Direct Answer
A distribution line is the local part of the electric grid that carries power from a distribution substation toward customers. Distribution systems usually include primary feeders, lateral branches, protective devices, voltage-control equipment, distribution transformers, secondary conductors, and customer service connections.
Distribution engineering is different from simply sizing a wire. Engineers review voltage drop, phase imbalance, conductor and transformer loading, fault current, fuse/recloser coordination, switching options, vegetation and weather exposure, underground-cable condition, DER backfeed, and restoration strategy because those factors determine the reliability customers actually experience.
Distribution Feeder at a Glance
How Distribution Lines Fit in the Power System
Distribution lines begin where bulk transmission has been stepped down to local utility voltage levels. The system then branches repeatedly so one substation can serve neighborhoods, commercial areas, campuses, industrial loads, farms, and critical facilities.
| System level | Main role | Typical engineering focus |
|---|---|---|
| Transmission | Move bulk power between major grid nodes | High-voltage transfer, stability, thermal limits, protection, ROW |
| Distribution | Deliver power locally from substations toward customer areas | Voltage regulation, feeder loading, fault isolation, restoration, vegetation and field exposure |
| Customer service | Connect utility secondary voltage to end-use equipment | Service capacity, transformer loading, voltage drop, metering and customer equipment interface |
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For the upstream bulk-power side, see Transmission Lines.
Primary and Secondary Distribution Voltage
Primary distribution is usually medium voltage between the substation and distribution transformers; secondary distribution is the lower-voltage network after the transformer that serves customer equipment.
| Distribution level | Role | Examples / notes |
|---|---|---|
| Primary distribution | Moves power from the substation through main feeders and laterals | Common U.S. nominal classes include roughly 4 kV, 12.47 kV, 13.2 kV, 13.8 kV, 24.9 kV and 34.5 kV, depending on the utility |
| Distribution transformer | Steps primary voltage down near the load | Pole-mounted, pad-mounted, vault or other utility-specific construction |
| Secondary distribution | Connects transformer secondary to customer services | Typical service voltages vary by country, customer class, and utility practice |
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Distribution does not mean low voltage. The primary side of a distribution feeder is often medium voltage and remains hazardous until a distribution transformer steps it down.
Distribution Feeder Anatomy
The feeder is the core electrical path that leaves the substation. From there, laterals, transformers, fuses, reclosers, regulators, capacitor banks, switches, and customer services create the full local distribution system.
| Component | Function | High-value engineering check |
|---|---|---|
| Main feeder | Carries power outward from the substation | Peak load, voltage profile, emergency loading, conductor/cable rating |
| Lateral | Branches from the feeder to smaller load groups | Fuse/recloser coordination, conductor size, outage exposure |
| Distribution transformer | Steps primary voltage down to secondary voltage | kVA loading, voltage drop, phase balance, protection and grounding |
| Recloser | Interrupts faults and can automatically reclose | Pickup, timing, shot sequence, coordination with fuses and upstream breaker |
| Fuse cutout | Protects transformer taps or laterals | Fuse link, minimum-melt/total-clear curves, transformer inrush and coordination |
| Voltage regulator | Changes taps to control feeder voltage | Setpoint, bandwidth, line-drop compensation, time delay and reverse-power behavior |
| Capacitor bank | Supplies local reactive power | kVAR, voltage, switching control, harmonics and voltage-rise effects |
| Switch / sectionalizer | Creates isolation and restoration points | Fault duty, switching capability, automation and alternate-feed path |
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Radial, Loop, and Network Distribution Systems
Feeder configuration determines how power flows, how faults are isolated, and how quickly customers can be restored after an outage.
| Configuration | How it works | Strength | Tradeoff |
|---|---|---|---|
| Radial | One normal source path feeds downstream loads | Simple, economical and easy to protect | Upstream faults can interrupt every downstream customer |
| Open loop | Two possible supply paths exist with a normally open tie | Improves restoration options | Requires switching plans and protection checks after reconfiguration |
| Network | Multiple sources and interconnected paths supply dense load areas | High service continuity | More complex protection, operation, fault current and planning |
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Why Normally Open Ties Matter
Many utilities build feeders with tie points that remain open during normal operation. After a faulted section is isolated, operators or automation can close a tie and restore unfaulted customers from another feeder—if the alternate feeder has enough capacity and the protection assumptions remain valid.
Voltage Drop and Voltage Regulation on Distribution Lines
Voltage drop is one of the defining distribution-system problems. A long feeder can have acceptable substation voltage and still deliver low customer voltage at the far end because current flows through conductor resistance and reactance.
- \(I\)Load current through the feeder section.
- \(R\)Conductor resistance over the feeder section.
- \(X\)Conductor reactance over the feeder section.
- \(\phi\)Load power-factor angle.
