4.4 System sizing
System sizing is the process of determining the size of the various system components, for instance the peak power rating of the solar array or the current rating of the controller. The selection of the components themselves is covered in section 5.5.
Please use the pv calculators to do these calculations. If this is not available for whatever reason then the calculations can be performed by hand or with an electronic calculator.
There are a number of steps to be followed. It may also be necessary to perform a number of iterations; if the result of the sizing process is not as expected it may be necessary to repeat the process a number of times.
4.4.1 Loads
The first part of the process is to calculate the daily energy requirement of the proposed system in Watt-hours per day.
4.4.1.1 Assessment
For each load determine the power rating in Watts. This may be found on the appliance or in the manufacturer’s data. The power rating may also be stated in kilowatts or kW where 1 kW = 1000 W. If this information is not available then appendix 5 gives approximate power ratings for common appliances. Now determine now many of each appliance is needed and the average daily hours of use. Enter these figures in the appropriate columns as shown in figure 17 or use this equation:
E = n x P x T
Where:
E is the energy requirement in Wh/day
n is the number of appliances
P is the power rating in Watts
T is the average usage in hours
For any mains voltage or AC appliances it is necessary to account forinverter efficiency. This is because some power is lost when an inverter converts low voltage DC into high voltage AC. Divide the result above by 0.9 (90%) unless you know the efficiency of the actual inverter that will be used.
Now total the results for all the loads:
ET = E1 + E2 + …
Where:
ET is the total energy requirement
E1, E2,… are the energy requirements of the individual loads
This will give a total figure for the energy requirement of the system.
For the example above the result is 732 Wh/day.
4.4.1.2 Optimisation
It is worth paying some attention to optimising the load, that is decreasing the energy consumption to its practical minimum. From the figures above it will be apparent which of the loads are the most significant in terms of energy consumption. Consider whether there are any gains to be made, for instance by using a smaller number or more efficient appliances, using 12 Volt instead of 230 Volt appliances or using a different energy source for some appliances.
Repeating the steps at sections 4.4.1.1 and 4.4.1.2 until the optimum is reached will pay dividends later in the process.
4.4.1.3 System voltage
At this stage it will help to decide on the system voltage, that is the voltage of the battery bank. The choice is normally dependent on the loads which it is necessary to power. If there are loads which are 12 Volt, then obviously it makes sense for the system voltage to be 12 Volts. However if there is a large 230 Volt requirement then it may be necessary to consider 24 Volts or even 48 Volts in order to obtain a suitable inverter.
There are no fixed rules for the choice of system voltage. On balance it is probably best to use 12 Volts unless there are compelling reasons to use a different voltage. This choice may also affect your choice of loads, and it may therefore be necessary to repeat the assessment and optimisation processes.
4.4.2 Solar array
The size of the solar array is determined by the daily energy requirement and the solar resource or insolation available to the system. The greater the energy requirement the larger the solar array needs to be and the greater the insolation the smaller the solar array.
4.4.2.1 Insolation
Insolation is a measure of the amount of solar energy falling on an area. The usual measure is kWh/m2/day. That is kilowatt-hours (thousands of Wh) per square metre per day.
Insolation data may be obtained from a variety of different sources such as meteorological agencies. Figure 18 is an example of an insolation map generated by NASA’s web site at http://eosweb.larc.nasa.gov/sse/. It is well worth subscribing to this service which is free at the time of writing.
If local data is not available then appendix 1 provides a set of global insolation maps derived from the NASA data which will provide sufficient accuracy for most purposes.
The aim is to determine a figure for ‘design insolation’. This is the minimum daily average insolation available to the system. The figure used should be from the month with the least insolation, based on whichever months of the year the system is intended to be used. The figure arrived at is likely to be between 1 and 6 kWh/m2/day.
For our holiday home example, assuming that it may be used at any time during the year, a figure of 3.5 kWh/m2/day is appropriate. This represents the lowest monthly average insolation for Portugal, from the insolation maps in appendix 1.
