How to Calculate Voltage Drop Across a Cable

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Voltage drop is the reduction in voltage between the source of the electric current and the load point. Voltage drop occurs due to the resistance of wires (conductors) in a circuit and can be computed as the product of the resistance across the cable and electric current. As an example, we will calculate voltage drop in the circuit wired with 250 feet of the number 13 copper conductor, if current at the load is 12 amperes.

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  1. 1

    Obtain the cable (conductor) type information from its specifications or elsewhere. Note the American wire gauge (AWG) system is used in the US to classify wires. Conductors are typically made either from copper or aluminium.

    In our case, it is a copper number 13 conductor (1.8mm in diameter).

  2. 2

    Navigate to the AWG (copper or aluminium) table and find the resistance per 1,000 feet value corresponding to the specific conductor.

    In our example, resistance of copper conductor (number 13) is 2.042 Ohms/1,000 feet.

  3. 3

    Multiply the resistance from Step 1 by the wire length (in feet) in the circuit and divide by 1,000 to obtain the circuit resistance. Resistance (circuit) = wire length (feet) x conductor resistance /1,000 feet.

    In our example, resistance (circuit)= 250 feet x 2.042 Ohms/1,000 feet =0.511 Ohms.

  4. 4

    Multiply the resistance (Step 3) and current (in amperes) to calculate voltage drop in the circuit. Voltage drop (volts)=Resistance (circuit) x Current.

    In our example, voltage drop= 12 amperes x 0.511 Ohms=6.13 volts.

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