Solar PV Cable UK: Types, Sizes, and Where to Buy

What is H1Z2Z2-K solar PV cable?

If you are planning any DIY solar installation — balcony system, off-grid setup, or a full roof array — the cable that connects your panels to your DC isolator and inverter must be H1Z2Z2-K. That designation is not just a product name; it is a specification that describes exactly what the cable is and what it can withstand.

Breaking down the code:

• H — Halogen-free outer sheath. In a fire, it produces less toxic smoke than PVC cable.
• 1 — Rated for single-core use (one conductor per cable, supplied as separate positive and negative).
• Z2 (first) — Cross-linked polyethylene (XLPE) primary insulation, UV-stabilised and temperature resistant.
• Z2 (second) — A second layer of XLPE insulation. This double-insulation is what makes the cable safe for DC circuits where a fault cannot be interrupted by a simple breaker.
• K — Fine-stranded flexible copper conductor, which makes it easier to route around panels, through trunking, and into connector bodies.

The cable is rated to 1,500V DC and is UV resistant, meaning it can be routed across rooftops, through exposed conduit, and in direct sunlight without the insulation degrading. That rating is not a luxury — string voltages on even modest domestic arrays can exceed 400V DC in cold weather.

Why standard cable is not suitable on the DC side

The most common mistake in DIY solar wiring is using twin-and-earth (T+E) — the grey flat cable used for most UK domestic ring circuits and lighting. It seems reasonable: it is widely available, cheap, and familiar. It is also completely wrong for DC solar wiring, for several reasons.

Voltage rating: Standard T+E is rated for 230V AC (or 400V three-phase AC). DC voltage behaves differently to AC — faults are harder to interrupt, arcs sustain longer, and the insulation stress is higher. A cable rated for 230V AC is not suitable for string voltages of 300–900V DC.

UV degradation: T+E uses PVC insulation, which breaks down when exposed to ultraviolet light. Roof-mounted cable runs that are not enclosed in conduit will show cracking and brittleness within a few years. H1Z2Z2-K uses XLPE, which is stabilised specifically for outdoor UV exposure.

Single insulation: T+E has a single layer of PVC around each conductor (with a bare earth). H1Z2Z2-K has two independent insulation layers. On the DC side of a solar system, double insulation is a requirement under BS 7671 Chapter 71 (Section 712), not a preference.

Regulatory compliance: Using T+E on the DC side of a solar installation is a violation of BS 7671 and MIS 3002. Any competent person inspection or insurance claim following a fire or fault would identify this immediately. Do not use it.

Choosing the right cable size

PV cable is sized by the cross-sectional area of the copper conductor, measured in mm². The three sizes you will encounter for domestic UK installations are 2.5mm², 4mm², and 6mm².

For the vast majority of UK domestic solar installations, 4mm² is the correct choice. A typical 4kW domestic array on a 48V off-grid system draws around 18–20A on the DC side. A grid-tied string inverter with a 4kW array at a string voltage of 350V would draw around 11–12A. Both sit comfortably within the 32A capacity of 4mm² cable with margin to spare.

If you are unsure, 4mm² is the sensible default. Sizing down to 2.5mm² is only worth considering for genuinely short runs on small systems. Sizing up to 6mm² is worth considering for longer cable runs — see the voltage drop section below.

Voltage drop: why it matters and how to calculate it

Voltage drop is the reduction in voltage along a cable caused by the resistance of the copper conductor. On a DC system, any voltage lost in the cable is energy that never reaches your inverter or battery. More practically, excessive voltage drop can cause inverters to operate outside their MPPT (maximum power point tracking) window, reducing output.

MIS 3002:2025 targets a maximum of 1% voltage drop for domestic solar installations. Up to 3% is acceptable for larger or longer-run systems, but 1% is the design target for a standard residential installation.

The formula for voltage drop in a two-conductor DC circuit is:

V_drop = (2 × length × current × resistivity) / cross-section

Where:

• Length is the one-way cable run in metres
• Current is the maximum operating current in amperes
• Resistivity of copper is 0.0172 Ω·mm²/m
• Cross-section is the conductor area in mm²

Worked example: 10m cable run, 9A operating current, 4mm² cable.

V_drop = (2 × 10 × 9 × 0.0172) / 4
       = (3.096) / 4
       = 0.774V

On a 350V string, that is 0.22% voltage drop — well within the 1% target. At 10m with typical domestic currents, 4mm² cable is not a concern.

Where it gets tighter: if that same 9A string has a 25m cable run, the voltage drop rises to 1.94V (0.55% on 350V). Still fine. But at lower string voltages — for example a 100V battery system drawing 30A over 20m on 4mm² cable — the numbers change significantly:

V_drop = (2 × 20 × 30 × 0.0172) / 4 = 5.16V

On a 100V battery system that is 5.16% — well above the 1% target. This is where sizing up to 6mm² (dropping the voltage drop to 3.44V / 3.44%) or shortening the cable run makes a practical difference.

AC cable vs DC cable: understanding the boundary

Your solar installation has two electrically distinct sides separated by the inverter. It is worth being clear on which cable standards apply where.