4.4.2.2 Efficiency
Having determined a figure for design insolation the efficiency of the battery charging process must be considered. There are two factors to take into account; power point efficiency and charge cycle efficiency.
Power point efficiency: As shown in figure 19, the peak power output of a solar panel is produced at the ‘knee’ of the output curve. In this example this is at 20 Volts, where the current is 3.5 Amps. Therefore the peak power output is:
20 x 3.5 = 70 Watts
However the battery charging voltage is likely to be between 13 and 14 Volts. From the graph we can see that at 14 Volts the current is approximately 3.75 Amps. This gives a power output of:
14 x 3.75 = 52.5 Watts
Hence the efficiency is:
52.5 / 70 = 0.75 or 75%
If you have a copy of the output curve for the particular solar panels which you intend to use, then calculate the efficiency as described. Otherwise use the figure of 0.75 as this is a reasonable approximation for the purposes of system sizing.
Charge cycle efficiency: The charge cycle efficiency is a measure of the proportion of the energy used to charge a battery which is returned when the battery is discharged. The actual efficiency of a particular battery may be obtained from the manufacturer, however an approximation will suffice. For this purpose assume an efficiency of 0.95 or 95%.
4.4.2.3 Sizing calculation
All the variables necessary to size the solar array are now known. Enter the values into the spreadsheet as shown in figure 20, or proceed as follows:
The sizing calculation is:
S = (E / i) / (epv x ebat)
Where:
S = Array size in peak Watts or Wp
E = Daily energy requirement in Wh/day from section 5.4.1
i = Insolation in kWh/m2/day
epv = Power point efficiency
ebat = Charge cycle efficiency
It can be seen that the result of this calculation is not dependent on the system voltage, as it refers to the power output of the solar array rather than the current or voltage.
Figure 20 gives the result of this calculation for the example holiday home system.
4.4.3 Battery
Battery sizing is the process of ensuring that there is sufficient battery capacity to support the loads during such times as there is insufficient energy available from the solar array. As battery capacity is relatively cheap, it may be thought that it is impossible to have too great a battery capacity. This is a fallacy, as it is important to ensure that the battery is fully charged on a regular basis to prevent damage through sulphation. An overly large battery in comparison to the size of the solar array will not reach full charge as it will require a greater charging current than the solar array can deliver.
Battery capacity is measured in Ampere-hours (Ah) at the system voltage, and is derived as a function of the daily energy requirement, the ‘holdover’ requirement and the ‘depth of discharge’ limit.
4.4.3.1 Holdover
The holdover is simply defined as the number of days that the load is required to operate without any charging input from the solar array. It should be noted, however, that at most latitudes there is no such thing as a day with no sun. Even overcast winter days can provide some useful charging input, so no system will ever be required to operate entirely from the battery for the holdover period.
The required holdover period is determined by the security of supply required. There is no hard and fast rule, but for general applications such as lighting and domestic purposes a figure of 3 days should be adequate. For more critical application such as medical refrigeration a period of 7 days should be considered. If there is another source of charging input such as a diesel or wind generator then it is possible to reduce the holdover. If a modular battery system is chosen then it will be relatively easy to increase the battery capacity at a later date should it prove necessary.
4.4.3.2 Depth of discharge
The depth of discharge is the proportion of the battery’s capacity that can be used by the loads without recharging. It is the opposite of the state of charge; a depth of discharge of 80% is equivalent to a state of charge of 20%.
The design depth of discharge is determined by the type of battery used and the expected life, balanced against the cost of the battery. A leisure type battery will typically be used to a depth of discharge of between 30% and 50%, whereas a deep-cycle or traction battery will be discharged to between 50% and 80%. This should be considered a practical maximum, as there is no type of lead-acid battery which can safely be completely discharged on a regular basis.
