Solar cables and MC4 connectors: cross-section, connectors, crimping

Direct current flows between solar panels and the inverter; in domestic systems, this is often several hundred volts. The cables and connectors used for this are separate components with their own specific requirements. This page explains which cable is suitable, how the cross-sectional area affects losses, and what is important to consider with MC4 connectors.

Since the introduction of the EN 50618 standard, solar cables for the DC side have been labelled H1Z2Z2-K. According to Helukabel, they are weather-, UV- and ozone-resistant, designed for temperatures ranging from −40 to +90 °C, halogen-free and flame-retardant (Solar Cable White Paper, 2023). According to Helukabel, the cross-sectional area should normally be at least 4 mm².

According to the data sheet, the solar cables in the Athilex range are rated for 1.5 kV DC, have a tinned copper strand and are cut to length by the metre; they can be found under ‘Solar cables’:

If the cable lies on or against a surface, the data sheet specifies lower values; for a 6 mm² cable, these are 67 A or 57 A respectively. The minimum bending radius for fixed installation is five times the cable diameter.

The currents in a module string are well below the current-carrying capacity of both cross-sections. The losses are the decisive factor: a cable has electrical resistance, and power is lost as heat along its length. The longer and thinner the cable, the greater the loss.

Calculations are based on the resistance R = length / (58 × cross-sectional area) for copper and the power loss P = R × current². The length refers to the combined length of the supply and return cables. For example: A cable is 20 m away from the inverter, with a current of 13 A flowing through it.

For a string with 4.5 kWp, this amounts to around 0.6 per cent and 0.4 per cent of the power output respectively. This is an approximate value for 20 °C; the cable data sheet specifies slightly higher conductor resistances (4 mm²: 5.09 Ω/km, 6 mm²: 3.39 Ω/km), and during operation, losses increase with temperature. For short distances, 4 mm² is sufficient; for long distances, such as from the roof to the cellar, 6 mm² is recommended. The electrical contractor will carry out the design.

MC4 is originally a connector manufactured by Stäubli. Many other manufacturers offer connectors described as ‘MC4-compatible’. Stäubli states the following in its installation instructions: Never connect the plug or socket of a Stäubli connector to the socket or plug of another manufacturer. According to Stäubli, connections made in this way lose their IEC and UL certification. The standards governing the installation of PV systems, IEC 62548 and IEC 60364-7-712, also require that mated plugs and sockets be of the same type and from the same manufacturer.

In practice, this means: if a Stäubli MC4 or MC4-Evo 2 is fitted to the module, the corresponding Stäubli connector must be used on the cable. According to Stäubli, Stäubli MC4 and Stäubli MC4-Evo 2 connectors are compatible with one another. Before purchasing, check which connector is fitted to the module by examining the module itself or consulting its data sheet.

Stäubli connectors are available for various cross-sections and cable diameters. For the MC4-Evo 2, the installation instructions specify the following sealing inserts:

The outer diameter of your cable is specified in its data sheet. Different ranges apply to the original MC4; these are set out in Stäubli’s installation instructions. Plugs and sockets for 4 and 6 mm² are the same; 10 mm² has its own. The range includes sealing inserts I and II; the 4 and 6 mm² solar cables (5.4 and 6.2 mm respectively) are compatible with insert I.

• Who: According to Stäubli, only a qualified electrician or a person trained in electrical engineering may install the connectors and commission the system.
• Tools: Stäubli stipulates that only tools approved by Stäubli are to be used. These include the crimping pliers with the appropriate locator, against which the stranded wire stops when inserted; the test probe, used to check that the contact is seated correctly; and the assembly and release tool.
• Stripping and tightening: For the MC4-Evo 2, the instructions specify a stripping length of 6.0 to 7.5 mm and a tightening torque for the cable gland of 4.0 Nm for 4 mm² and 3.5 Nm for 6 mm²; the table in the instructions for the cable used is authoritative.
• Install once: According to Stäubli, a connector is installed only once and is not modified subsequently.
• Do not disconnect under load: Connectors are fitted in a de-energised state and must not be disconnected under load; a release tool is required to open them.

Tools and accessories are listed under ‘Tools for PV installation’.

Y-connectors are used to connect two module strings in parallel to a single input on the inverter. According to Stäubli, such distribution blocks must not be plugged directly into the inverter connection; a cable with connectors must be used in between.

If several strings are connected in parallel, in the event of a fault a reverse current may flow through a string that exceeds the modules’ capacity. Several module manufacturers therefore stipulate that a string fuse must be fitted from three parallel strings onwards. The reverse current rating specified in your module’s data sheet is decisive. Stäubli offers cables with built-in fuses for this purpose, available in the range with 15 and 20 A.

To find out how modules, inverters and storage systems work together, see ‘Planning a PV system’; to find out which inverter is suitable, see ‘Hybrid, string or micro inverters: which one is right?’.