Voltage Drop Limits by Application
| Application | Max Recommended Drop | Why This Limit |
|---|---|---|
| NEC feeder circuits | 3% | NEC 210.19(A) Informational Note — recommended, not mandatory |
| NEC total (feeder + branch) | 5% | Combined drop from panel to outlet should not exceed 5% |
| Solar panel to charge controller | 2-3% | Every lost volt reduces charging current and system efficiency |
| Battery to inverter (DC) | 2-3% | Low-voltage DC systems are extremely sensitive to drop |
| Well pump or motor feeder | 3% | Low voltage causes motor overheating and reduced starting torque |
| Lighting circuits | 3% | Visible dimming at the far end of long runs |
| EV charger circuit | 3% | EVSE may reduce charge rate if supply voltage drops too low |
The NEC 3% and 5% values are recommendations, not code requirements. But they exist for good reason. A motor fed through a 5%+ drop circuit gets less voltage than it was designed for. It draws more current to compensate, overheats, and fails early. Lights dim noticeably. Electronics may malfunction or refuse to operate.
How to Calculate Voltage Drop
- Identify the current (amps) and distance (feet). Use the actual expected load current, not the breaker rating. If the circuit will carry 20A, use 20A — not 30A just because you have a 30A breaker. For the distance, measure the one-way run from the source (panel, battery, charge controller) to the load. The calculator doubles it internally because current must flow out and back.
- Pick your maximum drop percentage. 3% is standard for most applications. Use 2% for critical or low-voltage DC circuits where every tenth of a volt matters — like solar panel runs and battery connections.
- Read the recommended gauge from the calculator. The result is the minimum AWG that keeps voltage drop under your target. A smaller gauge number means thicker wire. If the calculator recommends 6 AWG, using 8 AWG (thinner) will exceed your drop target. Using 4 AWG (thicker) will reduce the drop further.
- Cross-check ampacity. The recommended gauge must also handle the current without overheating. For most residential and solar applications, voltage drop is the controlling factor on long runs — the wire will be thicker than ampacity alone requires. On short runs at high current, ampacity is the controlling factor instead.
- Account for temperature. Copper resistance increases about 4% for every 10C above 20C. Wire in a hot attic or exposed conduit in summer sun may need one gauge larger than the calculator suggests.
Example: Running Wire from Solar Panels to a Battery Shed
A common off-grid scenario: four 400W solar panels on a ground mount, 100 feet from the battery shed where the charge controller lives. The array produces about 35A at 48V (panels wired in series for a 48V string).
At 48V, a 3% drop target allows only 1.44V of loss. With 35A over 100 feet (200 feet round trip), you need 6 AWG copper wire. That is a thick, expensive run — about $1.50-2.00 per foot for each conductor, so roughly $400-600 for the pair.
Compare the same panels wired at 24V (two parallel strings of two series panels): now they produce 70A at 24V. The 3% drop limit is 0.72V. The recommended wire gauge jumps to 2/0 AWG — much heavier and more expensive. The wire cost alone might triple.
This is why experienced solar installers wire panels in high-voltage strings (often 150-600V) and use MPPT charge controllers that step down the voltage near the battery. A 150V string carrying 11.7A only needs 10 AWG for the same 100-foot run at 3% drop. The MPPT controller handles the conversion to 48V battery charging voltage efficiently.
If you are designing the full solar and battery system, our solar panel and battery sizing calculator helps you figure out the right number of panels and batteries. Then come back here to size the wire runs.
Already know your panel output and want to convert watts to amps for the wire calculation? Our solar watts to amps calculator does that conversion at any voltage.
Worked Examples
Running 12V DC Lighting to a Detached Barn
Context
You want to power 12V LED strip lights in a barn 120 feet from your battery bank. The lights draw 8 amps total. You want less than 3% voltage drop.
Calculation
Round-trip distance: 120 x 2 = 240 feet
Required resistance: (12 V x 0.03) / 8 A = 0.045 ohms total
Resistance per foot needed: 0.045 / 240 = 0.000188 ohms/ft
10 AWG copper is 0.00102 ohms/ft → 240 ft x 0.00102 = 0.245 ohms → drop = 8 x 0.245 = 1.96 V = 16.3% — far too high
You need 2 AWG (0.000162 ohms/ft) → drop = 8 x 0.039 = 0.31 V = 2.6%
Interpretation
At 12V and 120 feet, even 10 AWG wire loses 16% of the voltage. You need thick 2 AWG cable, which is expensive. This is why most long runs use 24V or 48V systems.
Takeaway
Switching to a 48V system would let you use 10 AWG for this same run. To size the full solar-plus-battery system for the barn, use our solar panel and battery sizing calculator.
Sizing Wire for a 250-Foot Well Pump Circuit
Context
A well pump on a 240V, 15A circuit is located 250 feet from the main panel. You need the voltage drop under 3% to meet NEC recommendations.
Calculation
Round-trip distance: 250 x 2 = 500 feet
Allowable drop: 240 x 0.03 = 7.2 V
Required resistance: 7.2 / 15 = 0.48 ohms over 500 ft = 0.00096 ohms/ft
10 AWG (0.00102) is marginal. 8 AWG (0.000641) gives: 500 x 0.000641 x 15 = 4.81 V = 2.0% drop
Interpretation
8 AWG keeps the drop at 2%, well within the 3% target. Going up to 6 AWG would be overkill for this current, but 10 AWG is borderline at 3.2%.
Takeaway
At 240V, long runs are far more forgiving than at 12V. But the pump's starting surge draws 5-7x the running current — check the startup impact with our locked rotor amps calculator.
Frequently Asked Questions
Glossary
Voltage Drop
The reduction in voltage between the power source and the load, caused by the resistance of the wire. NEC recommends keeping drop under 3% for branch circuits and 5% total for feeder plus branch combined.
Circular Mil
A unit of wire cross-sectional area used in electrical engineering. Larger circular mil values mean thicker wire with lower resistance. AWG is the practical sizing system derived from circular mil measurements.
Round-Trip Distance
The total length of conductor from source to load and back. Current flows through both the positive and negative conductors, so voltage drop occurs over twice the physical distance between the panel and the device.
Want to know how much energy your solar panels actually deliver after wire losses? Our solar panel output calculator accounts for system losses including wiring. Try it now →
Voltage drop is the hidden tax on every long wire run. It wastes energy as heat, reduces equipment performance, and can shorten the life of motors and sensitive electronics. Spending more on heavier wire up front saves money in lost energy over the life of the installation. For any permanent wiring — especially at permit-required voltages — have a licensed electrician verify the wire gauge, routing, and connections. These results are estimates based on standard copper resistance values at 20C. Actual drop varies with wire temperature, connection quality, and conductor material.
Last updated:
Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.