Most PV cable is bought in one of three sizes: 4 mm², 6 mm² or 10 mm². The short answer is that 4 mm² covers the majority of residential string runs, 6 mm² is for longer runs or higher-current strings, and 10 mm² belongs on DC trunk lines, combiner-to-inverter runs and commercial arrays. But "short answer" is where installs go wrong, because size is set by three independent constraints — thermal capacity, voltage drop, and mechanical/connector limits — and the binding one is often not the one people check.
This guide works through all three in order, with the arithmetic shown so you can run it against your own string.
The three constraints that actually set the size
A cable is adequately sized only when it passes all three tests. Failing any one of them is a defect, no matter how the other two look.
- Thermal — the conductor must not exceed its rated temperature at the worst ambient it will ever see. For H1Z2Z2-K this is a 90 °C conductor in a 30 °C reference ambient.
- Voltage drop — the resistance of the run must not eat an unacceptable share of the string voltage, because every volt lost in the cable is energy you never sell.
- Mechanical and connection — the conductor must be robust enough for the installation method and must physically fit the connector and terminal range you are using.
Sizing by current alone is the most common error: on short residential runs, voltage drop usually governs; on long or high-current runs, thermal capacity governs; and either way a connector that cannot accept the conductor size can invalidate the choice entirely.
Step 1 — Thermal capacity, and why roofs are not 30 °C
Typical current-carrying capacities for single-core H1Z2Z2-K (EN 50618 / 2PfG 1169 style construction), free in air, 30 °C ambient:
| Cross-section | Typical capacity at 30 °C | At 50 °C (×0.82) | At 60 °C (×0.71) | At 70 °C (×0.58) |
|---|---|---|---|---|
| 2.5 mm² | 41 A | 34 A | 29 A | 24 A |
| 4 mm² | 55 A | 45 A | 39 A | 32 A |
| 6 mm² | 70 A | 57 A | 50 A | 41 A |
| 10 mm² | 98 A | 80 A | 70 A | 57 A |
| 16 mm² | 132 A | 108 A | 94 A | 77 A |
Two things to take from this table. First, these are reference values — always confirm against the current edition of the standard table and the manufacturer's own data sheet, since construction and test basis vary. Second, and more important: the derating column is the one that matters in practice. A cable clipped to a roof surface in summer, or run in a conduit on a sun-facing wall, can sit well above 60 °C. A 4 mm² conductor that looks comfortable at 55 A becomes a 32 A conductor at 70 °C.
For most modern modules the design current is modest — a 14 A Isc module with the usual 1.25 safety factor gives about 18 A — so thermal capacity is rarely the binding constraint on a residential string. It becomes binding on parallel strings, on trunk lines, and anywhere the cable is bundled or in a hot enclosure. If you are also working in imperial units, the AWG to mm² conversion chart covers the cross-walk, but note that a conversion is not a substitute for re-checking the capacity table.
Step 2 — Voltage drop: the arithmetic that decides most residential runs
For a two-wire DC circuit the drop is:
ΔU = 2 × I × L × ρ / A
where I is the design current in amps, L the one-way run length in metres, ρ the conductor resistivity (use 0.023 Ω·mm²/m for a copper conductor near 90 °C — the 20 °C figure of 0.0175 flatters the result), and A the cross-section in mm². The leading 2 accounts for the return conductor.
Worked examples, 600 V DC string, 18 A design current:
| Run (one way) | 4 mm² | 6 mm² | 10 mm² |
|---|---|---|---|
| 20 m | 4.1 V — 0.69 % | 2.8 V — 0.46 % | 1.7 V — 0.28 % |
| 40 m | 8.3 V — 1.38 % | 5.5 V — 0.92 % | 3.3 V — 0.55 % |
| 60 m | 12.4 V — 2.07 % | 8.3 V — 1.38 % | 5.0 V — 0.83 % |
| 100 m | 20.7 V — 3.45 % | 13.8 V — 2.30 % | 8.3 V — 1.38 % |
Read the 4 mm² column downwards and you can see the decision point: 4 mm² is fine at 20 m and unacceptable at 40 m once a 1 % DC limit applies. The usual design targets are ≤1 % from string to combiner, ≤2 % for the whole DC side; some designers allow a little more, but anything above 2 % on the DC side is throwing away yield permanently.
That loss is not theoretical. Voltage drop in percent is very nearly power loss in percent, so a 2 % drop costs about 2 % of annual generation for the life of the array. On a 5 kW system producing roughly 7,000 kWh a year, that is about 140 kWh a year, every year, for 25 years — normally far more than the price difference between 4 mm² and 6 mm².
One practical note: doubling the cross-section halves the drop, but going from 4 mm² to 6 mm² is only a 33 % reduction. If your numbers are already close to the limit, jumping straight to 10 mm² is often cheaper than discovering the problem after installation.
