Three-Phase Solar Panel Integration for UK Factory Power Supplies
Key Takeaway: Nearly all UK factories operate on 415V three-phase supplies — a fundamentally different electrical environment from the domestic installations most solar guides describe. Integrating solar correctly requires purpose-built three-phase inverters, CT metering on all phases, a formal G99 DNO application, and careful attention to load balancing. Done properly, a 100–500kW factory solar system reduces energy costs by £25,000–£120,000 per year.
The UK manufacturing sector runs almost exclusively on three-phase electricity. A 415V three-phase supply can deliver the high currents demanded by CNC machines, compressors, conveyor motors, and industrial HVAC — loads that would trip a domestic single-phase supply instantly. Yet the vast majority of solar installation guides, online calculators, and even some installer proposals are written with domestic or small-commercial single-phase systems in mind.
That mismatch matters. Installing solar incorrectly on a three-phase factory supply causes real-world problems: phase imbalance that damages sensitive equipment, inverter protection trips that cut generation at peak production hours, and DNO rejection that delays or prevents connection. This guide covers everything a factory owner, energy manager, or electrical engineer needs to know about three-phase solar integration — from the physics of phase rotation to completing a G99 application.
Understanding Three-Phase Power in UK Factories
UK industrial premises receive electricity as a three-phase, four-wire system: three live conductors (L1, L2, L3) plus a neutral. The voltage between any live conductor and neutral is 230V (previously 240V — both figures appear in existing factory documentation). The voltage between any two live conductors is 415V. This 415V line-to-line voltage is what powers high-power three-phase motors and equipment.
Under the current wiring colour convention introduced by BS 7671 in 2004 (harmonising with IEC standards), the three live phases are identified as brown (L1), black (L2), and grey (L3). Older installations still in service may use red, yellow, and blue. Phase rotation — the sequence in which each phase reaches its voltage peak — is always L1, L2, L3 in a correctly installed UK supply, and reversal of rotation causes three-phase motors to run backwards.
Why Phase Balance Matters for Solar
A balanced three-phase system means each phase carries the same current and the loads are distributed equally. Most factory machinery — CNC machining centres, air compressors, cooling towers — draws current from all three phases simultaneously, naturally helping to maintain balance. The problem arises when solar generation is added phase by phase rather than in a balanced three-phase output.
Important: Single-Phase Solar on a Three-Phase Supply
A single-phase solar inverter connected to just one phase of a three-phase supply pushes generation onto that phase only. If other loads are not drawing sufficient current from that phase, the excess flows back to the grid. More critically, the generation is invisible to the other two phases — when factory machinery on those phases starts, the supply meter shows high import on L2 and L3 even while L1 is exporting. DNOs enforce limits on how much phase imbalance the network can tolerate. Exceeding those limits triggers DNO intervention or automatic disconnection.
CT (Current Transformer) Metering: The Foundation of Accurate Monitoring
CT metering is non-negotiable for factory solar systems. A Current Transformer is a measurement device that clamps around a live conductor and produces a small secondary current proportional to the primary (load) current — typically a 5A secondary signal for every 100A, 200A, or 500A primary current, depending on the ratio specified. Three CTs — one per phase — installed on the main incomer cables give the inverter or energy management system real-time visibility of exactly how much power each phase is importing from or exporting to the grid.
Without per-phase CT metering, a solar system has no way of knowing which phase has the highest load and therefore where self-consumption is greatest. The system cannot limit export intelligently, cannot report accurate ESOS/SECR consumption data, and cannot feed a battery storage EMS with the data it needs for load-shifting decisions.
