Voltage Drop Calculator
Calculate voltage drop, find minimum wire size, or max run length for DC, single-phase, and three-phase AC solar circuits per NEC standards — free, no signup.
Voltage Drop Calculator for Solar
Select circuit type, wire size, material, and run length. Get voltage drop percentage, pass/fail status, power loss, annual energy loss, and recommended wire size.
NEC 690.8 solar multiplier is applied automatically in Solar Options below. This override is for other adjustments only.
- Base current—
- NEC 690.8 × 1.25—
- Design current—
- Phase factor—
- One-way length—
- Resistance (Ω/kft)—
- Formula—
- Voltage Drop—
| Wire Size | VD% | VD (V) | Power Loss |
|---|
Dashed lines show NEC thresholds: 2% DC limit · 3% AC limit · 5% max total. Vertical marker shows your wire run length. Highlighted line = selected wire size.
NEC Chapter 9 Resistance Reference
Conductor DC resistance at 75°C (Ω per 1,000 ft) · NEC Table 8
| Wire Size | DC Resistance Ω / 1000 ft | DC Resistance Ω / 1000 m | VD per 100 ft at 10A (1-phase) | VD per 100 ft at 30A (1-phase) |
|---|
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What This Tool Covers
The Voltage Drop Calculator determines voltage loss in DC string and AC output circuits for solar installations per NEC requirements. Enter your wire size, material, run length, current, and system voltage, and it returns voltage drop in volts and percent, pass/fail status against NEC limits, power loss, annual energy loss, and the recommended wire size if you fail.
Inputs Accepted
- • Circuit type: DC string or AC output
- • Wire size (AWG, from 14 AWG to 750 kcmil)
- • Wire material: copper or aluminum
- • One-way wire run length (feet)
- • Circuit current (amps)
- • System voltage (V)
Outputs Generated
- • Voltage drop in volts (V)
- • Voltage drop as percentage (%)
- • Pass/fail against NEC 2% DC and 3% AC limits
- • Power loss in watts (W)
- • Annual energy loss in kWh
- • Recommended minimum AWG to pass NEC limits
Features
NEC Pass/Fail Status
Instantly see whether your wiring configuration meets NEC Article 690 recommendations: 2% for DC circuits, 3% for AC circuits. A clear pass/fail flag makes inspection prep straightforward.
Energy Loss Quantification
Voltage drop translates directly to wasted energy. The calculator converts power loss (W) into annual kWh loss so you can put a dollar value on wire undersizing.
Recommended Wire Size
If your current wire size fails, the tool calculates and displays the minimum AWG (for copper or aluminum) that would bring voltage drop below the NEC limit.
How It Works
Select Circuit Type and Wire Parameters
Choose DC string or AC output circuit. Select your wire material (copper or aluminum) and AWG size. The calculator loads the corresponding resistivity from NEC conductor resistance tables.
Enter Run Length and Electrical Values
Enter the one-way run length in feet (the calculator doubles it to get total conductor length for DC circuits, or uses the appropriate multiplier for AC). Input circuit current in amps and system voltage.
Calculate Voltage Drop
Using the conductor's resistance per 1,000 feet (from NEC Chapter 9 Table 9), the calculator computes the voltage drop across the total conductor length at the given current. This result appears in volts and as a percentage of system voltage.
Check Against NEC Limits and Get Recommendation
The tool compares calculated drop to the 2% DC or 3% AC threshold. If the circuit fails, it iterates through larger AWG sizes to find the minimum passing size and reports it alongside the pass/fail status.
Use Cases
Long DC String Runs
Rooftop arrays with combiner boxes far from the inverter can have DC runs exceeding 100 feet. Verify that the home-run wire size keeps voltage drop below 2% before the installation crew pulls wire.
Inverter-to-Panel AC Wiring
AC output circuits from a string inverter to the main panel can be long in homes where the inverter is in a garage or detached structure. Check that 3% AC voltage drop limit is met before sizing conduit.
Energy Loss Cost Justification
When a customer pushes back on upsizing wire, calculate the annual kWh loss from the undersized wire, multiply by the utility rate, and show the cost. Often the payback on larger wire is under 3 years.
Calculation Methodology
Voltage Drop (DC)
VD = 2 × L × R × I ÷ 1000 (R = resistance per 1,000 ft for AWG, from NEC Ch.9 Table 9)
Voltage Drop Percentage
VD% = (VD ÷ System Voltage) × 100
Power Loss
P Loss (W) = VD × I
Annual Energy Loss
Annual kWh = P Loss × Peak Sun Hours × 365 ÷ 1000
Pro Tips
Use Isc for DC string voltage drop. NEC 690.8 requires sizing DC conductors at 125% of Isc. Use 1.25 × Isc as the current input for DC string calculations to ensure code compliance in your wiring, not just voltage drop.
Aluminum is 61% as conductive as copper. If you switch to aluminum to reduce cost, you'll need a larger AWG to achieve the same resistance. The calculator handles this automatically when you select the wire material.
2% is a recommendation, not a hard NEC requirement. NEC 210.19(A) and 215.2(A) recommend voltage drop limits but don't mandate them as code violations. However, many AHJs enforce them during inspection, and energy loss from exceeding them is real regardless of code status.
Check both ampacity and voltage drop independently. A wire may have sufficient ampacity (current-carrying capacity) but fail voltage drop — or vice versa. Always verify both. The larger AWG from either check is the one to use.
Frequently Asked Questions
What NEC voltage drop limits apply to solar?
NEC recommends 2% voltage drop for branch circuits (including DC strings) and 3% for feeders (including AC output circuits). The combined recommended limit is 5% end-to-end. These are in NEC 210.19(A) informational notes and 215.2(A). Many utilities and AHJs enforce these as conditions of interconnection or permit approval.
Should I use one-way or round-trip length?
Enter one-way (single conductor) length. For DC circuits, the calculator automatically doubles it to account for the return conductor. For single-phase AC, it also doubles. This is the standard convention — entering the total wire length would double-count.
What current should I use for DC string calculations?
For NEC compliance, use 1.25 × Isc (short-circuit current from the panel spec sheet). For actual expected voltage drop during normal operation, use Imp (maximum power current). The tool lets you input either — use Imp for energy loss estimates and 1.25 × Isc for code compliance checks.
Does temperature affect voltage drop calculations?
Yes. Conductor resistance increases with temperature (copper's resistance coefficient is approximately 0.393% per degree Celsius). The NEC Chapter 9 Table 9 values are at 75°C. In high-temperature conduit installations, resistance is higher and voltage drop increases. Use the wire size calculator for temperature-derated ampacity, then verify voltage drop separately.
Is voltage drop the same as power loss?
Voltage drop (V) and power loss (W) are related but different. Power loss = voltage drop × current (P = VD × I). Voltage drop is measured in volts; power loss in watts. A circuit can have a small voltage drop percentage but significant watt loss if current is high.
Does this calculator work for microinverter systems?
Yes, for the AC trunk cable from microinverters to the panel. Microinverter systems don't have DC strings, so select AC output circuit type. Enter the total AC current from all microinverters on the branch and the trunk cable run length to the breaker panel.
How much does an oversized wire reduce annual energy loss?
Going from 10 AWG to 8 AWG (one size up) reduces resistance by approximately 37%. On a 200-foot, 10-amp DC circuit, this might reduce annual energy loss from 180 kWh to 113 kWh — a savings of 67 kWh/year, worth $10–$20 annually. Over 25 years, that's $250–$500 in recovered generation.
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