For a standard 5kW 48V solar power system, the direct answer for your main DC battery-to-inverter run is 2/0 AWG copper THHN wire in conduit, protected by a 150A Class T fuse. For the solar array strings, use 10 AWG PV wire wired in series to keep current low and voltage high. This guide walks through the exact sizing math, system architecture, and code-compliant decision paths to wire this system without a second trip to the electrical supply house.
The System Block: Source to Load Wiring Architecture
Before pulling any wire, you need a clear mental model of the power flow. In a DC-coupled hybrid or off-grid system, the electrical wiring for solar panels follows a strict source-to-load sequence. Every transition point requires a specific overcurrent protective device (OCPD) and disconnect.
- PV Array (Source): Solar panels wired in series strings output high-voltage, low-current DC.
- PV Disconnect & MPPT Charge Controller: Strings feed into a DC disconnect, then the MPPT controller, which steps the high PV voltage down to the battery bank charging voltage.
- Battery Busbars & Bank: The controller outputs to a common busbar. The battery bank connects to this same busbar. Code requirement: A fuse or breaker must be installed on the positive ungrounded conductor between the busbar and the battery bank.
- Inverter/Charger: Draws 48V DC from the busbars to invert into 120/240V AC. It also manages AC charging from a generator or grid.
- Critical Loads Subpanel (Load): The inverter's AC output feeds a dedicated subpanel. Neutral and ground are bonded only at the main service entrance; the inverter subpanel must have an isolated neutral bar and a dedicated ground bar bonded to the grounding electrode system.
Solar Array Wiring: Series vs. Parallel Consequences
How you wire your solar panels dictates your wire gauge, conduit fill, and charge controller selection. The choice between series and parallel wiring fundamentally alters the Voltage (V) and Amperage (Ah/A) output.
The Math: 4x 400W Panels (Vmp 40V, Imp 10A)
| Configuration | Voltage (Vmp) | Current (Imp) | Total Power | Wire Size Required |
|---|---|---|---|---|
| 4 in Series | 160V | 10A | 1600W | 10 AWG PV Wire |
| 4 in Parallel | 40V | 40A | 1600W | 6 AWG PV Wire (or dual 10 AWG runs) |
The Verdict: Always wire solar panels in series (or series-parallel strings) to maximize voltage and minimize current. High voltage allows the MPPT charge controller to operate efficiently while keeping the DC current low. Low current means you can use standard 10 AWG PV wire (rated for 600V/1000V and UV resistant) instead of expensive, stiff 6 AWG or 4 AWG wire required for high-current parallel arrays. Furthermore, modern MPPT controllers (like the Victron SmartSolar 150/35) require the PV input voltage to be significantly higher than the battery voltage to begin charging; a parallel 40V array will struggle to charge a 48V battery bank.
Battery Bank Sizing: C-Rates, DoD, and the Peukert Penalty
For a 5kW inverter, your battery bank must handle massive DC current surges. This is where battery chemistry and discharge limits dictate your wiring and bank size.
Charge and Discharge Limits
- Depth of Discharge (DoD): LiFePO4 batteries can safely be discharged to 80-90% DoD daily. Lead-acid (AGM/Gel) should not exceed 50% DoD without severely shortening cycle life.
- C-Rate (Discharge Limit): The C-rate defines the maximum safe continuous draw. A 100Ah battery with a 1C rating can output 100A continuously. A 0.5C rating limits it to 50A.
The Peukert Effect: Why We Mandate Lithium
Peukert's Law states that the faster you draw current from a battery, the less total capacity it delivers. This is devastating for lead-acid batteries but negligible for lithium.
Let's look at a 48V 100Ah battery bank powering a 4000W load (drawing ~83A):
- Lead-Acid (Peukert exponent k ≈ 1.3): At an 83A draw, a 100Ah lead-acid battery will actually only yield about 55Ah of usable capacity before voltage collapse.
- LiFePO4 (Peukert exponent k ≈ 1.05): At an 83A draw, a 100Ah LiFePO4 battery yields roughly 95Ah of usable capacity.
Because of the Peukert penalty, sizing a lead-acid bank for a 5kW inverter requires massive oversizing and parallel runs of 4/0 AWG cable. LiFePO4 eliminates this penalty, allowing us to use a single 48V 100Ah server-rack battery and manageable wire gauges.