Step 3 — Mechanical and standard minimums
Standards set a floor that has nothing to do with current. EN 50618 and IEC 62930 both permit 1.5 mm² as the smallest PV cable, but 1.5 mm² on a roof is a fragile choice: it kinks, it suffers at clamp points, and it leaves no margin for a future string extension. In practice 4 mm² is the sensible residential floor, and that floor — not the thermal calculation — is why 4 mm² dominates residential BOMs.
Mechanical minimums to hold onto:
- 4 mm² minimum for exposed roof or wall runs where the cable is clipped, tied or subject to wind movement.
- 6 mm² where runs exceed roughly 30–40 m at typical residential currents, or where multiple strings are paralleled.
- 10 mm² and above for DC trunk lines, combiner-to-inverter runs, and commercial/utility arrays where current and distance both climb.
- Where the cable is buried or at risk of rodent damage, consider armoured construction and size on the same arithmetic — see armoured vs unarmoured solar cable.
Step 4 — The connector limit people forget
A conductor size can be electrically perfect and still be unusable because the connector will not accept it. Standard MC4 crimp contacts are built for roughly 1.5–6 mm² and are typically rated around 30 A. A 10 mm² conductor does not fit a standard MC4 contact; it needs a larger connector family or a proper transition to a terminal or junction block.
This matters because forcing a conductor into an undersized contact creates a high-resistance joint — which is exactly where PV fires start. Two further checks worth doing at the same time:
- Parallel strings: two strings in parallel means a fault in one can push roughly 2× Isc back through the healthy string's cable. Check both the cable and the connector rating against that, not just against normal operating current.
- Mixed sizes in one string: joining 4 mm² and 6 mm² mid-string is sometimes done for a long tail run, but every joint is a future failure point in a DC circuit that cannot be switched off under load. Prefer to change size at a junction box.
The decision table
| Situation | Recommended | Why |
|---|---|---|
| Residential roof, run under ~25 m, single string | 4 mm² | Drop well under 1 %, fits standard MC4, robust enough |
| Residential roof, run 25–50 m | 6 mm² | Keeps drop near or below 1 % where 4 mm² exceeds it |
| Large-format modules, high-current strings | 6 mm² | More thermal margin at roof temperatures |
| Two or more strings paralleled | 6 mm² | Fault-current and thermal margin |
| Combiner to inverter, DC trunk | 10 mm²+ | Current sums across strings; distance usually long |
| Commercial / utility array | 10–35 mm² | Both constraints bind; calculate, do not assume |
Five mistakes worth avoiding
- Sizing on current only. On short runs the drop governs, and it is the cheaper constraint to satisfy before installation rather than after.
- Using the 20 °C resistivity. 0.0175 Ω·mm²/m understates the drop on a conductor that runs near 90 °C; use 0.023.
- Ignoring ambient derating. A roof in summer is not a 30 °C test chamber; apply the correction before you call a size adequate.
- Substituting CCA on the same table. Copper-clad aluminium has markedly higher resistance per mm², so a copper size table does not transfer — see the CCA reality check.
- Forcing oversized conductor into standard MC4. Use the correct connector for the size, or change the size at a proper junction.
FAQ
Q1: Is 4 mm² enough for a residential solar installation? A: For a single string with a run under roughly 25 m at typical residential currents, yes — 4 mm² usually lands below 1 % voltage drop and is the practical mechanical minimum. Beyond about 40 m at 18 A, 4 mm² exceeds 1 % and 6 mm² becomes the better choice.
Q2: Can I use 6 mm² everywhere instead of calculating? A: It is a defensible simplification for residential work and costs little more, but it is not correct everywhere: long trunk runs and paralleled strings can still exceed the drop or thermal limits at 6 mm², and no amount of oversizing fixes a connector that cannot accept the conductor.
Q3: What voltage drop should I design for? A: Keep the string-to-combiner section at or below 1 % and the whole DC side at or below 2 %. Every percent of drop is roughly a percent of annual yield lost for the life of the array, so the cable cost is usually trivial against the energy cost.
Q4: Does a larger cable always mean a better installation? A: No. Beyond the point where drop and thermal limits are satisfied, extra copper buys nothing, adds stiffness and weight on the roof, and can push you outside the connector range. Size to the constraint, not to a slogan.
Q5: Does the cable size affect which connector I can use? A: Yes, and it is a hard limit rather than a preference. Standard MC4 contacts cover roughly 1.5–6 mm²; 10 mm² requires a larger connector family or a transition to a terminal block. Never crimp a conductor into a contact that was not designed for it.
Sourcing note
🔴 Disclaimer: current-carrying capacities, derating factors and voltage-drop limits depend on the applicable standard edition, installation method and local code; the values above are typical reference figures for H1Z2Z2-K style cable and must be confirmed against the current standard table and the manufacturer's data sheet before design sign-off. This article is engineering guidance, not a design certification.
For cable construction and standard background behind these sizes, see the H1Z2Z2-K / EN 50618 market guide. Conductor material affects the same arithmetic — compare bare copper, tinned copper and tinned CCA before assuming a copper table applies.
