Quick Answer: What Is Medium Voltage?
Medium voltage (MV) refers to electrical power systems operating between 1 kV and 35 kV (IEC definition) or 1 kV to 69 kV (IEEE/ANSI definition). It is the voltage class used to distribute power from substations to buildings, factories, and neighborhoods. Medium voltage sits between low voltage (below 1 kV, used inside buildings) and high voltage (above 35-69 kV, used for long-distance transmission). Most industrial and commercial facilities receive power at medium voltage and transform it down to low voltage for use by equipment.
Voltage Range Definitions by Standard
| Standard / Region | Low Voltage | Medium Voltage | High Voltage |
|---|---|---|---|
| IEC 60038 | Up to 1 kV | 1 kV - 35 kV | Above 35 kV |
| IEEE / ANSI | Up to 1 kV | 1 kV - 69 kV | Above 69 kV |
| China (GB) | Up to 1 kV | 1 kV - 40.5 kV | Above 40.5 kV |
| Europe (CENELEC) | Up to 1 kV | 1 kV - 36 kV | Above 36 kV |
| US (NEC practice) | Up to 1 kV | 2.4 kV - 69 kV | Above 69 kV |
Common Medium Voltage Levels Worldwide
| System Voltage | Equipment Rating | Regions | Typical Use |
|---|---|---|---|
| 3.3 kV | 3.6 kV | UK, India, older systems | Industrial motors, mining |
| 6.6 kV | 7.2 kV | UK, India, Japan, mining | Large motors, industrial |
| 10 kV | 12 kV | China (standard) | Urban/rural distribution |
| 11 kV | 12 kV | UK, India, Middle East, Australia | Primary distribution |
| 13.8 kV | 15 kV | North America | Distribution, industrial |
| 20 kV | 24 kV | France, Belgium, parts of Europe | Distribution |
| 22 kV | 24 kV | Thailand, parts of SE Asia | Distribution |
| 33 kV | 36 kV | UK, India, Middle East, Africa | Sub-transmission |
| 35 kV | 40.5 kV | China | Sub-transmission, large industrial |
Why Medium Voltage Matters
Medium voltage exists because of a fundamental law: power loss is proportional to the square of the current (Ploss = I²R). By increasing the voltage, the current decreases proportionally, dramatically reducing energy losses and cable costs.
Consider delivering 1 MW of power over 1 km:
| Voltage | Current | Cable Size | Power Loss |
|---|---|---|---|
| 400 V (LV) | 1,443 A | 6 x 300 mm² parallel | ~80 kW (8%) |
| 10 kV (MV) | 58 A | 1 x 25 mm² | ~3 kW (0.3%) |
The MV solution uses a fraction of the cable and wastes far less energy. This is why any facility above approximately 300-500 kW receives power at medium voltage.
Types of Medium Voltage Equipment
Circuit Breakers
MV circuit breakers are the primary protection and switching devices:
- Vacuum Circuit Breakers (VCB) — dominant at 12-24 kV, with outdoor pole-mounted (ZW32) and indoor models up to 40.5 kV
- SF6 Circuit Breakers — compact gas-insulated design for space-constrained installations
- Specialized Breakers — CES for capacitor switching, CEG for generator protection
Switchgear
MV switchgear assemblies house breakers, bus bars, disconnectors, and protection relays:
- Metal-Enclosed Switchgear — ASN series, 12 kV to 40.5 kV, up to 5000 A / 50 kA
- Gas-Insulated Switchgear (GIS) — GSN series, compact and maintenance-free, including eco-friendly nitrogen option
Ring Main Units (RMU)
Ring main units are compact switchgear for underground cable distribution networks, used extensively in urban areas. The XGN15-12 RMU offers modular switch/fuse configurations for flexible network design.
Load Break Switches
SF6 load break switches sectionalize overhead distribution feeders and automate fault isolation. The FZW28-12 boundary switch integrates SF6 insulation with intelligent fault detection for smart grid applications.
Contactors
Vacuum contactors handle frequent switching at medium voltage — motor starting, capacitor bank switching, and transformer energization. Built for high-duty-cycle operations that circuit breakers are not designed for.
Medium Voltage Applications by Industry
| Industry | Typical MV Voltage | Equipment Used |
|---|---|---|
| Utility Distribution | 10-33 kV | Switchgear, RMU, reclosers, pole-mounted VCB |
| Industrial / Manufacturing | 6.6-13.8 kV | Switchgear, MCC, VCB, contactors |
| Commercial (Data Centers, Hospitals) | 11-13.8 kV | Compact switchgear, GIS |
| Mining | 6.6-33 kV | Explosion-proof switchgear, VCB |
| Oil & Gas | 6.6-13.8 kV | Hazardous area switchgear, generator breakers |
| Renewable Energy | 20-35 kV | Collector switchgear, transformer protection |
| Rail Transit | 10-35 kV | Traction substations, GIS |
| Water / Wastewater | 6.6-11 kV | Pump motor starters, VCB, contactors |
Key Medium Voltage Standards
- IEC 62271-200 — Metal-enclosed switchgear
- IEC 62271-100 — High-voltage circuit breakers (applies to MV)
- IEC 60038 — Standard voltages (defines MV range)
- IEEE C37.20.2 — Metal-clad switchgear (North America)
- GB 3906 — Chinese standard for MV switchgear
- GB 1984 — Chinese standard for MV circuit breakers
NAIJI Electric: Full Medium Voltage Product Range
NAIJI Electric manufactures a complete range of medium voltage equipment covering 12 kV to 40.5 kV across 10 product categories and 29 models:
- Outdoor VCB: ZW32-12, ZW32-24
- Indoor VCB: VN3-12E, VN2-24E, VN1-40.5E, CE-12, CEi-12, CVG series — up to 50,000 operations
- Switchgear: ASN3-12, ASN2-24, ASN1-40.5, ASN3i-12, ASN550
- GIS: GSN3-12 (nitrogen), GSN1-40.5L
- RMU: XGN15-12
- Load Break Switches: FZW28-12, FLW36-12
- Contactors: CEC, CEC/F
- Specialized: CES 40.5 kV, CEG generator breaker
- Low Voltage: GCS, GGD, EZMX1 ACB, EZM1 MCCB
Contact the NAIJI Electric engineering team for product selection, technical specifications, or a project quotation.