How does the installation of microgeneration affect the rated current of a consumer unit?
By Michael Peace CEng MIET MCIBSE
When a generator is connected in parallel with the public distribution network, it is important that the current rating of the consumer unit is not exceeded. This article looks at the problem, explores different solutions, and explains why it is important to consider overload co-ordination and the rated current of assemblies.
What are low voltage switchgear and controlgear assemblies?
The product standard for low voltage switchgear and controlgear assemblies is the BS EN 61439 series. This series of standards covers a wide range of low voltage assemblies from distribution boards to switchboards and assemblies for various applications, such as, domestic ‘household’ installations, marinas and camping sites.
In this article, the term “distribution board” covers consumer units and similar low voltage switchgear and controlgear assemblies.
Low voltage switchgear and controlgear is defined in BS EN 61439-1:2021 as:
“3.1.1 low-voltage switchgear and controlgear assembly
combination of one or more low-voltage switching devices together with associated control, measuring, signalling, protective, regulating equipment, with all the internal electrical and mechanical interconnections and structural parts, as defined by the original manufacturer, which can be assembled in accordance with the original manufacturer’s instructions.
Note 1 to entry: Throughout this document, the term assembly(s) is used for a low-voltage switchgear and controlgear assembly(s).
Note 2 to entry: The term “switching device” includes mechanical switching devices and semiconductor switching devices, e.g. soft starters, semiconductor relays, frequency converters. The auxiliary circuits may also include electro-mechanical devices, e.g. control relays, terminal blocks, and electronic devices, e.g. electronic motor control devices, electronic measurement and protection devices, bus communication, programmable logic controller systems.”
What is the rated current of a distribution board?
The product standard for “distribution boards intended to be operated by ordinary persons (DBO)”, is BS EN 61439-3. DBOs are more commonly known in the UK as consumer units. “Intended to be operated by ordinary persons” means, for example, switching operations and replacing fuse links, such as, in domestic (household) applications. This article focuses on DBOs.
The rated current of the assembly is exactly that - it is the maximum current rating of the equipment.
It is required to be marked using the symbol InA, followed by the current rating, for example, InA 100 A.
The rated current of the assembly (InA) is the maximum load current the distribution board is designed to manage and distribute, Regulation 536.4.202 of BS 7671:2018+A2:2022+A3:2024 refers. InA should be equal to, or greater than, the maximum sum of the design current of all the circuits that are loaded simultaneously. As determined by the electrical installation designer, diversity may be considered, Regulation 311.1 of BS 7671:2018+A2:2022+A3:2024 refers. The rated current of the distribution board is not to be exceeded if further circuits are added in the future.
What is rated diversity factor?
Like applying a grouping factor for cables, manufacturers of distribution boards specify a derating factor for distribution boards called the rated diversity factor (RDF). RDF is applied where adjacent circuit-breakers are continuously loaded.
The tests to BS EN IEC 61439-3 enable the RDF to be calculated and declared in the information supplied with the distribution board, for example, in the installation instructions.
Fundamentally, RDF is a derating factor applied to outgoing circuits to account for the mutual heating effect of continuously and simultaneously loaded adjacent circuits. For example, a ten-way distribution board has an RDF of 0.5. Four adjacent lighting circuits using 6 A circuit-breakers are judged to be continuously and simultaneously loaded. The load current (Ib) for each lighting circuit should not exceed 0.5 x 6 = 3 A. In this example, 6 A is where the “In” and “Inc” of the circuit-breaker have been tested in the distribution board as being equal ratings.
It is important to note that RDF to BS EN IEC 61439 is not applied to the InA of the distribution board and must not be confused with electrical installation maximum demand diversity or assumed loading factors in BS EN IEC 61439 series.
RDF and assumed loading factor (ALF) are different parameters. RDF is fundamentally a group rated factor and ALF is a factor to determine the design current in the absence of other design information by the electrical installation designer.
For a detailed explanation of RDF, see this excellent IET Wiring Matters article by Joe Cannon.
Can I connect a generator such as a photovoltaic (PV) system to a distribution board?
It is common to see a distribution board rated at 100 A protected with the DNO fuse which is also rated at 100 A. If a generator, such as a PV system which is connected in parallel with the DNO supply, is connected to a distribution board, the amount of current available is increased and the designer shall select a distribution board with the appropriate rated current.
How does the connection of a generator in parallel with the public distribution network affect the rated current of the distribution boards?
Chapter 55 of BS 7671:2018+A2:2022+A3:2024 provides requirements for generators. Regulation 551.7.2 states that the rated current of the protective device, plus the rated current of the output of the generating set, shall not exceed the rated current of the assembly, as shown below.
Where the generating set is connected via a low voltage switchgear and controlgear assembly then:
(v) The assembly shall be selected such that:
InA ≥ In +Ig(s)
where:
InA is the rated current of the assembly.
In is the rated current or current setting of the incoming circuit overcurrent protective device, either incorporated within the low voltage switchgear and controlgear assembly or upstream of it.
Ig(s) is the rated output current of the generating set or sets.
NOTE: InA is defined in BS EN IEC 61439-1 section 5.3.1: “Rated current of an assembly (InA) refers to the current that the busbars can distribute in a particular arrangement.”
