Every wire has resistance. The longer the run and the smaller the wire, the more voltage you lose before electricity reaches the load. This voltage drop calculator estimates the voltage loss in your circuit based on the wire size, conductor material, cable length, and anticipated load current. Use it to determine the proper wire size for an electrical circuit and keep voltage drop within safe limits.
This calculator is a planning aid for estimation of the voltage drop. It does not replace review by a licensed contractor or engineer.
Calculate Voltage Drop for Any Circuit
Enter your source voltage, current in amps, conductor size, cable length, and conductor material. The calculator estimates percent voltage drop and voltage at load so you can verify your wire choice before pulling cable.
Results work for common voltages: 120V, 240V, 480V, or any custom value. The tool covers single-phase, three-phase, and DC circuits. Whether you are wiring a residential branch circuit or sizing cable for a solar array, the same core inputs apply.
How the Voltage Drop Calculator Works
The calculator uses Ohm's law and standard resistance values for copper and aluminum conductors. It multiplies the resistance of a conductor (in ohms per 1,000 ft) by the current flowing and the round-trip wire length, then divides by source voltage to find percent voltage drop.
Conductor size and AWG
AWG stands for American Wire Gauge. It is the standard sizing system for electrical wire in North America. A smaller AWG number means a larger wire with lower resistance.
Common sizes range from 14 AWG (light-duty residential circuits) up to 4/0 AWG and beyond for heavy feeders. If your project uses metric wire sizes in mm², convert to AWG or enter the conductor cross-section directly if the calculator supports it.
Cable length and ampere inputs
Cable length is the one-way distance from the power source to the load, measured in feet. The calculator doubles this value internally to account for the full circuit loop (supply and return conductors).
Ampere input is the anticipated load current your circuit will carry. Use the fully loaded value or the nameplate rating of the equipment. For motor circuits, the NEC may require multiplying by 1.25 to account for starting current.
Copper vs. aluminium conductor
Copper is a better conductor than aluminum. It has lower resistance per foot at the same gauge, which means less voltage drop for the same wire size and circuit length.
Aluminum is lighter and less expensive. It is common in larger feeder cables and utility service entrances. When using aluminum, you typically need a larger wire to match the same voltage drop performance as copper.
The calculator lets you select copper or aluminum so the correct resistance values are applied automatically.
DC Voltage Drop Calculator
For direct current circuits, voltage drop depends only on resistance. There is no reactance component. The DC voltage drop calculator uses this simplified formula:
Voltage Drop = (2 × Length × Current × Resistance per foot) ÷ 1,000
This is the standard approach for battery banks, solar panel wiring, automotive circuits, and low-voltage LED runs. The constant 21.2 (for copper) or 12.9 (for aluminum) appears in some versions of the formula as a conductivity factor. Our calculator handles this lookup for you based on conductor material.
Single phase and three phase circuits
For single-phase AC circuits, the formula structure is similar to DC, but it uses impedance (resistance and reactance combined) instead of resistance alone. The round-trip multiplier remains 2.
For three-phase circuits, the multiplier changes to √3 (approximately 1.732) instead of 2. This reflects the phase relationship between conductors. Select your circuit type in the calculator and the correct multiplier is applied.
How do you calculate volt drop in 240V? The same way as any other voltage. Enter 240 as your source voltage, provide the load current, wire size, and length, and the calculator returns the voltage drop in volts and as a percentage.
Direct current vs. alternating current
DC circuits experience only resistive losses. AC circuits add reactance, which depends on conductor spacing, conduit type, and frequency. For most building wiring at 60 Hz with standard conduit, reactance is small compared to resistance, especially in smaller wire sizes.
The practical difference: AC voltage drop calculations are slightly more complex, but the calculator handles both. Just choose DC or AC and the correct formula runs behind the scenes.
Calculate Your Voltage Drop Step by Step
Enter your circuit voltage and electrical load
- Enter the source voltage (for example, 120V, 208V, 240V, or 480V).
- Enter the electrical load as current in amps. If you only know wattage, divide watts by voltage to get amps.
- Select the circuit type: DC, single-phase AC, or three-phase AC.
Select wire size and conductor material
- Choose the conductor material: copper or aluminum.
- Select the wire gauge (AWG) you plan to use, or start with a common size and adjust.
- Enter the one-way cable length in feet from the panel or source to the load.
- Click calculate. The tool returns voltage drop in volts, percent voltage drop, and voltage at load.
