Electrical Cable Size: Current, Voltage Drop, and Safety

Choosing a cable size is one of the most consequential decisions in any electrical installation, and it is also one of the easiest to get wrong by focusing on only one factor. A cable must carry its load current without overheating, it must keep voltage drop within limits so equipment works properly, and it must allow the protective device to clear a fault quickly enough to be safe. A size that satisfies one of those checks can fail another, especially on long runs. The Electrical Cable Size Calculator helps you test a size against those checks together, but the result is guidance only. Local electrical codes and a licensed electrician govern the final selection, and cable work on supply-connected installations should only be carried out by qualified people.

This guide explains the three main checks in plain terms, shows a worked example with checked arithmetic, and lists the mistakes that most often lead to undersized cables. Use it to understand the process and to have an informed conversation with your electrician, not as a substitute for proper design.

Check one: current-carrying capacity

Every cable has a current rating that depends on its conductor size, its insulation type, and how it is installed. A cable buried in thermal insulation, run in a hot roof space, grouped with other loaded cables, or enclosed in conduit cannot lose heat as easily as a single cable clipped to a wall in free air, so its safe current is lower. That reduction is applied through derating factors: one factor for ambient temperature, one for grouping, one for thermal insulation, and so on, depending on the code. The design current, after allowing for the load and any diversity that the code permits, must be no greater than the cable's derated rating. The protective device rating also fits into this picture: the device must protect the cable against overload, so the device rating and the cable rating are chosen together, not independently.

Conductor material matters as well. Copper is the usual choice for most installations because of its conductivity and joint reliability. Aluminium is used for larger sizes in some applications, but it needs a larger cross-section for the same current and careful jointing. Insulation type matters because different materials tolerate different maximum operating temperatures. A cable with insulation rated for ninety degrees can carry more current than the same size with insulation rated for seventy degrees, all else being equal, but the terminations and equipment it connects to must also be suitable for that temperature. These are details a qualified designer checks from tables in the applicable code; the calculator helps you see the direction of each effect, but the code tables are the authority.

Check two: voltage drop

Voltage drop is the voltage lost along the cable due to its resistance, and for alternating current circuits, its reactance as well. The longer the run and the higher the current, the larger the drop. The load at the far end then receives less than the supply voltage, which can cause motors to run hot or fail to start, lights to dim, heaters to underperform, and electronic equipment to misbehave. Codes typically limit voltage drop to a small percentage of the supply voltage, often around three to five percent depending on the circuit type and the code. The limit applies to the whole path from the supply point to the load, not just to one cable section, so a long submain plus a long final circuit can exceed the limit even when each section looks fine on its own.

Voltage drop is where cable size often has to increase beyond what current capacity alone would suggest. Doubling the length doubles the drop, so a cable that is perfectly adequate for a ten-metre run may be inadequate for a forty-metre run to the same load. The remedy is a larger cross-section, which lowers resistance and therefore drop, or a redesign such as moving the distribution board closer to the load or splitting the load across circuits. The calculator lets you test this directly: keep the load the same, increase the length, and watch the voltage drop rise and the recommended size step up. That sensitivity is the most useful thing a cable calculator teaches, because it breaks the habit of choosing size from current alone.

Check three: fault protection and earth fault loop impedance

The third check concerns what happens during a fault. When a live conductor touches an exposed metal part or another conductor, the fault current must be high enough, and flow for a short enough time, for the protective device to operate within the time the code requires. Fault current depends on the impedance of the whole fault loop, including the supply, the line conductor, and the protective conductor back to the source. A long, small cable has higher impedance, so fault current is lower and disconnection is slower, which is less safe. Increasing the cable size, including the protective conductor where required, lowers loop impedance and improves disconnection. This check is sometimes called the earth fault loop impedance check, and it is verified by calculation during design and by measurement after installation, using instruments and procedures that qualified people are trained to use.

Short-circuit withstand is related: the cable must survive the fault current until the device clears it, without insulation damage. That check uses the same kind of heating logic as earthing conductor sizing, relating fault current and time to the conductor area. For most final circuits protected by modern breakers, the device clears quickly enough that this check is easily met, but for larger circuits and slower devices it must be checked explicitly. Again, the calculator can flag a problem, but the code and the device data govern.

