Voltage Drop

Connecting a load to any electrical circuit will result in the voltage at the load reducing.

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Resistance of Wires

Although most theoretical models and drawings of electrical circuits assume the wiring has zero resistance, real wiring does have a small resistance.

When any load is connected, current flowing through the small resistance of the wires causes a voltage to appear across the wires, hence the voltage at the load is less than that at the origin of the circuit.