Voltage Drop in Moving Cable Systems

The choice of conductor size is frequently influenced by consideration of the voltage drop in the circuit.Economic factors dictate that a cable having the smallest conductor

size be chosen for any installation.However,care must betaken to ensure that the use of a cable having small conductors not result in such a large voltage drop from the power supply to the utilization equipment that the equipment not function properly.

It is common in moving cable systems for the cable to join a power supply to a motor.Assuming that the motor is designed to operate at the nominal supply voltage,voltage drops larger than 5% can result in decreased motor life.

Voltage drop can be calculated using the following equations.

For single-phase circuits,

Voltage drop,

V_d = Vs - Vr = (2×L×I×R)/1000

For three-phase circuits,

Voltage drop between conductors,

V_d = Vs - Vr = (√3×L×I×R)/1000

where  Vs = voltage at the supply end

Vr = voltage at the receiving end

L = cable length (feet)

I = current (amps)

R = conductor resistance (ohms/1000 ft)

The conductor resistance used in these equations should be the resistance at the cable’s operating temperature.Industry standards normally provide resistance values at some reference temperature such as 20°C or 25°C.These values can be converted to resistance at the operating temperature by the use of the following equation.

Resistance at operating temperature,

Ro = Rr [1+0.00393(To -Tr )]

Where  Rr = resistance at reference temperature

To = operating temperature (°C)

Tr = reference temperature (°C)

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