There are two independent reasons to choose a larger wire gauge. The first is ampacity — the wire must not carry more current than its insulation can handle continuously without overheating. The second is voltage drop — the wire must not create so much resistance over its length that the voltage at the far end drops below the useful operating threshold. These two constraints are independent. A wire that passes the ampacity check might still fail the voltage drop check on a long run. A wire that passes the voltage drop check might fail the ampacity check if it runs through a hot conduit in direct summer sun. The correct wire size is the larger of the two requirements — whichever constraint is binding for your specific circuit.
Solar installations add a third layer of complexity that standard wire sizing tools miss entirely: NEC Article 690 treats solar source circuits as continuous loads, requiring that conductors be sized at 125% of the maximum circuit current. For a panel string with an Isc (short-circuit current) of 11.5A, the required conductor ampacity is 11.5 x 1.25 = 14.4A minimum. A conductor rated for exactly 14A is not sufficient. This derating applies to all solar source circuits and, per NEC 690.8(B), to charge controller output circuits as well.
NEC Table 310.15(B)(2)(a) requires that conductor ampacity be derated when the ambient temperature exceeds 86 degrees Fahrenheit (30 Celsius). Metal conduit mounted on a south-facing roof in Phoenix, Arizona, can easily reach 140 degrees Fahrenheit on a summer afternoon. A THWN-2 conductor in that conduit has its ampacity derated to 71% of its 90-degree-Celsius rating — a 12 AWG conductor that could otherwise carry 25A is limited to about 17.75A in that environment. Skipping this derating in a hot climate is a code violation and a potential fire hazard. Our calculator applies this derating automatically when you enter your actual ambient temperature.
Select your circuit type (source, output, battery, or AC), enter the current in amps, the one-way run length, the system voltage, the maximum ambient temperature, and your voltage drop tolerance. The calculator applies NEC 690.8 continuous-duty derating (x1.25) to find the minimum required ampacity, then looks up the AWG wire size that meets that derated ampacity after temperature correction. Separately, it calculates the minimum AWG required to keep voltage drop at or below your target percentage. The final recommendation is the larger of the two — the binding constraint. Results include the metric mm² equivalent, the actual voltage drop at the recommended gauge, and the voltage at the far end of the circuit.
What is NEC 690.8 and how does it affect solar wire sizing?+
NEC Article 690 covers photovoltaic power systems. Section 690.8 establishes how to size conductors for solar source circuits (panel strings to controller) and output circuits (controller to battery). The key rule: because solar panels are considered continuous power sources — they produce current for extended periods whenever the sun is shining — all solar conductors must be sized at 125% of the maximum circuit current. The maximum circuit current for a source circuit is the panel string’s short-circuit current (Isc). For a panel with an Isc of 11.5A, the conductor must be rated for at least 11.5 x 1.25 = 14.4A. This is more conservative than standard NEC continuous load derating (which is also 125%) and applies to both the source circuit and the charge controller output circuit per NEC 690.8(B).
What is voltage drop and why does it matter in solar systems?+
Voltage drop is the reduction in voltage that occurs when current flows through a wire that has electrical resistance. The drop equals current times resistance (V = I x R). In solar systems, voltage drop in the panel-to-controller wiring means the controller receives a lower voltage than the panels are producing — and the MPPT controller can only capture power equal to the voltage and current it actually receives, not the panel’s theoretical maximum. A 3% voltage drop in the panel wiring means losing 3% of potential solar production every day of the year, which compounds to a meaningful annual energy loss. The NEC does not set a maximum voltage drop for PV circuits, but industry best practice is 1-2% for source circuits and 2-3% for DC output circuits. For 12V systems especially, keep source circuit voltage drop below 2% — at 12V, a 3% drop is only 0.36 volts but represents significant wasted power at low panel voltage.
