Voltage drop in solar circuits and how to limit it

Every foot of conductor has resistance, and resistance turns a slice of your solar production into waste heat before it ever reaches the panel. That loss is voltage drop, and while the NEC treats it mostly as a recommendation, controlling it is the difference between a system that delivers its rated energy and one that quietly bleeds a few percent all day, every day.

Is it actually required?

The NEC addresses voltage drop primarily through informational notes (in Article 210 and elsewhere) suggesting a 3% drop on a branch or feeder and 5% total from service to load. These notes are recommendations, not mandatory rules. However, many AHJs, utilities, and installers enforce a stricter 2% target on PV circuits as good design practice, and some jurisdictions make it a local requirement. Confirm what your AHJ enforces.

Why 2% instead of 3%

Solar is unusual: the circuit runs near capacity for hours every day, so the energy lost to voltage drop accumulates in a way it doesn't for an intermittent load. A 2% target on the longest run keeps lifetime energy loss modest and leaves margin for the AC side, where drop between the inverter and the point of interconnection also eats production.

The DC side: use Vmp, not Voc

For DC voltage-drop math, use the maximum power voltage, Vmp, at the maximum power current, Imp — not the open-circuit Voc. Voc is the no-current condition where no power flows and no meaningful drop occurs. The array actually operates at Vmp/Imp, so that's the operating point where the drop matters for energy.

The calculation

The single-phase voltage-drop formula is:

VD = 2 × K × I × L ÷ CM

The leading 2 accounts for the current going out and returning. Divide the resulting VD by the circuit voltage to get the percentage.

Example: a 40 A AC inverter output on #8 copper (16,510 CM) over a 90 ft one-way run at 240 V: VD = (2 × 12.9 × 40 × 90) ÷ 16,510 = 5.6 V, or 2.3% of 240 V. Upsizing to #6 copper (26,240 CM) drops it to about 1.5% — a clean pass under a 2% target.

Three ways to limit it

• Upsize the conductor — the most common fix; each size up roughly cuts the drop proportionally to the circular-mil increase.
• Shorten the run — relocate the inverter or combiner closer to the array or service.
• Raise the voltage — higher string voltage (within 690.7 limits) means lower current for the same power, and drop scales with current.

Don't let it fight ampacity

Remember that the conductor you pick for voltage drop still has to satisfy ampacity and termination limits — sometimes voltage drop drives you larger than ampacity alone would. Check the final size against the 310.16 ampacity tables and the 110.14 termination cap so the conductor passes every test, not just the drop.

OneLine Studio is a design aid, not a permitting authority. Confirm the enforced code and any local voltage-drop amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the design before you file or energize.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.