1. Solar Cables
The DC side cabling is one of the most sensitive parts of a solar power system. High voltage is present here, the wires run on the roof, exposed to UV radiation, heat, precipitation, and faults are often not immediately obvious. The problem usually appears slowly: in the form of overheating, increased contact resistance, insulation failure, or even fire. Therefore, DC cabling is not "general electrical work," but a specifically PV-specific field.
PV1-F cables – what makes a cable "solar"?
In solar systems, PV1-F marked cables are typically used. The essence is not the name, but the properties:
- outdoor, UV-resistant design
- high heat resistance
- double insulation
- designed for continuous DC voltage
The conductor is mostly fine-stranded copper, which is more flexible and better withstands bending and vibration. The insulation and sheath are typically cross-linked (e.g., XLPE), which provides better resistance to aging and thermal stress.
An important piece of data is the cable's rated voltage (e.g., 1000 V DC or 1500 V DC). This should not be adjusted to the average operating voltage, but to the worst-case scenario, i.e., the increased Voc string voltage in cold weather.
UV and heat resistance – why aren't "any cables" suitable?
DC cables running on the roof are exposed to direct sunlight and extreme heat. In summer, the temperature in the space under the panel can easily reach 70–80 °C.
If the cable is not UV-resistant:
- it becomes brittle,
- it cracks,
- water gets under the insulation.
- If the cable is not sufficiently heat-resistant:
- material aging accelerates,
- insulation resistance decreases,
- the risk of short circuits and breakdowns increases.
Heat is also important from an electrical point of view: the resistance of copper increases with temperature. In hot weather, the same current causes greater losses, so PV systems need to be dimensioned more conservatively than indoor networks.
Selection of Cross-Section
Cable sizing is based on three main criteria:
Current carrying capacity: The cable must safely withstand the maximum operating current. When designing, not only the Imp value but also the short-circuit current (Isc) and its correction factors must be taken into account.
Voltage drop: The longer the cable, the greater the loss. This directly reduces production. The goal is generally a DC voltage drop below 1%.
Installability and compatibility: A cable that is too thick is harder to bend and can be problematic with MC4 connectors and cable glands.
For small systems, a cross-section of 4–6 mm² is common, but the specific value must always be calculated.
Calculation of Voltage Drop
On the DC side, the calculation is simple because we work with a purely ohmic approximation.
- Basic formula: ΔU = I · R
- Conductor resistance: R = ρ · L / A
- For PV systems, it is important to note that the DC circuit has both a forward and a return path, so: ΔU = I · ρ · 2L / A
- Percentage voltage drop: ΔU% = (ΔU / U_operating) · 100
Important: the operating voltage is around Vmpp, not Voc.
Example
- Current: 11 A
- Distance: 25 m (one way)
- Cross-section: 4 mm²
- Resistivity of copper: 0.0175 Ω·mm²/m
ΔU ≈ 2.4 V
If the string operating voltage is 550 V:
ΔU% ≈ 0.44%
This is a good value. However, for longer distances or higher currents, a 6–10 mm² cable may be justified.
The point is that cable sizing is not an empirical tip, but can be easily calculated.
Outdoor Wiring Rules
For DC cabling, it's not just about what kind of cable it is, but also how it is routed.
Important principles:
- The cable should not hang loosely on the roof.
- It should not touch sharp sheet metal edges or tile edges.
- It should not lie on extremely hot surfaces.
- Use UV-resistant fasteners.
- Do not fasten too tightly (the cable expands and moves).
The most common "invisible fault" is chafing. If the cable vibrates for years, rubbing against a rail or sharp edge, the sheath slowly wears through. This doesn't happen overnight – but when it does, it's too late. Large, untidy cable loops should also be avoided. They are not only aesthetically disturbing but also unfavorable in terms of lightning induction and interference. The goal is always a short, tidy, mechanically protected routing.
2. MC4 Connectors
The MC4 connector is one of the most sensitive points of the DC side of a solar power system. From the outside, it appears to be a simple plastic connector, but in reality, it is a high-voltage, high-current electromechanical connection designed for outdoor environments. A significant proportion of DC side failures and fires are almost always due to installation errors related not to the panels or the inverter, but to the connections.
A characteristic of DC systems is that they are continuously under voltage during the day, and there is no zero crossing as in AC current. Therefore, a weakening contact does not "clear itself" but gradually deteriorates.
Inside the MC4, there is a crimped metal contact, which is held under continuous preload by a spring mechanism. This spring ensures that contact pressure is maintained even in the event of thermal expansion or vibration. The outer casing is UV-resistant plastic, equipped with a seal. However, waterproofing only works if:
- the cable is properly crimped,
- the connector is fully snapped together,
- the seal is not damaged.
Even a single geometric imperfection is enough to increase contact resistance.
Crimping
The quality of an MC4 connection is primarily determined by crimping. This is not a simple compression, but a controlled cold forming process during which the copper conductor and the contact establish a material-like connection. If the crimping is inadequate, the connection may work initially, but will start to overheat in the long run. On the DC side, this is particularly dangerous because heat generation is proportional to the square of the current. Even a small increase in contact resistance can cause significant overheating.
Important professional principle:
MC4 connections should not be soldered. Soldering creates a brittle connection that can crack due to thermal expansion and vibration.
Mixing Manufacturers
The term "MC4" has become an industry standard, but this does not mean that all MC4-compatible connectors are identical. Different manufacturers use different contact geometries and spring forces. If connectors from different manufacturers are connected, the connection may appear mechanically stable, but the contact surface may not be optimal. This can lead to increased contact resistance, which over time causes overheating.
The good practice is simple: use the same manufacturer and type within a string.
