Voltage Drop Calculator
Calculate voltage drop, load voltage, and wire-size requirements for DC, single-phase, and 3-phase circuits.
Voltage drop calculator
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Quick answer
What is voltage drop?
Voltage drop is the reduction in voltage that occurs as current flows through a conductor’s resistance or impedance. The load sees supply voltage minus that drop.
What is a common voltage-drop target?
3% is a commonly used design target for many branch-circuit and low-voltage applications, while 5% is often used as a combined feeder-plus-branch guideline. Check the requirements that apply to your installation. These are planning targets, not universal legal limits.
How to use the voltage drop calculator
- Choose Voltage drop, Wire size, or Maximum length.
- Set DC, single-phase, or three-phase and the system voltage (12 V through 480 V, or custom).
- Enter amps or watts, copper or aluminum, and one-way length in feet or meters.
- Set a 1%, 2%, 3%, or 5% target — or a custom percent.
- Read volts dropped, percent, voltage at the load, and whether the run is within target.
What is voltage drop?
Current through a wire produces a voltage loss along the run. Lights can dim, motors can run hot, and electronics can brown out if the load voltage falls too far. A voltage drop percentage calculator compares that loss to the supply: drop % = (Vdrop ÷ Vsupply) × 100. Low-voltage systems (12 V and 24 V) show a much larger percent for the same ohms and amps.
Voltage drop formula
Resistance comes from NEC Chapter 9 Table 8 (uncoated, 75 °C, ohms per 1,000 ft). Length is one-way. The factor 2 accounts for outbound and return conductors.
- DC / single-phase:
Vdrop = 2 × I × R × Lwith L in thousands of feet - Balanced three-phase:
Vdrop = √3 × I × R × L Drop % = (Vdrop ÷ Vsupply) × 100Vload = Vsupply − Vdrop
AC results are a resistance-based estimate at power factor 1.00. If you set a lagging power factor, the tool adds an approximate PVC-conduit reactance term (R·cosφ + X·sinφ). That is still a planning model, not a full impedance study.
DC voltage drop
A DC voltage drop calculator uses the two-conductor resistance path only. There is no power-factor term. This is the right model for 12 V, 24 V, and 48 V DC lighting, solar, and vehicle circuits when you enter one-way length.
Single-phase voltage drop
The single-phase voltage drop formula is the same 2 × I × R × L path used for DC. It is the usual model for 120 V and 240 V branch circuits and many 230 V supplies. Treat the result as a voltage-drop calculation to check against your design target.
Three-phase voltage drop
For a balanced three-phase run the path factor is √3 instead of 2. A 3 phase voltage drop calculator therefore shows slightly less drop than the same amps, AWG, and length on single-phase. Unbalanced or high-reactance feeders need a more complete model.
12V voltage drop
A 12 volt voltage drop calculator is unforgiving: 0.77 V on a 12 V circuit is already about 6.4%. Keep 12 V LED and DC runs short, or step up to a larger AWG. Use the 12 V preset, then switch to Wire size if the percent exceeds your target.
24V voltage drop
The same ohms and amps on 24 V cut the percent in half versus 12 V. A 24 volt voltage drop calculator is still more sensitive than 120 V. Landscape lighting and control circuits often need a larger conductor or a shorter one-way length.
120V and 240V voltage drop
At 120 V, 15 A on 12 AWG for 50 ft is about 2.4% — a common branch-circuit check. Doubling voltage to 240 V halves the percent for the same amps, wire, and length. That is why a 240 V tool circuit can travel farther before it exceeds a 3% target.
How wire size affects voltage drop
Larger conductors (smaller AWG numbers, then kcmil) have lower ohms per 1,000 ft. The comparison table next to the results shows nearby sizes. Wire size by voltage drop is not the same as ampacity. Use the wire gauge calculator for minimum copper AWG from load current, then return here to check the run.
Copper vs aluminum
Aluminum has higher resistance than copper of the same size, so voltage drop is larger. A copper/aluminum toggle uses the matching Table 8 values. Do not substitute aluminum for copper without also checking terminations and ampacity.
Real examples
Numbers below come from the same engine as the calculator (copper, 75 °C, power factor 1.00 unless noted).
