Voltage Drop & Cable Sizing
A practical guide to voltage drop and how it drives cable sizing: why long, heavily loaded cables lose voltage, how cross-section and length affect it, the typical percentage limits, and how UAE conditions push designers toward larger conductors.
Voltage drop is the gradual loss of voltage along a cable as current flows through it. Every conductor has resistance, and that resistance turns a small part of the supply voltage into heat over the length of the run. By the time power reaches a distant load, the voltage available is slightly lower than at the source — and if the cable is too small or too long, that loss becomes a problem.
Controlling voltage drop is one of the main reasons a cable may need to be larger than the current rating alone would suggest. Excessive voltage drop causes dim lighting, motors that run hot and struggle to start, and electronics that misbehave. In the UAE, with long cable runs in large developments and high ambient temperatures that already force derating, voltage drop is a routine and important part of cable sizing.
How it works
Voltage drop depends on four main factors: the current flowing, the length of the cable run, the conductor's resistance per metre (which falls as cross-section increases), and, on AC circuits, the cable reactance. In simple terms, more current and longer cables increase the drop, while a larger conductor reduces it. This is why a long circuit often needs a bigger cable than a short one carrying the same current.
Voltage drop is usually expressed as a percentage of the nominal supply voltage. Wiring codes set limits to keep equipment working correctly — commonly around 3% for lighting and about 5% overall from the origin of the installation to the load, though the exact figures depend on the standard and the type of circuit. The cable is sized so the calculated drop stays within these limits.
Cable selection therefore involves two checks that must both pass. First, the conductor must have enough current-carrying capacity (after derating) so it does not overheat. Second, the voltage drop over the full route must be within the allowed percentage. On short circuits the current rating usually governs the size; on long runs the voltage-drop check often forces a larger conductor than thermal capacity alone would need.
Manufacturers publish a millivolt-per-amp-per-metre value for each cable size, which makes the calculation straightforward: multiply this figure by the design current and the route length to get the drop, then convert to a percentage. For three-phase circuits a line-to-line factor is applied. Designers also keep the load balanced across phases, since an unbalanced load raises the drop on the most loaded phase.
The UAE climate makes this discipline stricter. High ambient and ground temperatures reduce a cable's current rating, and grouping many cables together reduces it further, so conductors are already derated before voltage drop is considered. Long feeders to remote pump rooms, lift motors, chillers and external lighting must all be checked, and the final cable size is the larger of the two requirements — adequate current capacity and acceptable voltage drop.
Main types
In the UAE
- In Abu Dhabi, cable sizing and voltage-drop limits follow the Department of Energy / distribution company wiring regulations, which are based on the principles of IEC 60364 for both current capacity and permissible voltage drop.
- High UAE ambient and ground temperatures force significant derating of cable current ratings, so conductors are often larger than in cooler climates even before the voltage-drop check is applied.
- Large UAE developments have long feeder routes to pump rooms, lifts, chillers and external lighting, making voltage-drop calculation a routine compliance step, alongside coordination with the protective device and earthing.
How GPR applies this
GPR sizes cables for residential, commercial and industrial projects across Abu Dhabi by checking both current-carrying capacity and voltage drop over the full route, applying correct derating for the local climate, grouping and installation method. We balance loads across phases, upsize feeders to remote plant where the drop demands it, and coordinate the final cable size with protective devices and earthing so installations meet Department of Energy wiring regulations and perform reliably in service.
Frequently asked questions
What causes voltage drop in a cable?
The resistance (and on AC, reactance) of the conductor. As current flows, a small part of the supply voltage is lost as heat along the cable, so less voltage reaches the load.
What is an acceptable voltage drop?
Wiring codes commonly limit it to about 3% for lighting and around 5% overall from the origin to the load, though exact values depend on the standard and circuit type.
How do I reduce voltage drop?
Use a larger conductor cross-section, shorten the route where possible, balance loads across phases, and on long feeders increase the cable size beyond the minimum needed for current capacity.
Does voltage drop or current capacity decide cable size?
Both must be satisfied. On short runs current capacity usually governs; on long runs the voltage-drop limit often forces a larger conductor than thermal capacity alone.
Why does the UAE climate affect cable sizing?
High ambient and ground temperatures, and grouping of cables, reduce the current rating (derating), so conductors are larger before voltage drop is even considered.