Three-Phase Solar Inverter Options
The inverter is the critical component that converts DC power from the solar array into grid-synchronised AC power. For factory installations, the inverter must produce balanced three-phase output — not a single-phase output, and not three independent single-phase outputs running without co-ordination.
| Inverter Type | Phase Output | Typical Size | Best Application |
|---|---|---|---|
| Three-phase string inverter | Balanced 3-phase | 15–100 kW | 50–500 kW systems |
| Three-phase central inverter | Balanced 3-phase | 100 kW–2 MW | Large industrial (>500 kW) |
| Three-phase microinverter arrays | Per-panel 3-phase | Variable (panel-level) | Complex roofs, partial shading |
| Multiple single-phase (not recommended) | 3× single-phase, unco-ordinated | Variable | Not suitable for factory supply |
Three-Phase String Inverters: The Factory Standard
For the majority of UK factory solar installations in the 50–500 kW range, three-phase string inverters are the default choice. They connect strings of series-wired solar panels on the DC side and produce a single balanced three-phase AC output. Multiple units can be operated in parallel for larger systems. Key manufacturers and models used extensively on UK factory installations include:
- SMA Sunny Tripower X — 15–50 kW per unit, widely specified on UK commercial projects, strong G99 compliance documentation
- Fronius Symo GEN24 — 3–25 kW per unit, hybrid-ready for battery integration, excellent monitoring via Fronius Solar.web
- Sungrow SG series — 30–110 kW per unit, highly competitive on cost, strong MPPT performance
- Huawei SUN2000 — 30–100 kW, AI-powered MPPT optimisation, integrated with FusionSolar monitoring platform
- GoodWe GW series — 30–80 kW, increasingly popular on mid-size factory systems, good smart meter integration
Central Inverters for Large Industrial Sites
For factories with roof areas above 2,000 m² and systems above 500 kW, central inverters offer lower cost per kilowatt and simplified installation with fewer AC combiner circuits. The trade-off is that a single central inverter represents a single point of failure — a failure that takes the entire system offline rather than just one string. For critical manufacturing sites, a cluster of large string inverters (4–8 units of 50–100 kW each) typically provides better resilience than a single central unit.
DNO Grid Connection for Three-Phase Factory Solar
Every factory solar system that generates more than 16A per phase — that is, essentially every commercial installation above approximately 11 kW — must obtain formal permission from the Distribution Network Operator (DNO) before connecting. The governing standard is ENA Engineering Recommendation G99.
G99 vs. G98: Which Applies to Factory Solar?
G98 applies to single-phase generation below 3.68 kW — domestic solar. G99 (ENA Engineering Recommendation G99 Issue 1 Amendment 6, 2023) applies to all generation above 16A per phase on any phase of a low-voltage connection, and all generation connected to a high-voltage supply regardless of size. Nearly all factory solar systems fall under G99. The distinction matters because G99 requires a formal application and DNO approval before connection; G98 is a simpler notification process.
G99 Technical Requirements for Three-Phase Generators
G99 sets specific technical requirements that three-phase factory solar systems must meet. The inverter manufacturer's datasheets and G99 compliance documentation must demonstrate:
- Balanced three-phase output: Generation must be distributed equally across L1, L2, and L3. Inverters with proven three-phase balance are specified for exactly this reason.
- Power factor: Typically required to be 0.95 lagging or leading (absorbing or producing reactive power). Some DNOs require unity power factor or specify a reactive power curve.
- Frequency ride-through: The inverter must remain connected and generating during frequency deviations between 47.5 Hz and 51.5 Hz, only disconnecting at extremes (below 47 Hz or above 52 Hz).
- Voltage ride-through: Must remain connected during voltage dips and rises within specified thresholds per the Low Voltage Ride Through (LVRT) requirements.
- Anti-islanding protection: Must disconnect within 5 seconds of detecting a loss-of-mains condition, preventing energisation of a de-energised network section during a fault.
- Rate of Change of Frequency (RoCoF): Must disconnect if frequency changes at more than 1 Hz per second (some DNOs require 0.5 Hz/s for larger installations).
The G99 Application Process Step by Step
-
1
DNO Pre-Application Enquiry: Submit a brief technical summary (system size, connection point, inverter type) to the DNO (e.g. Western Power Distribution, UK Power Networks, SP Energy Networks). The DNO confirms whether the network has available capacity and advises on likely connection requirements. Many DNOs provide an online portal for this step. No fee at this stage.
-
2
Formal G99 Application: Submit the full application form with single-line diagram, inverter G99 compliance documentation, proposed protection relay settings, and site electrical details. Application fees are typically £500–£2,000 depending on DNO and system size.
