Part 5 – Cable Selection by Ampacity
Introduction
After calculating the Full Load Current (FLC), the next step in the cable sizing process is selecting a preliminary cable size. This stage is often misunderstood because many engineers assume that choosing a cable with a current rating slightly higher than the calculated load current is sufficient. In reality, this is only the starting point of the design process.
The first cable selected is known as the Preliminary Cable Size or Trial Cable Size. It is not the final cable recommendation. This cable must still pass several engineering checks, including derating, voltage drop, power loss and short-circuit withstand verification before it can be approved.
Professional electrical design follows a sequence of verification steps rather than selecting a cable directly from a manufacturer's catalogue. This approach ensures that the cable remains safe and reliable under actual operating conditions throughout the design life of the project.
Whether calculations are performed manually or verified using OneCalcApp.com, engineers should always understand that preliminary cable selection is only the first engineering checkpoint.
What Is Cable Ampacity?
Cable Ampacity is the maximum continuous electrical current that a cable can carry under specified installation conditions without exceeding the permissible operating temperature of its conductor and insulation.
In simple terms, ampacity answers the question:
"How much current can this cable safely carry continuously without overheating?"
Every conductor generates heat whenever electrical current flows through it. This heating is caused by the electrical resistance of the conductor and is commonly referred to as I²R (Joule) Heating.
If the heat generated inside the conductor exceeds the cable's ability to dissipate that heat to the surrounding environment, the conductor temperature will continue to rise.
Excessive temperature can lead to:
- Insulation ageing
- Reduction in cable life
- Loss of dielectric strength
- Premature insulation failure
- Fire hazards
- Unexpected plant shutdown
Therefore, current carrying capacity is one of the primary criteria used for selecting an electrical cable.
Why Cable Ampacity Is Important
The inverter output cable carries the complete AC output current continuously whenever the solar power plant is generating electricity.
Unlike cables used for intermittent loads, inverter cables in utility-scale solar plants may operate near their rated current for several hours every day.
If the selected cable has insufficient ampacity:
- The conductor temperature will increase.
- Voltage drop will rise.
- Power losses will increase.
- Insulation ageing will accelerate.
- Cable service life will reduce.
- Protective devices may trip unexpectedly.
- Plant availability may decrease.
On the other hand, selecting a cable with excessive ampacity increases:
- Cable cost
- Installation cost
- Cable tray size
- Cable gland size
- Termination cost
- Overall project cost
The objective of professional cable sizing is therefore to find the optimum cable size rather than simply the largest available conductor.
How Manufacturers Determine Cable Ampacity
Cable manufacturers do not assign current ratings randomly. The published ampacity values are obtained through standardized testing and calculations based on internationally accepted standards.
Ampacity depends on the balance between:
- Heat generated inside the conductor.
- Heat dissipated to the surrounding environment.
When these two become equal, the conductor reaches its steady-state operating temperature.
Manufacturers determine ampacity by considering:
- Conductor material
- Conductor cross-sectional area
- Insulation type
- Maximum conductor operating temperature
- Installation method
- Ambient temperature
- Thermal characteristics of the surrounding medium
These values are then published in manufacturer catalogues for reference.
Factors Affecting Cable Ampacity
Cable ampacity is not a fixed value. The same cable can carry different currents depending on where and how it is installed.
The major factors affecting ampacity include:
1. Conductor Material
Copper has lower electrical resistance than aluminium and therefore carries more current for the same cross-sectional area.
2. Conductor Cross-Sectional Area
Larger conductors have lower resistance and greater surface area for heat dissipation, allowing them to carry higher current.
3. Insulation Type
Different insulation materials have different maximum operating temperatures.
For example:
- PVC insulation generally operates at a lower permissible conductor temperature.
- XLPE insulation allows a higher continuous operating temperature.
Higher permissible temperature generally results in higher current carrying capacity.
4. Installation Method
Heat dissipation varies significantly depending on whether the cable is:
- Installed in free air
- Installed in a cable tray
- Installed in a conduit
- Buried underground
- Installed in ducts
- Installed in trenches
A cable installed in free air usually has better cooling than one installed in a conduit.
5. Ambient Temperature
Higher surrounding temperatures reduce the cable's ability to dissipate heat.
As ambient temperature increases, cable ampacity decreases.
6. Cable Grouping
When several loaded cables are installed close together, they heat each other.
This mutual heating reduces the current carrying capacity of every cable in the group.
7. Soil Thermal Resistivity
For underground installations, soil acts as the cooling medium.
Dry soil has higher thermal resistance than moist soil, resulting in lower cable ampacity.
Preliminary Cable Selection Procedure
Professional engineers generally follow the following procedure:
Step 1
Calculate the Full Load Current.
↓
Step 2
Refer to the approved cable manufacturer's current rating tables.
↓
Step 3
Select the smallest cable whose rated current carrying capacity exceeds the calculated Full Load Current.
↓
Step 4
Treat this cable only as the trial cable.
↓
Step 5
Continue with derating calculations before confirming the final cable size.
Why the First Selected Cable Is Not the Final Cable
This is one of the biggest misconceptions among beginners.
The current rating published in manufacturer catalogues assumes reference installation conditions.
Actual project conditions may differ due to:
- High ambient temperature
- Multiple parallel cables
- Underground installation
- Poor ventilation
- Increased burial depth
- High soil thermal resistivity
Because of these differences, the cable selected from the catalogue may no longer be capable of carrying the required current safely.
Therefore, engineers apply derating factors in the next stage of the design process.
Only after derating is completed can the cable be confirmed or rejected.
Common Mistakes During Preliminary Cable Selection
Many cable sizing errors originate at this stage.
Some common mistakes include:
- Selecting a cable based only on Full Load Current.
- Ignoring installation method.
- Ignoring insulation type.
- Assuming manufacturer ampacity is valid under all conditions.
- Selecting cable based only on previous projects.
- Ignoring future expansion requirements.
- Using different manufacturers' tables interchangeably without verification.
Professional engineering practice requires every cable selection to be verified using the project-specific design conditions.
Engineering Best Practices
To improve design reliability:
- Always use the approved manufacturer's ampacity tables.
- Verify that the cable construction matches the project specification.
- Confirm conductor material before selection.
- Ensure the insulation type is suitable for the installation environment.
- Treat the first selected cable only as a preliminary choice.
- Continue with derating calculations before final approval.
- Document all assumptions used during cable selection.
How Onecalcapp Helps Engineers
During detailed engineering, designers often compare multiple cable sizes before selecting the optimum conductor.
Performing these comparisons manually using manufacturer tables can be time-consuming.
The OneCalcApp.com AC Cable Sizing Calculator helps engineers quickly compare different conductor sizes, organize design inputs and verify current carrying capacity before proceeding to derating and voltage drop calculations.
While the calculator simplifies repetitive tasks, engineers should always validate the final design using the approved project specifications and applicable standards.
Summary
At this stage, we have:
- Calculated the Full Load Current.
- Selected a preliminary cable based on rated ampacity.
- Understood that this is only a trial cable.
The next and most critical step is to determine whether this cable can still carry the required current under actual project conditions.
Engineering Takeaway
Preliminary cable selection is only the first ampacity checkpoint.
A cable that passes the basic rated-current comparison cannot yet be considered the final selected cable.
The trial cable must next be checked against actual installation conditions using appropriate derating factors before the final cable size is confirmed.
Calculate. Verify. Design with confidence.