Written by: Grant Seeman – National Technical Manager
A practical series on the four inspection questions and 15 tests in SANS 10142-1
Part 1: Conductors and their current-carrying capacity
The first inspection question reads:
“Conductors are of the correct rating and current-carrying capacity for the protective devices and connected load.”
Clause 8.5.2 requires this statement to be confirmed with a “Yes”. A “No” answer prevents the issuing of the test report.
This question cannot be answered simply by deciding that the conductor “looks big enough”. The conductor, connected load, protective device and actual installation conditions must be considered together.
Why is this important?
A conductor carrying more current than it can safely handle can overheat, damage its insulation and create a risk of premature failure or fire.
The circuit-breaker or fuse must protect the conductor. If the protective device is rated higher than the conductor’s effective current-carrying capacity, an overload could continue without being disconnected soon enough.
What must be confirmed?
Clause 6.2.1.1 requires the maximum estimated load not to exceed the conductor’s current-carrying capacity.
The registered person must therefore establish that:
- The maximum estimated load is suitable for the conductor.
- The protective-device rating does not exceed the conductor’s effective current-carrying capacity.
- The conductor is suitable for the circuit voltage and installation conditions.
- The voltage drop remains within the permitted limit.
- The conductor is suitable for the expected mechanical and short-circuit stresses.
The standard test report does not require the full-load current of every circuit to be measured and recorded. The load must, however, be properly assessed from the connected equipment, design information and intended use of the circuit. A current measurement may provide useful supporting information, particularly on an existing installation, but it does not replace the required assessment.
How is the inspection carried out?
Work through the installation circuit by circuit and identify:
- The circuit-breaker or fuse rating.
- The conductor material, insulation type and cross-sectional area.
- The connected or maximum estimated load.
- The installation method and circuit length.
- The operating and environmental conditions.
Select the correct current-carrying-capacity table for the conductor and installation method. Apply all relevant correction factors, including ambient temperature, cable grouping, burial conditions, neutral imbalance, harmonics and direct solar radiation.
The basic relationship is:
Maximum estimated load ≤ effective conductor current-carrying capacity
and:
Protective-device rating ≤ effective conductor current-carrying capacity
Also confirm that the conductors are adequately protected against short-circuit current. Determine the PSCC at the circuit origin and verify that the protective device has sufficient breaking capacity. For PVC-insulated copper conductors, the note to Clause 6.7.3.2 gives a practical guide that the conductor’s cross-sectional area in square millimetres should be at least numerically equal to the PSCC in kiloamperes.
Where this comparison is not satisfied at the circuit origin, calculate the PSCC at the relevant points of consumption in accordance with Clause 6.7.3.3. For PVC-insulated conductors not exceeding 4 mm², Clause 6.7.3.4 regards the first 5 m of a single-phase circuit and the first 10 m of a multiphase circuit as fault-free for this purpose. The impedance of these concessionary lengths may not, however, be used when determining the required breaking capacity of the upstream protective device.
Also confirm that the voltage drop from the point of control to the relevant point of outlet or fixed appliance does not exceed the permitted 5%. The voltage-drop calculation must be carried out after the applicable correction factors and load power factor have been considered.
Do not inspect only the conductor connected at the circuit-breaker. Where accessible, check local isolators, junctions and points of consumption for smaller conductors, branches or alterations.
Common mistakes
Common mistakes include:
- Using an uncorrected tabulated conductor rating.
- Ignoring grouped cables, thermal insulation or high ambient temperatures.
- Assuming a conductor size always permits the same circuit-breaker rating.
- Increasing the circuit-breaker rating to prevent nuisance tripping.
- Checking only the conductor visible inside the distribution board.
- Ignoring excessive voltage drop on long circuits.
What if the result is unacceptable?
The defect may require a larger or more suitable conductor, a lower-rated protective device, a reduced or divided load, or a change to the installation method.
Regulation 9(3) of the Electrical Installation Regulations requires the registered person to refuse to issue the Certificate of Compliance until the fault or defect has been rectified.
Practical tip
Do not ask only:
“What size is the wire?”
Ask:
“What current can this conductor safely carry in the way and place in which it has actually been installed?”
Key takeaway
A conductor is correctly rated only when the maximum estimated load, protective device, installation method, correction factors, short-circuit capacity and voltage drop have all been considered together.
