This overview on standards for system installation and wiring for solar home systems and is an extract of the publication: Quality Standards for Solar Home Systems and Rural Health Power Supply: Photovoltaic Systems in Developing Countries, February 2000.
Comment: The CENELEC committee CLC BTTF 86-2 recently (06/98) drafted a standard-proposal entitled "Photovoltaic Systems–Solar Home Systems, Safety and Performance Test Requirements and Procedures", which was kindly made available by TÜV-Rheinland. This proposal mainly provides recommendations for system installation, wiring, earthing/grounding and system documentation. The technical specifications given in this proposal are not very precise yet; therefore, most of the following text proposals are taken from other sources.
Stranded and flexible insulated copper wires and cables must be used for all outdoor and indoor installations. Indoor installation of the lighting distribution system might be performed with solid wires, if appropriate and common practice.
External cables should be specifically adapted to outdoor exposure (see IEC 60811). Especially the outer insulation must be sunlight (UV)-resistant, weatherproof and designed for underground installation. Preferably rubber-coated and PE-coated cables shall be used. (H07 R-NF, NYCWY, RV 0.6/1kV, DV 0.6/1kV, VV-K 0.6/1kV, RVkO. 6/lkV, according to VDE 0281, 0282 and CENELEC HD-361). US-American standard cable types are USE (Underground Service Entrance), SE (Service Entrance) and UF (Underground Feeder). The following types are recommended in IEEE P 1374 “Draft guide for terrestrial PV power system safety”: USE-2, USE, RHW, RHH, SE, RHW-2, XHHW-2.
The temperature resistance of all interconnecting wires and cables should be > 85° C.
The minimum acceptable cross-section of the wire in each of the following sub-circuits is as follows:
- From PV generator to charge regulator: 2.5 mm2 (American AWG 13)
- From charge regulator to battery: 4 mm2 (American AWG 11)
- From charge regulator to loads: 1.5 mm2 (American AWG 15)
Notwithstanding the above minimum wire-size requirements, all wiring must be sized to keep line voltage losses to less than 3% between PV generator and battery, less than 1% between battery and charge regulator, and less than 5% between battery and load, all of them at the maximum current conditions. The minimum cross-section must also allow the circuit to operate within the capacity rating of the wire.
For copper cables and 12 V of nominal voltage, the following formula can be used:
A[mm2 ] = 0.3 * l * IMax[A] / ΔV [%]
where A is the minimum cross-section of the cable, l the single (one-way) length of the cable, IMax the maximum current and ΔV the allowable voltage loss.
Comment: Alternatively, the following formula can be used to calculate the voltage-loss on a given cable:
ΔV [V] = 2 * l [m]* IMax[A] / (56 m / Ω mm2 * A[mm2])
For example, the maximum length of a 2.5 mm2 cable connecting one 50 Wpeak PV module (IMax = 3 A) with the battery is approx. 8 m at a tolerable voltage loss of 3%. For a 4 mm2 cable, the maximum length is 13 m, and for a 6 mm2 cable, it is 20 m.
Coloring of the Wire
All wiring shall be colour-coded and/or labelled. The following conventions shall be followed for two-conductor DC wiring in PV power systems:
Positive: Red or brown
Negative: Black or blue
Earth conductors, either separate or as a third wire in 3-core cables, if present, must be green-yellow.
Protecting the Wire
All exposed wiring must be in UV-resistant conduits or be firmly fastened to the building and/or support structure. Cable binders, clamps and other fixing material must also be UV-resistant, preferably made of polyethylene. (Comment: Polyvinyl Chloride (PVC) is definitely not UV-resistant and will become brittle within less than one year in direct sunlight).
Wiring through roofing, walls and other structures must be protected through the use of bushings. Wiring through roofing must be sealed (waterproof).
Holes through roofing materials should be avoided wherever possible. Cables through roofing shall be contained in purpose-made roof-entry boxes, or proper UV-resistant glands, which shall form a weatherproof seal to prevent leakages. In corrugated roofs, holes for cables are to be drilled at the top of corrugations. All holes in roofing shall be thoroughly sealed and made waterproof with UV-resistant silicone sealant or an equivalent method.
Where wires or cables are fixed to or passed through particularly flammable materials (thatch, etc.), they shall either be flame-retarding cables or shielded in non-flammable conduits. Fittings need to be fastened to suitable supports, which may need to be provided if not already present. No conduit or fitting shall be attached directly to thatch or any other non-supportive surface.
