Engineering Recommendation (EREC) G99

Guidance and requirements outlined in Engineering Recommendation (EREC) G99, which is essential for generators connecting to public distribution networks after April 27, 2019.


Engineering Recommendation (EREC) G99 covers the requirements for the connection of generation equipment in parallel with public distribution networks and is a legal requirement for certain generators connecting on or after 27 April 2019. A new EREC G99 standard application form is also now available on the Energy Networks Association (ENA) website. The information below provides guidance on all aspects of the connection process for compliance with these requirements. We require Part 4 of the G99 generator standard application form to be completed at the initial submission stage if your power generating module is rated at 150 kW or greater.

If you require guidance on the relevant G99 forms for your installation, please see our G99 application form guide.

Are there any exemptions?


Currently Electricity Storage Technology is exempt from some of the requirements of G99 but there are other G99 requirements that electricity storage needs to comply with. Please consult EREC G99 section 2.1 for further information. If you are making modifications to an existing installation, exemptions may apply. All installations will be dealt with on a case by case basis. Please notify gandpconnections@enwl.co.uk of all planned changes to an existing installation.

How are generation types classified?


Type classification also depends on whether you are installing synchronous or asynchronous generation onto the network. Synchronous power generating modules are classified separately, as in the example below:

To support our customers when completing the G98/99 application form we have published a guidance document with an example of a completed form. You can find the documents at the bottom of this page.



This 1.2 MW power generating facility  consists of 3 x 400kW synchronous power generating units corresponding to 3 x 400kW power generating modules, meaning that Type A requirements are applicable.

For asynchronous generation, as demonstrated in the diagram below, the PGF consists of 3 x 2kW inverter connected generating units which are aggregated in order to form one 6kW module, rather than being treated separately as in the synchronous example.


Generators are now classified into different types, of which there are four. In Great Britain, these four are as follows:

  • Type A: 0.8kW to < 1MW and connecting at a voltage <110kV
  • Type B: 1MW to <10MW and connecting at a voltage <110kV
  • Type C: 10MW to <50MW and connecting at a voltage <110kV
  • Type D: ≥ 50MW or connecting at a voltage ≥ 110kV

Each type carries a different set of requirements with which the generation must be compliant. The requirements of each type are cumulative, i.e. Type D generation must comply with the requirements of Type A, B and C generation in addition to the requirements of the Type D category.

What do I need to do?

It is your responsibility to ensure that you are fully compliant with all the necessary requirements for your type classification. All the requirements for each type are outlined in these documents from the Energy Networks Association. For your connection, you will need to provide a Power Generating Module Document (PGMD) which summarises how your generation is compliant. The document will need to be completed and updated through all stages of the project to ensure all data has been submitted to us, the DNO. This document lists all compliance requirements and the acceptable forms of evidence, specifically:

  • S -Simulation study
  • P -Design data
  • MI -Manufacturers’ Information
  • D -Copies of correspondence or other documents
  • T -results of, and/or witness, tests or monitoring
  • TV -Indicates type test reportsIt also contains a compliance declaration, which you the generator must sign.

You can find more information regarding EREC G89 and G99 in our FAQs.


Compliance with the latest Harmonic Standard

Harmonic distortion limits and compliance requirements have changed with the implementation on 17th June 2020 of Engineering Recommendation G5 Issue 5 2020 entitled “Harmonic voltage distortion and the connection of harmonic sources and/or resonant plant to transmission systems and distribution networks in the United Kingdom”. Further information on how this affects your connection can be sought from generationenquiries@enwl.co.uk.


I have a query regarding an existing or future connection and G99 compliance

If your scheme is already in construction and you have an allocated project manager, you can contact them with any queries regarding G99 compliance. Additionally you can also email generationenquiries@enwl.co.uk with any questions.

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