Electrical cable sizing is one of those decisions that can look simple on a drawing but has major consequences once a project is operating. Selecting a cable based only on the equipment's rated current is not enough. The installation method, ambient temperature, voltage drop, grouping, fault conditions and future loading can all influence the final cable size.

This is particularly important in the UAE, where electrical installations can operate in demanding environmental conditions. High ambient temperatures, long cable routes, dense service areas and large commercial or industrial loads can all affect cable performance.

A properly sized cable helps maintain reliable equipment operation, control energy losses and reduce unnecessary thermal stress. An undersized cable, on the other hand, can create overheating, excessive voltage drop and premature insulation deterioration.

This guide explains the main factors engineers and contractors should consider when sizing electrical cables for UAE projects.

Why Correct Cable Sizing Matters

The primary purpose of cable sizing is to ensure that a conductor can safely carry the required electrical current under the actual installation conditions.

When a cable carries current, it generates heat because of the resistance of the conductor. If this heat cannot be dissipated effectively, the cable temperature can rise beyond its permitted operating limit.

Cable sizing therefore has two important objectives:

Carry the required load current safely

Keep voltage drop within acceptable limits

But practical cable selection often involves additional considerations such as short-circuit withstand capability, installation conditions, mechanical protection and future expansion.

A cable that appears adequate based on ampacity alone may therefore be unsuitable once all these factors are considered.

Start With the Load

The first step is to understand what the cable will supply.

Engineers normally establish the design current from the connected load, considering the supply voltage, phase configuration and expected operating conditions.

For a three-phase load, for example, the current depends on the power requirement, system voltage and power factor.

The important point is that cable sizing should be based on the design load, rather than simply copying the nameplate current of one item of equipment.

For systems containing motors, transformers, HVAC equipment or other loads with starting characteristics, the design should also consider starting or inrush conditions where applicable.

Loads that operate continuously may also require different considerations from intermittent loads.

Ambient Temperature Is Critical in the UAE

One of the most important factors in UAE cable selection is temperature.

Cable current-carrying capacity is normally based on a specified reference ambient temperature. When the actual ambient temperature is higher, the conductor has less ability to dissipate heat.

This can require a temperature correction or derating factor.

The issue becomes especially important for:

Outdoor cable routes

Rooftop installations

Cable trays exposed to direct sunlight

Plant rooms with poor ventilation

Utility areas

Industrial facilities

Electrical rooms with high heat loads

A cable that can carry a particular current under standard reference conditions may need to be increased in size when installed in a significantly hotter environment.

This is why UAE projects should not simply copy cable sizes from projects located in cooler climates.

Installation Method Changes Cable Capacity

The same cable can have different current-carrying capacities depending on how it is installed.

For example, cables installed:

In free air

On cable trays

In conduits

Buried underground

In ducts

Closely grouped together

may have different thermal characteristics.

Heat generated by one cable can also affect neighbouring cables.

When multiple loaded cables are installed together, the ability of the group to dissipate heat can decrease. A grouping or bunching correction factor may therefore need to be applied.

This is particularly relevant in large commercial and industrial projects where cable trays can contain numerous power circuits.

Voltage Drop Must Be Checked

Ampacity is only half of the cable-sizing decision.

Voltage drop can become a significant issue when cable routes are long.

As current travels through a cable, the conductor's resistance causes a reduction in voltage between the supply and the load.

Excessive voltage drop can result in:

Poor motor performance

Difficulty starting motors

Reduced lighting performance

Equipment malfunction

Increased current in some applications

Lower overall system efficiency

Long feeder circuits and remote loads therefore deserve particular attention.

For example, a cable may have sufficient current-carrying capacity but still be increased in size because its voltage drop is too high.

This is one reason simply using an ampacity table is not always enough.

Conductor Material and Cable Construction

Copper and aluminium are both widely used conductor materials, but their electrical and mechanical characteristics differ.

Copper generally provides lower electrical resistance for a given conductor size, while aluminium can offer weight and cost advantages for larger installations.

The selected cable should also be appropriate for the intended environment and installation method.

Other considerations include:

Insulation type

Rated conductor temperature

Number of cores

Armouring requirements

Fire performance

Mechanical protection

Indoor or outdoor installation

Exposure to moisture or chemicals

Cable construction should therefore be selected as part of the overall electrical design rather than independently from it.

Short-Circuit Withstand Capability

Normal operating current is not the only current a cable must survive.

During a short circuit, very high current can flow for a short period before the protective device disconnects the fault.

The cable must be capable of withstanding the resulting thermal and mechanical stresses for the relevant fault-clearing time.

This means cable selection should be coordinated with the protective device and the available fault current.

A cable with adequate continuous ampacity may still require evaluation for short-circuit withstand capability.

Cable Sizing for Motors and HVAC Equipment

Motors and HVAC equipment deserve additional attention because their electrical behaviour can differ from simple resistive loads.

Motor starting can produce significantly higher current than normal operating current.

