Voltage drop
Volts lost along a cable's resistance under load — why the far end of a long run can misbehave.
What it is
Every cable has some resistance. When current flows through it, some voltage is 'used up' along the way, so the voltage at the load is a little lower than at the supply. That loss is voltage drop.
Why it matters
Voltage drop grows with current and with cable length, and shrinks with bigger conductors. On a long, loaded run it can leave too little voltage at the far end — dim lights, a contactor that won't hold in, a motor that struggles.
It only shows up under load, which is why you measure with the load running. A sudden step in voltage along the run points to a high-resistance joint (which also heats); a gradual drop points to cable size or length.
On site
Always measure under load. A bad joint produces a localised drop and heat; an undersized or over-long run produces a gradual drop. The remedy differs — repair the joint, or address the conductor sizing/length.
Safety first
Measure under load and treat circuits as live. A high-resistance joint that drops voltage also overheats — a fire risk to rectify.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Voltage drop on a long cable run
Equipment at the end of a long run misbehaves — dim lights, a contactor that won't hold, a motor struggling — because volt-drop along the cable leaves too little voltage at the load.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
Related definitions
Ohm's law & power
The relationship between voltage, current and resistance — and how it gives you power.
Series vs parallel circuits
In series, current is shared and voltage divides; in parallel, voltage is shared and current divides.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.