What size cable and fuses do you need for solar panels?

Cables and fuses are the cheapest parts of a solar system and the ones most often got wrong. A cable that is too thin wastes your solar power as heat and can overheat. A fuse of the wrong size, or one made for household AC instead of DC, may protect nothing. This guide explains both in plain words, with a worked example, for anyone wiring a shed, cabin, van, boat or backup battery in the US or the UK.

The short version

• Cable size depends on the current it carries and how far it runs. The same 400 W is about 33 A at 12 V but about 8 A at 48 V, so 12 V systems need much thicker cable.
• The cable must carry the current without overheating (its ampacity) and keep the voltage drop, the power lost in the wire, to about 2 to 3 %.
• Panel wiring outdoors uses special solar cable: H1Z2Z2-K (standard EN 50618) in the UK and Europe, PV wire or USE-2 in the US.
• Every fuse or breaker must be rated for DC at your system's highest voltage. DC is harder to switch off than AC.
• A fuse protects the cable: above the normal current, no higher than what the cable can carry. The main one sits in the battery's positive cable, close to the battery.
• Building wiring follows the NEC in the US and BS 7671 in the UK. For anything connected to a house's wiring, use a qualified electrician.

Current, voltage and why 12 V needs thick cable

Electricity in a cable is like water in a hose. Voltage (V) is the pressure, current (A, amps) is the flow, and power (W, watts) is the two multiplied: watts = volts × amps (more in our solar glossary). For the same power, a lower voltage needs more current. Our calculation: 400 W at 12 V is 400 ÷ 12 ≈ 33 A. At 24 V it is about 17 A, at 48 V about 8 A.

The power lost in a cable is its resistance times the current squared (loss = R × I²), as Victron Energy's white paper on controllers puts it. Double the current and the loss is four times bigger, so, the paper concludes, doubling the voltage lets you use a quarter of the cable cross-section for the same loss. That is why bigger systems move to 24 V or 48 V, and why panels are often wired in series on an MPPT controller (see MPPT vs PWM controllers).

Two separate questions for every cable

1. Will it overheat? Every cable has a maximum continuous current, its ampacity or current rating, which depends on the insulation, the air temperature and how the cable is laid. One UK maker's 1.5 kV DC solar cable is rated, at 60 °C (140 °F) surrounding air:

• 4 mm²: 55 A as a single cable in free air, 44 A for two cables touching on a surface;
• 6 mm²: 70 A in free air, 57 A for two touching;
• 10 mm²: 98 A in free air, 79 A for two touching.

Bundled cable carries less because it cannot shed heat. Use the datasheet line that matches how you will lay it, and your wiring code's derating rules.

2. How much power will it waste? That is the voltage drop, and at low voltage it usually decides the size.

Voltage drop explained simply

A cable's resistance (in ohms, Ω) "uses up" some voltage, like friction in a long hose lowers the pressure at the nozzle. The lost voltage is:

Voltage drop = current × resistance of the cable, counting both wires, out and back.

The resistance of a copper cable is:

Resistance = 0.0172 × total length in metres ÷ cross-section in mm² (at 20 °C)

The 0.0172 is standard annealed copper's resistance in ohm × mm² per metre (0.017241 in the US National Bureau of Standards copper wire tables, now NIST). Real flexible cable is a little higher; its datasheet gives the maximum per kilometre. Resistance also rises by 0.393 % per °C, according to the same tables. Our calculation: a cable at 70 °C (158 °F) has about 20 % more resistance than at 20 °C (68 °F).

To express the drop as a share of the voltage: drop % = voltage drop ÷ system voltage × 100.

How much is acceptable? Victron's wiring guide says to "aim for a voltage drop below 2.5%". Our own checks allow up to 3 %. You may also see 3 % and 5 % quoted from BS 7671; those figures are for mains circuits in buildings supplied from the public network, not a rule written for low-voltage solar DC.

A worked example: one array, two ways to wire it

Our calculation from the cited figures. You have four 100 W "12 V" panels. Each gives its full power at 18 V and 5.56 A (the example panel in Victron's white paper). The controller is 10 m (33 ft) from the panels, so the cable is 20 m (66 ft) out and back. You start with 4 mm² solar cable, whose datasheet gives a maximum resistance of 5.09 Ω per kilometre at 20 °C, so 20 m is 0.102 Ω.

Wiring A: all four panels in parallel, at 18 V. The currents add: 4 × 5.56 = 22.2 A.

• Voltage drop: 22.2 A × 0.102 Ω = 2.26 V.
• As a share: 2.26 ÷ 18 = 12.6 %. You lose about 50 W of your 400 W in the cable, before it is even warm.

Wiring B: all four in series, at 72 V (needs an MPPT controller; our controller guide shows this exact string can pass a 100 V limit on a −20 °C morning). The voltages add, the current stays 5.56 A.

