DC wiring in a solar system operates under different rules than the AC wiring most electricians are accustomed to. DC arcs do not self-extinguish at zero crossings the way AC arcs do, which means loose connections and undersized wire are far more dangerous on the DC side. Voltage drop matters more because every watt lost in the wiring is a watt your panels produced but your battery never received. And NEC Article 690 adds solar-specific requirements for conductor sizing, overcurrent protection, disconnects, and labeling that go beyond standard residential electrical work.
This guide covers DC wire sizing from panels to charge controller and charge controller to battery, conduit and connector selection, grounding requirements, overcurrent protection, NEC 690 compliance, and the labeling requirements that many DIY installers overlook until inspection day.
Why Voltage Drop Matters More on DC Systems
A solar panel producing 40V at 10A delivers 400W. If voltage drop in the wiring consumes 2V, the charge controller receives 38V at 10A = 380W. You just lost 5% of your production permanently. Over a year, a 5% wiring loss on a 5 kW system wastes 300–500 kWh — energy your panels generated but your batteries never stored.
The target for DC solar wiring is 2–3% voltage drop. Some installers aim for 1.5% on long runs. The formula is the same as for AC: VD = (2 × L × I × ρ) / A, where L is one-way length in meters, I is current in amps, ρ is resistivity (0.0175 ohm·mm²/m for copper), and A is the conductor cross-section in mm². In AWG terms: VD = (2 × K × I × D) / CM with K = 12.9 for copper.
DC voltage drop is a bigger deal than AC voltage drop for two reasons. First, DC system voltages are lower — a 48V battery bank means 2% drop is less than 1 volt, which sounds small but represents real power loss on high-current circuits. Second, MPPT charge controllers have a minimum input voltage to operate. If voltage drop pushes the panel voltage below the controller's MPPT window, the controller either reduces output or shuts down entirely.
The practical fix is straightforward: use adequately sized wire and keep wire runs as short as possible. Place the charge controller close to the battery bank (where the high-current, low-voltage DC wiring is shortest) and accept the longer run from panels to controller at higher voltage where the same percentage drop loses fewer watts.
VD = (2 × K × I × D) / CM
K = 12.9 (copper), I = current (A), D = one-way distance (ft), CM = circular mils
Target: 2–3% maximum
Example: 30A at 50 ft on 6 AWG (26,240 CM):
VD = (2 × 12.9 × 30 × 50) / 26,240 = 1.48V (3.1% at 48V)
DC Wire Sizing Calculator
Calculate wire gauge and voltage drop for 12V, 24V, and 48V DC circuits. Solar PV, RV, marine, and automotive applications with NEC ampacity verification and cost estimates.
Wire Sizing for Solar Circuits
There are three DC circuits in a typical solar installation, each with different sizing requirements: panel to combiner box (if used), combiner to charge controller, and charge controller to battery bank.
Panel to charge controller: Current is determined by the number of parallel strings. Each string carries the panel short-circuit current (Isc). NEC 690.8 requires sizing conductors at 125% of Isc. If each panel has Isc = 11A and you have 2 parallel strings, the circuit carries 22A and must be sized for 27.5A minimum. For USE-2 or PV Wire rated at 90°C, 10 AWG handles 40A in free air. This circuit can use smaller wire if the panel voltage is high (longer strings = higher voltage = lower current for the same power).
Charge controller to battery: This is usually the highest-current circuit because the controller steps down voltage and steps up current. A 3,000W array charging a 48V battery bank pushes up to 62.5A. At 125% sizing: 78A minimum conductor ampacity. That requires 4 AWG copper minimum, and you should go larger based on voltage drop if the run exceeds 10 feet.
Use only wire types rated for the application. PV Wire (formerly USE-2/RHW-2) is required for exposed rooftop wiring. THWN-2 is acceptable in conduit. Battery cables should be fine-stranded welding cable or battery cable rated for the amperage. Solid conductors are not permitted for DC connections that experience thermal cycling.
Connectors, Conduit, and Junction Boxes
MC4 connectors are the industry standard for panel-to-panel and panel-to-home-run connections. They are weatherproof, rated for 1000V DC, and tool-less to connect (but require a special tool to disconnect). Use only MC4-compatible connectors from the same manufacturer — mixing brands can create poor contact and arc faults. Crimp MC4 connectors with the proper die set; improvised crimps are the leading cause of rooftop DC fires.
