Solar Cable Size Guide: 4 mm vs 6 mm vs 10 mm (and GM Cable 3/29 Price in Pakistan Today)

On Solar Works Pakistan projects we use GM Cable and Fast Cables. This page explains which cable goes where, why size matters, and how your engineer chooses it.

> Safety note. Solar DC cables carry high voltage whenever the sun is up, and they cannot be switched off from the grid. Cable selection, routing, termination and protection must be done by a qualified electrician or solar engineer. Undersized or badly crimped cables are a common cause of solar fires.

Cable price in Pakistan today

Solar DC cable (per metre or per roll)

Household AC wiring cable

Prices are usually quoted per coil (roll) for house wire and per metre or per roll for solar cable. Ask whether a price is for a full coil and what length the coil is.

Diagram

Thicker cable, less voltage drop

Panel stringsSolar DC cable

→

CombinerDC breaker + SPD

→

InverterCorrect DC size

→

BatteryHeaviest cable, short run

→

Main DBAC cable sized to inverter output

What does "3/29" mean?

Pakistani house wiring is often sold by an older strand code. "3/29" means 3 strands of 0.029-inch copper wire. By simple arithmetic that is about 1.3 mm² of copper, a small conductor, and it is generally used for lighting and light loads. Larger codes such as 7/29, 7/36 and 7/44 have more or thicker strands and carry more current.

These are AC building wires. They are fine for the right house circuits, but they are not designed for the solar DC side: they lack the UV-resistant, double-insulated construction and DC voltage rating that solar strings on a hot roof need.

DC solar cable vs AC cable

Using ordinary house wire on the roof leads to cracked insulation under Punjab's summer sun, then leakage, insulation faults on the inverter and eventually a fire risk.

Why voltage drop matters

Every cable has resistance. Current flowing through it loses some voltage as heat. That lost voltage is energy your panels produced but your inverter never receives. Too much drop also heats the cable and its connections.

Voltage drop rises with:

• cable length (longer runs lose more);
• current (more panels in parallel means more current);
• smaller cross-section (thinner cable has more resistance).

Designers usually aim to keep DC voltage drop to a small percentage of the string voltage; the exact target is set in the design and the inverter manufacturer's guidance.

A worked example (illustrative only)

For a two-wire DC run, voltage drop is roughly:

Drop (V) = 2 × length (m) × current (A) × 0.0175 ÷ cable size (mm²)

(0.0175 is the approximate resistivity of copper in ohm·mm²/m.)

For example, if a string carries 13 A over a 20-metre one-way run:

• With 4 mm² cable: 2 × 20 × 13 × 0.0175 ÷ 4 = about 2.3 V.
• With 6 mm² cable: 2 × 20 × 13 × 0.0175 ÷ 6 = about 1.5 V.

On a 400 V string, both are well under 1%. Double the distance, or combine two strings into one cable, and the difference becomes significant. These numbers are only to show the logic; your engineer works from the actual panel datasheet, cable datasheet and site layout.

4 mm vs 6 mm vs 10 mm: which do you need?

Modern high-wattage panels produce more current than older ones, so a cable size that was enough a few years ago may now be marginal. Two other checks always apply:

1. Current rating. The cable must carry the maximum circuit current with a margin, after derating for heat, sun exposure and bunching in conduit. Use the manufacturer's datasheet, not a general chart.
2. Protection. The breaker or fuse protecting the cable must be rated at or below what the cable can carry. That is why cable size and breaker selection are designed together.

On the AC side, the inverter output cable is sized for the inverter's maximum AC current, the distance to the distribution board and the protection device. On larger three-phase and industrial systems, armoured multi-core cables are often used.

Good installation practice

• Use proper MC4 connectors of the same type on both halves, crimped with the correct tool. Mismatched connectors are a known failure point. See MC4 connectors and earthing.
• Run cables in UV-resistant conduit or trunking, clipped so they do not rest on hot roof surfaces or sharp edges.
• Keep positive and negative of the same string close together to reduce loop area, which helps with surge protection.
• Label strings at the combiner and inverter.
• Protect strings with DC-rated breakers or fuses and surge protection. Our earthing and surge protection guide explains the full layout.
• Understand how your strings are built before choosing cable; see solar panels in series vs parallel.

Why we use GM Cable and Fast Cables

Cable is one of the lowest-cost parts of a solar system and one of the hardest to replace once it is buried in conduit. A cable with less copper than its label claims will run hot and lose energy for the life of the system. We use GM Cable and Fast Cables for our projects, with sizes chosen by our in-house engineers and listed in your quotation and BOQ. Manufacturer warranty is per product.

Tips when buying cable yourself:

• Buy from a trusted supplier, and check the printed markings along the cable (brand, size, rating).
• Be wary of a price far below the market; it usually means less copper.
• Do not mix AC house wire into DC solar runs.

For complete protection, see our breakers and protection range, or have our own teams handle the full job through our solar installation services.

Get today's price or a free site survey

Need today's price on GM Cable or Fast Cables, or want the right cable sized for your system?

• WhatsApp or call: 0336 666 4321
• Email: info@solarworks.pk
• Head office: Main Boulevard, Sargodha Rd, Block F FDA City, Faisalabad
• Lahore branch: 2nd Floor, Jeff Heights, Gulberg, Lahore
• Hours: 9:00 AM to 6:00 PM, daily

We serve all of Punjab, and site surveys are normally free. Contact us.

Frequently asked questions

What is the GM Cable 3/29 price in Pakistan today?

Cable prices change with copper prices, the dollar rate and stock, so we do not publish a fixed figure. WhatsApp 0336 666 4321 for today's rate on GM Cable 3/29 and other sizes.

Is 4 mm or 6 mm cable better for solar panels?

Neither is "better" in general. 4 mm² is often enough for a single string on a short run, while 6 mm² reduces voltage drop on longer runs or with higher-current panels. Your engineer should calculate it for your layout.

When do I need 10 mm solar cable?

10 mm² is used where current is higher or the run is long, for example when strings are combined or the inverter is far from the roof. It also appears on some DC links in smaller systems.

Can I use 3/29 or 7/29 house wire for solar panels?

Not on the DC panel side. House wire is not built for high DC voltage or constant sun exposure. Use proper double-insulated solar DC cable there, and house wire only for suitable AC circuits.

What happens if solar cable is too thin?

It loses more energy as heat, runs hotter and can damage connectors and insulation over time. In the worst case it becomes a fire risk, especially if the protection device is not matched to the cable.

Which cable brands does Solar Works Pakistan use?

We use GM Cable and Fast Cables. The exact sizes and lengths are listed in your quotation.