• Voltage drop: 5.56 A × 0.102 Ω = 0.57 V.
• As a share: 0.57 ÷ 72 = 0.8 %. Same cable, about 3 W lost.

What would Wiring A need to get under 3 %? With the same datasheet, 16 mm² (1.24 Ω/km) still gives 3.06 %, so you would need 25 mm². In US sizes, using the resistance table in Victron's guide, 6 AWG gives 3.2 % (just over) and 4 AWG gives 2.0 %. For Wiring B, even 10 AWG gives about 0.5 %.

That is the whole lesson in one example: same panels, same distance, and a cable roughly six times thicker if you stay at low voltage.

mm² and AWG side by side

UK and European cable is sold by cross-section in square millimetres (mm²). US cable uses AWG (American Wire Gauge), where a smaller number means a thicker wire; above 1 AWG come 1/0, 2/0 and so on. From Victron's conversion table: 10 AWG = 5.26 mm², 8 AWG = 8.36 mm², 6 AWG = 13.3 mm², 4 AWG = 21.1 mm², 2 AWG = 33.6 mm², 1/0 AWG = 53.5 mm², 2/0 AWG = 67.4 mm². Metric sizes go 4, 6, 10, 16, 25, 35, 50, 70 mm², so there is rarely an exact match. Always round up.

Which cable type

Panel wiring (outdoors, in sun). In the UK and Europe, use solar cable to EN 50618, marked H1Z2Z2-K. One maker's datasheet shows what that means: rated 1.5 kV DC, conductor allowed up to 120 °C (248 °F), finely stranded tinned copper, halogen-free, double insulated. In the US, NEC section 690.31 names single-conductor PV wire and USE-2 for wiring within the array. Check which cable types your code edition allows for your inverter and location. Ordinary household cable is not made for years of sun on a roof.

Battery and inverter cables (short, high current). Victron's guide advises flexible, finely stranded copper rather than coarse-stranded, stiff cable, with tinned marine cable where it is damp.

The cable's voltage rating must cover the highest voltage it will see; on the panel side, the string's cold-weather open-circuit voltage.

Fuses and breakers: what they are for

A fuse is a deliberate weak link that melts and breaks the circuit if too much current flows; a circuit breaker does the same with a resettable switch. Their job is to protect the cable from overheating if something shorts. So a fuse must be:

• above the normal current, or it will blow in everyday use. US practice under NEC section 690.8 treats solar currents as continuous and adds 25 % margins, as the IAEI (the electrical inspectors' association) and EC&M explain: first on the panels' short-circuit current, then again for the fuse and cable.
• no higher than the cable's current rating, or the cable could overheat before the fuse blows. Victron's guide says a fuse must match the rating of the device or the cable, whichever is lower.

Why it must be DC-rated

Household mains is AC (alternating current): it reverses direction 100 times a second in the UK (50 Hz) and 120 in the US (60 Hz), passing through zero each time, which helps the spark (the arc) in a blowing fuse go out. Batteries and panels make DC (direct current), which never passes through zero. Mersen, a fuse maker, puts it plainly: "The absence of natural voltage zero makes the interruption of DC faults more difficult than the interruption of AC faults."

Mersen adds that an AC fuse may be used on DC only if its maker publishes its DC ratings. So check:

• DC voltage rating at least your system's highest voltage. Victron notes that most DC fuses are fine at 12 and 24 V "but they are not necessarily suitable for 48 V and higher".
• Interrupt rating, the biggest fault current it can safely break. Lithium batteries can deliver huge short-circuit currents; Victron calls it mandatory that at least one fuse can break the battery bank's maximum short-circuit current, and lists battery fuse types from about 1,000 A to 200,000 A.
• On the panel side, solar fuses marked gPV (IEC 60269-6) or listed to UL 2579. Solar arrays give only small fault currents; one maker's datasheet notes its solar fuses break currents as low as 1.35 times their rating.

Where fuses go

1. Main battery fuse, in the positive cable, as close to the battery as practical, protecting the thick battery cable. Boats have their own marine standards with stricter distance rules.
2. Controller to battery, in the positive cable, sized for that cable.
3. Battery to inverter, unless it shares the main fuse; the inverter's manual gives the size.
4. Between strings of panels. A string is a set of panels wired in series. With three or more strings in parallel, the others can feed a fault in one string, so each needs its own fuse, no bigger than the maximum series fuse rating on the panel's datasheet. The IAEI describes when one or two strings can go without.
5. Panel isolator, a DC breaker or switch to disconnect the panels for work. In the UK, BS 7671 Section 712 requires isolation on the DC side of an inverter, as summarized by Voltimum, an electrical trade site.

When you need an electrician

In the US, the National Electrical Code (NFPA 70) is a model code that states and towns adopt, often with changes and some years later. Article 690 covers solar; in the 2023 edition the off-grid (stand-alone) rules moved from 690.10 to 710.15, according to the IAEI. Permits and licensing depend on your state and local building department. Anything connected to a home's wiring or breaker panel should be done or checked by a licensed electrician.

