A voltage drop calculator estimates how many volts are lost in the wires between a power source and a load. Formula: V_drop = current (A) x resistance (ohms) x length factor. A 15-amp load at the end of 200 feet of conductor at 0.002 ohms per foot loses about 6 volts.
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How to use the voltage drop calculator
- Enter the current in amperes.
- Enter the resistance of one conductor, either per unit length or for the whole one-way run.
- Enter the one-way distance from source to load, or 1 if the resistance is already the one-way total.
- Choose single-phase/DC or three-phase, add the supply voltage, and read the drop and percentage.
How the voltage drop calculator works
The voltage drop calculator applies Ohm’s law, V = I × R, to the wire. It multiplies the current in amperes by the resistance of one conductor, by the one-way length and by a circuit factor: 2 for single-phase and DC, √3 for balanced three-phase. This is the same form as the NEC-style estimate VD = 2 × K × I × L ÷ CM. If you enter resistance per foot or per meter, enter the one-way length in the same unit; if you enter the resistance of the whole one-way conductor, leave the length at 1.
Count both conductors
Current flows out on one conductor and back on the other, so a single-phase or DC circuit has twice the wire length of the one-way run. The voltage drop calculator now does the doubling for you: enter 100 feet for a load 100 feet away and it uses 200 feet of conductor. Do not double the distance yourself, or the result will be twice the true drop. Forgetting the return conductor is the most common error in hand calculations, and it halves the answer. Three-phase circuits use a factor of √3 instead of 2.
Voltage drop calculator worked example
A 15-amp load sits 100 feet from the panel on copper wire with about 0.002 ohms per foot per conductor. You enter 100 feet, and the voltage drop calculator uses the 200-foot round trip: 2 × 15 × 0.002 × 100 = 6.0 volts. On a 120-volt circuit that is 5% of the supply. With wire of 0.003 ohms per foot, the drop rises to 9 volts, or 7.5%.
| Current (A) | Resistance | One-way length | Drop |
|---|---|---|---|
| 10 | 0.25 Ω (one-way total) | 1 | 5.0 V |
| 15 | 0.00319 Ω/ft | 50 ft | 4.785 V |
| 20 | 0.00205 Ω/ft | 75 ft | 6.15 V |
| 15 | 0.003 Ω/ft | 100 ft | 9.0 V |
| 15 | 0.002 Ω/ft | 100 ft | 6.0 V |
How much drop is acceptable?
A common design target in North America is no more than about 3% on a branch circuit and 5% in total from the service to the farthest outlet. On 120 volts, 3% is 3.6 volts and 5% is 6 volts. These figures come from informational guidance rather than mandatory rules in many codes, but excessive drop makes motors run hot, lights dim and electronics misbehave. Low-voltage systems such as 12-volt solar or LED lighting are far more sensitive: a 1-volt drop is more than 8% of 12 volts.
Finding wire resistance
Resistance depends on the conductor material, its cross-sectional area, its length and its temperature. Resistance = resistivity × length ÷ area, so doubling the length doubles the resistance and doubling the area halves it. Copper has lower resistivity than aluminum, which is why aluminum conductors are one or two sizes larger for the same job. Wire manufacturers and code tables list ohms per 1,000 feet or per kilometer for each size; divide by 1,000 to get ohms per foot or per meter, and use the value for the expected operating temperature.
Reducing voltage drop
The voltage drop calculator makes the options clear. Use a larger conductor to cut resistance, shorten the run by moving the panel or load, raise the supply voltage so the same power needs less current, or split the load over more circuits. Doubling the voltage for the same power halves the current and quarters the percentage drop.
Tips
- Enter the one-way distance; the tool doubles it for single-phase and DC.
- Use resistance at operating temperature.
- Compare the drop with the supply voltage as a percentage.
- Have a licensed electrician design and check real installations.
For study and planning only. Check lab, engineering and safety work against your course, instrument or a qualified professional.
Related converters and calculators
Related calculators:
- Ohm’s law calculator: V, I, R
- Electrical resistance converter: ohms
- Electricity calculator: energy cost
- Resistor calculator: resistors
Sources
OpenStax College Physics 2e, 20.2: Ohm’s law; OpenStax College Physics 2e, 20.3: Resistance and resistivity.
Formula and method
V_drop = current (A) x resistance (ohms) x length factor.
Ohm-law style product with an optional one-way length multiplier. Simplified educational model.
Questions about this calculator
What formula does it use?
Drop = 2 × current × resistance × one-way length (√3 instead of 2 for three-phase).
Do I double the length?
No. Enter the one-way distance; the tool doubles it for single-phase and DC circuits.
What drop is acceptable?
A common target is 3% on a branch circuit and 5% in total.
How do I reduce voltage drop?
Use larger wire, a shorter run or a higher voltage.
Is it a substitute for an electrician?
No.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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