Common Voltage-Control Tools
| Tool | What it changes | Main caution |
|---|---|---|
| Substation LTC | Changes transformer tap ratio for the feeder bus | One setpoint affects multiple feeders and load conditions |
| Line voltage regulator | Changes voltage locally along a feeder | Control settings must match feeder direction and loading |
| Capacitor bank | Supplies reactive power and can improve voltage / reduce current | Can cause overvoltage or harmonic issues if misapplied |
| DER volt/VAR controls | Uses inverter reactive capability to influence local voltage | Requires coordinated interconnection settings and system studies |
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Distribution feeders are often unbalanced. Single-phase laterals, residential loads, rooftop solar, EV charging, and small commercial loads can cause one phase to have worse voltage or loading than the three-phase average suggests.
Distribution Line Protection and Fault Isolation
Distribution protection is built around selective isolation. The system should remove the smallest practical faulted section while keeping healthy parts of the feeder energized.
Why Reclosers Reclose
Many overhead distribution faults are temporary. A tree branch, animal contact, or lightning-related flashover may clear after the circuit opens. A recloser can trip, wait, and re-energize the feeder automatically. If the fault is gone, service returns without crew intervention.
Fuse and Recloser Coordination
Protection strategy varies by utility. Some schemes try to save lateral fuses by having an upstream recloser clear a temporary fault first. Others intentionally allow the lateral fuse to isolate its own branch so the main feeder remains energized. Either strategy requires coordinated time-current characteristics and realistic minimum/maximum fault currents.
End-of-Line Fault Current Matters
Fault current usually decreases as distance from the source increases because feeder impedance increases. Engineers must verify that protective devices remain sensitive enough to detect faults at the end of long laterals while staying coordinated for faults closer to the substation.
For the broader calculations behind this review, see Short Circuit Analysis and Overcurrent Protection.
Overhead vs. Underground Distribution Lines
Underground distribution reduces exposure to some overhead hazards, but it does not eliminate outages and usually costs more to install, locate, and repair.
| Factor | Overhead | Underground |
|---|---|---|
| Weather exposure | High exposure to trees, wind, ice, lightning, wildlife and vehicles | Lower exposure to wind/trees but vulnerable to flooding, excavation and insulation failure |
| Fault visibility | Many failures are visible during patrol | Faults often require testing and locating equipment |
| Repair time | Often faster if damage is visible and accessible | Can take longer because cable must be located, excavated and repaired |
| Installation cost | Usually lower | Usually higher |
| Future modifications | Often easier to add taps, devices or reconductor | Can be more constrained by ducts, vaults and spare conduit |
| Aesthetics / ROW | More visible | Lower visual impact after construction |
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Distribution Reliability, Outages, SAIDI, SAIFI, and CAIDI
Distribution reliability is usually measured at the system, feeder, or customer level. IEEE 1366-2022 is the active IEEE guide defining distribution reliability indices and factors that affect their calculation, while a revision project is underway.
| Index | What it represents | How to interpret it |
|---|---|---|
| SAIDI | System Average Interruption Duration Index | Average total interruption duration experienced by customers over the reporting period |
| SAIFI | System Average Interruption Frequency Index | Average number of sustained interruptions experienced by customers |
| CAIDI | Customer Average Interruption Duration Index | Average restoration duration per sustained customer interruption |
| MAIFI | Momentary Average Interruption Frequency Index | Frequency of qualifying momentary interruptions |
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Common Distribution Outage Causes
| Cause | Typical impact | Mitigation / review |
|---|---|---|
| Vegetation | Temporary or permanent faults, conductor damage | Vegetation management, ROW access, inspection cycles |
| Storm / wind / ice | Broken conductors, pole failures, tree contact | Hardening, construction standards, sectionalizing and restoration plans |
| Lightning | Flashover, arrester operation, equipment damage | Grounding, arresters, insulation coordination |
| Wildlife | Phase-to-phase or phase-to-ground fault | Guards, covers, equipment arrangement |
| Underground cable failure | Permanent fault requiring locating and repair | Cable condition, joints, terminations, spare strategy |
| Vehicle / construction damage | Pole, cabinet or underground damage | Physical protection, marking, coordination with civil work |
| Aging equipment | Connector, transformer, insulator or switch failure | Inspection, replacement programs, condition monitoring |
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Distributed Energy Resources and Modern Distribution Lines
Modern distribution systems are increasingly active rather than purely one-way. Rooftop solar, utility-scale distribution-connected solar, batteries, generators, and inverter-based resources can change net feeder load, voltage profile, fault current, and power-flow direction.
Reverse Power Flow
When local generation exceeds local load, power can flow back toward the substation. That can affect regulator controls, protection assumptions, transformer loading, voltage rise, and how utilities define normal feeder operating states.
