Voltage Drop: Why Voltage Changes Along a Circuit

A current-carrying path has electrical characteristics.

Real conductors and connections have resistance. In a simple resistive model, current through that resistance produces a voltage difference along the path. For AC circuits, impedance can include reactance as well as resistance.

The voltage available at the load can therefore differ from the source voltage while current flows. That is a physical relationship, not automatically a finding that a wire needs replacement or that a particular connection is defective.

A meaningful example identifies the source, the current, the complete path and the assumed electrical characteristics. A meaningful property evaluation also considers the equipment and actual operating conditions.

Further reading: 9.4 Ohm's Law · 9.3 Resistivity and Resistance · Short-Circuit Current Calculations and electrical formulas

Length, conductor size, material, temperature and load.

For a uniform conductor at a specified temperature, resistance grows with length and decreases with cross-sectional area. Materials have different resistivities. Temperature changes can also affect resistance.

Current matters too: for an unchanged resistance, higher current produces a larger voltage drop. A motor starting or equipment changing operating state can alter the current and the voltage behavior.

Connections are part of the path. Deterioration or other abnormal conditions can contribute to unwanted voltage differences and local heating. The formula alone cannot distinguish those conditions from ordinary designed behavior.

Further reading: 9.3 Resistivity and Resistance

Worked example: 0.2 ohm around the full loop.

Assume a fictional 120 V source, 10 A of steady current and 0.2 Ω of total resistance in the supply-and-return path. The drop is 10 × 0.2 = 2 V. The modeled voltage at the load is 118 V.

Relative to the 120 V source, 2 ÷ 120 × 100 is about 1.67%. The 0.2 Ω already includes the full loop; it is not a one-way conductor value to be used without the return path.

The same path at 20 A would have a 4 V drop in this resistance-only model. That comparison assumes the resistance is unchanged. It does not establish that either operating condition is suitable for a real circuit.

Further reading: 9.4 Ohm's Law · 9.3 Resistivity and Resistance

Dimming lights can have more than one cause.

A momentary change when equipment starts, persistent low voltage and a new intermittent flicker are different observations. Supply conditions, load behavior, connections and other equipment issues can produce similar symptoms.

A larger-than-expected change, unusual heat, damage or recurring equipment trouble needs attention to the actual installation. Do not assume a particular connection is the cause because the symptom matches an example online.

Record what you can observe without opening equipment: which lights or devices change, what else is operating and whether the behavior is new. That pattern can help your Mister Safety professional organize diagnostic work.

Circuit planning and fault diagnosis have different jobs.

A new long circuit starts with the intended load and route, equipment requirements and applicable design criteria. Diagnosing an existing problem starts with the observed condition and the installation as it exists.

A percentage such as 3% is not a universal safety certificate or a stand-alone legal rule for every circuit. Applicable requirements, design recommendations and manufacturer limits need to be evaluated for the particular project.

Neither a low calculated percentage nor a familiar wire size proves that an installation is safe. Voltage drop is one part of the complete electrical assessment.

Questions you may be asking.