DC side (panels → DC isolator → inverter DC input): H1Z2Z2-K or equivalent solar PV cable. Sized per the guidance above. Installed per BS 7671 Section 712.

AC side (inverter AC output → consumer unit / grid connection): Standard domestic cable to BS 7671 Part 5. For most domestic installations this is 6mm² or 10mm² twin-and-earth (or SWA armoured cable if buried — see the cable burial guide for depths and requirements). This work must be carried out or signed off by a Part P registered electrician in England and Wales.

The inverter is the boundary. Do not use H1Z2Z2-K on the AC side (it is not rated for AC circuits) and do not use T+E on the DC side (as covered above).

Where to buy PV cable in the UK

Cable prices vary enormously depending on where you buy. Here is a rough hierarchy from cheapest to most expensive:

Trade electrical wholesalers are typically the cheapest source for cable. CEF (City Electrical Factors), Edmundson Electrical, and Rexel all stock H1Z2Z2-K in various sizes and lengths. If you have a trade account, prices are often 30–40% lower than retail. Some branches will open trade accounts for sole traders and self-builders without requiring a VAT number.

Solar-specific suppliers such as Sunstore and Bimble Solar stock H1Z2Z2-K, often with the option to buy pre-terminated lengths with MC4 connectors already crimped on. This is worth considering if you are not confident crimping your own connectors, though you pay a premium for the convenience.

Amazon stocks H1Z2Z2-K from multiple sellers but prices are generally higher than trade, and quality control varies. If buying from Amazon, look for cable with TUV certification marking and check the seller's reviews carefully.

Colour conventions

PV cable is typically supplied in red (positive) and black (negative) for single-core solar DC cable. Some suppliers also offer blue for negative. Maintain consistent colour conventions throughout your installation — mixing up positive and negative in a DC string will not trip a breaker. Label each end of every cable run during installation; it is far easier than tracing cables after the fact.

String Fuse Sizing for DIY Systems

String fuses are a safety component that protects individual strings of panels when multiple strings are connected in parallel. Understanding when you need them — and how to size them — is important for any DIY system with more than one string.

When are string fuses needed?

String fuses protect against reverse current: if one string underperforms (due to shading, partial failure, or a fault), current from the other strings can flow backwards through it. This reverse current can exceed the panel's rated tolerance and cause damage or a fire.

The general rule is:

• 2 strings in parallel: String fuses are generally not required. Most panels are rated to tolerate reverse current from one other string without fuse protection.
• 3 or more strings in parallel: String fuses are required on each string. With three or more strings, the combined reverse current can exceed the panel's maximum series fuse rating.

Check your panel datasheet — it will specify the Maximum Series Fuse Rating (sometimes labelled "Max Overcurrent Protection" or similar). This value is the ceiling for your string fuse rating, not the target.

How to size string fuses

The standard approach is to rate string fuses at 1.5× to 2× the panel's short-circuit current (Isc), while staying within the Maximum Series Fuse Rating shown on the datasheet.

Worked example: A panel with Isc = 13A and a Maximum Series Fuse Rating of 20A.

• Minimum fuse: 1.5 × 13A = 19.5A → round up to 20A (the datasheet maximum)
• A 15A fuse would also be acceptable (between 1.5× and 2× Isc, within the ceiling)
• A 25A fuse would exceed the Maximum Series Fuse Rating — do not use it

DC-rated fuses only

String fuses must be DC-rated. This is not optional. AC fuses are designed to interrupt alternating current, which naturally crosses zero volts 50 times per second. DC current does not cross zero — when an AC fuse attempts to interrupt a DC arc, it may fail to extinguish the arc, causing the fuse holder to overheat or catch fire.

Look for fuses marked with a DC voltage rating (e.g., "1000V DC" or "1500V DC") and a DC interrupting capacity. Solar-specific fuses from manufacturers such as Littelfuse (PVSC series), Mersen, and Bussmann (PV-series) are widely available from UK solar suppliers and electrical wholesalers.

Fuse holder options

For DIY installations, MC4-inline fuse holders are the most practical option. These:

• Connect directly into the MC4 string cable with no tools needed (other than an MC4 crimping tool for the connectors)
• Are weatherproof and UV-stabilised — suitable for outdoor and roof use
• Accept standard 10×38mm cylindrical solar fuses (confirm the fuse holder's fuse size before ordering)
• Allow fuse replacement without disturbing cable routing or trunking

Inline MC4 fuse holders are available from Bimble Solar, Sunstore, and various Amazon sellers. Always check the seller's specification to confirm the holder is rated for DC voltage at your string voltage, not just AC.

String combiner boxes are an alternative for larger systems, housing multiple fuse holders in a single weatherproof enclosure with labelled inputs — useful if you have four or more strings.

Further reading

If you are installing PV cable in a run between a garage or outbuilding and roof-mounted panels, you will likely need to bury some of it. Cable burial has its own depth requirements and conduit rules under BS 7671 Regulation 522.8.10 — the cable burial depth guide covers these in detail.

For the connectors that attach to the end of your PV cable, the MC4 connectors guide covers the IEC 62852 interoperability standard, counterfeit connector risks, and crimping technique.

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