Otis Singleton
Calculating the maximum electrical demand of an installation is a complex process that requires careful consideration of various factors. The maximum demand refers to the peak or highest electrical power required by the consumer, typically measured in half-hour intervals over a month. While there is no single formula that applies to every scenario, a commonly used approach is to take the largest-rated circuit and add 40% of the total of the remaining circuits. This method provides a realistic estimate for domestic installations. Other factors to consider include the rated current of the consumer unit, the potential addition of generators and solar systems, and the diversity factor, which accounts for the fact that not all circuits will be used simultaneously. To accurately determine the maximum demand, data loggers or smart digital power meters can be employed to monitor and analyse power consumption patterns.
| Characteristics | Values |
|---|---|
| Definition | Maximum demand refers to the peak or highest electrical power required or demanded by the consumer. |
| Time Interval | Usually measured in half-hour intervals over a complete month. |
| Formula | Max demand = The units of kWh used over a period of time, divided by the time period. |
| Diversity Factor | The total connected load multiplied by diversity gives the maximum demand. |
| Calculation Methods | OSG guide values, rule of thumb method, custom diversity factor with an additional usage factor. |
| Rated Current | The rated current of an assembly (consumer unit) is required to be provided on the consumer unit designation label. |
| Additional Supplies | Extra incoming supplies like generators and solar systems need to be incorporated into the rated current of the consumer unit. |
| Industry Guidelines | Following the guidelines in the electrician's design guide will result in a low, realistic calculation of maximum demand. |
| Installation Design | The installation design may allow for diversity, accounting for circuits not being loaded to the maximum. |
| Data Logger | Using a data logger can help determine the actual maximum demand by measuring the load currents. |
| Load Characteristics | The designer's knowledge of the installation's load characteristics can help fine-tune the maximum demand figure. |
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What You'll Learn
Diversity factors
The diversity factor is the ratio of the sum of the individual non-coincident maximum loads of various subdivisions of the system to the maximum demand of the complete system. It is calculated using the following formula:
> Diversity factor = Sum of total demands ÷ Maximum demand on feeder
For example, if a distribution feeder serves 5 houses, each with a peak demand of 5 kW, and the feeder peak is 20 kW, the diversity factor is calculated as 20/25 or 0.8. The diversity factor is always greater than 1.
It is important to note that the diversity factor is not an exact science, as there are multiple varying factors to consider, such as the life span of the electrical installation and the specific use case. Additionally, the diversity factor does not affect the energy but only impacts the power, providing a correction factor to lower the total power load.
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Rated current
When additional supplies, such as generators or solar systems, are incorporated into an installation, these contributions must be accounted for in the rated current of the consumer unit (CU). The rated current of the assembly (InA) should be equal to or exceed the combined values of the rated current (In) and the rated output current of the generating set(s) (Ig(s)). This relationship can be expressed by the formula: InA ≥ In + Ig(s).
It is important to note that the rated current of an assembly is not always the same as the rating displayed on the incoming device. This distinction arises because the rated current of an assembly considers the current setting of the overcurrent protective device, which may be incorporated within the low voltage switchgear and control gear assembly or positioned upstream.
To calculate the rated current accurately, one must consider the type of installation, such as residential, commercial, or industrial. This consideration is essential because the applicable diversity factors, which account for the likelihood of all equipment operating at full load simultaneously, vary depending on the installation type. By applying these diversity factors, a more realistic estimate of the maximum demand can be obtained.
Additionally, it is worth mentioning that the rated current calculation should also take into account the potential for changes in the way the installation is used over time. The diversity factors applied initially may become irrelevant if the usage patterns or load characteristics of the installation change significantly. Therefore, it is advisable to fine-tune the calculations based on the expected load characteristics whenever possible.
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Time intervals
The time interval of 30 minutes is a standard used by TNB (Tenaga Nasional Berhad) to calculate the maximum demand, which is the peak or highest electrical power required by the consumer. During this time interval, the units of kWh used are divided by the time period to determine the maximum demand. For example, if a kWh meter reads 18,200 kWh at 8 am and 18,500 kWh at 8:30 am, the calculation would be as follows:
18,500 kWh - 18,200 kWh = 300 kWh used in 30 minutes
300 kWh / 0.5 = 600 kW maximum demand from 8 am to 8:30 am
This process can then be repeated for the next 30-minute interval, and so on, to determine the maximum demand for each period.