It is a fallacy that lead-acid batteries last longer when regularly deeply discharged. The life of a lead-acid battery is shortened by each charge / discharge cycle, by an amount roughly proportional to the depth of discharge of that cycle.
4.4.3.3 Sizing calculation
Once you have arrived at figures for the holdover and depth of discharge, enter these into the spreadsheet (figure 20). Alternatively follow this method:
The sizing calculation is:
C = (E x h / d) / V
Where:
C = Battery capacity in ampere-hours (Ah)
E = Daily energy requirement in Wh/day from section 4.4.1
h = Holdover in days
d = Depth of discharge expressed as a decimal
V = System voltage
See figure 20 for the results of the battery sizing calculation for our example.
4.4.4 Allowing for expansion
Depending on the use that the system is going to be put to, you may need to consider the possibility of future expansion. In this case, the controller in particular should be sized to meet the future maximum size of the solar array. Batteries and solar panels can be added to, although it’s always best to use only components that are the same as the ones already installed.
In the case of a later expansion of battery capacity, it is important to consider whether the existing battery is close to the end of its life. If it is, then it would be better to replace the entire battery with one of larger capacity, rather than add to an old battery, only to have to replace the existing cells a short time later.
4.4.5 Hybrid systems
It is common to combine solar power with other forms of generation, either renewable or conventional. Any supplemental battery charging source must be connected directly to the battery terminals and not via the solar charge regulator. The most common types of hybrid systems are covered briefly here.
4.4.5.1 Wind turbines
Wind turbines offer a good complement to solar photovoltaics. After all, there aren’t many days which are neither sunny nor windy. In order to calculate the solar component of such a system it is necessary to know the monthly output of the wind turbine. Once this is known, then the solar part of the system can be sized as above, performing a separate calculation for each month of the year, and subtracting the daily contribution from the wind turbine (monthly output divided by number of days in the month) from the load requirement. You will also need to use the insolation figure for that particular month.
4.4.5.2 Diesel generators
In many systems a diesel or petrol engined generator is used either to ensure security of supply or to supplement the solar output during the winter months. The generator is usually wired through a changeover relay to replace the inverter when running, and also to a battery charger so that the batteries will be replenished at the same time.
The battery charger chosen should be a model designed for this type of application, and sized in consultation with the generator manufacturers. The maximum sized battery charger that a given generator can operate will be significantly smaller than the generator rating suggests.
Some inverters are capable of remotely starting a generator when the battery needs charging, thus allowing the system to be completely automated. Some incorporate a battery charger with automatic switching between inverter and generator.
4.5 Component selection
The selection of the components of a solar power system is determined by their electrical characteristics. However there are other factors including price, availability and the necessity for any parts to fit in the space available.
4.5.1 Solar array
The solar array consists of more than just the solar panels. There is also the support structure and cabling to consider. The selection of the solar array depends on the particular installation as follows.
4.5.1.1 Solar Panels
The prime consideration when choosing solar panels is usually their cost per Watt. This is the price of the solar panel divided by the peak wattage, for example if a 50 Wp solar panel costs R2300.00, then it is said to cost R46.00 per watt.
This is not the only consideration however. The following points should also be taken into account:
- Physical size. It is likely that thin-film solar panels will be bigger than crystalline solar panels for the same power rating. Likewise a large number of small solar panels is likely to occupy a larger area than a small number of large solar panels. The space available for mounting may therefore determine which solar panels are chosen.
- Support structure. If it is intended to purchase a ready-made support structure then they may only be available for certain combinations of solar panels. Also a support structure for many small solar panels is likely to cost more than one for a few larger solar panels. The greater flexibility may, however, outweigh this disadvantage.
- Cabling and installation. Again there will be more cabling for a larger number of solar panels. This will increase the time needed for installation and the quantity of cable required.
- Fit to system. How closely it is possible to meet the system requirements should be considered. For instance, if a minimum of 60 Watts at 12 Volts is needed, then three 20 Watt solar panels would be a better fit than two 50 Watt solar panels, and may be cheaper. However, if the system were 24 Volts, then four 20 Watt solar panels would be needed, which would be more expensive.
