
DC
CIRCUIT INFORMATION: CURRENT  AMPERES
LENGHT     FEET
NOMINAL VOLTAGE  VOLTS

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