PV Cables from Roof to Basement: Safe Routing

When connecting a photovoltaic system, many people underestimate the cable route. The PV cable transports the direct current from the roof to the basement, usually to the inverter, sometimes to a hybrid inverter with battery storage. A direct connection is possible and even makes sense in many houses. However, it must be short, protected, waterproof and electrically clean.

PV cables are not normal house cables. High direct voltages can be present on the DC side. Damaged insulation, moisture or poor plug connections can trigger arcs that are more difficult to break than with alternating current. Therefore, the cable route is not just a matter of a few watts of output, but also of operational safety, fire protection and ease of maintenance.

The most important thing in brief

• PV cables can be led directly from the roof into the basement, for example via empty pipes, installation shafts, disused chimneys or a professionally sealed core drilling.
• Special solar cables are required for the DC side, often H1Z2Z2-K according to EN 50618. They are designed for UV radiation, moisture, temperature changes and permanent electrical load.
• The cable cross section depends on the cable length, current strength and voltage drop. 4 mm² and 6 mm² are typical sizes, long distances can be 10 mm² or require more.
• DC and AC lines must be planned separately. PV string lines do not belong in the same pipe with 230 V lines or network cables.
• Fedthroughs need moisture and fire protection. Particularly wall, roof and Ceiling breakthroughs are typical weak points.
• Connection and commissioning are the responsibility of a qualified electrician. This protects the system, building and insurance coverage.

Short answer: Is a direct connection from the roof to the basement possible?

Yes. PV cables may be routed directly from the roof to the basement if they are mechanically protected, correctly dimensioned, permanently sealed and laid in accordance with the applicable electrical engineering regulations. The best way is usually a short, straight cable route through your own empty conduit or a suitable shaft. Improvised openings, bending radii that are too narrow, missing fire barriers, unsuitable cables and a cross-section that is too small become critical.

A good practical rule is: the shorter the DC path from the module to the inverter, the lower the voltage drop, material costs and error area. The direct route to the basement is not automatically risky. It only becomes risky if it is implemented without calculation and a protection concept.

Choosing the right PV cable

In a PV system, the cable is part of the generation system. Normal installation cables are not intended for permanent outdoor use on the roof. For the DC side, solar cables are used that can withstand high DC voltages, UV radiation, ozone, rain, frost and summer heat.

The cable type H1Z2Z2-K used according to EN 50618. Such cables are flexible, halogen-free, double-insulated and specially developed for photovoltaic systems. This is important if cables are routed over roof hooks, through empty conduits, along the facade or through a shaft into the basement.

The cross section is the second major adjustment screw. For short distances, 4 mm² is often sufficient. For a typical path from the roof to the basement, 6 mm² is more realistic in many residential buildings. For long cables, high currents or low string voltage, 10 mm² can make sense. A larger cross-section costs a little more, but reduces losses and heat generation.

Easily calculate the voltage drop

The voltage drop is caused by the resistance of the line. The longer the cable and the higher the current, the more voltage is lost along the way. For a rough DC estimate with copper you can calculate like this:

Voltage drop in volts = 2 x cable length in meters x current in amperes x 0.0178 / cross section in mm²

The “2 x” stands for forward and return conductors. Example: With a single cable length of 25 m, 12 A string current and 6 mm² copper, the voltage drop is around 1.78 V. With a 400 V string voltage this is around 0.45%. With only 120 V string voltage it would be around 1.5%. That is why general statements such as “6 mm² is always enough” are inaccurate. The specialist company must evaluate the length, current, voltage, temperature and type of installation together.

As a target value, many installers plan the DC line losses to be as low as possible, often less than 1%. Higher values ​​may occur in difficult buildings, but should be calculated carefully.

This is how the PV cable gets safely from the roof to the basement

The best cable route is short, dry, accessible and mechanically protected. In practice there are four typical variants.