What is the difference between AWG and mm2 wire sizing?+
AWG (American Wire Gauge) is the standard wire size system used in the US. In AWG, smaller numbers mean larger wire — 4 AWG is larger than 12 AWG. Most US solar panels and charge controllers reference AWG for wire specifications. Millimeter squared (mm2) is the metric standard used in Europe and by many panel and component manufacturers who sell globally. Many solar panel datasheets sold in the US (especially those manufactured in China or Europe) specify wire sizes in mm2. Common conversions: 2.5 mm2 = approximately 14 AWG, 4 mm2 = approximately 12 AWG, 6 mm2 = approximately 10 AWG, 16 mm2 = approximately 6 AWG, 35 mm2 = approximately 2 AWG. Our calculator displays both the AWG recommendation and the metric equivalent to resolve this ambiguity.
What wire type should I use for solar panel wiring outdoors?+
USE-2 (Underground Service Entrance, dual-insulated, 90C) is the NEC-required wire type for solar source circuit wiring exposed to outdoor conditions and UV radiation. It is the same wire that connects via MC4 connectors at the panel junction boxes. USE-2 is rated for direct burial and outdoor exposure without conduit. THWN-2 (Thermoplastic Heat and Water-resistant Nylon-coated, 90C) is acceptable inside weatherproof conduit but must be protected from UV if run outdoors without conduit. Standard THHN wire is not UV-rated and will crack and fail within a few years of outdoor exposure. For underground conduit runs, THWN-2 inside schedule 40 PVC conduit is the most common approach. For roof surface wiring, USE-2 in a conduit strapped to the roof is the standard NEC-compliant installation.
How does temperature affect wire ampacity?+
Wire ampacity (the maximum current it can carry without exceeding its temperature rating) is specified at a standard ambient temperature of 86 degrees Fahrenheit (30 Celsius). When the ambient temperature is higher — as in metal conduit on a sun-exposed roof — the wire generates the same amount of heat at a given current, but the ambient heat makes it harder to dissipate that heat into the surroundings. The wire temperature rises higher for the same current, approaching and potentially exceeding the insulation’s temperature rating. NEC Table 310.15(B)(2)(a) provides mandatory derating factors for conductors in elevated ambient temperatures. At 104F (40C), a 90C-rated conductor must be derated to 91% of its rated ampacity. At 140F (60C), it must be derated to 71%. In Phoenix, Arizona, a south-facing metal conduit can easily reach 140F in summer, requiring significant derating. Always measure or estimate the actual conduit temperature, not just the air temperature.
Can I use aluminum wire for solar installations?+
Aluminum wire is allowed by the NEC for conductors 4 AWG and larger in most applications. Aluminum has lower conductivity than copper — an aluminum conductor must be approximately two AWG sizes larger to carry the same current as copper. 4 AWG aluminum carries about the same as 6 AWG copper. For large battery bank runs, feeder circuits, and panel array homerun cables, aluminum can be significantly cheaper than copper at large gauges. A 350 MCM copper conductor might be replaced by a 500 MCM aluminum conductor for the same ampacity. The key requirements for aluminum wiring: use aluminum-rated connectors and lugs (not copper-only lugs), apply anti-oxidant compound at all connections, and use AA-8000 series aluminum wire for smaller gauges. For most source circuits and small wire gauges (10 AWG and smaller), copper is standard and aluminum is not typically used.
What voltage drop should I target for each circuit?+
Recommended targets: Source circuit (panels to controller) — 1-2% for 12V systems, 2-3% for 24V and 48V systems. Lower is always better; the production loss from voltage drop accumulates over the life of the system. Output circuit (controller to battery) — 1-2%, same reasoning. Battery to inverter — 1-2% maximum; this is often a short run so voltage drop is usually controlled by the wire gauge needed for ampacity. AC output circuit (inverter to loads) — NEC 210.19(A) informational note suggests 3% as a general target with 5% total from the source to the furthest outlet. High loads like electric stoves and dryers should ideally have less than 3% drop on their dedicated circuits. For any DC circuit at 12V, voltage drop is critically important — 3% of 12V is only 0.36V, but it directly reduces the power available at the load.