Figure 21 shows the solar module selection function of the sizing spreadsheet. Enter the voltage and peak power rating of the chosen solar panels and the number of solar panels required is calculated. This can easily be calculated manually if the spreadsheet is not available. By entering the details of all the available solar panels and multiplying the result by the cost per solar module the optimum solar modules can be selected. Remember to check that the selected solar modules will fit in the space earmarked for them.
4.5.1.2 Support structure
The selection of a support structure is dependent on the results of the site survey. If it is intended to purchase a structure then this should be considered as part of the process of selecting the solar panels. The angle of tilt should be able to be set at the correct angle for the system. The correct angle from the horizontal is normally equal to the angle of latitude at the location where the system is to be installed.
It is perfectly feasible to manufacture a support system on site from steel or aluminium or even wood. Galvanised perforated steel angle is ideal. The design of the structure must take account of the worst case wind loading, which will be significant, especially if the structure is roofmounted.
4.5.1.3 Cabling
It may be possible to purchase ready-made cables known as solar array interconnects. These are short cables cut to the correct lengths to connect the solar panels together and are resistant to ultra-violet light. Alternatively it is possible to make these on site; this is covered in the section on installation.
4.5.2 Battery
There are many options for the system battery, and for systems with a battery requirement of much more than a few hundred Amp-hours it is best to seek the advice of a specialist battery supplier. Batteries are available as individual cells or as ‘monobloc’, that is a number of cells in a single battery in the same way as a car battery.
4.5.2.1 Configuration
A battery can be made up in many ways. For example a 24 Volt, 200 Amp-hour battery may be configured as, for example:
- Four 12 V, 100 Ah monoblocs in series / parallel.
- Four 6 V, 200 Ah monoblocs in series.
- Twelve 200 Ah cells in series.
For low cost domestic applications the normal choice is leisure batteries. These are usually 12 Volt monoblocs with a capacity between about 60 Ah and 110 Ah.
Remember that, as the battery capacity is expressed in Amp-hours, twice the number of cells will be required for the same capacity at 24 Volts than at 12 Volts. For example, a 12 V, 100 Ah battery may consist of a single monobloc. Two of these monoblocs in series would constitute a 24 V, 100 Ah battery.
Figure 24 shows the battery selection table of the sizing spreadsheet.
4.5.2.2 Lifetime
The life of a battery is normally expressed in two ways. ‘Life in float service’ is the life of a battery in years if it is always on charge and never discharged. This is the practical maximum life. ‘Cycle life’ is expressed as a number of cycles to a particular depth of discharge, e.g. 300 cycles to 80% d.o.d. This is sometimes available from the manufacturer in the form of a graph or table showing the cycle life versus the depth of discharge such as that in figure 23.
Determining the life of a battery in a solar power system is not straightforward. It is difficult to accurately predict the number or depth of discharges as this is determined by the weather and the usage pattern. An approximation can be made by dividing the number of days in the year (365) by the number of days holdover in the system, so a system with 3 days holdover would perform approximately 120 cycles annually.
From this information can be determined the expected life of the battery, for example a battery with a cycle life of 1000 cycles to 50% d.o.d., in a system designed for 3 days holdover to 50% d.o.d., will last around 8 to 8½ years. The optimum battery life is reached when the cycle life is equal to the life in float service. Any further increase in capacity will not extend the life of the battery.
4.5.2.3 Sealed or vented
Traditional lead acid batteries usually include vented caps which allow lost electrolyte to be replenished. Many modern batteries are sealed. Sealed batteries come in three types; liquid, AGM and Gel. Many of the batteries which are generally referred to as ‘gel’ batteries are in fact AGM batteries, where the electrolyte (acid) is contained in an absorbent material between the plates, preventing spillage. In a true gel battery the electrolyte is a jelly. As a general principle AGM batteries can provide a higher current where gel batteries have a longer cycle life, although this is by no means universal.