Installation shaft or supply shaft

An existing shaft saves drilling and keeps the cable route in the building. Beforehand, it must be checked whether there is enough space, whether the shaft remains dry and whether the PV cables can be routed separately from other lines. Own empty pipes create order and make later maintenance easier.

Disused fireplace

A fireplace that is no longer in use can work if it is truly out of order, dry, continuous and structurally suitable. Soot, moisture, sharp edges or unclear fire areas speak against this. A camera inspection in advance saves a lot of trouble.

Core drilling through the exterior wall

If no shaft is available, core drilling is often used. It needs a suitable empty pipe, a permanent seal, clean fastening and, if possible, an outward slope. Temporary silicone solutions age quickly and are not a reliable solution for a PV system.

Routing along the façade in a protective conduit

Sometimes the pipes run down the outside of the facade and only enter the house in the basement area. This can be technically clean if UV-resistant protective tubes, stable fastenings and weather protection are included. You can find more basic information in the article Laying PV cables: regulations and practical tips.

Fire protection and insulation: there is no improvisation here

PV string lines carry direct current. Dangerous DC arcs can occur if cables are damaged or connectors are poorly made. Fire protection therefore begins on the roof and ends at the inverter.

• Cables must not rub loosely on roof surfaces.
• Sharp metal edges need edge protection.
• Cable ties and protective tubes in outdoor areas must be UV-resistant.
• Plugs must not be left permanently in water or snow.
• Wall and ceiling penetrations must be permanently sealed.
• Appropriate fire protection insulation must be planned for fire-resistant components.

In the building, empty pipes or cable ducts protect against mechanical damage. Documentation also counts: string, polarity, cable length and bushings should be clearly marked. This helps with maintenance, troubleshooting and later expansions.

Correct connection to the inverter in the basement

The inverter is the transition between the DC and AC sides. It is often best kept in the basement because the room is dry, accessible and close to the meter cabinet. Nevertheless, the device needs air, distance and an environment that matches the manufacturer’s specifications. Damp walls, dust, poor ventilation or excessively high temperatures shorten the service life.

Three points are particularly important when connecting:

1. Suitable connectors: Similar-looking PV connectors are not automatically approved across manufacturers. Crimping and combination must match the manufacturer’s specifications.
2. Clean string assignment: Plus, minus, polarity, open-circuit voltage and MPP tracker assignment are checked before commissioning.
3. Clear AC planning: Others apply behind the inverter Cables, protective devices and connection conditions than on the DC side.

Anyone planning a storage system should consider the inverter, storage system and meter cabinet together. Short distances make assembly and maintenance easier. Matching: PV system structure: modules, inverters and storage.

Technical orientation: cross section, length and material

The following table is a guide. The actual design must match the system, type of installation, temperature and string voltage.

Simple cable length	Typical string current	Frequently useful cross section
up to 10 m	up to approx. 10 A	4 mm²
10 to 25 m	10 to 15 A	6 mm²
25 to 40 m	12 to 20 A	6 to 10 mm²
over 40 m	system-dependent	individual design

Copper is common in PV string lines because it offers low resistance and good processing properties. Pay attention to the manufacturer data sheet, type identification, temperature range, UV resistance and reputable certifications. Cheap no-name cables are bad business for a system that is supposed to run for 20 years or more.

Can you use existing cable ducts?

Existing cable ducts can be used if they are dry, continuous, sufficiently large and suitable for fire protection. PV cables should not be disorganized with AC cables, network cables or heating pipes. Your own empty pipes are the better solution.

A shaft that appears empty can kink on the inside, contain old fastenings or border fire compartments. Therefore it should be checked in advance. If it fits, it saves time, material and visible cable routes. If not, a new, clearly planned route is usually better.

Which standards and regulations apply?

Several sets of rules are relevant for PV cables from the roof to the basement. The focus is on DIN VDE 0100-712 for photovoltaic power supply systems. DIN VDE 0100-520 also plays a role for cable and wiring systems. On the grid connection side, there are also the requirements of the grid operator, TAB, VDE application rules and the low-voltage connection regulations.