Conduit is required for DC wiring that runs along building surfaces, through attics, or underground. Use metallic conduit (EMT or rigid) where required by local code, or PVC Schedule 40 for underground runs. NEC 690.31 specifies that PV source circuits inside buildings must be in metallic raceway or have a rapid shutdown system. This requirement exists because DC wiring inside a structure cannot be de-energized by the utility disconnect — the panels produce voltage whenever the sun is shining.
Junction boxes for DC circuits must be rated for the voltage and accessible for inspection. Label every junction box with "WARNING: SOLAR CIRCUIT — ENERGIZED IN DAYLIGHT" per NEC 690.56. This labeling is not optional and is one of the most common inspection failures on DIY installations.
For battery connections, use properly sized ring terminals or bus bars with stainless steel hardware torqued to manufacturer specifications. Battery terminal connections experience thermal cycling and vibration. A loose battery terminal at 60A+ DC generates heat, corrodes, and can ignite hydrogen gas vented by lead-acid batteries.
Grounding and Bonding for Solar Systems
NEC 690 requires both equipment grounding and system grounding for solar installations. Equipment grounding bonds all metal frames, racking, junction boxes, and equipment enclosures to the ground bus. This provides a fault current path that trips the overcurrent device if a hot conductor contacts the frame. Without equipment grounding, a ground fault energizes the racking and creates a shock and fire hazard.
System grounding (grounding one current-carrying conductor) is required for some system configurations. In a grounded system, one DC conductor is bonded to ground at a single point, typically through a ground-fault protection device (GFPD). Ungrounded systems are permitted under NEC 690.35 with specific requirements for disconnect and fault detection equipment.
Panel frame grounding uses WEEB (washer, equipment grounding bonding) clips or lay-in lugs that bond each panel frame to the racking. The racking bonds to a ground bus via a bare or green equipment grounding conductor sized per NEC 250.122. For most residential systems, 6 AWG copper equipment grounding conductor is sufficient.
The grounding electrode system connects to ground rods, a concrete-encased electrode (Ufer ground), or the building's existing grounding electrode system. For a standalone solar ground mount, drive two ground rods at least 6 feet apart and bond them to the equipment ground bus.
Overcurrent Protection for DC Circuits
Every DC circuit in a solar system needs overcurrent protection (fuses or breakers) sized to protect the conductors. NEC 690.9 requires overcurrent devices on all PV source and output circuits. The device rating must be at least 125% of Isc (matching the conductor sizing requirement) and must not exceed the conductor ampacity.
For panel strings, use DC-rated fuses in a combiner box. Each string gets its own fuse. The fuse rating is typically 15A or 20A for residential panels (based on 125% of Isc). Use only fuses rated for the system voltage — AC fuses cannot safely interrupt DC arcs. A 600V DC fuse for a 48V system has massive voltage margin, which is correct. The fuse must be able to interrupt a fault at the maximum system voltage.
Between the charge controller and battery, use a DC-rated breaker or fuse sized for the maximum charge current at 125%. A 60A charge controller on a 48V system needs a minimum 75A DC breaker. DC breakers are different from AC breakers — they are specifically designed to interrupt DC arcs, which do not naturally extinguish at zero crossings. Using an AC breaker on a DC circuit is a code violation and a fire hazard.
Battery bank fusing protects against short circuits in the battery cables. A short circuit on a large lead-acid bank can deliver thousands of amps — enough to melt cables, cause fires, and create explosive arc flash conditions. Install a Class T fuse or DC-rated breaker as close to the battery positive terminal as possible. This fuse must interrupt the maximum available fault current from the battery bank, which can be 5,000–10,000A for large banks.
NEC 690 Labeling Requirements
NEC 690.56 requires permanent, weatherproof labels on multiple components of a solar installation. Missing labels are the most common inspection failure on DIY solar installations because they are easy to overlook and inspectors check them carefully.
Required labels include: the main service panel ("WARNING: SOLAR ELECTRIC SYSTEM — MULTIPLE POWER SOURCES"), every DC disconnect, every junction box containing DC conductors, the utility meter (so line workers know the system has a second power source), and the inverter. Labels must be reflective, weatherproof, and use permanent adhesive.
NEC 690.53 requires a label at the PV disconnect listing: DC operating current, DC operating voltage, DC maximum circuit current, DC maximum circuit voltage, and maximum available short-circuit current. This information must be calculated from your specific panel specs and string configuration, not generic values.
Rapid shutdown labeling (NEC 690.12) is required for systems on buildings. The label at the main disconnect must indicate the rapid shutdown type and the location of the initiation device. Rapid shutdown reduces rooftop DC voltage to 30V within 30 seconds of activation, protecting firefighters who may need to access the roof.