Cable Size and Voltage Drop: Why Distance Matters

An applied extra to Cobler's Electricity Fundamentals course.

You are cutting tiling at the far corner of the house, the drill on the end of two extension cords daisy-chained together, and it just will not bite the way it did in the garage. It sounds strained, it slows under load, the cut takes longer. When you coil the cords up afterwards, they are warm. Not the plug, the whole length of the cable, warm along its run. Same drill, same 13 A socket it was plugged into in the garage. The only thing that changed was the distance.

Most of us shrug and blame the tool. The warm cord is trying to tell you something more interesting than that. It has a name, voltage drop, and once you have felt it you will never quite unsee it.

Why does distance change anything if the socket is still 230 V?

Because the cable is not a neutral pipe that delivers the socket's voltage untouched to the far end. The cable is itself a resistor, wired in series with whatever you plug in, and every metre of copper adds a little more of it.

In volts and amps the course set out the one relationship that governs this: Ohm's law, voltage equals current times resistance, V = I × R. A thin wire has a small resistance per metre, so small you never notice it on a short run. Stretch that wire out over twenty or thirty metres and the little resistances add up into something real. Push a current through that accumulated resistance and, by V = I × R, some voltage gets used up inside the cable itself. It never reaches the drill. That used-up voltage is called voltage drop, and it is exactly the reading you would get if you measured across the two ends of the cord while the drill was running: a few volts, quietly missing.

The drill at the end does not see 230 V any more. It sees 230 V minus whatever the cable ate on the way. A motor handed less voltage delivers less power, so it labours, and a labouring motor pulls more current to compensate, which drops even more voltage in the cable. That is the strained sound.

So where does the missing voltage go? It becomes the warmth in the cord

The voltage does not vanish. It turns to heat, spread along the whole length of the cable.

This is the satisfying part. Voltage drop is not an abstraction on an electrician's calculator. It is the warmth you felt when you coiled the cord up. Those missing volts, multiplied by the current flowing, are watts of power being dissipated as heat in the copper, the same way a kettle element or an old filament bulb turns electricity into heat on purpose. The extension cord is doing it by accident, because it is too thin for the job over that distance. The warm cord is the voltage drop, made touchable. Feel a cord that is warm along its length under load and you are feeling energy that was supposed to reach your tool leaking out as heat instead.

Coiling makes it worse, incidentally, because a coiled cable cannot shed that heat to the air. Hold that thought, because it is the whole basis of a rule we will reach in a moment.

The two levers that set voltage drop: length and thickness

Voltage drop is set by two things you can actually change, and both behave simply.

The first is length. Drop is proportional to how far the current has to travel. Double the run and you double the drop. Two cords daisy-chained is twice one cord, which is why the corner of the house behaved worse than the garage.

The second is cross-section, the thickness of the copper. Drop is inversely proportional to it. Double the cross-sectional area and you halve the drop, because fat copper has lower resistance per metre. Going from a common 2.5 mm² cable to a 4 mm² one cuts the voltage drop by roughly 40% for the same run.

Put those together and cable sizing stops being mysterious. It is a two-question decision, and only two: how many amps will this cable carry, and how far does it have to go. Amps decide the minimum thickness the cable needs just to carry the load without overheating. Distance can then force it thicker still, because a cable that is fat enough for the heat can still be too thin for the length. An electrician sizing a long run is not being cautious for its own sake. They are answering the second question.

The rules of the trade, in plain terms

Wiring standards put a ceiling on how much voltage you are allowed to lose in the cable. Under BS 7671, the British wiring standard that underpins Malaysian practice, the limit is 3% for lighting circuits and 5% for power circuits, measured from the origin of the supply (TotalSkills, BS 7671 voltage-drop guide). Malaysian residential wiring is commonly held to a single 4% overall figure, in line with the Suruhanjaya Tenaga (Energy Commission) Guidelines for Electrical Wiring in Residential Buildings, which follow MS IEC 60364. On a 230 V supply, 4% is about 9 volts. That is the entire budget the cable is allowed to spend between your meter and your socket.

A few percent sounds generous until you remember the levers. A long run burns through it fast, which is why electricians size up the cable on anything that travels far. It is cheaper to lay thicker copper once than to explain why the socket in the back bedroom runs everything weakly.

It is also why heavy appliances get their own fat circuit rather than sharing. A storage water heater, an oven, and especially an EV charger pull heavy current continuously for a long time. A 7 kW single-phase home charger draws around 32 A for hours on end. Malaysian guidance requires it on a dedicated final circuit, and sizing typically starts at 6 mm² copper for that 32 A load (Suruhanjaya Tenaga Guidelines on Electric Vehicle Charging System, 2025). That thickness is partly about carrying the amps without cooking, and partly about keeping the voltage drop inside budget over the run to wherever the car parks. Big continuous load plus real distance is precisely the combination that eats voltage.

Why a cable rated "X amps" sometimes is not

There is a second reason installers upsize, and it is the coiled-cord problem again. A cable's rated current assumes it can shed its heat freely. Bundle several cables together in one trunking, or run them through a Malaysian ceiling void baking at 45 °C, and they cannot. So the rating is derated, multiplied by factors below 1 to reflect the conditions it actually lives in.

The factors stack. A grouping factor for bundled cables is often around 0.8 to 0.9, and a temperature factor for a hot ceiling around 0.87. Apply both and a cable rated 100 A carries only about 78 A safely (ECalPro, cable derating under IEC 60364). A cable that looks big enough on the packet can be too small once it is bundled with its neighbours and sweating in a roof space. This is not electricians padding the invoice. It is the same physics as the warm cord: a conductor that cannot lose its heat has to carry less current, so you give it more copper.

What this looks like across a whole building

The extension cord is the pocket version of a problem that scales up to an entire site. Long feeders to a far wing, distribution boards at the end of long cable runs, circuits that were fine when the building opened and are now feeding loads nobody planned for.

It shows up two ways, and both are things a facility team can feel. The far ends of the site collect voltage complaints: equipment that runs weak or trips on hot afternoons when everything is loaded, the building-scale version of the drill in the corner. And the cabling itself runs warm, sometimes warm enough to feel at a distribution board, because undersized or ageing conductors are dropping voltage and dumping it as heat exactly where you do not want it. A board that is warm to the hand under load is telling you the same thing the cord did. Left alone, that heat is the front end of the story that ends in voltage sags tripping equipment at the weak end of a plant.

The fix is not guesswork, because voltage drop is one of the few electrical problems you can point a meter at directly. You measure the voltage at the origin and at the far end under real load, and the difference is the drop, in volts, on the table. From there it is the two levers: a shorter path if the layout allows, or thicker copper if it does not. Cable sizing was only ever a two-question decision. The building just has more cables asking the same two questions the extension cord did.

This is an applied extra to Cobler's Electricity Fundamentals course. It builds directly on Volts and Amps and Ohm's law, and on how current flows through a series path in What Is an Electric Circuit.

Cobler builds CobiNeural, a smart operation platform that measures voltage, current and power across a Malaysian building or factory so warm boards and weak far ends become numbers you can act on. Book a demo to see your own readings on one screen.