What this means in practical terms for homeowners is that the DC side must be protected against electric shock, short circuits, overloads, arc flash risks and mechanical damage. Cables, connectors and protective devices must match the maximum voltage and current. Connection and commissioning belong in the hands of a registered electrical specialist company. The article Registering a PV system without an electrician: What really worksis relevant to the legal side.

DC and AC cabling: the difference

The DC cabling runs from the solar modules to the inverter. It transports direct current and can continue to carry voltage when exposed to light. A switched off house fuse does not automatically de-energize the PV strings.

The AC cabling begins behind the inverter and carries alternating current towards the house network, meter location and feed. Both areas require different types of cables, protective devices and markings. Clean separation helps with troubleshooting, maintenance, fire protection and electromagnetic compatibility. If you use multiple strings, you should clearly label the assignment to the MPP tracker. More about this: MPP tracker explained.

Typical errors when laying

A lot of damage is caused by small negligence. Bending radii that are too narrow squeeze the insulation. Normal cable ties become brittle on the roof. Poor wall penetrations allow moisture into the insulation or masonry. Mixed connectors can cause contact problems. A lack of labeling makes subsequent maintenance unnecessarily difficult.

Another point is surge protection. Long cable routes and exposed roof areas increase the importance of proper protection planning. You can find out more about this in the guide Surge protection for PV systems.

Practical checklist before installation

1. Really measure the cable path from the roof to the inverter, not estimate it.
2. Check string current, string voltage and MPP tracker assignment.
3. Calculate cross section based on length, current and voltage drop.
4. Select the appropriate solar cable, empty pipe, seal, fastening and connector.
5. Plan roof, wall and ceiling ducts with moisture and fire protection.
6. Permanently mark strings, polarity and separation points.
7. Have polarity, open-circuit voltage and insulation values documented.

This documentation sounds dry, but it is valuable if an inverter is later replaced, a storage unit is added or an error is searched for.

Sources and further specialist information

• DKE/VDE: DIN VDE 0100-712 for photovoltaic power supply systems
• VdS 3145: Photovoltaic systems, recommendations for damage prevention
• DGUV Information: Use on photovoltaic systems
• LAPP: technical information on H1Z2Z2-K solar cables according to EN 50618

Conclusion: A direct connection is possible with proper planning

Running PV cables from the roof to the basement is feasible and often makes sense. The direct route saves cable length, reduces losses and brings the inverter into the technical area. However, good results can only be achieved if the cable type, cross section, protective tube, sealing, fire protection and connection are planned together.

The most important question is not: “Where does the cable somehow fit through?” It is better: “Is this path short, dry, protected, calculated and still comprehensible later?” If so, the direct connection from the roof to the basement is a strong solution. If not, another cable route is worth it.

FAQ: PV cables from the roof to the basement

How should PV cables be laid from the roof to the basement?

PV cables should be routed in suitable empty conduits, cable ducts or shafts. They need mechanical protection, UV and moisture resistance as well as professional sealing of bushings. A qualified electrician will take care of the connection and measurement.

Which cable is used from the roof to the inverter?

On the DC side, special solar cables are used, often H1Z2Z2-K according to EN 50618. These cables are designed for photovoltaic systems, outdoor areas, temperature changes and high DC voltages.

How long can the cable from the roof to the inverter be?

There is no fixed maximum length. The line should be as short as possible. For longer distances, the cross section and voltage drop must be calculated.

Are PV cables allowed to lie in the same pipe as power or network cables?

PV string lines should be routed separately from AC lines, data lines and other installations. Own empty pipes increase safety, clarity and ease of maintenance.

Can a disused chimney be used for PV cables?

Yes, if the fireplace is out of service, dry, continuous and structurally suitable. The shaft should be checked beforehand, ideally with a camera.

Can I lay PV cables from the roof to the basement myself?

You can prepare and discuss the cable route with the specialist company. Electrical design, connection, measurements and commissioning belong to the electrician.