If the result exceeds your target percentage, try a larger wire size and recalculate.
Wire Size and Voltage Drop Formula
The core formula for single-phase circuits:
VD = (2 × L × I × R) ÷ 1,000
Where:
- VD = voltage drop in volts
- L = one-way length in feet
- I = current in amps
- R = resistance in ohms per 1,000 ft (from NEC Chapter 9, Table 8 or Table 9)
Percent voltage drop = (VD ÷ source voltage) × 100
For three-phase, replace the 2 with 1.732.
Electrical resistance and wire gauge
Every wire gauge has a published resistance per 1,000 ft. Smaller wire (higher AWG number) has more resistance. For example, 12 AWG copper has roughly 1.93 ohms per 1,000 ft, while 10 AWG copper has about 1.21 ohms per 1,000 ft.
Resistance also increases with operating temperature. NEC tables often list values at 75°C, which is the standard rating for most building wire insulation.
Ampacity and carrying capacity
Ampacity is the maximum current a wire can safely carry without overheating. It depends on wire gauge, insulation type, and installation conditions like conduit fill and ambient temperature.
Voltage drop and ampacity are separate limits. A wire might satisfy ampacity requirements but still produce excessive voltage drop on a long run. Always check both. The right wire size is the one that meets ampacity ratings and keeps voltage drop within acceptable limits.
How to Size Wire to Reduce Voltage Drop
Voltage drop per 100 ft varies by wire size and load. The rule of thumb for voltage drop: keep it under 3% for branch circuits and under 5% total for the combined feeder and branch circuit. These are recommendations, not hard code limits, but following them prevents performance problems.
To minimize voltage drop:
- Use a larger wire (lower AWG number) to reduce resistance.
- Shorten the cable run where possible.
- Switch from aluminum to copper for lower resistance per foot.
National Electrical Code voltage drop limits
The National Electrical Code (NEC) recommends, in informational notes to sections 210.19(A) and 215.2(A), that branch circuit voltage drop not exceed 3% and that the total drop (feeder plus branch) stay at or below 5%. These are not mandatory requirements, but most inspectors and engineers treat them as design targets.
Some local jurisdictions enforce stricter voltage drop limits. Always check your local amendments.
Choosing the right AWG cable size for your circuit
Start with the minimum wire size that satisfies ampacity for your current load. Then run the voltage drop calculation. If percent voltage drop exceeds 3%, step up one wire gauge and recalculate.
Example: A 20A load on a 120V circuit with a 150 ft run using 12 AWG copper might show over 5% voltage drop. Stepping up to 10 AWG or even 8 AWG can bring the drop under 3%.
What size wire do I need to avoid voltage drop? There is no single answer. It depends on your circuit voltage, load current, and cable length. The calculator lets you test different combinations in seconds.
Calculate Voltage Drop for Long Cable Runs
Long cable runs are where voltage drop becomes a real problem. A wire that works fine at 50 ft may produce excessive voltage drop at 200 ft. Equipment may underperform, lights may dim, and motors may overheat.
For runs over 100 ft, always calculate before selecting wire. For very long runs (300 ft and beyond), you may need to jump two or three wire sizes above what ampacity alone requires.
Electrical conduit and impedance considerations
When conductors run inside metal conduit, impedance changes slightly compared to open-air or PVC conduit installations. Metal conduit introduces a small amount of additional reactance due to magnetic effects.
For most residential and light commercial work, this difference is minor. For large conductors in steel conduit on long runs, use the impedance values from NEC Chapter 9, Table 9 rather than simple DC resistance. The calculator estimates voltage drop for typical installations. If conduit fill, derating, or unusual installation conditions apply, consult with a licensed contractor or engineer.
When to Calculate Your Voltage Drop Before Wiring
Calculate voltage drop before you buy wire and before you pull cable. Changing wire after installation is expensive and time-consuming.
Situations where voltage drop matters most:
- Detached garages, barns, or outbuildings far from the panel
- Well pumps, irrigation systems, or outdoor lighting on long runs
- Solar panel arrays with extended home runs to the inverter or battery bank
- Subpanel feeders running across large commercial buildings
- Any circuit where the one-way length exceeds 50 ft with a significant electrical load
Even short runs deserve a quick check if the load is heavy relative to the wire size. This free voltage drop calculator gives you the answer in seconds, helping you choose the right wire and avoid problems before they are buried in the wall.