- 15 A · 12 AWG · 50 ft · 120 V: 2.9 V · 2.41% · load 117.11 V
- 20 A · 12 AWG · 100 ft · 120 V: 7.72 V · 6.43% · load 112.28 V
- 20 A · 10 AWG · 100 ft · 120 V: 4.84 V · 4.03% · load 115.16 V
- 12 V DC: 0.77 V · 6.43% · load 11.23 V
- 24 V DC: 0.77 V · 3.22% · load 23.23 V
- 240 V single-phase: 7.26 V · 3.03% · load 232.74 V
- Three-phase 208 V: 6.69 V · 3.21% · load 201.31 V
Common mistakes
- Entering round-trip distance instead of one-way length.
- Using the wrong wire gauge or mixing AWG with measured diameter.
- Ignoring supply voltage — percent drop is Vdrop ÷ Vsupply.
- Using current from the wrong load assumption.
- Treating voltage drop and ampacity as the same calculation.
- Assuming 3% is always a mandatory legal limit.
- Ignoring conductor material or temperature.
- Forgetting that 12 V and 24 V systems are more sensitive to the same voltage loss.
How to reduce voltage drop
- Increase conductor size (use Wire size mode).
- Shorten the one-way run or move the supply closer.
- Raise system voltage when the equipment allows it (12 V → 24 V).
- Split the load across more than one circuit.
- Use copper instead of aluminum when the design allows.
Then confirm the breaker size calculator and circuit load calculator still match the conductor and load. More electrical calculators live in Calculator Tools.
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Voltage drop FAQ
What is a normal voltage drop?
Many designers use 3% as a branch-circuit or low-voltage target and 5% as a combined feeder-plus-branch guideline. Those are planning targets, not a single legal limit for every installation.
How do I calculate voltage drop?
Use Vdrop = 2 × I × R × L for DC or single-phase, or √3 × I × R × L for balanced three-phase. R is ohms per 1,000 ft and L is one-way thousands of feet. Then divide Vdrop by supply voltage for the percent.
Does voltage drop depend on wire size?
Yes. Larger conductors have lower resistance, so the same amps and length drop fewer volts. That is why 20 A on 100 ft at 120 V is about 6.43% on 12 AWG and about 4.03% on 10 AWG.
How does wire length affect voltage drop?
Drop scales with one-way length. Doubling the run doubles the volts lost if current and wire size stay the same. Enter one-way distance, not the round-trip.
Is 3% voltage drop acceptable?
3% is a commonly used design target, not a universal mandatory limit. Check the requirements that apply to your installation, then confirm ampacity and overcurrent protection separately.
How do I calculate voltage drop on a 12V circuit?
Use the DC mode. The same ohms produce a much larger percent on 12 V than on 120 V. 10 A on 12 AWG for 20 ft is 0.77 V — about 6.4% of 12 V.
How do I calculate voltage drop on a 24V circuit?
Use DC mode at 24 V. The same 10 A, 12 AWG, 20 ft run is still 0.77 V, but that is about 3.2% of 24 V — half the percent of the 12 V case.
What is the voltage drop formula for single-phase?
Vdrop = 2 × I × R × L, with L in thousands of feet. Percent = (Vdrop ÷ Vsupply) × 100. The factor 2 is the outbound and return conductors.
What is the voltage drop formula for three-phase?
For a balanced three-phase run, Vdrop = √3 × I × R × L. That path factor is smaller than the single-phase 2, so the same amps and AWG drop slightly less voltage.
Does aluminum wire have more voltage drop than copper?
Yes, for the same size. Aluminum has higher Table 8 resistance, so 15 A on 12 AWG for 50 ft at 120 V is about 2.90 V on copper and 4.77 V on aluminum.
What wire size reduces voltage drop?
A larger conductor — a smaller AWG number, then kcmil — reduces drop. Wire size mode returns the smallest supported size that meets your percent target. That is not an ampacity or “safe wire” result.
Does voltage drop affect motors and lights?
Yes. Excess drop can dim lamps, reduce motor starting torque, and brown out electronics. Low-voltage lighting is especially sensitive because a small volt loss is a large percent of 12 V or 24 V.