Unlocking SCE’s Subtransmission System: A Strategic Path to Full Capacity Deliverability

Sub-transmission systems play a critical role in supporting medium- to large-scale generation and energy storage projects. Operating at voltage levels between 60 kV and 138 kV, these networks serve as an intermediate tier between the high-voltage transmission grid and local distribution systems. Their capacity and configuration make them especially valuable for accommodating new resource interconnections, including renewable generation and strategically located energy storage. 

Operational Control in California 

Within California, operational responsibility for sub-transmission facilities varies by utility: 

• Pacific Gas & Electric (PG&E) and San Diego Gas & Electric (SDG&E) have transferred operational control of most of their sub-transmission-level assets to the California Independent System Operator (CAISO).
• Southern California Edison (SCE) has retained operational control of most of its sub-transmission facilities. Unlike the other two investor-owned utilities, SCE continues to manage, plan, and study these assets independently, outside of CAISO operational authority. 

SCE Sub-transmission System Characteristics 

SCE operates an extensive and diverse sub-transmission network that is well-positioned to support new electrical infrastructure development: 

The utility maintains over 40 distinct sub-transmission systems, consisting of: 

• Thousands of miles of 66 kV lines,  
• Hundreds of miles of 115 kV lines, and 
• Hundreds of 66 kV and 115 kV substations. 

These voltage levels determine the scale of projects that can be reliably interconnected per SCE designation: 

• 66 kV systems can support projects up to 112.5 MW. 
• 115 kV systems can support projects up to 200 MW. 

Because of the geographic reach and electrical capabilities of these systems, SCE’s sub-transmission network represents one of the largest and most versatile interconnection environments available in the state with generation interconnections accommodated via SCE’s FERC-approved Wholesale Distribution Access Tariff (WDAT). 

Deliverability Considerations within the SCE Metro Area 

Interconnections within SCE’s Metro Area footprint, which comprises 23 of the over 40 total sub-transmission systems, offer particularly significant benefits. These systems generally exhibit: 

• Higher load densities 
• Stronger network topology 
• Better access to downstream transmission paths 

As a result, projects interconnected in these Metro Area sub-transmission systems are more likely to achieve Full Capacity Deliverability Status (FCDS). 

The Role of Energy Storage in Sub-transmission Planning 

Energy storage resources have a unique opportunity within the SCE sub-transmission network. When appropriately sited and sized, storage facilities can: 

• Alleviate local congestion 
• Enhance system reliability 
• Improve deliverability outcomes for both standalone storage and hybrid generation-plus-storage projects 

Strategically deploying energy storage in these areas can significantly increase the likelihood of securing FCDS—an essential requirement for participating in capacity markets and securing long-term resource adequacy value.

EPE’s Unique Expertise in the SCE Sub-transmission System

EPE possesses a level of insight into the SCE sub-transmission network that is unmatched outside of SCE itself. This expertise is rooted not only in decades of analytical work, interconnection support, and system modeling across SCE’s 66 kV and 115 kV systems, but also in the experience of former SCE transmission planning staff now at EPE.

This includes team members who previously served as SCE’s own subject matter experts in transmission planning, with direct responsibility for: 

• Subtransmission planning and expansion 
• Generation and Load Interconnection assessments 
• Grid reinforcement strategy 
• System upgrade prioritization 
• Transmission–subtransmission interface studies 

Their firsthand knowledge of SCE’s planning methodologies, study processes, and system behavior provides EPE with unique capabilities in: 

• Identifying optimal interconnection points 
• Anticipating system constraints 
• Guiding project developers through SCE’s technical requirements 
• Crafting successful strategies for achieving FCDS 

EPE is uniquely positioned to help developers navigate the complexities of interconnecting within the SCE territory under SCE’s WDAT. Our level of system-specific expertise enables EPE to provide highly accurate guidance, strategic planning, and technical analysis for project developers. No organization outside SCE itself possesses a deeper or more practical understanding of the SCE sub-transmission system. To find out how we can help you navigate the interconnection process in SCE, reach out to our team today using the form below.