Solar Voltage Drop Calculator

Solar voltage drop: formula, examples, and common mistakes

The formula

A PV DC circuit is purely resistive; there is no phase angle, power factor, or reactance to account for. The drop on the two-wire run is:

VD = 2 × R × I × L ÷ 1000
Power loss = VD × I

Where R is the DC resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 (uncoated, 75°C), I is the operating current in amps, and L is the one-way distance in feet. The factor of 2 is the round trip: the positive and return conductors each carry full current and each drop voltage. Power loss is the resistive heat in the conductors (VD × I, equal to I² × R), the production that never reaches the inverter. Unlike the AC calculation, there is no √3 term for three-phase and no K-factor approximation; R comes straight off Table 8, and AC Table 9 (which adds reactance and skin-effect resistance) does not apply to DC.

Worked example

A 48 V off-grid array feeds a charge controller 40 ft away at 20 A operating current on #8 AWG copper. Find the voltage drop.

R(#8 Cu) = 0.778 Ω/1000 ft
VD = 2 × 0.778 × 20 × 40 ÷ 1000 = 1.24 V
VD% = 1.24 ÷ 48 = 2.59%
Power loss = 1.24 × 20 = 24.9 W
Voltage at load = 48 − 1.24 = 46.76 V

At 2.59% this clears the 3% maximum but misses the 2% target, and nearly 25 W is burning in the wire. Stepping up to #6 AWG copper (R = 0.491 Ω/1000 ft) brings the drop to 0.79 V, or 1.64%, and cuts the loss to about 16 W. The calculator runs this search automatically and reports the next size that lands under 2%.

Common mistakes

• Using Isc × 1.25 for the drop. That sizes ampacity and the overcurrent device, not voltage drop. Evaluate drop at the operating current (Imp, or Imp × parallel strings).
• Applying an AC formula to the DC side. The √3 multiplier and the K-factor are AC constructs. On the DC array conductors there is no phase or power factor; use the resistive form above.
• Entering round-trip distance. The ×2 round trip is already in the formula. Enter the one-way run; entering total conductor length roughly doubles the reported drop.
• Sizing by volts instead of percent on low-voltage banks. A 1 V drop is trivial at 400 V and severe at 24 V. Always evaluate against percentage, and tighten the target on low-voltage off-grid systems.

NEC references

NEC Chapter 9, Table 8 supplies DC resistance and circular mils per conductor size. NEC 690.8 governs PV circuit current and conductor sizing and is the basis for using the operating current (not the Isc-based sizing current) when evaluating drop. The 3% and 5% figures in NEC 210.19(A)(1) and 215.2(A)(1) are informational-note recommendations for AC branch circuits and feeders; the NEC does not impose a voltage-drop limit on PV circuits, so the 2%/3% targets used here are design practice for efficiency. Equipment listing and instructions remain enforceable under NEC 110.3(B).

Frequently asked questions

What voltage drop is acceptable for a solar PV system?

The common solar design targets are no more than 2% on the DC source and output conductors and no more than 3% total from the array to the inverter or charge controller. These are efficiency targets, not NEC requirements: the NEC does not impose a voltage-drop limit on PV circuits, and the 3%/5% figures in 210.19 and 215.2 are non-mandatory informational notes written for AC branch circuits and feeders. Every percent lost in the DC wiring is production you paid for in modules and never sell or store, so PV designers hold a tighter budget than general premises wiring. Equipment manufacturer instructions can set their own limit and are enforceable under NEC 110.3(B).

Which current do I use, Imp or Isc?

Use the operating current, Imp, for a single source circuit, or Imp multiplied by the number of parallel strings for a combined PV output circuit. Do not use the Isc times 1.25 times 1.25 value from NEC 690.8. That inflated figure sizes the conductor ampacity and overcurrent device; it is the maximum the circuit could ever carry, not the current the array actually delivers. Voltage drop must be evaluated at the real operating current, so using the 690.8 sizing current would overstate the drop and push you to oversize the wire unnecessarily.

Why is voltage drop a bigger deal on low-voltage off-grid solar?

Voltage drop is an absolute number of volts; what changes between systems is how large that drop is as a percentage of the source. A 2 V drop is 0.5% on a 400 V grid-tie string and a crippling 4.2% on a 48 V battery bank. Off-grid 12 V, 24 V, and 48 V arrays run at high current for their power, and a small percentage is a large absolute drop, so they reach the 2% target at far shorter distances and force much larger conductors than a high-voltage grid-tie string. Size low-voltage DC runs by percentage, keep the runs short, and step up the conductor early.

Should I enter one-way or total circuit length?

Enter the one-way distance, the conductor length from the array to the inverter or charge controller. The calculator multiplies by 2 for the round trip, since the positive and return conductors each carry full current and drop voltage. Entering the round-trip length double-counts and roughly doubles the reported drop. This is the most common input error on any voltage-drop tool. If your PV run is 40 ft from the roof to the inverter, enter 40, not 80.

Does this handle temperature correction for hot rooftop conduit?

No. It holds the published 75°C DC resistance from NEC Chapter 9, Table 8. PV conductors in conduit in direct sun run hotter than 75°C, and hotter copper or aluminum has higher resistance, so the real drop trends slightly above the reported figure. Holding 75°C is therefore conservative for a design target: it will not understate the drop by more than the temperature term, and correcting resistance downward is the unsafe direction for a tool that does not measure conductor temperature. For a precise high-temperature analysis, apply the Table 8 Note 2 correction to the resistance separately.

Why doesn't this use the K-factor or √3 formula?

Those terms belong to AC. The √3 multiplier accounts for three-phase line-to-line relationships, and the K-factor is an approximation of AC resistance per circular mil. A PV DC circuit has no phase, no power factor, and no reactance, so the calculation reduces to pure resistance: VD = 2 × R × I × L ÷ 1000, with R taken directly from NEC Chapter 9, Table 8 DC resistance. The AC output circuit from a string inverter to the panel is a separate AC calculation; use the AC Voltage Drop Calculator for that leg.

Related tools

• DC Voltage Drop Calculator: general two-wire DC drop for battery banks, telecom, and automotive, against the NEC 3%/5% notes.
• DC Wire Size Calculator: sizes a DC conductor for both ampacity and a target voltage drop at once.
• Wire Ampacity Calculator: allowable conductor ampacity with temperature and conduit-fill adjustments per NEC 310.16.
• AC Voltage Drop Calculator: for the AC output circuit from a string inverter to the panel.

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