Single Line Diagram for EV Charging Station: Complete Guide
Understand the electrical architecture of an EV charging station — from utility connection to charger output — with a detailed SLD breakdown and component guide.
EV Charging Electrical Design & Infrastructure Market (2026)
The global EV charging infrastructure market is projected to reach $65.7 billion in 2026 and grow to over $405 billion by 2035 (22.4% CAGR). A well-designed single line diagram (SLD) is the foundation of every successful charging station project. Proper electrical design for ev charging station ensures safety, reliability, and code compliance.
Source: IEA 2026, BloombergNEF, Grand View Research
A Single Line Diagram (SLD) is the blueprint for your EV charging station’s electrical system. It shows how power flows from the utility grid through transformers, switchgear, and distribution panels to each charger. Getting the SLD right is essential for safety, reliability, and code compliance.
In this guide, we’ll break down the complete single line diagram for a typical DC fast charging station, explain each component, and show how everything connects together.
📋 Table of Contents
What is a Single Line Diagram?
A Single Line Diagram (also called a one-line diagram) is a simplified representation of an electrical power system. It uses standard symbols to show each major component and how they’re connected by a single line (representing all three phases of a 3-phase system).
💡 Why SLDs Matter for EV Charging
- Design verification: Ensures the electrical system meets load requirements
- Safety: Shows all protection devices and their coordination
- Code compliance: Required by NEC, IEC, and local regulations
- Utility approval: Needed for grid connection applications
- Maintenance: Essential reference for technicians working on the system
- Expansion: Makes future upgrades easier to plan
Typical SLD for a 10-Stall DCFC Hub
Here’s what a complete single line diagram looks like for a 10-stall × 150kW DC fast charging station:
Component-by-Component Breakdown
Let’s go through each major component in the SLD, starting from the utility connection and working toward the chargers.
MV Side Components (Utility to Transformer)
Transformer
LV Side Components (Transformer to Chargers)
Medium Voltage Side Design Considerations
Switchgear Configuration
For medium voltage, you have several switchgear options:
| Switchgear Type | Typical Use Case | Cost Level |
|---|---|---|
| Ring Main Unit (RMU) | Small sites, single transformer, up to 1,000 kVA | Low |
| Load Break Switch + Fuses | Medium sites, single transformer | Low-Medium |
| Vacuum Circuit Breaker (VCB) | Large sites, multiple transformers, high fault current | High |
| Gas-Insulated Switchgear (GIS) | Space-constrained sites, indoor installations | Very High |
💡 GOHOSLF Recommendation
For most EV charging stations up to 2,000 kVA, a Ring Main Unit (RMU) with load break switches and fuses provides reliable, cost-effective protection. For larger sites with multiple transformers, upgrade to VCB switchgear for better protection coordination.
Fault Current Considerations
The available fault current at your site determines the interrupting rating required for your switchgear and breakers. This depends on:
- Utility system short circuit capacity
- Transformer impedance (typically 4-6% for distribution transformers)
- Cable length and size between utility and transformer
Typical fault current at LV side for a 1,500 kVA transformer with 5% impedance: ~43 kA sym.
Low Voltage Side Design Considerations
Busbar Rating
The LV main busbar must be sized for the total connected load, including future growth. For a 1,500 kVA transformer at 400V, the rated current is 2,165A. Specify a busbar with at least 2,500A rating for this size.
Charger Grouping Strategy
Grouping chargers on multiple feeders provides several benefits:
- Fault isolation: One fault only takes down one group, not all chargers
- Maintenance flexibility: Work on one group while others remain operational
- Cable sizing: Multiple smaller cables are easier to install than one huge cable
- Load balancing: Easier to balance phases across groups
We recommend grouping 3-5 chargers per feeder for sites with 8+ chargers.
Voltage Drop
Proper cable sizing ensures voltage drop stays within acceptable limits (typically ±5% of nominal at full load). For long cable runs to chargers, you may need to increase cable size.
Where: L = cable length (m), I = current (A), R = cable resistance (Ω/km), cos φ = power factor, V = nominal voltage (V)
Protection & Safety Components
📈 Protection Device Hierarchy & Coordination in EV Charging SLD
Figure: Five-level protection hierarchy in a typical EV charging station SLD. Proper coordination ensures that a fault at any level trips only the nearest upstream breaker, minimizing downtime.
Overcurrent Protection
Every circuit needs proper overcurrent protection. In an EV charging station, you typically have multiple levels:
- MV side: Fuses or circuit breaker for transformer primary
- LV main: Air circuit breaker (ACB) or MCCB for total load
- Feeder level: MCCBs for each charger group
- Charger level: Built-in protection in each charger
Breakers must be properly coordinated so that a fault at a charger trips only that charger’s breaker, not the entire site.
Ground Fault Protection
Ground fault protection is required by code (NEC 230.95 for services >1000A). For EV charging stations with 3-phase 4-wire systems, ground fault protection is especially important due to harmonic neutral currents.
Surge Protection
Install surge protective devices (SPDs) at multiple levels:
- Class I: At transformer LV side (for lightning protection)
- Class II: At main distribution panel
- Class III: At sensitive equipment (controls, monitoring)
Isolation & Lockout/Tagout
Each charger group should have a visible disconnect or breaker that can be locked out for maintenance. This is critical for technician safety when working on chargers.