In general, it is better to use a traditional vented battery wherever possible. Reasons for using a sealed battery include:
- Freedom from explosive gasses
- Maintenance free
- Ease of transporting
However these are outweighed in many applications by the significantly higher cost and frequently shorter lifespan, especially at high ambient temperatures.
4.5.3 Controller
The selection of the controller is determined by four factors; the system voltage, the solar array (input) current, the load (output) current and the type of battery. If it is not possible to find a controller with the correct specifications, then it may be necessary to change the system voltage and repeat the sizing calculations.
4.5.3.1 Voltage
The chosen controller must be able to operate at the system voltage chosen in the section on sizing. Many controllers can operate at more than one voltage; some automatically select the correct voltage.
4.5.3.2 Solar Array current
The maximum solar array current is the short circuit current (Isc) of an individual solar panel multiplied by the number of solar panels in parallel. For example, in a 12 Volt system with four solar panels with a short-circuit current of 4 Amps each, the solar array current is 16 Amps. For a 24 Volt system with the same solar panels, there are only two in parallel so the solar array current is 8 Amps.
4.5.3.3 Load current
If the system has low voltage DC loads, such as lamps, then it is generally necessary to wire these via the controller. For this purpose it is necessary to select a controller with a battery protection or low voltage disconnect facility which automatically disconnects the loads. The load or output current is determined by calculating the maximum number of appliances which are likely to be switched on at the same time and adding up the current consumption of them all.
4.5.3.4 Battery type
If it is decided to use sealed batteries, then it is important that the controller chosen is suitable. Vented batteries require a higher voltage ‘boost charge’ periodically which would damage a sealed battery. Suitable controllers have a facility which disables this boost charge.
4.5.4 Inverter
There are four things to consider when choosing an inverter; input, output, power rating and waveform.
4.5.4.1 Input
As with the controller, the input voltage of the inverter is determined by the system voltage. Larger inverters tend to be more expensive or have a lower rating in 12 Volt versions than 24 or 48 Volt, so it may be worth considering a higher system voltage if there is a lot of AC load.
4.5.4.2 Output
The output voltage and frequency are determined by the input voltage of the appliances that the system is designed to power. This is generally determined by the mains supply of the country that it is to be installed in or where the appliances were bought. For example, in Europe the output would need to be 230 Volts at 50 Hertz, whereas in the USA it would be 110 Volts at 60 Hertz and in South Africa it would be 220 Volts at 50 Hertz. It is very important that this is correct.
4.5.4.3 Power rating
The power rating is the maximum continuous power that can be supplied to the loads. This is determined by adding up the power consumption in Watts of all the appliances that are likely to be switched on at any one time. Many inverters have a large overload capacity, which means that they can provide substantially more than the rated output for short periods of time. This is useful where motors have to be started, particularly in refrigeration systems.
4.5.4.4 Waveform
The choice between a modified sine wave and pure sine wave inverter is not straightforward. The advantages of a modified sine wave are:
- Low cost
- High overload capacity
- High efficiency
And of a pure sine wave are:
- Low noise
- Compatible with all appliances
Recent advances have reduced the cost and increased the efficiency of pure sine wave inverters so that the differences are less clear-cut, especially in the case of larger inverters.
The types of appliances that are incompatible with a modified sine wave inverter are those with a crude power supply, such as certain small battery chargers and those requiring a noise-free supply, such as music equipment.
4.5.5 Appliances
The selection of most appliances will already have been considered during the initial estimation stage of the design process. With the information now available it should be possible to decide upon the actual appliances to be used.
4.5.5.1 Lighting
There are many types of lamp to choose from, but the most important choice is whether the lighting is to be low voltage DC or mains voltage AC. The advantages of choosing AC are:
- Reduction in cabling cost
- Local availability of lamps
- May use existing wiring
Whereas the disadvantages are:
- Cost and complexity of inverter
- Safety in places where electricity is unfamiliar
- Higher power consumption
Generally, AC lighting will normally be chosen in larger systems, where an inverter is already required for other loads.
