Sizing Cable Correctly: Current, Distance and Voltage Drop
A pump at the far end of a site, fed by a cable sized to match its breaker. The pump runs, but draws more current than expected, runs hot, restarts with difficulty, and the cable is warm along its length. Nothing burned, the breaker never tripped. The problem is that the run was long and the cable was chosen on current alone — which is half the calculation, not all of it.
1. The Three Governing Factors
Cable size is set by three independent criteria, and the final size is the largest that satisfies all of them — not the smallest that satisfies one:
- Current carrying capacity: the maximum continuous current the cable can carry without conductor temperature exceeding the limit for its insulation. The thermal criterion.
- Voltage drop: the voltage lost along the cable must not impair the operation of loads at its end. The performance criterion, and often the governing one.
- Short circuit withstand: the cable must survive the large thermal energy passing through it in the brief moments before the protective device clears the fault, without damaging its insulation. Checked against the protective device characteristics and prospective fault level.
2. Why Voltage Drop Usually Governs Long Runs
Every conductor has resistance proportional to length and inversely proportional to cross-section. Current through it drops voltage equal to current times resistance. On a short run the drop is negligible; on a long run total resistance grows and so does the drop, however thermally adequate the size may be.
The practical result: over a long distance a given size may be entirely adequate for current yet produce an unacceptable voltage drop, forcing the designer up one size or more purely for voltage. That is not over-specification — it is correct calculation.
What Voltage Drop Does to Loads
- Motors: doubly harmed. Low voltage reduces starting torque so starting is sluggish, and the motor draws higher current to maintain output, running hot and degrading its insulation over time.
- Lighting: dimming and instability, with flicker or shortened life on some drivers.
- Heating appliances: noticeably reduced output and longer run times.
- Electronics and controls: unpredictable behaviour at voltage limits, possibly self-protective shutdown.
The General Principle on Limits
The accepted engineering principle is that voltage drop from the supply point to the load terminals should stay within a small percentage of nominal voltage, typically with a tighter allowance for sensitive lighting circuits and slightly more latitude for power circuits. Exact percentages differ between codes, installation types and loads, so take them from the local code in force rather than from general figures. A useful mental rule: every extra metre spends part of the voltage budget, and the budget must be accounted from the distribution board all the way to the last point on the circuit, not across one section.
3. Derating Factors
Tabulated current capacities assume ideal reference conditions. Any departure reduces the real capacity, and factors multiply together, so the combined reduction can be severe:
| Condition | Effect on capacity | Physical reason |
|---|---|---|
| Grouping cables together | Significant reduction, growing with the number | Each cable heats its neighbours and blocks their cooling |
| High ambient temperature | Reduction growing with temperature | Less temperature difference available to shed conductor heat |
| Contact with or burial in thermal insulation | Large reduction, worst when fully surrounded | Insulation traps heat around the cable |
| Enclosed conduit and trunking | Depends on method and conductor count | Reduced ventilation |
| Direct burial in ground | Depends on soil type, moisture and depth | Varying soil thermal resistivity |
| Crowded multi-layer trays | Reduction increasing with layers | Heat build-up and poor airflow |
4. Copper vs Aluminium
| Aspect | Copper | Aluminium |
|---|---|---|
| Conductivity per size | Higher | Lower, so needs a larger size for the same current |
| Weight per ampere | Heavier | Lighter, an advantage on large feeders |
| Bending | Better | Stiffer, needs larger bend radii |
| Terminations | Standard and straightforward | Critical: needs specified lugs, compounds and torques, and no direct contact with copper |
| Creep and oxidation | Less sensitive | Creeps under pressure and forms an insulating oxide, weakening joints and causing heating |
Practically: aluminium is a legitimate, widely used material for feeders and large runs where weight and economy matter, but it is unforgiving in termination. Most aluminium problems are in the joints, not the conductor: an improperly made aluminium termination heats progressively until it becomes a hazard. For small final circuits and terminations, copper remains the usual choice.
5. Why Undersized Cable Is a Fire Risk
This is the most serious point. The misconception is that an undersized cable means merely reduced performance. In reality, a cable carrying more than its capacity becomes a heating element distributed along its length:
- Heat generated rises with the square of the current, so a modest current increase means a large heating increase.
- Sustained heat accelerates insulation degradation, leaving it brittle, cracked and no longer insulating.
- Degraded insulation leads to earth leakage and then to a short circuit between conductors, possibly in a concealed location inside a wall or a trunking full of combustible material.
- Worst of all, the process is silent and gradual. No smell, no spark at first; the circuit appears normal for years until it fails once.
The Illusion That the Breaker Protects the Cable
- An overload protective device is selected to protect the cable, so its rating must be coordinated with the cable's derated capacity — not with the load's demand.
- If the breaker is larger than the cable's capacity, the cable can run continuously in an overheating regime while the current never reaches a level that trips the device. Protection is effectively absent.
- Small sustained overload is precisely the most dangerous case: the breaker never trips and the cable slowly deteriorates.
- Increasing breaker size to stop nuisance tripping, instead of addressing the load or enlarging the cable, is among the most dangerous practices in electrical work.
6. The Correct Order of Thinking
- Establish the real expected load, its characteristics and starting current, plus sensible spare capacity.
- Record the actual route length, not straight-line distance — real routes are far longer.
- Record installation conditions precisely: grouping, ambient temperature, containment, thermal insulation, burial.
- Select a size that meets capacity after all derating factors are applied together.
- Check voltage drop over the complete route and increase the size if needed.
- Check short circuit withstand in coordination with the protective device.
- Confirm the protective device actually protects the chosen cable, and that the protective earth conductor is adequate.
- Have the whole result reviewed by a qualified electrical engineer against the local code before installation.
Conclusion
The correct size is the largest demanded by three tests: thermal capacity, voltage drop and short circuit withstand. On long runs voltage drop usually governs, so do not be surprised when the calculation demands a size that looks generous. Record real installation conditions and apply derating, understand that the breaker protects the cable rather than excusing a small one, and remember that undersized cable is not a performance issue but a silent fire risk. In any real design, the local code and a qualified engineer are the authority.
