Electrical Three-Line Diagram

An Electrical Three-Line Diagram is a detailed engineering drawing that shows the AC electrical architecture of a solar PV system using three separate lines representing the three phases of the power system—Phase A, Phase B, and Phase C. Unlike a one-line diagram, which simplifies the electrical network into a single representative line, a three-line diagram displays phase-by-phase connections, conductor routes, grounding paths, protection devices, and inverter outputs in a fully accurate format.

Three-line diagrams are required for certain commercial, industrial, and utility-scale solar projects—especially those involving three-phase inverters, transformers, switchgear, protection relays, and interconnection with medium-voltage grids. They play a crucial role in utility applications, engineering reviews, and compliance with NEC, IEEE, and utility interconnection rules.

Solar engineering teams often create three-line diagrams during the advanced design and permitting stages using tools supported in planning environments like the Solar Project Planning Hub.

Key Takeaways

An Electrical Three-Line Diagram shows phase-by-phase AC wiring for commercial and utility-scale solar systems.
It displays conductors, breakers, grounding, inverters, transformers, and switchgear in detail.
Required for advanced engineering, utility interconnection, and NEC compliance.
Essential for C&I, hybrid, microgrid, and utility-scale PV system design.
Forms the backbone of electrical engineering deliverables for EPC teams and inspectors.

What Is an Electrical Three-Line Diagram?

An Electrical Three-Line Diagram (3LD) is a representation of the full AC electrical system showing:

All three phases (A, B, C)
Conductors and conduit paths
Switchgear and disconnects
Panelboards and breakers
Transformer primary and secondary connections
Inverter AC outputs
Monitoring equipment
Grounding and bonding paths
Protection devices (OCPDs, fuses, relays)

It is more detailed than a one-line diagram, which is often used for residential or simple commercial systems. A three-line diagram is essential when the project includes:

Three-phase service
Multiple parallel inverters
Commercial switchgear
Medium-voltage connections
Backup power systems
Complex grounding

It forms the backbone of the EPC workflow for accurate construction, inspection, and interconnection engineering.

Related foundational concepts include Electrical Single-Line Diagram, Stringing & Electrical Design, and Inverters.

How an Electrical Three-Line Diagram Works

1. Each phase is represented separately

Phase A, Phase B, and Phase C each have their own dedicated line on the diagram.

2. Conductors are shown individually

Hot, neutral, equipment grounding conductors, and bonding wires are all displayed.

3. Protection devices are mapped

Breakers, disconnects, relays, fuses, and OCPDs are clearly identified.

4. Inverters feed into the distribution system

Showing phase rotation, current flow, conductor sizing, and point of interconnection.

5. Transformers and switchgear are included

With winding configurations (Δ, Y) and primary/secondary details.

6. Grounding system is explicitly shown

Including electrodes, bonding jumpers, and equipment grounding.

7. Load flow and current paths are traceable

This helps engineers perform fault current calculations and coordination studies.

Types / Variants of Three-Line Diagrams

1. Commercial Rooftop PV Three-Line Diagram

Includes multiple string inverters feeding a three-phase panelboard.

2. C&I Ground-Mount Three-Line Diagram

Shows combiner panels, homerun conductors, switchboards, and medium-voltage gear.

3. Utility-Scale Three-Line Diagram

Includes central inverters, pad-mounted transformers, medium-voltage feeders, reclosers, and substation interconnection.

4. Hybrid or Storage-Integrated Three-Line Diagram

Includes battery inverters, transfer switches, and load management equipment.

5. Microgrid Three-Line Diagram

Displays DERs, controls, loads, and isolation mechanisms.

How It’s Measured / What It Represents

Three-line diagrams aren’t “measured” with units, but they display critical engineered values including:

Voltage levels (208V, 400V, 480V, 600V, MV ranges)
Conductor sizes (AWG/kcmil)
Breaker ratings (A/kAIC)
Inverter AC outputs
Transformer winding types
Grounding components
Current paths and load flow

Engineers use these diagrams to validate NEC compliance, conductor ampacity, and interconnection requirements.

Typical Values / Ranges Shown on a 3-Line Diagram

Voltages

208V / 240V / 277V / 480V (Low-voltage systems)
600V–35kV (Medium-voltage utility interconnection)

Breaker Sizes

20A–600A (Commercial LV)
800A–1200A (Switchgear)

Conductor Sizes

#10–#2 AWG (small inverters)
1/0–500 kcmil (high-power AC feeders)

Transformer Ratings

45 kVA – 1,500 kVA (C&I)
2.5 MVA – 10 MVA (Utility)

Practical Guidance for Solar Designers & Electrical Engineers

1. Start with the Single-Line Diagram

Use the Electrical Single-Line Diagram before expanding to a three-line design.

2. Use accurate inverter AC output values

Ensure AC current, voltage, and breaker sizing match manufacturer datasheets.

3. Verify conductor ampacity

Include temperature ratings, derating factors, and conduit fill.

4. Confirm phase rotation

Incorrect rotation can cause severe equipment issues in three-phase systems.

5. Include grounding and bonding details

Critical for NEC compliance and inspector approval.

6. Incorporate utility-requested symbols

Different utilities require specific symbols for reclosers, CTs, relays, etc.

7. Export designs for interconnection

Three-line diagrams are often required in utility interconnection packets—use the Solar Project Planning Hub for workflow management.

Real-World Examples

1. Commercial Rooftop 120 kW System

A three-line diagram shows ten 12 kW inverters feeding a 480V three-phase panelboard with 150A breakers.

2. Utility-Scale 5 MW Ground Mount

The diagram includes central inverters, pad-mounted transformers, MV feeders, and a recloser feeding a substation.

3. Storage-Integrated Microgrid

A three-line diagram maps battery inverters, PV inverters, critical loads, and an automatic transfer switch.

Releated Terms

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