MV Cable

What is an MV cable?

Answer: MV means medium voltage. An MV cable carries electrical power at the voltage rating specified for a medium-voltage network. In a solar farm, it commonly connects step-up transformers, collection switchgear and the project substation. Selection depends on the network voltage, grounding arrangement, fault duty, installation conditions and compatible accessories; conductor size alone is insufficient.

“Medium voltage” does not define one universal cable rating. Country conventions, network requirements and product standards differ. The public catalog for IEC 60502-2:2014 with Amendment 1:2024 describes extruded-insulation cables rated from 6 kV to 30 kV, with corresponding maximum system voltages from 7.2 kV to 36 kV. That is the scope of this standard, rather than a definition of every MV cable worldwide. Its catalog includes a July 2026 correction; the complete standard and locally applicable requirements must be checked for a project.

Where it fits in a solar installation

A common power path is:

PV modules → DC circuits → inverter → low-voltage AC circuit → step-up transformer → MV collection cable → switchgear or substation → grid connection.

The MV circuit usually carries power after the transformer has raised the inverter’s AC output voltage. It is different from module string wiring and from the inverter’s low-voltage AC feeder. Actual equipment arrangements vary, so use the project’s approved single-line diagram to identify the circuit boundary.

A higher transmission voltage allows the same three-phase power to flow at a lower current, for a given power factor. That can reduce resistive losses for otherwise comparable conductors. It also changes insulation, protection, access and installation requirements; it does not make an MV circuit automatically preferable for every rooftop project.

What the cable construction tells you

Construction varies by product. For example, Nexans’ 18/30 kV solar cable family describes aluminum conductors, XLPE insulation, copper-wire screens and an LDPE sheath. This is manufacturer-reported information for that family, not a specification for all MV cables.

Cable voltage notation must be interpreted using its product standard. Do not substitute a network’s nominal voltage for the manufacturer’s complete rating or infer suitability from a familiar voltage label alone.

How engineers size an MV cable

The governing requirement may be thermal loading, voltage performance, fault duty or installation constraints. A useful design record separates these checks:

1. Network conditions: nominal and maximum voltage, grounding arrangement, protection coordination and fault duration.
2. Continuous loading: current at the relevant operating conditions, including the circuit’s reactive-power requirements.
3. Thermal installation: soil thermal resistivity and temperature, burial depth, grouping, ducts, air exposure and other route conditions.
4. Electrical performance: voltage change, conductor losses and any additional losses represented in the study.
5. Fault withstand: conductor and screen duties against the project’s calculated faults and clearing times.
6. Physical installation: pulling limits, bend radii, accessory dimensions, water protection and route-specific mechanical protection.

Rated ampacity is tied to stated conditions. A catalog value for a single circuit in one soil condition cannot establish capacity for grouped cables in a different trench. Similarly, an armored construction does not by itself establish that direct burial without additional protection is acceptable.

A three-phase voltage-change example

For a balanced three-phase circuit, a simplified steady-load approximation is:

ΔU ≈ √3 × I × L × (R cos φ + X sin φ)

Here, I is current in amperes, L is the one-way route length in kilometers, R and X are per-phase resistance and reactance in ohms per kilometer, and φ is the power-factor angle. ΔU is the line-to-line voltage change in volts for the stated load convention. Resistance must represent the relevant conductor temperature. Schneider Electric’s Electrical Installation Guide distinguishes this expression from the factor-of-two expression for a single-phase circuit.

Hypothetical example: assume an 11 kV balanced load circuit carrying 100 A over 1 km, R = 0.15 Ω/km, X = 0.10 Ω/km and a lagging power factor of 0.95. The expression gives approximately 30.1 V, or 0.27% of 11 kV. Under these assumed resistances, three-phase conductor heating is 3 × I² × R × L = 4.5 kW.

These are illustrative calculations, not results for a named cable or an approved design. They exclude additional losses such as screen and dielectric losses. A solar export circuit may produce voltage rise; network conditions, reactive-power direction and cable charging can require a fuller load-flow model. There is no universal utility acceptance threshold implied by this example. See line losses for the distinction between power loss and energy lost over time.

Temperature and installation limits need their own checks

Continuous operating temperature and short-circuit temperature are different limits. Prysmian’s Australian solar cable brochure lists 90°C continuous operation and 250°C short-circuit temperature for one 19/33 kV MV construction. The higher figure is not an allowable continuous or general emergency operating temperature.

The same brochure gives different bending limits during installation and in the final position. Use the exact cable’s instructions rather than a universal multiple of cable diameter. Service life also depends on the installed system and operating conditions; an insulation material name does not establish a guaranteed lifetime.

Procurement, commissioning and handover

Ask the responsible engineer and installer to document:

• The approved cable specification and the assumptions behind its sizing.
• The cable, joint and termination manufacturers’ compatibility requirements.
• The screen bonding and grounding design, including its protection implications.
• The actual installed route, joint locations and any approved design changes.
• The project-specific inspection and test plan, acceptance criteria and results.

MV installation and testing require qualified specialists. Test methods, voltages and procedures must follow the applicable project requirements and equipment instructions; this definition is not a field testing procedure.

For a solar project team, these records connect cable procurement to the wider balance of system and engineering handover. When evaluating SurgePV’s solar engineering workflow, confirm which project documents and exports are available, and identify the external engineering checks the project still requires.