DC Solar Cable Calculator
Enter system voltage, current and run length to get the smallest cable that passes both ampacity and voltage drop.
Step-by-step
- Enter valid values to begin.
Voltage drop uses VD = 2 × K × I × L / CM, the same formula used for DC and single-phase AC. Ampacity uses NEC Table 310.16 with the ambient correction from 310.15(B). PV wire and exposed rooftop conditions (up to 70°C+) need the 690.31 adjustments — use the Cable Size calculator for those.
DC Solar Cable Sizing: Why 12 Volts Is Brutal on Wire
Core Engineering Principles
This is where voltage drop bites hardest. On a 12 V system, a 3% drop is only 0.36 V, and a battery cable that loses half a volt is running at 4% loss while the inverter sees 11.5 V and starts alarming. Because the current at 12 V is four times what it would be at 48 V for the same watts, the same wire loses sixteen times the power. That’s why every serious off-grid install is 24 V or 48 V and why the charge controller run to the battery bank is always the shortest cable you can manage.
The formula is the same one used for AC single-phase: VD = 2 × K × I × L / CM, with L as the one-way distance. For solar there are two checks. Voltage drop usually controls on short, high-current runs like the battery cable, and ampacity usually controls on long, higher-voltage PV strings. The page runs both and tells you which one sets the size. For PV source circuits the NEC multiplies the short-circuit current by 125% twice, 1.25 × 1.25 = 1.56, because the irradiance can exceed the rating and the circuit is continuous.
PV source circuit: Idesign = Isc × 1.25 × 1.25 • Battery / controller: I × 1.25 (continuous)
NEC & Standard References
NEC 690.8(A) sets the maximum PV circuit current as 125% of Isc, and 690.8(B) applies another 125% for conductor and overcurrent sizing, so the conductors must carry 156% of Isc. 690.31 covers PV wiring methods; exposed single-conductor PV wire and USE-2 are rated 90°C wet. 310.16 gives ampacities and 310.15(B) gives the ambient correction. Sunlit rooftop raceways run much hotter than the air, so use a higher ambient. Article 706 covers energy storage and battery cable sizing, and the manufacturer’s installation manual often specifies the cable length and size for the inverter. The 3% voltage drop value is only a recommendation in 210.19(A) IN 4, but many inverter and controller manuals require 1–2%.1. Design current: 50 × 1.25 = 62.5 A. Ampacity: 6 AWG at 75°C = 65 A, so 6 AWG passes.
2. Allowed drop: 24 × 3% = 0.72 V.
3. Required area: 2 × 12.9 × 50 × 15 / 0.72 = 26,875 CM. 6 AWG is 26,240, just short, so voltage drop needs 4 AWG (41,740 CM).
4. Actual drop on 4 AWG = 19,350 / 41,740 = 0.46 V (1.93%).
Ampacity said 6 AWG, voltage drop said 4 AWG. Drop controls.
- Get the length right. It’s the one-way distance from the controller to the battery.
- Use a fuse or breaker rated for DC. AC breakers can’t break DC arcs reliably. Look for a DC rating of at least the system voltage.
- Crimps matter. Heavy battery cables need hydraulic or properly rated lugs and heat-shrink; a poor crimp is the usual cause of melted battery terminals.
- Don’t use cheap copper-clad wire. It carries less than the table assumes. Use real copper, preferably fine-stranded.
- Hot roof, hot wire. Rooftop PV wire runs at 60–70°C. Use the ambient field and check the 90°C column.