Worked example with sample values

Take a sample single-phase circuit supplying a load of three thousand watts at two hundred and thirty volts. The load current is power divided by voltage, which is three thousand divided by two hundred and thirty, which equals about thirteen point zero four amperes. Suppose the circuit is a twenty-metre run, one way, so the total conductor length out and back is forty metres, and suppose the cable is copper with a resistance of about seven point four one ohms per kilometre for a two point five square millimetre conductor, a typical planning figure. Convert to ohms per metre: seven point four one divided by one thousand equals zero point zero zero seven four one ohms per metre. The resistance of the loop is zero point zero zero seven four one multiplied by forty, which equals about zero point two nine six ohms. Voltage drop is current multiplied by resistance, which is thirteen point zero four multiplied by zero point two nine six, which equals about three point eight six volts. As a percentage of two hundred and thirty volts, that is three point eight six divided by two hundred and thirty, multiplied by one hundred, which equals about one point six eight percent. That is within a typical five percent limit, so voltage drop would not force a larger size in this sample case, provided the rest of the path from the supply is also within limits.

Now extend the same sample load to a sixty-metre one-way run, giving a loop length of one hundred and twenty metres. Loop resistance becomes zero point zero zero seven four one multiplied by one hundred and twenty, which equals about zero point eight eight nine ohms. Voltage drop becomes thirteen point zero four multiplied by zero point eight eight nine, which equals about eleven point six volts, or about five point zero four percent of two hundred and thirty volts. That is at or above a typical five percent limit, so the sample design would need a larger cable, a shorter route, or a redesign, even though the current rating of the two point five square millimetre cable might still be adequate for thirteen amperes. This contrast is the heart of cable sizing: the same load, the same cable, and a different length give a different answer. These sample figures use simplified resistance values and ignore reactance and temperature effects, so treat them as an illustration of the method, not as a design for a real circuit.

Common mistakes to avoid

The most common mistake is sizing from a current table alone without checking voltage drop or fault loop impedance. The second is forgetting derating: a cable rated for twenty amperes in free air may only be safe at fourteen or fifteen amperes when grouped, insulated, or in a hot space. The third is using the load's running current for a motor circuit without allowing for starting current and duty, where the code may require a different approach. The fourth is ignoring the protective conductor: the line and protective conductors are a pair for fault purposes, and an undersized protective conductor can undermine an otherwise adequate line conductor. The fifth is poor installation: cables crushed by fixings, bent too tightly, buried in insulation without derating, or run alongside hot pipes will not perform as their tables promise. The sixth is reusing an old cable for a new, larger load without rechecking all three checks, because the old cable was sized for the old load and the old route.

Also watch for voltage drop across the whole installation, not just the final circuit. A submain that already drops two percent leaves only three percent for the final circuit under a five percent total limit. Designers keep a voltage drop budget across sections, and a calculator result for one section should be read in that context. If you are adding a load to an existing installation, ask your electrician to check the cumulative drop and the fault loop impedance at the new point, not just the new cable in isolation.

Using the calculator well

Enter the real load, the real length measured along the cable route rather than the straight-line distance on a plan, the supply voltage and phase, the installation method that best matches reality, and the ambient conditions. Then read all three checks, not just the headline size. Try the next size up and the next size down to see which check changes the answer, and try a longer route to see how sensitive the result is to measurement error. If the result is borderline on any check, the larger size is usually the sensible choice, because cable is hard to replace once installed, while the cost difference between adjacent sizes is often small compared with the cost of rewiring. Keep a record of the inputs and the result for each circuit, because that record is useful for future additions and for fault finding.

For related work, the Single Phase Power Calculator helps you derive current from power and voltage, and the Three Phase Power Calculator does the same for three-phase loads. You can also read the related guide on parallel battery connections at /blog/battery-series-parallel-calculator-guide/ for a different kind of conductor and connection reasoning. Throughout, remember the limit that applies to every cable guide: the calculator is guidance only, local codes and a licensed electrician govern, and only qualified people should carry out electrical installation work.