-
3
DNO Network Impact Assessment: The DNO assesses whether local network infrastructure (transformers, cables, protection devices) can accommodate the additional generation. If network reinforcement is required, the DNO issues a quote for that work — costs can range from zero to £50,000+ depending on local network conditions.
-
4
Site Assessment and Protection Testing: For larger systems, the DNO may require a site visit to verify protection relay settings and witness commissioning tests. The installer must demonstrate correct operation of anti-islanding, over/under voltage, and over/under frequency protection before the system is approved to generate.
-
5
Connection Agreement and Energisation: Once approved, a formal Connection Agreement is signed. The installer energises the system, completes commissioning records, and notifies the DNO. Timescales for a straightforward factory connection are typically 4–8 weeks from formal application submission, though network reinforcement can extend this to 3–6 months.
Do Not Energise Before G99 Approval
Connecting a solar system to the grid before formal G99 approval is a breach of your Connection Agreement with the DNO. The DNO has the right to require immediate disconnection and may charge for any network investigation or remediation costs. Always obtain written confirmation of approval before commissioning.
CT Metering for Three-Phase Factory Solar
Accurate CT metering is the nervous system of a factory solar installation. Without it, the inverter cannot implement export limitation (a common DNO requirement), the energy management system has no data to optimise battery charging, and compliance reporting for ESOS and SECR cannot be completed accurately.
Three-Phase CT Configuration
The standard configuration places one CT on each phase of the main incomer — the cable between the DNO's supply cut-out and the factory's main LV switchboard (MLVS). This location measures the net import/export at the grid boundary point: positive values indicate grid import; negative values indicate export. The three CTs feed a combined data logger or smart meter that calculates real power (kW), apparent power (kVA), reactive power (kVAr), and power factor on each phase independently.
CT Specification Checklist for Factory Installations
Selecting the Correct CT Ratio
CT ratio selection requires knowledge of the factory's maximum demand current. This can be read from the half-hourly metering data (available from the meter operator or energy supplier), from the main fuse or circuit breaker rating, or from a temporary clamp-meter survey. As a general guide:
| Maximum Demand (per phase) | Recommended CT Ratio | Typical Application |
|---|---|---|
| Up to 80A | 100:5A | Small factory or sub-board |
| 80–160A | 200:5A | Light manufacturing, sub-board |
| 160–320A | 400:5A | Medium factory main incomer |
| 320–480A | 600:5A | Large industrial incomer |
| 480–640A | 800:5A | High-demand manufacturing |
| 640A+ | 1000:5A or 1500:5A | Large industrial, HV-connected |
Load Balancing and Phase Distribution
The goal of load balancing in a factory solar system is to maximise self-consumption — using generated solar power directly within the facility rather than exporting it at the low Smart Export Guarantee (SEG) rate. The challenge is that factory loads are dynamic: CNC machines start and stop, compressors cycle, process lines ramp up and down. The solar system must respond intelligently to this constantly shifting load profile.
Export Limitation: The DNO's Primary Requirement
Most DNOs approve factory solar connections on the condition that net export to the grid is limited — either to zero (zero-export) or to a defined maximum. Export limitation is achieved by the inverter reading real-time CT data and throttling its output when generation would otherwise exceed site consumption. Modern three-phase string inverters from SMA, Fronius, Sungrow, and Huawei all support export limitation via CT feedback as a standard feature.
Zero-export systems never export to the grid. This simplifies the G99 application in some cases (the DNO may be satisfied with a simpler technical assessment) and avoids the need for a Smart Export Guarantee agreement with the energy supplier. The trade-off is that generation is curtailed on weekends, bank holidays, or during production downtime — periods when solar generation could otherwise be generating revenue or bank hours in a battery.
Energy Management Systems for Phase Balancing
For factories with battery storage or complex load profiles, a dedicated Energy Management System (EMS) provides the intelligence layer that simple inverter export limitation cannot. The EMS reads per-phase CT data, weather and solar forecast data, factory production schedules, and electricity tariff structures to optimise when the battery charges and discharges, ensuring maximum self-consumption across all three phases simultaneously.
