Q&AElectrical contracting often presents problems which, when solved by

experts and shared in a forum, can be turned into learning opportunities. Working in co-operation with the ECA(SA) and other experts, Vector now

provides such a forum. Our expert for this issue is Cecil Lancaster.

Experts

AnswerYes, it is common practice. SANS 10142-1 describes some conditions though:

Question

I have a 400 kW machine to feed at 0,8 power factor TP/N – a distance of 127 m. Is it possible to parallel up three smaller sub-main feeder cables rather than using two larger cables? I would like to do this because of the difficulty in manhandling them on site. If so, what are the formulas with regard to voltage drop, short circuit protection and the harmonics in the three parallel cables?

AnswerYou can use either GP wire (single core PVC insulated cables) or multi-core cables provided they are installed and terminated correctly. The decision will ultimately be influenced by site conditions, cost and practicality. Refer to 6.3.7 of the code for the requirements regarding terminations.

Question

This concerns a large house that has two distribution boards, one downstairs and the other on the first floor, between 15 and 20 m apart. Which is the better cable to use for sub-mains, three-core 25 mm SWA or single insulated tails? Also, what is the best method of terminating them?

AnswerAll multi-core cables should be suitable for installation in wet ground, provided that they comply with all other requirements. For underwater (submersed) installation, it may be better to use marine cable, suitably protected. When in doubt, consult with the cable manufacturer.

Question

What power cables are suitable for direct burial in ground which may be prone to water logging?

AnswerWe recommend that you install a separate earth conductor or ECC, especially in corrosive conditions, but using the armouring is permitted. Refer to 6.12.1 of the code, and particularly the accompanying note which reads:

Question

Must I run a separate protective earth cable when installing a steel wire armoured cable, or will the armoured part of the cable suffice?

AnswerIt is dealt with in the wiring code:

Question

Can PVC flex be installed outdoors?

Send your questions, and queries to the editor at vector@ee.co.za

AnswerClarify please: What is meant by a bonding cable? Any conductor can be extended by proper and suitable connection.

Question

Are there any circumstances where bonding cable can be extended by connection?

6.2.1.3 Except in the case of a ring circuit, there shall be no branches, tee-offs or socket-outlets along parallel conductors and, where two or more conductors are connected in parallel, the conductors shall:a) have the same characteristics in respect of conductor material, nominal cross-sectional area, length and, where applicable, insulation and cable construction, andb) follow the same route and have the same installation method(s).6.2.9.2.1 Where two or more conductors are connected in parallel in the same phase or pole of the system, measures shall be taken to ensure that the load current is shared equally between them.

The formulas you ask for are in the code, see 6.2 of SANS 10142-1. Should that information not be sufficient, we would recommend the assistance of a cable manufacturer in the design.

Where the armouring of a cable is used as the earth continuity path, the resistance of the earth continuity path shall not exceed the appropriate value given in table 8.1; it may be necessary to replace some of the steel wires with tinned copper ones or to use a supplementary earth continuity conductor.

This should be read together with:6.3.7.3 Any armouring or sheathing shall be terminated in or on equipment. Armour wires shall be terminated by a clamp or gland in such a way thata) pressure is not applied to insulating material, andb) there is an earth tag washer (or similar) for connecting to the earth continuity conductor.It is also advisable to make the user aware of the risk and recommend regular confirmation of the reading.

6.1.11 Flexible cords shall not be used as part of the electrical installation, except where a) required by the relevant product solely for termination or connection of moving parts,b) specified in the product standard,c) used as single cores in conduits,d) used in an authorized wiring system, ore) needed for the connection of luminaires, provided that each connection is limited to one luminaire and to a maximum length of 3 m. Amdt 1