It is important to note that the calculation of maximum demand is not a precise science, and various factors, such as the likely use of the installation, must be considered. Additionally, the guidelines provided in resources like the Electricians Design Guide can be helpful in determining a realistic calculation.
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Domestic vs non-domestic premises
Calculating the maximum electrical demand for a premises is a complex process that requires careful consideration of various factors, including equipment ratings, likely usage, and contributing factors such as use, loading, and operational demands. While there is no single correct way to calculate maximum demand, there are important differences between domestic and non-domestic premises that must be considered.
For domestic premises, such as houses, the calculation of maximum demand involves assessing specific features such as kitchen ring mains, electric showers, immersion tanks, electric cookers, and electric heating. It is unlikely that all sockets or appliances will be used simultaneously, so a diversity factor is applied to account for this. A common method is to take the largest rated circuit and add 40% of the total of the remaining circuits. This provides a more realistic value for domestic installations.
Non-domestic premises, on the other hand, include shops, offices, hotels, and B&Bs, and may have different electrical demands and considerations. For example, a shop or office may have different power requirements than a house, with a higher demand for lighting or computer equipment. Additionally, the floor area served is an important consideration for non-domestic premises, as there is a limit to how much power can be dissipated in a small space without overheating. The 32A per 100m2 rule may be applied to calculate maximum demand in such cases.
It is crucial to accurately determine the type of premises or locations to apply the appropriate calculations and considerations. The inclusion of electric vehicle charge points, for instance, can significantly increase the maximum demand and may require additional notification to the distributor. Additionally, the life span of the electrical installation should be considered, as changes in usage patterns over time may render the initially applied diversity factors irrelevant.
In conclusion, while the calculation of maximum demand involves similar principles for domestic and non-domestic premises, the specific factors and considerations differ. Domestic premises often focus on typical household appliances and the application of diversity factors, while non-domestic premises may involve different types of equipment and considerations such as floor area and overheating.
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Load currents
Calculating the maximum demand involves considering the load currents of various circuits and appliances within a premises. For example, in a domestic setting, circuits may include kitchen ring mains, electric showers, immersion tanks, electric cookers, and electric heating. Each of these contributes to the overall load current and, subsequently, the maximum demand.
Diversity is a concept that acknowledges that not all circuits or appliances will be operating at full capacity simultaneously. It provides a more accurate representation of the maximum demand by considering the likelihood of certain loads being active at the same time. The On-Site Guide offers diversity factor calculations to account for this. For instance, a 32 Amp ring final circuit for standard 13-amp socket outlets is unlikely to have all sockets in use simultaneously.
To calculate the load currents, one can refer to the IET On-Site Guide, which provides guidance on applying diversity to different types of loads and premises. A basic example for a cooker in a dwelling is provided by Appendix A of the OSG: the first 10 Amps + 30% of the remaining load. So, 10 Amps + 7.43 Amps = 17.43 Amps total (after diversity). This calculation considers that not all elements of the cooker, such as the oven, grill, and hob rings, will be in use simultaneously.
Additionally, the rated current of the consumer unit and its assemblies must be considered. The rated current, denoted as (InA), may differ from the rating shown on the incoming device and can be found on the consumer unit designation label. When adding supplies like generators and solar systems, these contributions to the rated current must be incorporated.
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Frequently asked questions
The formula for maximum demand is: Max demand = The units of kWh used over a period of time, divided by the time period. For example, if you have used 300kWh of power in half an hour, the maximum demand for that period is 300kWh divided by 0.5, which equals 600kW.
Maximum demand refers to the highest electrical power required by the consumer, usually measured in half-hour intervals over a complete month. You can use a digital power meter to measure the greatest amount of electrical power consumed in any 30-minute period.
Calculating maximum demand is influenced by the type of premises, the equipment ratings, and the likely use of the installation. For example, in a domestic property, an upstairs 32A ring circuit will likely never reach 20A, whereas in a big kitchen, it is more likely to be higher.