Overheating and Fire Risk
The increase in contact resistance is a self-reinforcing process. Overheating reduces the contact surface, which further increases resistance and causes even greater heat generation. On the DC side, if the contact breaks or becomes unstable, an arc discharge can occur. Since there is no zero crossing, the arc does not extinguish naturally.
Therefore, the MC4 connector is not a "minor detail" in the system. A poorly executed connection can remain invisible for years, then suddenly cause serious damage. The key to prevention is precise execution: proper crimping, use of compatible components, and mechanical strain relief of the cables.
3. Series and Parallel Connections
One of the most important questions regarding the DC side of a solar power system is how to connect the panels. Series and parallel connections are not merely cabling decisions, but rather voltage, current, and protection concepts. This is where it is decided whether the system functions in a way that is properly "interpretable" for the inverter, and whether it remains stable in the long term.
Forming Strings – The Logic of Series Connection
The term "string" refers to a group of solar panels connected in series. In a series connection, voltages add up, while the current remains the same across each panel. This allows for the generation of hundreds of volts of DC from panel voltages of around 30–50 V for the inverter.
- its voltage is the sum of the panel voltages,
- its current is the same as the current of the individual panels,
- and its power is the product of the two.
One fundamental characteristic of a series connection is that the weakest panel determines the behavior of the entire string. If a panel is shaded, dirty, or faulty, the current of the entire string decreases. This is why identical orientation, tilt angle, and similar shading conditions must be ensured within a string.
The length of the string is not chosen based on the number of panels, but by calculation. The following must be considered:
- the maximum open-circuit voltage (Voc) of the panels in cold weather,
- the operating voltage (Vmpp),
- and the DC input limits of the inverter.
A string that is too long can exceed the inverter's permissible voltage, while a string that is too short may not reach the optimal operating range of the MPPT.
Parallelization – When is it Justified?
In a parallel connection, the currents of the strings add up, while the voltage remains unchanged. This is typically used when a single string does not provide sufficient power, or when the inverter's input current is greater than what one string can provide.
Important distinction: parallelization is always done with strings, not individual panels.
However, parallel connection introduces a new phenomenon: the possibility of reverse current. If one string behaves differently (shaded, shorter, faulty), the other string can feed current into it. This can cause overheating and overload.
Therefore, parallelization is not just about connecting, but also about protection.
The Role of String Fuses
The purpose of a string fuse is to prevent dangerous reverse currents in parallel strings. In normal operation, the fuse does not intervene. In the event of a faulty string, however, it trips before the cable or the panel's internal wiring is damaged. It is common design practice that for two parallel strings, it can often be omitted, but for three or more parallel strings, fusing is necessary.
When selecting a fuse, the panel's short-circuit current (Isc) and the number of parallel strings are decisive.
The Problem of Unequal Strings
One of the most common mistakes is connecting strings with different characteristics. This can occur in several ways:
- different number of panels on one MPPT,
- different orientations,
- partial shading on only one string,
- mixing different panel types.
In such cases, the strings do not want to operate at the same operating point. The inverter's MPPT is forced into a compromise, which causes power loss. In a parallel connection, this is even more severe, as the current distribution will not be even.
A classic mistake is when a longer and a shorter string are paralleled "because the voltage is almost the same." Electrically, this can lead to unstable operation.
The good practice is simple: only parallel strings of the same length, same orientation, and same panel type to one MPPT input.
5. How Optimizers Work
The advent of power optimizers fundamentally changed the classical string-based approach. In a traditional system, the inverter searches for a common operating point for an entire string, meaning all series-connected panels behave "as a team." If one panel produces less, the entire string adapts to it. In an optimizer system, this logic breaks down to the panel level. Each panel receives its own regulation. However, this does not automatically mean that the system will be better; it simply operates differently.
An optimizer is a small electronic unit that connects directly to the panel. Its task is to ensure that the panel always operates at its own maximum power point, regardless of the state of the rest of the string.
If a panel in a traditional string becomes shaded, the performance of the entire string decreases. In an optimizer system, the underperforming panel does not drag down the others. The loss "stays local."
However, it is important to clarify a misunderstanding: the optimizer does not increase the nominal power of the panel. It does not boost it, nor does it magically create extra watts. It merely ensures that the given panel operates as well as possible given its own conditions.
When Does It Make Sense to Use?
Typical situations include, for example, when intermittent shading appears on the roof, surfaces with different orientations are placed within the same string, or the roof geometry does not allow for the formation of identically behaving panel groups. In such cases, the optimizer can localize the problem, so it does not affect the entire system.
However, if the panels have the same orientation, are unshaded, and can be easily separated into different MPPTs, then the optimizer generally does not provide a significant added benefit. In such cases, it only increases the complexity of the system.
What is the Price of Flexibility?
An optimizer is an active electronic device. This means that more components are placed on the roof, the system's complexity increases, and its operation is more tied to a specific manufacturer's technology. A well-designed, homogeneous string system operates with fewer points of failure. Therefore, an optimizer is not a default solution, but a targeted tool for specific situations.
Optimizer or Multiple MPPTs?
An inverter with multiple MPPTs solves discrepancies at the string level. For example, if there are two roof surfaces with different orientations, they can be connected to separate inputs, so both operate at their own optimal power point. This is a simple and robust solution.
An optimizer, on the other hand, provides an advantage when the discrepancy occurs within the same string and cannot be separated into different MPPTs. In such cases, the classic string logic can no longer adapt properly.
The professionally correct approach, therefore, is always to start the design with string logic and MPPT allocation. If this does not result in a stable and efficient system, then the optimizer comes into play as a targeted solution, not an automatic choice.