IEEE 1547 Context
IEEE 1547-2018 remains the active IEEE standard for interconnection and interoperability of distributed energy resources with electric power system interfaces, and IEEE has an active revision project underway. The standard covers requirements such as abnormal-condition response, reactive-power capability, voltage/power control, power quality, islanding, interoperability, and testing that directly affect distribution-connected DER.
A feeder that was designed around one-way load flow can behave very differently after large amounts of rooftop solar, batteries, EV charging, or other DER are added. Voltage regulation and protection should be re-evaluated when feeder behavior changes materially.
Distribution Line Engineering Review Checklist
A strong review follows the feeder from the substation to the farthest customer and checks electrical performance, protection, physical condition, and restoration capability together.
| Review item | Verify | Risk if missed |
|---|---|---|
| Source and voltage class | Substation bus, transformer, feeder voltage and grounding | Wrong equipment or study basis |
| Peak / seasonal loading | Feeder, lateral, transformer, switch and conductor loading | Overheating, low voltage, accelerated aging |
| Phase balance | Per-phase current and voltage | One phase can become overloaded while average looks acceptable |
| Voltage profile | Substation, mid-feeder, far-end and long-lateral voltage | Customer voltage complaints and equipment performance issues |
| Protection coordination | Breaker, recloser, fuse and sectionalizer behavior | Too many customers interrupted or fault not cleared selectively |
| Minimum fault current | End-of-line and high-impedance fault sensitivity | Protection may not detect remote faults |
| Switching / restoration | Normally open ties, alternate feeds and capacity after transfer | Restoration plan cannot be executed safely |
| DER impacts | Reverse power, voltage rise, regulator behavior and fault contribution | Legacy assumptions become invalid |
| Physical exposure | Vegetation, wildlife, flood, wildfire, road crossings, access | Reliability dominated by field conditions |
| Asset condition | Poles, connectors, insulators, cable joints, transformers and switches | Unexpected equipment failure and extended outage |
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Distribution Engineering References
- IEEE 1366-2022 — Electric Power Distribution Reliability Indices Active IEEE guide defining distribution reliability indices and calculation factors for distribution systems, substations, circuits, and defined regions; IEEE also has an active P1366 revision project.
- IEEE 1547-2018 — Interconnection and Interoperability of Distributed Energy Resources Active IEEE standard covering DER interconnection performance, abnormal-condition response, reactive-power capability, voltage/power control, islanding, power quality, interoperability, and testing; an active revision project is underway.
- U.S. Department of Energy — Electricity Grid Backgrounder Public overview of transmission, substations, distribution feeders, protective equipment, vegetation management, grid automation, and resilience measures.
Frequently Asked Questions
What is a distribution line?
A distribution line carries electricity from a distribution substation toward end users through primary feeders, laterals, transformers, protective devices, secondary conductors, and customer services.
What voltage are distribution lines?
Primary distribution is commonly medium voltage and varies by utility. Common U.S. nominal classes include roughly 4 kV through 34.5 kV, while secondary distribution is the lower-voltage service supplied after a distribution transformer.
What is the difference between a feeder and a distribution line?
A feeder is the main distribution circuit that leaves a substation and carries power toward the load area. “Distribution line” is the broader term that can include the feeder, laterals, taps, transformers, secondary circuits, and services.
What is the difference between transmission and distribution lines?
Transmission lines move bulk power over longer distances at higher voltage between major grid nodes. Distribution lines deliver stepped-down power locally from substations toward customer loads.
Why do distribution lines have reclosers?
Many overhead faults are temporary. Reclosers interrupt the fault and can automatically re-energize the feeder after a short delay, restoring service if the fault has cleared.
What causes voltage drop on a distribution feeder?
Voltage drop is caused by current flowing through feeder resistance and reactance. Longer lines, higher current, smaller conductors, poor power factor, and phase imbalance can worsen far-end voltage.
Are underground distribution lines more reliable?
Underground systems avoid many tree and wind exposures, but they can still fail because of cable insulation, joints, water, excavation damage, or equipment faults. Repairs can also take longer because failures are harder to locate and access.
What are SAIDI and SAIFI?
SAIDI measures average sustained outage duration experienced by customers over a reporting period, while SAIFI measures average sustained interruption frequency. Both are widely used distribution reliability indices.
How does rooftop solar affect distribution lines?
High DER penetration can reduce net load, create reverse power flow, raise local voltage, change regulator behavior, and alter protection assumptions. Utilities review these effects during interconnection and feeder planning.
Summary and Next Step
Distribution lines are the local delivery system between substations and customers. Their performance depends on far more than conductor size: feeder topology, phase balance, voltage control, transformer loading, reclosers and fuses, switching flexibility, DER penetration, vegetation, weather, cable condition, and restoration planning all affect customer reliability.
The strongest way to understand distribution engineering is to follow the actual feeder from source to customer and ask three questions at every section: can it carry the load, can it hold acceptable voltage, and can the correct protective device isolate a fault without unnecessarily interrupting healthy customers?