Electrical Cable General Requirements: General Considerations
Selecting the right electrical cable for industrial, commercial, or utility projects is more than just picking a size and type, it requires careful consideration of safety, performance, and compliance with industry standards. Poor selection can lead to overheating, voltage losses, nuisance tripping, or even catastrophic failures under fault conditions.
In this guide, we’ll cover general cable selection requirements with a focus on:
- Thermal short circuit capacity
- Voltage drop limits
- Current rating factors
- Loop impedance for earth faults
These are based on common project specifications, DEP 33.64.10.10-Gen., and IEC / IEE wiring regulations, and can be applied to HV (High Voltage), LV (Low Voltage), and control cables.
1. Thermal Short Circuit Capacity (HV Cables)
Thermal short circuit capacity defines how much fault current a cable can withstand for a short duration (typically 1–3 seconds) before its insulation or conductor is damaged.
Key Considerations:
- Depends on conductor cross-sectional area, material (copper/aluminum), and insulation type.
- Must match the maximum fault current that could occur at the installation point.
- Often calculated using IEC 60949 or IEEE 242 (Buff Book) methods.
Practical Tip: Always coordinate with the source switchboard’s short circuit breaking capacity and take into account fault limiting devices like fuses, relays, and circuit breakers.
2. Voltage Drop Requirements
Voltage drop occurs when electrical energy is lost as heat due to resistance in the cable. Excessive voltage drop can lead to:
- Reduced motor torque
- Dim lighting
- Malfunctioning control systems
Standard Limits (DEP 33.64.10.10-Gen.):
For A.C Cables:
- < 5% based on continuous maximum current loading and rated voltage.
- < 20% of rated equipment voltage during motor starting.
For D.C Cables:
- < 5% from minimum system voltage at the distribution board to minimum equipment operation voltage.
Example:
A 3-phase motor drawing 100 A at 0.85 PF over a 100 m cable with R = 0.2 Ω/km and X = 0.08 Ω/km:
Voltage drop ≈ 3.8% (acceptable for continuous load).
3. Maximum Cable Loop Impedance for Earth Faults
Loop impedance must be low enough to ensure the fault current is high enough to trip protective devices within the required time.
- Requirement: Earth fault clearance time ≤ 1s under solid earth fault conditions.
- Reason: Ensures conductor temperatures remain within safe limits and prevents insulation damage.
- Application: Especially important for long-distance cables where resistance is higher.
Tip: For critical systems, verify using on-site earth fault loop impedance testing.
4. Cable Current Rating
The current rating (ampacity) of a cable depends on:
- Conductor material (copper or aluminum)
- Insulation type (PVC, XLPE, EPR)
- Installation method (buried, tray, conduit)
- Ambient temperature and soil conditions
4.1 Rating Factors (Ampacity Adjustment)
Onshore Installations:
- Depth of laying: As per IEE Wiring Regulations 16th Edition (1991)
- Shallow burial = higher heat dissipation
- Deep burial = reduced cooling, lower rating
- Soil thermal resistivity: Poor heat dissipation reduces ampacity.
- Grouping of cables: Multiple cables in close proximity require derating.
Example Ampacity Reduction Factors:
| Condition | Factor |
|---|---|
| 40°C Ambient Air | 0.91 |
| 45°C Ambient Air | 0.87 |
| Buried at 1.0 m depth | 0.95 |
| Three cables touching | 0.85 |
5. Coordination with Protection Devices
Cable selection must align with the short circuit breaking capacity of upstream switchgear:
- Check CB/fuse ratings against maximum fault levels.
- Consider fault limiting characteristics of devices (current limiting fuses, electronic trip CBs).
- Match fault clearance time to cable thermal limit.
6. Motor Starting Considerations
When cables supply motors, account for starting currents (6–7 times full-load current for DOL starting):
- Ensure voltage drop during starting < 20% to avoid failure to start.
- Consider soft starters or VFDs for long cable runs to reduce drop.
7. Summary Table – General Cable Requirements
| Parameter | Requirement | Reference |
|---|---|---|
| Thermal Short Circuit Capacity | ≥ Max fault current for duration | IEC 60949 / IEEE 242 |
| Voltage Drop – AC Continuous | < 5% rated voltage | DEP 33.64.10.10-Gen |
| Voltage Drop – AC Motor Start | < 20% rated voltage | DEP 33.64.10.10-Gen |
| Voltage Drop – DC Continuous | < 5% min system voltage | Project Spec |
| Loop Impedance | Fault cleared ≤ 1s | IEC 60364 |
| Cable Ampacity | As per installation derating | IEE Wiring Regs |
8. Final Thoughts
Selecting the right cable is not just about matching current, it’s about ensuring thermal safety, voltage stability, and fast fault clearance.
For industrial projects, especially with HV feeders, long LV runs, and critical motor loads, engineers must:
- Check thermal short circuit capacity.
- Limit voltage drop for both normal and starting conditions.
- Verify earth fault loop impedance meets tripping requirements.
- Apply correct derating factors for installation conditions.
Getting these right ensures a safe, compliant, and reliable electrical installation.