MCPD vs NRMM: what they are and how they apply to diesel generators
The Medium Combustion Plant Directive (MCPD) regulates stationary combustion plant with a rated thermal input of 1–50 MWth under the UK Environmental Permitting Regulations. It sets emission limit values (ELVs), monitoring/testing requirements and record‑keeping for new and existing plant. What applies depends on plant size, installation date, and duty (standby vs prime/continuous).
NRMM Stage V covers non‑road mobile machinery such as mobile and hire generators used on construction and other temporary sites. Engines must have valid UKCA/CE type‑approval, compliant aftertreatment, and documentation available on site. Generators exporting to the grid or enrolled in DSR can also fall under Specified Generator controls; see our grid rules overview: G99/G98 compliance. For project options and configurations, view our commercial diesel generators.
Standby vs prime/continuous duty: which rules apply
Duty classification drives compliance, sizing and controls:
- Emergency standby (e.g., hospitals, data centres): often benefits from reduced requirements when used only during outages. Routine testing is typically capped (around 50 hours/year under Specified Generator backup provisions; confirm with your regulator). Participating in peaking/DSR usually removes backup‑only status.
- Prime/continuous duty and most construction power: must meet applicable ELVs and usually require monitoring and periodic testing. Clear duty definition helps specify the engine, aftertreatment and controls; see ISO 8528 duty ratings for UK usage profiles.
Permitting thresholds, aggregation and timescales
- Scope: MCPD applies at 1–50 MWth (thermal input is the fuel energy rate, not electrical output). As a guide, a 1,000 kVA (~800 kWe) diesel is roughly 2–2.5 MWth depending on load and efficiency.
- Aggregation: multiple engines discharging to a common stack can be aggregated, pushing the combined plant above 1 MWth and into permitting. Your regulator may also consider other technical links when assessing aggregation.
- Regulators: EA (England), SEPA (Scotland), NRW (Wales), NIEA (Northern Ireland).
- Timescales: new plant permits typically take 8–16 weeks, longer if dispersion modelling is required. Existing plant follow staged deadlines set in the legislation. Early engagement avoids programme slips; see our case studies.
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Exemptions and limited‑hours derogations
- Backup‑only units may qualify for reduced requirements when used solely during power outages, with routine test hours capped and logged. Keep on file: a signed declaration of use, test schedule, outage evidence, and hour logs. Using the set for peaking/export can remove the exemption.
- Other limited cases include R&D, safety‑critical operation, offshore and transitional scenarios. Local planning conditions and Air Quality Management Areas (AQMAs) may still impose stack height, testing windows or abatement. Read permit conditions carefully and keep records up to date.
Emission limits and control technologies for diesel gensets
Key pollutants and typical controls:
- NOx: selective catalytic reduction (SCR with AdBlue) can deliver large NOx reductions when exhaust temperature, dosing and catalyst management are correct.
- Particulate matter (PM): diesel particulate filters (DPF) trap soot; design for regeneration strategy (passive/active) and monitor backpressure.
- CO and hydrocarbons: diesel oxidation catalysts (DOC) reduce CO/HC and support DPF performance.
Plan for backpressure limits, DPF regeneration, AdBlue quality, and sensor calibration in the maintenance regime. Use ultra‑low sulphur diesel or HVO to reduce PM and protect catalysts; see our generator fuel efficiency guide. Engines from Cummins, Perkins, FG Wilson and Baudouin integrate well with modern aftertreatment when correctly specified.
NRMM Stage V for hire and mobile power
- Compliance: Stage V sets must carry valid UKCA/CE type‑approval and legible engine plates. Hire fleets should retain conformity documents and service records.
- Transition schemes: flexibility and transition stock are limited and time‑bound; check applicability before mobilising older units.
- On site: maintain an NRMM register, ensure each set is Stage V compliant, spill‑bunded and maintained. Local authorities can audit and enforce. For compliant temporary power, view our hire options and availability.
Stack height assessment and dispersion modelling
Low‑level exhausts risk local NO2 exceedances, especially in urban canyons and AQMAs. Many planning applications require screening; some need full dispersion modelling.
