Page type: primary-anchored (mirrors System Operator - Transmission Owner Code, 1 April 2026)
The System Operator - Transmission Owner Code (STC)
The STC is one of the 11 GB industry codes governing the electricity system. It is administered by NESO and governs the relationship between the System Operator (NESO) and Transmission Owners (NGET, SPT, SHET, offshore TOs, and competitively appointed TOs). Users -- generators, suppliers, demand customers -- are not parties to the STC; their relationship with the system is governed by the CUSC and Grid Code.
Legal foundation: Electricity Act 1989, s.9; ESO Licence SLC C7; Transmission Licence SLC B12.
Current version: Complete STC, 1 April 2026 (388 pages).
Source file: stc.md
What the STC does
The Grid Code tells Users what their equipment must do. The CUSC tells Users what they pay. The STC tells the SO and TOs how to work together to plan, build, and operate the physical network.
Three questions the STC answers:
- How do TOs provide services to NESO? -- Each TO makes its transmission system available, lets NESO direct its configuration, and provides system information (Section C)
- How does new transmission get built? -- NESO coordinates planning with TOs; construction is done through TO Construction Agreements (Section D)
- What standards must offshore TOs meet? -- Section K sets minimum technical criteria for OFTO systems
Structure at a glance
| Section | Title | What it does | Pages |
|---|---|---|---|
| A | Applicability | Code structure and precedence | 2 |
| B | Governance | STC Modification Panel, modification process | 60 |
| C | Transmission Services & Operations | TO services, outage planning, testing | 22 |
| D | Planning Co-ordination | Network planning, construction, TEC exchange, CATOs | 54 |
| E | Billing & Payment | Invoicing between SO and TOs | 4 |
| F | Communications & Data | Data exchange requirements | 5 |
| G | General Provisions | Liability, confidentiality, IP, force majeure | 14 |
| H | Disputes | Dispute resolution | 6 |
| I | Transition | Transitional provisions | 4 |
| J | Definitions | Defined terms | 55 |
| K | Offshore Technical Standards | Minimum criteria for OFTO systems | 25 |
Plus 15 Schedules (including TO Construction Agreement proforma at 52 pages) and Code Procedures (STCPs).
Section C: Transmission Services
Section C defines "Transmission Services" as three things (C.2.1):
- Physical availability -- making the system available so it can convey electricity
- Control -- enabling NESO to direct the configuration of the TO's system
- Information -- providing NESO with the data it needs to coordinate flows
Each TO must specify the technical limits of its services in a Services Capability Specification (C.3). When services are reduced below specification (e.g. due to equipment failure), the TO must follow the Services Reduction process (C.4) and develop Services Restoration Proposals (C.5).
NESO has the right to direct electricity flows across the system -- the "System Direction" power (C.6) -- but only for ESO functions (C.2.3).
Part Two covers outage planning: TOs submit Outage Proposals, NESO integrates them into Outage Plans for the whole system.
Section D: Planning and Construction
Section D governs how the transmission system is planned, developed, and built:
- Part One (Planning): Each TO must ensure its system meets Grid Code technical criteria (CC 6.1-6.4) and SQSS planning standards (D.Part1.2.2.6)
- Part Two (Construction): TO Construction Offers and Agreements -- the mechanism by which reinforcement works identified in CUSC connection offers get built
- Part Three (CATOs): Framework for Competitively Appointed Transmission Owners -- Ofgem's vehicle for introducing competition in onshore transmission
- Part Four (TEC Exchange): Temporary transfer of Transmission Entry Capacity between connection sites
- Part Five (ETI): Evaluation of Transmission Impact for new connections
- Part Six: TO-side implementation of CUSC User Commitment Methodology
Section K: Offshore Standards
Section K sets minimum technical, design and operational criteria that OFTOs must meet. It uses a two-track approach:
- Annex 1: Pre-2019 offshore systems (AC connected before 27 April 2019; DC before 8 September 2019) -- legacy technical standards
- Annex 2: Post-2019 offshore systems -- aligned with EU network codes (the Requirements for Generators and HVDC Systems network codes)
Section K also covers offshore-specific commercial matters: other charges (Part Three), interruption charges (Part Four), compensation payments (Part Seven), and replacement OFTO charges (Part Eight).
Governance
The STC Modification Panel comprises: - NESO: up to 2 representatives - Each onshore TO: up to 2 representatives each - OFTOs collectively: up to 2 representatives - CATOs collectively: up to 2 representatives - Panel Chairperson (no vote), Panel Secretary provided by NESO (no vote) - Authority may observe and speak but not vote
Modifications follow three routes: - Standard: Panel evaluates, produces STC Modification Report, Authority decides - Fast Track: Expedited process for urgent changes - Self-Governance: Panel decides, with 15-Business-Day appeal window
Relationship to other instruments
The STC sits within the four NESO-administered codes: - Grid Code -- TOs must comply with Grid Code technical criteria (referenced in Section D) - CUSC -- the User-facing code; Interface Agreements referenced across both codes; CUSC Section 4.1.2.3 references STC for offshore reactive power - SQSS -- TOs must meet SQSS planning standards (referenced in Section D) - STC -- the SO-TO interface (this page)
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Last updated: 2026-04-05