EMS platforms commonly deployed on UK factory solar+storage systems include:
- Sigenergy SigenStor: Integrated three-phase EMS built into the Sigenergy inverter-battery platform; AI-powered forecasting; strong commercial solar track record in the UK
- GivEnergy AIO: Three-phase compatible EMS with Octopus Agile tariff integration and half-hourly optimisation; widely deployed in UK commercial applications
- Fox ESS Manager: Cloud-based EMS platform with per-phase monitoring, time-of-use optimisation, and remote configuration; popular with larger commercial installers
- SMA Energy Management: Factory-grade EMS via SMA's HEMS controller; strong integration with SMA's Sunny Tripower inverter range and industrial automation systems
Phase Balancing with Battery Storage
A three-phase battery storage system significantly improves phase balancing. When L1 is generating excess solar but L2 and L3 are importing heavily, a three-phase battery can absorb the L1 excess and simultaneously discharge on L2 and L3 — effectively redistributing generation across phases in real time. This capability is only available with properly specified three-phase battery systems (such as the BYD BBox Commercial, Sungrow SBH, or Huawei LUNA2000 commercial series) — single-phase residential batteries cannot achieve this.
Connecting to Factory Distribution Boards
The point at which a solar inverter connects to a factory's electrical distribution determines both the technical performance of the system and the regulatory compliance requirements. All connection work must comply with BS 7671: 2018 (18th Edition) as amended by Amendment 2 (2022), which introduced significant changes to surge protection device (SPD) requirements relevant to solar installations.
Main LV Switchboard (MLVS) Connection
For factory solar systems above 50 kW, connection at the Main LV Switchboard is the standard approach. The solar AC output is terminated at a dedicated solar incomer way within the MLVS, upstream of the factory's sub-distribution circuits. This configuration means all factory loads are downstream of the solar connection point, maximising the proportion of solar generation that can be self-consumed before any export occurs.
Protection and Isolation Requirements
AC Isolator (Mandatory)
A dedicated, lockable AC isolator must be installed on each phase of the solar inverter output, accessible without tools by emergency services. The isolator must be clearly labelled "Solar PV Isolator" and its location must be marked on the electrical distribution board schedule. For three-phase systems, a three-pole isolator with lockable mechanism is required.
Overcurrent Protection Coordination
The upstream MCB or fuse in the MLVS protecting the solar feeder cable must be rated to carry the full inverter output current with an appropriate margin. For a 100 kW inverter producing 144A three-phase output, a 160A or 200A MCB is typically specified. Protection must be co-ordinated with the inverter's internal over-current protection to ensure discrimination — the MCB must not trip before the inverter's own protection operates.
Surge Protection Devices (SPDs): BS EN 61643-11
Amendment 2 to BS 7671 made SPDs mandatory for most new electrical installations, including solar additions to existing factory supplies. A Type 1+2 combined SPD must be installed at the point of connection between the solar system and the factory distribution board. Type 1 provides protection against direct lightning strike-induced transients; Type 2 protects against switching transients from inductive loads (motors, transformers). For three-phase systems, three-pole plus neutral SPDs are required.
Earth Fault Protection
RCD or RCBO protection is required on the AC output of each inverter. For three-phase systems, a four-pole (3P+N) RCD of appropriate sensitivity — typically 100 mA or 300 mA for factory environments — provides protection against earth fault currents that the inverter's own earth fault detection may not detect below the inverter disconnection threshold. The RCD must be Type B or Type A (Type AC is not suitable where DC components may be present in the fault current from the inverter).
DC Wiring Considerations
The DC side of a three-phase solar installation — the array wiring, string combiners, and DC isolators — must comply with BS 7671 Section 712 (Solar Photovoltaic Power Supply Systems). Key requirements include double-insulated DC cables rated for continuous outdoor UV exposure, string fusing or string protection where more than two strings connect in parallel, DC surge protection at the inverter DC input, and a clearly marked DC isolator between the array and each inverter. The DC isolator must be rated for the DC system voltage and current — typically 1000 VDC or 1500 VDC.
Get a Three-Phase Solar Feasibility Assessment
Our technical team will review your factory supply, maximum demand data, and roof area to provide a detailed three-phase solar integration proposal including G99 pre-application and CT metering specification.
Request Technical AssessmentThree-Phase Factory Solar: Frequently Asked Questions
Can I use single-phase inverters on a three-phase factory supply?