While LiFePO4 is inherently safer than NMC lithium-ion, thermal runaway can still occur if the Battery Management System (BMS) fails or if cells become severely unbalanced. Never parallel mismatched cells or batteries of different ages/chemistries. When wiring multiple 48V server-rack batteries in parallel, they must be the exact same model, firmware version, and state of charge (SoC) at the time of connection. Always use a BMS with cell-level balancing, charge exclusively with a lithium-profile MPPT/charger, and install a secondary contactor or thermal fuse for catastrophic over-current protection.
Inverter Sizing and DC Wire Gauge Calculation
The most critical electrical wiring for solar panels happens between the battery busbar and the inverter. A 5000W inverter pulling from a 48V bank generates immense heat if the wire is undersized. We size this wire using NEC Article 690 and Article 310 ampacity tables.
Step-by-Step Sizing Math
- Base Current: 5000W continuous load / 48V nominal battery = 104.1A.
- Inverter Efficiency Factor: Inverters are not 100% efficient. Assuming 90% efficiency at peak load, the DC draw increases. 104.1A / 0.90 = 115.6A.
- Low Voltage Cutoff Factor: As the battery drains, voltage drops to ~44V. 5000W / 44V = 113.6A (efficiency adjusted: 113.6 / 0.90 = 126.2A). We use the worst-case low-voltage draw: 126.2A.
- NEC Continuous Load Multiplier: NEC requires conductors to be sized at 125% of the continuous load. 126.2A * 1.25 = 157.75A minimum ampacity required.
Selecting the Wire and Fuse
Looking at the NEC Table 310.16 (75°C column, as most inverter terminals are rated for 75°C):
- 1/0 AWG THHN: 150A (Too small, fails the 157.75A requirement).
- 2/0 AWG THHN: 175A (Passes. Provides a safe margin).
Voltage Drop Check: 2/0 AWG copper has a resistance of 0.194 ohms per 1000 ft. For a 5-foot run (10 ft total round trip), the voltage drop at 126A is: (126A * 0.000194 ohms/ft * 10 ft) = 0.24V. This is a 0.5% drop on a 48V system, well below the recommended 1% maximum for DC battery-inverter runs.
OCPD Selection: The fuse must protect the wire. 2/0 AWG is rated for 175A. We select a 150A Class T fuse. Class T fuses are mandatory here because they have a high interrupting capacity (AIC) of 20,000A, which is necessary to safely clear a dead short across a massive LiFePO4 battery bank that can dump thousands of amps instantaneously.
Decision Tree: Picking Your Exact Wire and Breaker
Use this decision matrix to finalize your Bill of Materials (BOM) based on your target continuous AC load. Do not guess; follow the branch to your exact system voltage and wire size.
| Target Continuous AC Load | System Voltage | Max DC Current (w/ 1.25x NEC factor) | Required Copper Wire (THHN, 75°C) | Battery Fuse (Class T) |
|---|---|---|---|---|
| Up to 2000W | 24V DC | 115A | 1/0 AWG | 110A |
| 2000W - 6000W | 48V DC | 158A | 2/0 AWG | 150A |
| 6000W - 10000W | 48V DC | 260A | 4/0 AWG | 250A |
| > 10000W | Parallel Inverters | N/A (Split across units) | 2x 2/0 AWG (One per inverter) | 2x 150A |
The Final Concrete Pick for a 5kW System
If you are building the 5kW off-grid or hybrid system outlined in this guide, stop researching and order these exact components to ensure NEC compliance, safety, and optimal efficiency:
- Inverter/Charger: Victron MultiPlus-II 48/5000/70-50 (48V, 5000VA, 70A charger).
- Battery Bank: Single or Parallel 48V 100Ah LiFePO4 Server Rack Batteries (e.g., EG4 or SOK) with integrated 100A BMS.
- Battery-to-Inverter Wire: 2/0 AWG Copper THHN (Red and Black), kept under 5 feet in length, routed in 1.25-inch flexible metallic conduit.
- Battery Fuse: 150A Class T Fuse and block (installed within 7 inches of the battery positive terminal).
- PV Wire: 10 AWG UV-rated PV wire (Black and Red) with MC4 connectors for series-wired solar strings.
- DC Disconnect: 600V 30A DC disconnect switch between the PV array and the MPPT controller.
By strictly adhering to the 48V architecture and 2/0 AWG wire sizing, you eliminate the voltage drop issues that plague 12V/24V DIY solar builds, ensure your breakers trip correctly under fault conditions, and build a system capable of running heavy inductive loads like well pumps and compressor fridges without tripping the inverter's low-voltage cutoff.