If AC lighting is chosen then avoid ordinary filament light bulbs. Fluorescent lamps are the best choice as they produce far more light for the power they consume. Any type can be used, including low-energy light bulbs and strip lights, but if strip lights are chosen make sure that they are fitted with an electronic ballast.
There are a variety of low voltage DC lamps available owing to their use for leisure purposes such as boating and caravanning. For reasons of efficiency the first choice should always be fluorescent lamps. These are available in a wide range of styles, including striplight and 2D types.
In some cases fluorescent lamps may not be suitable, for example where a spotlight is required. In these cases a halogen lamp may be used. There are fittings which are designed for low voltage application available from caravan and boat suppliers. Another option is to use dichroic lamps and fittings. These are normally used with 12 Volts AC from a transformer, but they will work just as well from 12 Volts DC. If they are used in an AC system then an electronic transformer is better than a conventional one.
Despite their low cost, standard incandescent light bulbs should never be used, either in AC or DC applications. Their power consumption is 5 times that of a compact fluorescent lamp for the same light output, so would need five times as many solar panels and batteries to supply it.
4.5.5.2 Refrigeration
The choice of a refrigerator for solar power operation is subject to a particular set of conditions unlike any other appliance. The power consumption of a refrigerator is quite low when compared with many other devices, but the fact that it has to be powered continuously means that its energy consumption is quite high.
DC refrigerators:There are three basic types of DC refrigerators; compression, absorption and peltier effect.
Peltier effect coolers use a solid state heatpump and are usually small, designed for use in cars. They are quite inefficient, consuming a lot of power for their size, and are really only suitable for mobile applications such as motorhomes where they will be powered by the alternator on the engine most of the time.
The absorption type of fridge is commonly found in caravans, owing to its ability to operate from different energy sources, usually AC and DC power and bottled gas, and also in remote medical centres. While this type of fridge is quite effective when operating from gas or kerosene they should not be considered for constant electrical operation as their performance is poor.
Compressor type fridges are by far the most efficient, especially in lowvoltage form. They are more expensive than most other types, but should be the first choice as the extra cost will be repaid many times by their much lower energy requirement.
AC refrigerators:An AC refrigerator should only be considered if a suitable DC compressor refrigerator is not available. They are generally less efficient than their DC alternatives especially when inverter efficiency is taken into account.
A further consideration is that the starting current of an AC fridge compressor tends to be very high compared to its DC counterpart, and this can lead to problems operating from an inverter. Advice should be sought from the inverter manufacturer as to whether their inverter is suitable to operate the intended appliance.
4.5.5.3 Microwave ovens
If a microwave oven is to be part of the system, it should be noted that the rated power of the oven is usually the output power and shouldn’t be confused with the input power. This is normally to be found on the rating plate on the rear of the appliance and is significantly higher.
Low voltage microwaves are now available and may prove to be more suitable, despite their lower output power which may mean longer cooking times.
4.5.5.4 Other appliances
Other appliances should be chosen primarily on the basis of their power consumption. Greater emphasis should be given to those appliances which are likely to require the most energy, for example those with a high power rating or those which will be on for the longest period. See appendix 5 for a list of common appliances and their power ratings.
4.6 Wiring
4.6.1 Wiring diagram
A wiring diagram should be drawn for even the simplest system, as it helps to ensure that nothing has been overlooked. It will also assist with the cable sizing process and will be essential to the installation process.
The wiring diagram will be different for every system and is drawn with reference to the manufacturers instructions for the various system components. If they are not all available at first, draw a general diagram and fill in the details later.
Some example wiring diagrams are included in appendix 4, and the following rules will assist:
4.6.1.1 Array
If the system voltage and the solar panel voltage are the same (usually 12 Volts) then all the solar panels are wired in parallel. If the system voltage is greater than the solar panel voltage then the solar panels are wired in strings (pairs in series for 24 Volts, four for 48 Volts) and then the strings connected in parallel.
