CMP446 Explained

What does raising the TIA Threshold to 5 MW mean for connections, distributed generation and developers?

CMP446 is a recently approved Code Modification which raises the lower threshold for when an Evaluation of Transmission Impact Assessment (TIA) is required under the Connection and Use of System Code (CUSC). The modification has increased the capacity level at which this assessment is needed from 1 MW to 5 MW for connections in England & Wales. Scotland is not within the scope of CMP446 and the TIA threshold in Scotland remains 200 kW on the mainland and 50kW in the islands. This is due to transmission network constraints in Scotland.

This change is designed to speed up distribution-connected projects and reduce unnecessary TIA workload while protecting the transmission system.

What does CMP446 change?

CMP446 (implemented as WACM1) amends the CUSC to lift the trigger point for a TIA in England and Wales. Under the new rules, embedded generation projects with an export capacity below 5 MW are no longer automatically subject to a TIA before progressing to a connection offer. The only exception is where the local Grid Supply Point (GSP) has less than 1 kA of available fault-level headroom, in which case the threshold falls back to 1 MW.

It should be noted that the new threshold applies to export capacity – previously, DNOs had different interpretations, with some understanding it to be export and some installed. Now there is no specific limit on installed capacity (other than fault level ones).

For developers looking to understand how this plays out in practice, Blake Clough Consulting provides capacity scouting and site feasibility studies, for grid, land, and planning, to help projects take full advantage of this change. We are also helping clients to engage with DNOs regarding how this change can be accommodated for projects with current connection offers which previously underwent a TIA but is now no longer relevant.

Why was CMP446 introduced?

The original framework, with its 1 MW threshold, treated a wide range of relatively small capacity projects as if they might pose significant risks to the transmission system. Developers and distribution network operators argued that this approach was disproportionate: most sub-5 MW schemes have little measurable effect on transmission security. Nonetheless, these projects were being held up by lengthy assessments that added cost without clear benefit.

CMP446 was designed to remove this bottleneck, helping small projects move through the connection process more efficiently, lowering development overheads, and contributing to the wider net-zero drive by enabling faster deployment of local, low-carbon capacity.

Is it now more simple for sub-5 MW projects to secure a grid offer?

With CMP446 in place, projects under 5 MW have a more direct route through the DNO/IDNO connection process, with less duplication of effort and fewer external assessments standing between an initial application and a formal offer[LM3.1].
Furthermore, by avoiding a TIA, projects can also avoid TANM curtailment, and, importantly, avoid a lot of costs associated with transmission reinforcements.

What does CMP446 mean for delivery timelines?

While the raised threshold should remove one of the more common blockers to progress, it does not guarantee that every project will move quickly from application to energisation. Connection timelines are influenced by several other factors, such as the technical complexity of the site, the capacity of the local DNO to process applications, and whether planning, land rights or charging issues sit in a project’s path to energisation. CMP446 is best seen as a streamlining measure; it cuts out unnecessary referrals to the transmission system operator, reducing the likelihood of administrative back-and-forth. Overall project delivery programme length will still be determined by a wider set of project-specific and regulatory considerations, however.

GSPs where the new limit does not apply

The raised threshold only applies provided that there is appropriate fault-level headroom at the relevant GSP. This is defined within the code as fault level headroom above 1 kA.

NESO will publish an updated list of GSPs with fault level headroom issues periodically, as the list is fluid and expected to change through reinforcement works, network reconfiguration, or projects connecting. As of February 2025, the following GSPs in England and Wales have fault level headroom below 1kA:

What happens projects of < 5 MW capacity which have already undergone a TIA but not yet connected?

Ofgem’s decision makes clear that projects under 5 MW that have already gone through the TIA process, but have not yet connected, will no longer be subject to that assessment or the requirements that resulted from it.

Projects that have already connected remain subject to the technical and contractual arrangements included in their contract at connection. This provides relief to many projects that were delayed by transmission-level referrals and should materially improve delivery prospects for projects in the pipeline. Blake Clough is currently supporting some clients through the process of engaging with the DNO to remove the associated TIA works and fees for a sub-5 MW project. If you are also navigating this, then get in touch to understand how we can help you.

How does CMP446 interact with CP2030 capacity buckets?

Projects that are not subject to a TIA do not need to align with the CP2030 capacity buckets or strategic alignment assessments used in the reformed connections gating process (this includes some projects that have already been through a TIA).

This makes sub-5MW projects highly desirable, especially if they are a technology currently saturated (such as a BESS project), as they would be allowed to connect much earlier than larger projects. It should be noted, however, that despite technologies within the CP2030 process being treated separately, the CMP446 modification only applies in the case where the total export of a project is below 5 MW. This means that a project with two technologies, each under 5MW but totalling an export capacity of 5MW or above, will still be subject to a transmission impact assessment and CP2030 capacity buckets.

How will CMP446 impact projects greater than 5MW?

For projects above 5 MW, CMP446 has important financial implications. Previously, reinforcement costs triggered by sub-5 MW connections were shared more widely across all projects included in a TIA. Now, with smaller schemes bypassing that process, the burden of transmission reinforcement is concentrated on the 5 MW+ projects that [LM4.1]contributed to the triggering of reinforcement.

This means that developers of larger assets could see higher connection charges or more material reinforcement liabilities, as there are fewer smaller schemes contributing to those costs. [LM5.1]However, as liabilities are based on the size of projects triggering the work and the size of the component needing to be replaced, the impact of this will be small.

While CMP446 accelerates the route to market for sub-5 MW generation, it may also shift a proportion of the cost exposure upwards, making it essential for larger developers to factor this change into project appraisals, investment cases, and risk management strategies.

4.99MW or 4.94MW?

Under the previous 1MW TIA threshold a rounding rule was applied to round to the nearest 0.1 (for example, a 0.99MW project would be rounded to 1.0MW and trigger a TIA whereas a 0.94MW project would be rounded to 0.9MW and not trigger a TIA).

Previously it was understood that the new TIA threshold would work in the same way, but the modification legal text implemented under WACM1 does not include this rounding principle and the actual limit will be 4.99MW.

Next Steps

If you’re developing a sub-5MW scheme (or any distributed generation project), Blake Clough Consulting can help you turn CMP446 from a regulatory change into a delivery advantage. We offer feasibility assessments, capacity scouting, CP2030 alignment advice, grid offer due diligence and a range of other services. Get in touch now for a practical, project-specific review and clear next steps.