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.
Safety first
Measure under load to see real volt-drop. Treat the circuit as live while testing and isolate before any wiring work.
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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Cable too small / run too long for the load
Most likelyConductor size and length mean significant volt-drop under load, starving the far end.
- 2
High-resistance joint along the run
#2A loose or corroded joint adds resistance and drops voltage, often with local heating.
- 3
Overloaded circuit
#3More load than designed increases the current and the volt-drop.
- 4
Loose terminations at either end
Least likelyLoose terminals at the supply or load end create extra drop.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Measure voltage at the supply end and at the load end, under load, and compare.
Only a small difference between the two under load.
Little drop — the load problem is elsewhere, not volt-drop.
Significant drop under load — quantify it and find where it occurs.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Volt-drop on long run
→ step 2 - 2decision
Is there significant drop end-to-end under load?
Yes→ step 3No→ step 4 - 3decision
Is the drop a step at a joint (vs gradual)?
Yes→ step 5No→ step 6 - 4result
Little drop — the issue isn't volt-drop; look elsewhere.
- 5result
Bad joint — repair the high-resistance connection.
- 6decision
Are terminations tight and load within cable capability?
Yes→ step 7No→ step 8 - 7result
Run undersized for the duty — design change needed.
- 8result
Loose termination or overload — repair/rebalance.
Common mistakes apprentices make
- Measuring with no load and seeing no drop.
- Not localising whether the drop is gradual or at a joint.
- Overlooking a high-resistance joint that also runs hot.
- Ignoring that the circuit is simply overloaded.
When to stop & escalate
If the cable is genuinely undersized for the run/duty, the remedy is a design change (larger conductor or reduced load) — plan it rather than working around it. A hot joint should be repaired promptly.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
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.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.
Ohm's law & power
The relationship between voltage, current and resistance — and how it gives you power.
Voltage drop
Volts lost along a cable's resistance under load — why the far end of a long run can misbehave.
Cable current capacity & derating
Why the same cable can safely carry less current in some installations than others — it all comes down to heat.