4.6.1.2 Batteries
The batteries are wired in the same way as the solar array. For example, a number of 12 Volt batteries can be wired in parallel in a 12 Volt system or a number of 2 Volt cells in series to produce 12, 24 or 48 Volts.
4.6.1.3 Controller
A typical controller will have connections to the solar array, battery and DC loads. It will normally be mounted close to the battery as measurement of the battery voltage is important to its operation.
4.6.1.4 Inverter
If there is an inverter in the system, it is connected directly to the battery using substantial cables.
4.6.1.5 Appliances
Lights and other appliances are either connected to the inverter output, if they are mains voltage, or to the controller output if they are DC. This is to take advantage of the low-voltage disconnect voltage function of the controller. There are certain appliances, notably low-voltages refrigerators, which must be connected directly to the battery. Consult the manufacturer’s instructions for more information.
4.6.1.6 Circuit protection
Circuit protection is essential in any system to avoid fire caused by the high currents that a battery can deliver into a short-circuit. The basic principles are the same for both AC and DC systems, but in a DC system there is the added requirement to protect the user from electrocution.
DC systems:In a simple DC system where all the loads are connected via the controller, then the fuse incorporated into the controller may be sufficient. In larger systems it is necessary to incorporate a fuse on the battery positive terminal and ensure that all current from the battery has to pass through it.
In larger DC systems it may also be necessary to incorporate separate fusing for individual circuits. This protection can be normal 230 Volt fuses and circuit breakers for 12 and 24 Volt systems but higher voltages must use special DC fuses.
Provision should be made to earth the battery negative terminal in order to ensure that the system does not float at high voltages.
AC systems:The requirements for battery fusing are the same as in larger DC systems, but the loads should be fused using a normal 230 Volt consumer unit. This should incorporate a Residual Current Device to protect the users of the system. A good earth should be provided to the consumer unit in addition to that provided at the battery.
4.6.1.7 Wiring accessories
For AC systems, the wiring in the building should follow normal wiring practices, with reference to any regulations in force in the country in which the system is to be installed.
For DC systems where there is any wiring beyond the controller, the same applies with the following differences:
- Larger cable should be used wherever possible, for example 2.5 mm2 cable for lighting circuits instead of 1.5 mm2.
- Standard sockets should not be used to connect low-voltage appliances, because of the risk of them being connected to the mains in error. Plugs and sockets designed for DC should be used, or an obsolete system such as the British 15 Amp round-pin plug.
- Standard mains wiring accessories such as light switches and juction boxes can normally be used at voltages up to 30 Volts DC. Consult the manufacturers if in any doubt.
- A separate earth connection to each appliance is normally unnecessary as the negative is earthed.
4.6.2 Cable sizing
Unlike mains wiring, low voltage DC systems lose a significant part of the generated power in the cabling. That’s because a lower voltage means that the current is higher, and the power dissipated in the cable is proportional to the square of the current (section 2.1). This means that it is sometimes necessary to use a larger cable than is necessary to carry the current.
If you are using the sizing spreadsheet, enter the lengths of the cables you want to calculate the sizes of into the fields on the Design Data page and the results are displayed on the Components page. See figure 27.
If you are not using the spreadsheet, then calculate as follows:
A = L x I x 0.04 / V
Where:
A = Cross-sectional area of the cable in mm2
L = Length of cable in metres
I = Current in Amperes
V = Maximum permissible voltage drop in Volts
The maximum permissible voltage drop should be 5% of the system voltage, that is 0.6 V for a 12 Volt system and 1.2 V for a 24 Volt system.
Cable is available in various sizes depending on the country you are in. The cable used should be the same size or larger then the result of the calculation, never smaller. See appendix 3 for a conversion table between metric and other systems of cable sizing.