Holes that penetrate external walls shall slope slightly to prevent the ingress of water and be suitably sealed.
Conduits to battery boxes or battery enclosures shall not provide a route through which hydrogen gas may escape, leading to any area or device (relays, etc.) where there is a danger of sparks.
Cables must be joined by the use of junction boxes, screw-connectors, block-connectors or by crimping ferrules (with insulated heat-shrink sleeves). All stranded wires must be terminated with proper end-sleeves. Soldering in the field and the use of wire nuts are not allowed. The rated current-carrying capacity of each joint must not be less than the circuit current rating.
Junction boxes or enclosures must be dust- and waterproof, non-corrosive and electrically insulated (no metal boxes). Interior junction boxes shall have an IP protection of at least IP 32, and external junction boxes a minimum of IP 55 according to IEC 60529.
Fittings for lights, switches and socket outlets may be used as junction boxes where practical.
Careful attention shall be given to entries into enclosures and junction boxes, to provide good sealing, proper strain relief to ensure that the wiring connections themselves are not under tension and to prevent chafing and damage to the insulation.
Surface-mounted cabling shall be installed using appropriate fasteners at suitable intervals (15 to 20 cm) to prevent sagging.
Visible interior cabling or conduits shall be aesthetically tidy, and should not slant from the vertical or horizontal unless essential.
Underground cables shall be buried at least 0.5 m below the surface, mechanically protected either by a conduit or laid in a sand-bed without any sharp rocks or stones.
Health, Optional: Underground cables shall be a minimum of 1 m below the surface and be indicated with markers (coloured plastic tape, minimum 50 mm wide, or lining with bricks or slates, 0.3 m above the cable). Underground cables shall be used across all areas with vehicular traffic, in preference to overhead cables.
Suspended cables shall be mounted so that the lowest point is at least 2.8 m above ground level. The cable shall be held in position by suitable brackets and strain relief to prevent mechanical wear and any strain on the electrical connections.
Fuses and circuit-breakers shall:
- be rated for DC service
- have voltage ratings greater than the maximum circuit voltage
- have current ratings between 125% and 150% of the maximum design current for the circuit
- be marked with the rated capacity and circuit voltage.
Health of the Wire
For circuits where normal transient surges are expected (e.g. when starting motors, refrigerator-compressors, etc.), the fuses or circuit breakers should have an appropriate time-delay before activation.
Health: Every circuit connected to a battery shall have a circuit breaker for protection against high fault currents or overloading. In a two-conductor grounded system these shall be installed in the underground (positive) conductors. They shall be installed as close to the energy source as possible, to minimise the risks of electrical shock.
Health: Required fuses and/or circuit breakers may be integrated into the regulator-box or installed separately in a fuse or distribution box located near the regulator and battery. Each fuse or circuit-breaker shall be clearly marked with rated capacity and the circuit for which it is used.
Health: Distribution boards shall be weatherproof (hot-dipped galvanised). The boards shall be surface-mounted with front panel/door complete with neutral and earth bar, labelling rails, labels, nameplate and legend.
However, fuses or any other components which can cause sparking shall not be installed in a battery enclosure where there is a chance of explosion of hydrogen. No fuses or circuit breakers shall be installed in a grounded conductor.
Light switches shall be installed next to the entrance door of each room at approximately 1.2 m above finished floor level.
Only special DC switches are allowed. They shall be rated for the current and voltage of the circuit they disconnect. AC switches may only be used if their DC switching performance is known (and certified) and satisfies the current and voltage ratings of the circuit. AC ratings are not adequate, due to arcing.
Health: The switch outlet will be of industrial type, watertight and surface-mounted, 5 A rating.
All switches should include a clear visual indication of their state (ON/0FF or I/O). However, suitable pull switches may be acceptable for overhead light fittings.
Health: All appliances that are not hard-wired shall be connected to the wiring through socket outlets unless otherwise specified. AC sockets shall not be used. Only sockets designed for DC shall be used, and it shall not be possible to reverse the polarity. Sockets shall be clearly labelled DC and show the voltage, to inform unfamiliar users about the nature of the power supply. The larger diameter pin in a DC plug shall always be positive.
Warning: Mains sockets and plugs are not to be used under any circumstances. Any 12 V appliance with a mains-type plug attached constitutes an unacceptable safety risk to the user if the appliance is used in a 110 or 230 V outlet.
The warranty for wiring, installation and the performance of the entire system shall be one year (World Bank: 6 months) from the date of commissioning (or date of acceptance test).