This is not necessarily the busbar rating, and it is important to consider the assembly arrangement as a whole.
What is the problem?
The example illustrated in Figure 1 shows that if a PV system with a rated output current of 16 A is connected to a distribution board with a rated current of 100 A, protected by a 100 A fuse, the rated current of the assembly can be exceeded. The calculation below shows that the requirement of Regulation 551.7.2 is not met.
InA ≥ In + Ig(s)
100 A ≥ 100 A + 16 A = 116 A
Figure 1: Rated current of a distribution board not compliant with Regulation 551.7.2 (v)
What are the possible solutions?
One solution is to request that the DNO fuse is downgraded, in this example, to 80 A. This is not desirable as it limits the supply capacity which may be required.
Other options include:
- Regulation 551.7.2 of BS 7671:2018+A2:2022+A3:2024 permits the following solution: InA ≥ In + Ig(s) therefore, if the supply cut-out fuse is 100 A, this would require a minimum InA of 116 A. Some manufacturers offer distribution boards with an InA of 116 A and, of course, an InA exceeding 116 A would be suitable for this specific arrangement, for example, a distribution board rated at 125 A.
- A manufacturer may provide a consumer unit capable of distributing the total load available from both incoming supplies. For example, a manufacturer may state that the incoming supplies (100 A and 16 A) must be at opposite ends of the busbar. A 100 A busbar would be adequate, but the manufacturer then must ensure the consumer unit can distribute 116 A without overheating. Such an arrangement is likely be subject to diversity being applied to the outgoing circuits. Where this is the case, it is important to obtain a declaration from the manufacturer and append it to the electrical certification.
NOTE: InA is defined in BS EN IEC 61439-1 section 5.3.1. Rated current of an assembly (InA) refers to the current that the busbars can distribute in a particular arrangement. This is not necessarily the busbar rating, and it is important to consider the assembly arrangement as a whole. - Install an overcurrent protective device immediately upstream of the distribution board, and connect the PV system to the AC supply upstream of this overcurrent protective device, as shown in Figure 2.
InA ≥ In (ii)
100 A ≥ 100 A
Figure 2: Rated current of a distribution board compliant with Regulation 551.7.2 (v)
The solution to achieve overload co-ordination with the rated current of an upstream overcurrent protection device (OCPD) is highlighted as one method in the BEAMA bulletin “Overload protection of an RCCB or switch in an LV assembly to BS EN 61439-3”.
Other solutions proposed in Amendment 4:2026 to the BS 7671:2018 IET Wiring Regulations draft for public consultation (DPC), achieving an equivalent resultant degree of safety, are:
(i) InA ≥ ICLS (max), or
(ii) InA ≥ ITCL
where:
InA is the rated current of the assembly.
ICLS (max) is the Customer Limitation Scheme (CLS) maximum current the distribution board can possibly be required to distribute when the load is controlled by the CLS. A CLS can be a basic configuration of, say, a load shedding relay(s) or a more complex arrangement of interconnected control equipment.
A warning notice should be attached in a visible position on the distribution board identifying the maximum permitted connected load (InA), for example, where InA = 100 A “Total connected load not to exceed 100 A”.
ITCL is the total connected load without diversity. Diversity must never be used for load control or overload protection, Regulation 536.4.202 of BS 7671:2018+A2:2022+A3:2024 refers.
Will an overload protective device protect against small overload of long duration?
OCPDs are not designed to protect against small overloads of long duration.
It is important to note that fuses, especially the DNO fuses, offer limited overload protection. The DNO fuses are generally designed to be effective for short circuit faults rather than overloads.
Regulation 433.1.1 of BS 7671:2018+A2:2022+A3:2024 states that every circuit shall be designed so that a small overload of long duration is unlikely to occur. Note 2 of this regulation makes it clear that protection in accordance with this regulation may not provide protection in all cases, for example, where sustained overcurrents of less than I2 occur. I2 is the current causing effective operation of the overload protective device within the conventional time, as stated in the product standard.
What type of overload protective device should be installed upstream of the distribution board?
Circuit-breakers provide closer protection than fuses. The non-fusing current for a 100 A BS 88-2 fuse is 1.25 In, as opposed to the conventional non-tripping current of 1.13 In for a circuit-breaker.
Conversely, the fusing current for a BS 88-2 fuse is 1.6 In, as opposed to the conventional tripping current 1.45 In for a circuit-breaker.
Figure 3 shows a comparison of tripping curves for a 100 A Type B circuit-breaker (blue curve) and a 100 A BS 88 gG fuse (pink curve). The blue curve shows that the circuit-breaker provides closer protection than the equivalent rated BS 88 fuse.
Figure 3: Time/current curve comparison
Summary
When parallel generation is connected via an assembly, the designer must ensure the current rating of the assembly is suitably rated. There are many different methods the designer can apply to achieve this.
Circuit-breakers provide closer protection than fuses for overload conditions, however, it is important to remember that protective devices should not be relied upon to protect against small overloads of long duration.
Acknowledgements
- Frank Bertie
- Joe Cannon
- Jon Elliot
- Leon Markwell
- Peter Monfort (Arena training)
- Steven Devine