In the UK, the standard is BS 7671, the IET Wiring Regulations, now BS 7671:2018+A4:2026; Amendment 4 was published on 15 April 2026 and, according to EAL, adds a chapter on stationary batteries. Some electrical work in homes is notifiable, meaning building control must be told about it. Notifiable work depends on your location: England, Wales, Scotland and Northern Ireland each have their own rules. Check the current Approved Document P or ask your installer. A registered electrician can self-certify notifiable work.

A self-contained van, boat or portable system is usually outside these building rules, but the physics is the same.

What to check before you buy

1. Work out each run's current: panel side (short-circuit current × parallel strings), battery side (inverter watts ÷ battery voltage, or charger current). Add 25 %.
2. Measure the one-way distance and double it.
3. Pick a cross-section with a voltage drop under about 2 to 3 % and a current rating that suits how you lay it.
4. Check the cable's voltage rating covers the cold-weather panel voltage.
5. Choose fuses above the circuit current, at or below the cable's rating, with a DC voltage rating and enough interrupt rating; with three or more strings, respect the panel's maximum series fuse rating.
6. If the work touches a building's wiring, involve an electrician.

What can go wrong

• Thin cable on a long 12 V run. A big share of the power never reaches the battery.
• AC breaker in a DC circuit. It may fail to break a DC fault, or arc inside.
• Fuse bigger than the cable. The cable overheats before the fuse blows.
• No fuse at the battery. A short anywhere along the battery cable is fed by the full battery current.
• Too low an interrupt rating on a lithium battery. Victron warns such a fuse "may fail catastrophically".
• Loose or badly crimped lugs. A poor joint adds resistance in one spot and gets hot.
• Indoor cable used outdoors. Sunlight breaks down insulation not made for it.

What our quiz checks

When our quiz builds a parts list, cables and fuses must pass these checks. A part with a missing value never passes.

• Current rating: every cable, fuse, breaker and connector is checked at 1.25 × the circuit current. On the panel side the circuit current is the panels' short-circuit current times the number of parallel strings; on the battery side it is the larger of the charge and discharge currents.
• Voltage drop: at most 3 %, calculated for copper at 70 °C (158 °F), over the cable run. Unless you tell us otherwise, we assume 10 m (33 ft) one way from panels to controller and 1.5 m (5 ft) to the battery for a cabin or backup system, and 4 m (13 ft) and 1 m (3.3 ft) in a vehicle.
• Cable voltage: solar cable rated for the cold-weather voltage of your string.
• Fuse: at or above the circuit current, at or below the cable's rating, and with a published DC voltage rating at or above the circuit's highest voltage. A fuse with only an AC rating never passes.
• String fuses: added from three parallel strings. Each string gets its own fuse, rated at least 1.56 times the panel's short-circuit current (the two 25 % margins above, 1.25 × 1.25) and never above the panel's maximum series fuse rating. For UK systems we list two fuses per string, one in the positive cable and one in the negative. A UK installation guide from 2006 (BRE/DTI) calls for fuses in both string cables on arrays of four or more strings; we apply that from three. That guide is neither a law nor a current standard. For US systems we list one fuse per string, because the National Electrical Code allows a single device per string as long as all of them are on the same pole (all positive or all negative). Either way, your installer makes the final decision from the current code (BS 7671, Amendment 4:2026, or the NEC) and the instructions for your panels and combiner box.

Our methodology page lists the sources behind each rule. Other related guides: how big a solar battery you need and power station or DIY system.

Questions people ask

Can I use car fuses? Only if the maker publishes a DC voltage rating high enough for your system and an interrupt rating that covers your battery's short-circuit current. Check the datasheet; a fuse that suits a car's 12 V circuits may not suit a 48 V battery bank or a solar string.

Does the negative cable need a fuse? It depends on which cable.

• Battery cable: the usual practice is to fuse the positive cable, close to the battery, as Victron's guide describes. Systems with special earthing arrangements follow their own rules; ask an electrician.
• Panel strings: the answer differs by country. Mersen, a fuse maker, says the usual practice on a "floating" array (one where neither cable is connected to earth) is a fuse on each polarity, and the 2006 BRE/DTI guide for UK installations asks for fuses in both the positive and negative string cables. In the US, the 2017 National Electrical Code began allowing a device in only one of the two conductors, as the IAEI explains, provided all of them are on the same polarity. Your installer decides from the current code and the instructions for your panels and combiner box.

Can I use thicker cable than needed? Yes; it only lowers losses. Check that the fuse still protects it and the connectors fit it.

Next step

Tell us where and how you will use solar, and our quiz gives you a ready, checked parts list, with every cable and fuse sized for your runs and checked against DC ratings. Every number is explained.

Our results and guides are estimates and do not replace advice from a qualified electrician.