Typical inputs: exhaust mass flow, NOx emission rate (g/s), temperature, flue diameter/velocity, building geometry and receptor locations. Mitigation options include:
- Raising discharge height or relocating the outlet
- Adding SCR/DPF to reduce NOx/PM
- Relocating the set or re‑orienting the flue
- Limiting test windows to favourable met conditions
Early studies de‑risk permitting and help optimise cost versus performance on constrained sites.
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Record‑keeping, monitoring and routine testing
- Logs: run hours by mode (emergency, testing, maintenance), outages, maintenance actions, faults, fuel deliveries and sulphur certificates.
- Documentation: commissioning data, stack assessments/modelling, permits and any exemption declarations.
- Monitoring/testing: periodic emissions testing as required by your permit/regulator guidance; retain results for the specified period.
- Systems: remote run‑hour capture, alarms and version‑controlled records help demonstrate ongoing compliance.
A practical compliance roadmap from survey to sign‑off
- Survey and classify duty (standby, prime, continuous); capture load profile, step loads and starting currents.
- Right‑size kVA and select engine/alternator, aftertreatment (DOC/DPF/SCR), controls and ATS strategy.
- Define stack and acoustic design; check space for aftertreatment and access for maintenance.
- Prepare permit and planning packs (including screening/modelling where needed).
- Install to method statements; commission via FAT/SAT and site IST; verify protections and ELV performance.
- Handover O&M and record templates; implement a service plan with 24/7 support and compliance audits.
Sector guidance: healthcare, data centres, construction, manufacturing and utilities
- Healthcare and data centres: backup‑only strategies, controlled test windows, stack mitigation to protect local air quality; N+1 resilience and clean changeover.
- Construction and utilities: NRMM Stage V hire with site registers, spill control and auditable service records.
- Manufacturing and agriculture: frequent/prime duty requiring SCR/DPF, robust cooling, and a defined fuel strategy (ULSD or HVO) to meet ELVs and manage running cost.
Used sets, decommissioning and export compliance
- Buying used: confirm engine tier, hours, service history, and feasibility of DOC/DPF/SCR retrofit; check documentation and space for aftertreatment and flues. If resale abroad is possible, see our exporting used generators UK guide.
- Decommissioning: plan isolation, spill prevention, waste handling and documentation.
- Export: verify destination emissions rules and line up type‑approval, conformity documents and logistics paperwork early to avoid port delays.
How PowerTech Generators helps you stay compliant
PowerTech Generators provides end‑to‑end compliance support: new and used supply, Stage V hire, ATS, installation, commissioning, servicing and 24/7 support. We engineer SCR/DPF retrofits, fuel strategies and stack solutions around your duty and site constraints.
As an independent provider, we source Cummins, Perkins, FG Wilson, Baudouin and more to meet your specification. Get a free compliance review covering site survey, stack screening and permit support. We supply documentation packs and ongoing audits to keep you inspection‑ready.
FAQs
Do I need a permit for a standby generator?
Possibly. If thermal input is 1–50 MWth, MCPD can apply, but backup‑only units may qualify for reduced requirements with capped test hours. Confirm specifics with your regulator.
How do I estimate MWth from kVA?
Thermal input is fuel energy, not electrical output. As a rule of thumb, a 1,000 kVA (~800 kWe) diesel is roughly 2–2.5 MWth depending on efficiency and load.
Will enrolling in DSR affect my exemption?
Yes. Joining DSR or peaking typically removes backup‑only status and triggers stricter controls and timelines. Reassess permitting and emissions strategy before enrolling.
Can I run HVO without modifications?
Most modern diesel gensets accept HVO with minimal changes, but always check OEM guidance. Maintain fuel quality controls to protect aftertreatment and warranties.
What records should I keep for compliance?
Run hours by mode, outage evidence, maintenance logs, emissions test reports, permits, stack studies and fuel sulphur certificates. Retain for the period set in your permit.
How long does an EA permit take?
New plant permits often take 8–16 weeks, longer if dispersion modelling is required. Start early to align with build and commissioning milestones.