How do I calculate voltage drop manually?+
Voltage drop for a DC circuit: V-drop = 2 x I x R x L / 1000. Where I = current in amps, R = wire resistance in ohms per 1,000 feet (from conductor resistance tables), L = one-way run length in feet, and the factor of 2 accounts for both the outgoing and return conductors. For a 12 AWG copper conductor (1.98 ohms per 1,000 feet) carrying 11.5A over a 20-foot one-way run: V-drop = 2 x 11.5 x 1.98 x 20 / 1000 = 0.91V. On a 12V system, that is 0.91/12 = 7.6% — far too high. For an AC circuit, use the same formula but with the AC system voltage. The percentage drop equals V-drop divided by the nominal system voltage. Most practical solar calculations use NEC Table 9 for AC conductor resistance or published copper resistance charts for DC circuits.
Do I need conduit for solar wiring?+
It depends on the installation location. USE-2 wire can be run exposed outdoors on a rooftop without conduit, as it is rated for direct UV exposure and weather. However, it must be secured every 4.5 feet and at all supports per NEC 690.31. In many jurisdictions and AHJ interpretations, source circuit wiring on a rooftop must be run in conduit. Wiring inside a building, through walls, or anywhere it could be subject to physical damage must be in conduit or cable. For underground runs, schedule 40 PVC conduit with THWN-2 wire is the most common approach. Check your local AHJ requirements — they can be more restrictive than the NEC minimum. In practice, running wiring in conduit even where not required protects it from physical damage, makes it easier to replace wire without disturbing the installation, and makes the installation look more professional and pass inspection more easily.
What is the difference between the source circuit and the output circuit?+
The source circuit is the wiring between the solar panels and the charge controller. It carries the panel’s DC output at whatever voltage the array is configured for (12V, 24V, 48V, or higher for grid-tie systems). The current in the source circuit equals the Isc of the panel string times the number of parallel strings. The output circuit is the wiring between the charge controller and the battery bank. It carries whatever current the controller is delivering to the battery, which is determined by the controller’s output amperage rating. The output current is typically higher than the source circuit current on a 12V system because the MPPT controller converts higher array voltage to lower battery voltage, increasing the current in the process (power is conserved: higher current x lower voltage = same power). Both circuits are treated as solar circuits under NEC 690 and must be sized at 125% of their maximum current.
How many conductors can I fit in a conduit?+
NEC Table 1 in Chapter 9 and the conduit fill tables establish maximum fill percentages: for 3 or more conductors, conduit must not be more than 40% filled. Common guidelines for THWN-2 wire in schedule 40 PVC conduit: 1/2 inch conduit holds up to 4 x 12 AWG or 3 x 10 AWG; 3/4 inch holds up to 7 x 12 AWG or 4 x 10 AWG or 3 x 8 AWG; 1 inch holds up to 11 x 12 AWG or 7 x 10 AWG or 5 x 8 AWG; 1.25 inch holds up to 4 x 4 AWG or 3 x 2 AWG. When multiple circuits share a conduit and the total exceeds 3 current-carrying conductors, NEC 310.15(C) requires additional ampacity derating: 4-6 conductors = 80% ampacity, 7-9 conductors = 70%, 10-20 conductors = 50%. This can force larger wire sizes when multiple circuits share a conduit run.
What size wire do I need for a 400W solar panel?+
It depends on the run length and system voltage, but for a typical 400W panel with Isc around 11.5A: a source circuit run of 10-15 feet to a 12V charge controller requires 10 AWG (to keep voltage drop under 3% and meet NEC derating). A run of 25-30 feet at 12V requires 8 AWG or larger just for voltage drop. A run of 20 feet to a 48V system can use 14 AWG comfortably. Use the calculator and switch to 48V voltage for the longer runs to see how dramatically the required wire size decreases. The 400W panel produces the same power at 48V — it just requires far less current (and therefore far smaller, cheaper wire) over the run from the panel to the controller.
