Voltage Drop Calculator
Calculate voltage drop, wire size recommendations, and power loss for electrical runs
How to Use This Voltage Drop Calculator
- Select your system voltage (120V residential, 240V appliances, 12V DC for solar/RV)
- Choose single phase or three phase based on your circuit type
- Enter the current draw in amps (check equipment nameplate or breaker size)
- Input the one-way wire run distance to your load
- Select wire material (copper or aluminum) and current gauge
- Click 'Calculate Voltage Drop' to see if your wire size is adequate
Example: Running a 20A circuit 100 feet with 12 AWG copper at 120V: voltage drop is 3.96V (3.3%), delivering 116V at the load. This barely meets NEC recommendations—consider 10 AWG for better efficiency.
Tip: Always check the wire comparison table to see which gauge brings you under 3% drop for branch circuits.
Why Use a Voltage Drop Calculator?
Undersized wiring wastes energy as heat and can cause equipment malfunction, flickering lights, or even fire hazards.
- Size wire for new electrical runs before purchasing materials
- Troubleshoot why motors run hot or lights flicker at the end of long runs
- Plan solar panel wiring from roof to inverter (low voltage = critical calculations)
- Design RV or marine 12V systems where every volt matters
- Verify existing installations meet code recommendations
- Calculate energy losses for efficiency audits
Understanding Your Results
The NEC provides guidelines for acceptable voltage drop. Your results should fall within these ranges:
| Result | Meaning | Action |
|---|---|---|
| Under 3% | Excellent | Meets NEC recommendation for branch circuits—good to go |
| 3-5% | Acceptable | Within combined feeder + branch limit; consider upsizing for efficiency |
| 5-7% | Marginal | Equipment may underperform; upsize wire or shorten run |
| Over 7% | Excessive | Serious problems likely; motors overheat, lights dim visibly |
Meaning: Excellent
Action: Meets NEC recommendation for branch circuits—good to go
Meaning: Acceptable
Action: Within combined feeder + branch limit; consider upsizing for efficiency
Meaning: Marginal
Action: Equipment may underperform; upsize wire or shorten run
Meaning: Excessive
Action: Serious problems likely; motors overheat, lights dim visibly
Note: NEC recommendations (3%/5%) are not code requirements but are considered best practice. Some equipment requires tighter limits.
About Voltage Drop Calculator
Formula
Vd = (2 × K × I × D) / CM Where K is resistivity (10.8 for copper, 17.7 for aluminum), I is current in amps, D is one-way distance in feet, and CM is the circular mil area of the conductor. For three-phase, replace 2 with 1.732.
Current Standards: NEC Article 210.19(A) recommends maximum 3% drop for branch circuits, 3% for feeders, and 5% total from service entrance to final outlet. UL requires equipment be tested at ±10% of rated voltage.
Frequently Asked Questions
Why is voltage drop more critical in 12V systems?
The percentage impact is amplified at low voltages. A 3% drop at 120V loses 3.6V (still 116.4V—fine for most equipment). But 3% at 12V drops to 11.6V, which may be below the minimum operating voltage for sensitive electronics. This is why solar, RV, and marine installations often use oversized wire.
Should I use copper or aluminum wire?
Copper has better conductivity (61% better than aluminum) and is standard for branch circuits. Aluminum is used for large feeders and service entrances where cost savings justify the larger size needed. Aluminum requires special connectors rated for aluminum to prevent oxidation issues at connections.
What AWG wire size should I use for a 20A circuit?
For ampacity alone, 12 AWG is rated for 20A. But for long runs, voltage drop may require 10 AWG. At 100 feet with 12 AWG copper at 120V, voltage drop is 3.3%—barely acceptable. At 150 feet, it's nearly 5%, suggesting 10 AWG is the better choice.
How does three-phase power affect voltage drop?
Three-phase systems have inherently lower voltage drop than single-phase for the same power delivered because current is distributed across three conductors. The calculation uses √3 (1.732) instead of 2 as the multiplier, resulting in about 13% less voltage drop for equivalent loads.
What happens if I exceed the 5% total voltage drop?
Equipment may malfunction, overheat, or fail prematurely. Motors draw higher current trying to compensate, triggering breakers or burning out windings. LED lights may flicker or fail. Sensitive electronics may shut down or behave erratically. Energy bills increase as wasted power becomes heat in the wiring.