Understanding Voltage Drop in Electrical Installations

Voltage drop is the difference between the voltage at the power source (switchboard or distribution board) and the voltage that actually arrives at the equipment. Every electrical cable has some resistance, and when current flows through that resistance, a portion of the supply voltage is “used up” by the cable itself rather than reaching the load.

Think of it like water pressure in a garden hose. If you have a short, wide hose connected to the tap, the water sprays out with good pressure. But if you add a long extension or use a narrow hose, the pressure at the sprinkler drops noticeably. The tap pressure hasn’t changed — the hose itself is consuming some of the pressure.

In electrical terms, the “tap” is your switchboard at 230 V (or 415 V for three-phase), the “hose” is the cable, and the “sprinkler” is your equipment. If the cable is too long, too thin, or carrying too much current, the voltage at the equipment drops below what it needs to work properly.

Electrical equipment is designed to operate within a specific voltage range — typically ±10% of the nominal voltage. When voltage drops beyond acceptable limits, real problems occur:

• Motors lose performance: An induction motor's torque is proportional to the square of the supply voltage. A 10% voltage drop reduces available torque by roughly 19%, making the motor work harder, draw more current, and overheat. Motor starting becomes particularly difficult at reduced voltage.
• Lighting dims or flickers: Incandescent and halogen lights are very sensitive to voltage — a 5% drop reduces light output by approximately 15%. LED drivers may exhibit flickering or fail to maintain constant output at low voltage.
• Electronic equipment malfunctions: Computers, PLCs, variable speed drives, and other sensitive equipment may reset, throw errors, or produce unreliable outputs if the supply voltage is too low.
• Energy is wasted: The voltage “lost” in the cable is converted to heat (P = I²R). While typically small compared to the load, this wasted energy adds up over time, particularly for high-current circuits running 24/7.
• Other equipment on the same supply is affected: Heavy loads that cause significant voltage drop can affect the voltage seen by all other equipment connected downstream of the same distribution board.

Three main factors determine how much voltage is lost along a cable:

1. Cable length: The longer the cable, the more resistance the current must flow through, and the greater the voltage drop. This is the most common reason for voltage drop problems. Double the cable length, and you double the voltage drop.
2. Cable size (cross-section): Thinner cables have higher resistance per metre. A 2.5 mm² cable has roughly four times the resistance of a 10 mm² cable. Using a larger cable reduces voltage drop proportionally.
3. Current: More current flowing through the same cable causes more voltage drop. Voltage drop is directly proportional to current — double the current, double the voltage drop.

There are secondary factors too: the conductor material (aluminium has about 64% higher resistance than copper), the cable temperature (resistance increases with temperature), and for AC circuits, the cable’s reactance (which becomes significant for larger cable sizes and is affected by the load’s power factor).

The basic formula captures these relationships: Voltage Drop = Current × Resistance × Length. For AC circuits, reactance is added to resistance, and the power factor determines how the two components combine.

Electrical standards specify maximum voltage drop limits as a percentage of the supply voltage:

For a 230 V supply, 5% voltage drop means the voltage at the load can be as low as 218.5 V. For a 415 V three-phase supply, 5% means the load sees as low as 394.25 V.

In practice, well-designed installations aim for 2–3% total voltage drop, leaving margin for load growth and transient conditions. The “total” voltage drop is measured from the origin of the installation (typically the meter or main switchboard) to the furthest point of utilisation.

There are two approaches to dealing with voltage drop: calculate it during design, and measure it during commissioning or troubleshooting.

Calculation (design stage): During design, voltage drop is calculated using the cable’s impedance data from the applicable standard and the expected load current. This is the method used by ECalPro and other cable sizing software. The formula uses tabulated mV/A/m values (millivolts per ampere per metre) that account for the cable’s resistance and reactance.

Measurement (commissioning/testing): Voltage drop can be measured by taking voltage readings at the switchboard and at the load simultaneously while the circuit is under full load. The difference is the actual voltage drop. A simpler approach is to measure the voltage at the load under full load and compare it to the no-load voltage at the same point. Digital multimeters with ±0.5% accuracy are sufficient for this measurement.

If measured voltage drop exceeds calculated values, this may indicate poor connections (loose terminals add resistance), damaged cable, or higher-than-expected load current.

If you’re facing a voltage drop problem — either during design or in an existing installation — here are practical strategies to reduce it:

1. Use a larger cable: The most direct solution. Increasing the cable cross-section reduces resistance and therefore voltage drop. Going from 4 mm² to 6 mm² reduces voltage drop by roughly 33%.
2. Shorten the cable run: If possible, relocate the distribution board closer to the load, or route the cable by a shorter path. Every metre saved reduces voltage drop proportionally.
3. Use copper instead of aluminium: If aluminium cable was specified, switching to copper of the same cross-section reduces voltage drop by about 39%. However, this may not be cost-effective for large cable sizes where aluminium is significantly cheaper.
4. Increase the supply voltage: For industrial installations, using a higher distribution voltage (e.g., 415 V instead of 230 V for single-phase loads via a local transformer) dramatically reduces current and therefore voltage drop for the same power.
5. Check connections: In existing installations, loose or corroded connections can add significant resistance. Re-tightening terminal connections to the correct torque can improve voltage drop measurably.
6. Power factor correction: For highly inductive loads (motors, transformers), installing power factor correction capacitors reduces the reactive component of voltage drop and the total current drawn.

This guide has introduced the concept of voltage drop at a practical level. To go deeper into the engineering methodology — including the full impedance formula, the mV/A/m tabulated method, and multi-standard limits — continue to the Voltage Drop Calculation Guide.

To see how voltage drop fits into the complete cable sizing process, read the Cable Sizing Methodology guide where voltage drop is Step 6 of the 8-step procedure.

Or try the Voltage Drop Calculator to see instantly how cable size, length, current, and power factor affect voltage drop across four international standards.