What gauge wire should I use for a 12V to 120V inverter?+
For the DC input cable from battery to inverter: a 1,000W inverter at 12V draws up to 92A (1,000W / 12V / 0.90 efficiency). With 25% margin, plan for 115A. Minimum conductor: 1/0 AWG at 75C (rated 150A). For a 2,000W inverter: up to 185A, use 4/0 AWG. For a 3,000W inverter at 12V: up to 277A, use 300 MCM or consider switching to 24V or 48V to reduce current. For the AC output from inverter: a 1,000W inverter on 120V AC produces up to 8.3A AC (1,000W / 120V). Standard 12 AWG THWN-2 circuit wiring easily handles this. A 2,000W inverter produces up to 16.7A AC — use 10 AWG. Keep the DC battery-to-inverter cable as short as possible (under 18 inches ideally) and use 4/0 flexible welding cable for large inverters.
Why does my charge controller get hot at the output terminals?+
Heat at the controller output terminals almost always indicates undersized wiring or a loose connection — both are high-resistance points that generate heat when current flows through them. First, check the torque on all terminal connections — solar wire terminals frequently loosen from vibration and thermal cycling. Consult the controller manual for the specified torque (usually 20-25 inch-pounds for small terminals). Second, check that the wire gauge is adequate for the current. On an MPPT controller at 12V, the output current can be significantly higher than the input current from the panels. A 40A MPPT controller output at 12V carries 40A continuously — 14 AWG is completely inadequate. Use 8 AWG minimum for 30A controller output, 6 AWG for 40A, and 4 AWG for 60A controllers. Third, if the controller itself is hot (not just the terminals), check that it has adequate ventilation and is not in a confined space.
Where can I find the official NEC wire sizing tables?+
The
National Electrical Code (NFPA 70) contains the authoritative wire sizing tables including Table 310.15(B)(1) for ampacity of copper conductors, Table 310.15(B)(2)(a) for temperature derating factors, and Chapter 9 Annex B and C for conduit fill calculations. NEC Article 690 covers all photovoltaic system wiring requirements. The
US Department of Energy Solar Energy Technologies Office and the Solar Energy Industries Association (SEIA) publish installation guides that reference these tables in the context of solar applications. Most state electrical inspection offices accept the current NEC edition plus local amendments for permit applications.
Can I splice solar wire or does it need to be continuous?+
Splices in solar source circuit wiring must be made in weatherproof junction boxes accessible for inspection — they cannot be buried in walls or enclosed in conduit without a junction box. MC4 connector pairs at the panel or in the junction box are the standard field-installable connection for USE-2 wire. Indoor splices in the output circuit or AC wiring can be made in standard NEC-compliant junction boxes. All splices must maintain the wire’s ampacity — the splice connection must be rated for at least the current the conductor is rated for. For the battery-to-inverter DC cable, no splices are preferred — a single continuous run minimizes resistance and connection points. If a splice is unavoidable, use a properly rated battery lug or mechanical connector with the correct torque specifications, not wire nuts, which are not rated for high DC current applications and corrode in battery environments.
Legal Disclaimer and Editorial Transparency
The Solar Wire Size Calculator on USCalculators.com provides estimates for educational and planning purposes only. Wire ampacity values are based on NEC Table 310.15(B)(1) for copper conductors with 90C insulation in free air at standard ambient temperature. Temperature derating factors are from NEC Table 310.15(B)(2)(a). Actual required wire sizes may differ based on conduit fill derating, bundling derating, local AHJ requirements, and specific installation conditions not captured in this calculator.
All permanent solar electrical installations must comply with the current edition of NFPA 70 (National Electrical Code), applicable state codes, and local AHJ requirements. Permits are required for permanent installations in most US jurisdictions. Wire sizing shown does not substitute for review by a licensed electrician. See NFPA.org for the National Electrical Code.
Editorial policy: USCalculators.com is an independent educational resource. No affiliate or commercial relationship exists with any wire manufacturer or solar component supplier.