Technically possible, but strongly discouraged for factory installations. Using multiple single-phase inverters on a three-phase supply creates phase imbalance if generation is unequal across phases. UK DNOs and BS 7671 require balanced loading across phases; persistent imbalance above permitted limits causes protection trips, power quality issues, and may result in DNO disconnection. Always specify purpose-built three-phase inverters for factory solar systems. The only scenario where multiple single-phase inverters are occasionally used is where three separate single-phase sub-boards are being supplied from a common three-phase incomer and each inverter is sized and CT-limited to match consumption on its own phase — but this requires careful engineering design and explicit DNO consent.
What is G99 and when does it apply to factory solar?
G99 (ENA Engineering Recommendation G99 Issue 1 Amendment 6, 2023) is the UK standard governing the connection of generating plants to the distribution network. It applies to all generation above 16A per phase — meaning virtually all factory solar systems. G99 replaced the older G59 standard and sets requirements for power factor, frequency ride-through, anti-islanding protection, and phase balance. A formal G99 application to your DNO is mandatory before energising any qualifying factory solar installation. Failure to obtain G99 approval before connection constitutes a breach of the Connection Agreement and can result in forced disconnection at the customer's cost.
How do I prevent phase imbalance with solar on a three-phase system?
The primary solution is to use a properly specified three-phase inverter, which by design produces balanced output across all three phases simultaneously. CT metering on each phase enables real-time monitoring of per-phase import and export. Export limitation settings in the inverter or an external energy management system (EMS) can cap generation during periods of low factory load, preventing any single phase from pushing excessive current back towards the grid. Battery storage further smooths phase imbalances by buffering excess generation and redistributing it to high-demand phases in real time. For very large systems (above 500 kW), it is worth commissioning a power quality survey before and after installation to verify phase balance performance under different load conditions.
What CT ratio should I specify for my factory metering?
CT ratio selection is based on the maximum demand current on each phase. A common specification for factories drawing up to 400A is a 400:5A or 500:5A ratio. Smaller facilities typically use 100:5A or 200:5A. Always select a ratio where the expected operating current falls in the 20–120% range of the CT primary rating to maintain Class 1 accuracy. Operating a 400:5A CT at only 20A primary current (5% of rating) causes accuracy to degrade significantly — in that scenario, a 100:5A CT would give much better results. Split-core CTs allow installation without disconnecting the live supply, making them preferred for retrofit metering on existing factory incomers where a planned supply outage is difficult to arrange.
Can I add battery storage to a three-phase factory solar system?
Yes, and for most factories it is strongly recommended. Three-phase battery storage systems (such as the Sungrow SBH, Huawei LUNA2000, GivEnergy commercial units, or BYD BBox Commercial) connect at the same MLVS point as the solar inverter and provide balanced three-phase output. Battery storage increases self-consumption from a typical 35–40% to 65–80%, allows peak demand shaving to reduce half-hourly metering charges by £5,000–£15,000 per year, and provides grid resilience during outages. A combined G99 application covering both the solar inverter and battery system simplifies the DNO process and ensures both systems are covered under a single Connection Agreement. Battery additions to an existing solar system require a G99 modification application.
How does three-phase solar integration affect my electricity meter?
Factory electricity meters are typically half-hourly (HH) advanced meters that measure import and export on each phase independently. Once solar is commissioned, your supplier or meter operator will configure the meter to record export as well as import. For G99 connections, the DNO may require a dedicated generation meter (registered with Elexon on the BSC Register) in addition to the existing supply meter — particularly for systems above 50 kW seeking to participate in Smart Export Guarantee payments or grid services. CT-based monitoring systems (separate from the billing meter) provide the per-phase real-time visibility needed for EMS control, ESOS energy audits, and SECR carbon reporting. Note that SEG export payments are calculated from the generation meter or export register of the supply meter, not from the inverter's internal monitoring.
Ready to Integrate Solar with Your Factory's Three-Phase Supply?
Our MCS-certified installation partners have the G99 application experience, three-phase inverter expertise, and industrial CT metering capability to deliver compliant, high-performance factory solar systems across the UK.