SLD Variations by Site Size
Small Site (2-4 DCFC, up to 600 kW)
- Simple RMU or fused disconnect on MV side
- Single transformer (500-750 kVA K-13)
- Single LV panel with individual charger breakers
- No AHF required (depends on utility requirements)
- Simplest and most cost-effective configuration
Medium Site (6-10 DCFC, 600-1500 kW)
- RMU or simple VCB on MV side
- Single transformer (1,000-2,000 kVA K-13)
- Main LV switchboard with multiple feeder breakers
- Active harmonic filter recommended
- PFC optional (depending on utility PF requirements)
- Multiple charger groups for redundancy
Large Site (16+ DCFC, 2,000+ kW)
- MV switchgear with multiple VCBs
- Two or more transformers in parallel
- Main-tie-main LV switchboard configuration
- AHF system sized for total harmonic load
- Detuned PFC system
- Redundant design for high availability
- Medium voltage switchgear with metering and protection relays
Prefabricated Substation (All Sizes)
For faster deployment, consider a prefabricated (containerized) substation. The entire MV switchgear, transformer, and LV switchgear assembly is factory-built and tested, then delivered as a single unit.
Learn more about prefabricated substations for EV charging →
❓ Frequently Asked Questions
Q: What is a single line diagram and why is it important for EV charging?
A: A single line diagram (SLD), also called a one-line diagram, is a simplified representation of an electrical power system that uses standard symbols to show major components and their connections by a single line (representing all three phases). For EV charging stations, the SLD is essential because it serves as the design blueprint for safety, code compliance, utility approval, and maintenance. A well-designed SLD ensures proper protection coordination, correct equipment sizing, and provides a reference for all future modifications and troubleshooting.
Q: What are the essential components that must be shown in an EV charging SLD?
A: A complete EV charging SLD should include: utility service point, instrument transformers (CT/VT) for metering, MV switchgear or RMU, surge arresters, distribution transformer (K-factor rated), main LV breaker (ACB/MCCB), LV busbar, feeder breakers for charger groups, active harmonic filter (if applicable), power factor correction (if applicable), auxiliary distribution panel, surge protective devices (SPDs), and DC fast chargers. Each component should be labeled with its rating. The SLD for SLD ev charging projects should also show grounding, protection relay types, and cable specifications.
Q: What is protection coordination and why is it critical?
A: Protection coordination is the process of setting protective devices (breakers, fuses, relays) so that a fault at any point in the system trips only the nearest upstream device, leaving the rest of the system operational. For EV charging stations, this means that a fault at one charger should trip only that charger’s feeder breaker, not the main LV breaker. Proper coordination minimizes downtime — if one charger group has a fault, the other groups continue operating. Without coordination, a single fault could shut down the entire station, losing revenue from all chargers.
Q: What standards and codes govern EV charging station SLD design?
A: The primary standards are: (1) NEC (National Electrical Code) in the US, particularly Article 450 for transformers and Article 625 for EV charging; (2) IEC 61936 and IEC 60364 internationally; (3) IEEE standards for power systems including IEEE 519 for harmonic limits; (4) Local utility requirements for interconnection; (5) NFPA 70E for electrical safety in the workplace. A licensed electrical engineer must prepare and stamp the SLD for permit and utility approval. Always check with your local Authority Having Jurisdiction (AHJ) for specific requirements.
Q: What are the most common SLD design mistakes for EV charging?
A: Common mistakes include: (1) Using standard transformers instead of K-factor rated ones, leading to premature failure from harmonics; (2) Inadequate protection coordination, where a single charger fault trips the entire station; (3) Forgetting auxiliary loads like HVAC, lighting, and controls in load calculations; (4) Under-sizing neutral conductors for triplen harmonics; (5) Missing surge protective devices at multiple levels; (6) Not accounting for future expansion in busbar and switchgear ratings; (7) Improper ground fault protection sizing for 3-phase 4-wire systems with high harmonic neutral currents.
Q: Do I need MV switchgear or can I connect at LV?
A: It depends on your total load and the available utility service. For small sites under ~200 kW, you might connect directly to an existing low-voltage service. For most DC fast charging sites with 4+ chargers, you’ll need a medium-voltage service and transformer. The exact threshold depends on the utility’s available LV capacity at your site. As a rule of thumb, if your total load exceeds 200-300 kW, plan for MV service with a dedicated transformer. Always coordinate with your utility early in the design process.
Q: Who can prepare a professional SLD for my EV charging project?
A: A licensed professional electrical engineer (PE) must prepare and stamp the single line diagram for permit submittal and utility approval. For EV charging projects, it’s best to work with engineers who have specific experience with DC fast charging systems, as they’ll be familiar with the unique requirements like K-factor transformers, harmonic mitigation, and high-power distribution. Our team at GOHOSLF works with experienced electrical engineers to provide complete SLD packages tailored specifically for EV charging projects.
Need a Custom SLD for Your EV Charging Project?
Our engineering team can design a complete single line diagram and electrical system for your EV charging station — from utility connection to charger output.
Related EV Charging Infrastructure Resources
Designing, sizing, or deploying EV charging infrastructure? Explore our complete range of EV charging station equipment, from switchgear to chargers and everything in between.
- Switchgear & Protection: Vacuum circuit breaker (12kV-40.5kV), GGD low voltage switchgear, GHRM ring main unit
- Power Quality: GGJ power factor correction capacitor bank, harmonic mitigation solutions
- Soft Starters: Soft starter for motor and charger applications
- Transformers & Substations: ZBW prefabricated substation, distribution transformers