Solar Wire Voltage Drop Calculator
Enter string voltage, current, run length and conductor to get voltage drop, loss and the smallest wire for your drop limit.
Step-by-step
- Enter valid values to begin.
Resistance comes from NEC Chapter 9 Table 8 (uncoated stranded conductors at 75 °C) for AWG sizes, and from IEC 60228 class 2 maximum DC resistance at 20 °C for mm² sizes, then corrected to the entered temperature with α = 0.00323 (Cu) or 0.00330 (Al) per °C for AWG and 0.00393 / 0.00403 for mm². Aluminium mm² values are estimated as 1.64 × copper. Round-trip length is 2 × the one-way run. DC resistance only; ampacity, conduit fill, ambient derating and PV wire listings are separate checks (verify the NEC edition, Article 690 and 310).
Solar Wire Voltage Drop: What the Long Run Costs You
Core Engineering Principles
A PV string is a low-current, high-voltage circuit, but the wire still has resistance, and every ohm costs us twice because the current travels out on the positive conductor and back on the negative. That is why the formula uses the round-trip length of 2 × L. A 300 ft one-way run is 600 ft of copper. The drop is simply I × R, and it is deducted straight from the string voltage the inverter or MPPT sees. The power lost is I² × R, wasted as heat.
We usually aim for 2% or less on the DC side. Resistance climbs with temperature, about 0.4% per °C for copper, so a conductor at 75 °C carries roughly 20% more resistance than the same wire at 20 °C. Aluminium has about 1.64 times the resistance of copper at the same size. A higher string voltage cuts current and percentage drop for the same power, which is why long ground-mount runs favour longer strings. We size for ampacity first, then check drop, and take the larger conductor.
Ploss = I² × Rloop • R(T) = R75 × [1 + α (T − 75)], αCu = 0.00323
NEC & Standard References
NEC Article 690 covers photovoltaic systems. Section 690.8 sets the maximum circuit current at 125% of Isc and requires conductor ampacity of at least 125% of that value, which is about 156% of Isc. NEC Chapter 9, Table 8 lists DC resistance of stranded conductors at 75 °C, and Chapter 9 Note 2 gives the temperature coefficients. NEC Table 310.16 gives ampacity; voltage drop is a performance target, not a safety limit. IEC 60228 gives DC resistance for mm² sizes. Check the adopted edition.1. Table 8 resistance for 6 AWG: 0.491 Ω per 1000 ft.
2. Round trip = 2 × 300 = 600 ft, so R = 0.491 × 0.6 = 0.2946 Ω.
3. VD = 9.6 × 0.2946 = 2.83 V, which is 0.94% of 300 V.
4. Power loss = 9.6² × 0.2946 = 27.2 W; voltage at the load = 297.2 V.
5. Largest allowed loop resistance = 0.02 × 300 / 9.6 = 0.625 Ω, so the smallest conductor is 8 AWG (10 mm²).
6. Ampacity check: 1.25 × 1.25 × 10.2 = 15.9 A minimum after derating.
- Count both conductors. Forgetting the return leg understates the drop by half and the loss by half.
- Don’t size by drop alone. A wire that meets 2% can still fail the 690.8 ampacity check after derating.
- Hot roofs matter. Sun-heated conduit can reach 70–90 °C, adding real resistance.
- Loose terminations add drop. A hot MC4 connector is resistance you did not budget and a fire starter.
- Voltage drop lowers MPPT headroom. On cold mornings Voc is highest, so stay inside the inverter’s maximum input voltage.