Although protective devices and motor-control equipment are designed to accommodate these characteristics, the cable and associated equipment must still be selected appropriately.

UAE buildings often have substantial HVAC loads, making this especially important for:

Chillers

Pumps

Cooling towers

Air-handling units

Large ventilation systems

Mechanical plant

A good design considers both normal operating conditions and the actual starting and operating characteristics of the equipment.

Cable Sizing and Energy Efficiency

Correct cable sizing is also an energy-efficiency decision.

Every conductor has electrical resistance, and current flowing through that resistance produces power losses.

Using a larger conductor can reduce resistance and therefore reduce losses, particularly on heavily loaded or long circuits.

However, this does not mean that the largest possible cable should always be selected.

The designer must balance:

Initial cable cost

Installation cost

Energy losses

Voltage-drop requirements

Expected operating hours

Future load growth

Available space

Termination requirements

For circuits operating continuously at high load, investing in an appropriately sized conductor can sometimes produce meaningful long-term savings.

Don't Forget Future Expansion

Electrical systems rarely remain exactly as originally designed.

Commercial buildings may add equipment. Industrial facilities may increase production capacity. Telecom sites may add additional equipment. EV charging infrastructure may also increase electrical demand.

For this reason, designers should consider reasonable future expansion where the project requirements justify it.

However, spare capacity should be based on a realistic engineering assessment rather than arbitrary oversizing.

Oversizing every cable can increase material costs, installation difficulty and the size of associated switchgear and containment.

The goal should be appropriate capacity with a sensible engineering margin.

A Practical Cable-Sizing Workflow

A straightforward cable-sizing process can help reduce mistakes.

Step 1: Establish the design load

Determine the connected and demand load, operating pattern and relevant starting conditions.

Step 2: Calculate the design current

Determine the expected current based on the electrical system and load characteristics.

Step 3: Select a preliminary cable

Choose a suitable conductor material, insulation type and installation configuration.

Step 4: Apply correction factors

Consider ambient temperature, grouping, installation method and other relevant derating conditions.

Step 5: Verify voltage drop

Check the complete cable length and ensure the resulting voltage drop is within the project's permitted limit.

Step 6: Check short-circuit withstand

Verify that the cable can withstand the prospective fault current for the required protection-clearing time.

Step 7: Coordinate protection

Ensure breakers, fuses and other protective devices are properly coordinated with the cable's characteristics.

Step 8: Review practical installation requirements

Check cable tray space, bending radius, termination requirements, mechanical protection and accessibility.

Step 9: Consider future requirements

Where appropriate, allow for realistic future expansion without unnecessarily oversizing the entire installation.

Common Cable-Sizing Mistakes

Several mistakes appear repeatedly in electrical projects.

Choosing a cable only from the load current: This ignores voltage drop and installation conditions.

Ignoring UAE temperatures: High ambient conditions can significantly affect allowable current.

Ignoring cable grouping: Closely installed cables can operate hotter than individual cables.

Checking voltage drop too late: Increasing cable size after containment and equipment selections can create unnecessary redesign work.

Ignoring fault conditions: Continuous load current is different from short-circuit withstand requirements.

Using outdated tables or assumptions: Cable ratings and project requirements should always be verified against the applicable standards and project specifications.

Conclusion

Electrical cable sizing in the UAE should be treated as a complete engineering exercise rather than a simple current-rating lookup.

Load current, ambient temperature, installation method, grouping, voltage drop, short-circuit withstand, protection coordination and future requirements all influence the final selection.

The harsh environmental conditions found across many UAE installations make thermal considerations particularly important. At the same time, long cable routes and large HVAC and industrial loads can make voltage drop and energy losses significant.

The best cable is therefore not necessarily the cheapest option or the largest available size. It is the cable that safely meets the project's electrical, thermal, operational and economic requirements.

A disciplined sizing process at the design stage can help reduce overheating risks, improve equipment performance, control energy losses and avoid costly changes during construction.

Frequently Asked Questions

1. Why is cable sizing particularly important in UAE projects?

High ambient temperatures can reduce a cable's ability to dissipate heat, while large commercial, HVAC and industrial loads can create substantial electrical demand. Installation conditions and cable grouping can further affect the allowable current.

2. Is cable ampacity enough to determine the cable size?

No. Ampacity is only one part of the calculation. Voltage drop, short-circuit withstand, ambient temperature, grouping, installation method and protection coordination should also be checked.

3. Does a longer cable require a larger size?

Not necessarily, but longer cable runs increase voltage drop. If the calculated voltage drop exceeds the project's acceptable limit, a larger conductor or another suitable design solution may be required.

4. Why does cable grouping affect its current rating?

Multiple loaded cables installed close together generate heat in the same area. Reduced heat dissipation can increase conductor temperature, so a correction factor may be required to maintain safe operating conditions.

5. Can using a larger cable improve energy efficiency?

Yes, a larger conductor generally has lower resistance, which can reduce resistive losses. However, the additional cable cost should be balanced against expected energy savings, operating hours, voltage-drop requirements and the overall project economics.