Wire Voltage Drop Calculator
Wiring note: Voltage-drop math is only one wiring check. Ampacity, insulation rating, terminal limits, conduit fill, overcurrent protection, wet locations, and local rules must be confirmed separately.
Inputs
Result
How to use this calculator
Use this calculator to see whether a long wire run may waste energy or cause equipment voltage problems.
What the result means
The result estimates volts lost, percent drop, and power loss for the conductor and circuit assumptions entered.
What the result does not settle
It does not approve a wire size for safety, code, conductor temperature, or installation method.
Inputs that change the answer most
- Source voltage
- Current
- One-way length
- Copper or aluminum resistance
- DC, single-phase, or three-phase circuit type
- Allowed voltage-drop target for the equipment
Readable method
Voltage drop = circuit multiplier × current × conductor resistance per foot × one-way length. Percent drop = voltage drop ÷ source voltage × 100.
Before you act
Confirm ampacity, temperature correction, overcurrent protection, conductor insulation, terminal ratings, and local electrical requirements with qualified guidance.
How this is calculated
DC/single-phase: voltage drop = 2 × current × resistance per foot × one-way length. Three-phase: voltage drop = √3 × current × resistance per foot × one-way length. Percent = drop ÷ source voltage × 100.
Interpreting Voltage Drop
Voltage drop is the loss between the source and the load. Too much drop can waste energy, reduce equipment performance, make motors run hotter, and cause nuisance faults in inverters, chargers, pumps, or low-voltage electronics. The acceptable target depends on the circuit type, distance, load current, and equipment sensitivity.
Ways to Reduce Voltage Drop
- Use a larger conductor size for long runs or high-current circuits.
- Raise system voltage where appropriate, such as using higher-voltage battery or PV strings with listed equipment.
- Shorten wire runs by moving equipment closer together.
- Split loads across circuits instead of pushing all current through one long run.
Always verify conductor ampacity, insulation rating, conduit fill, temperature correction, and overcurrent protection separately. A wire size that solves voltage drop still has to satisfy code and safety requirements.
Assumptions and formula
Use these inputs as planning assumptions, not as a final design, tax filing, permit package, or equipment approval.
- source voltage
- current
- one-way conductor length
- copper or aluminum resistance
- DC, single-phase, or three-phase circuit type
Formula
Voltage drop = circuit multiplier × current × conductor resistance per foot × one-way length. Percent drop = voltage drop ÷ source voltage × 100.
Voltage Drop Planning Guide
Voltage drop is the voltage lost as current travels through conductors. It becomes more important on long runs, high-current circuits, low-voltage DC systems, and equipment that is sensitive to input voltage. A circuit can have acceptable ampacity but still perform poorly if voltage drop is excessive.
Practical examples
A long 12 V DC run to a pump or light can lose a meaningful share of voltage because the current is high. Moving to 24 V or 48 V, shortening the run, or using larger conductors can reduce the loss. For PV strings, higher voltage can reduce current and voltage drop, but equipment maximum voltage and code requirements still set limits.
What to check besides voltage drop
- Conductor ampacity and insulation temperature rating
- Overcurrent protection and disconnect ratings
- Conduit fill and temperature correction
- Terminal temperature limits
- AC vs DC voltage and wet-location requirements
Frequently Asked Questions
Is 3% always the limit?
No. It is a common design target, but the appropriate limit depends on circuit type, equipment tolerance, efficiency goals, and applicable code guidance.
Can I just oversize wire?
Larger wire can reduce voltage drop, but it must still fit terminals, conduit, breakers, and installation constraints.
Sources
Source notes
Use these as starting points when the page affects a purchase, design, tax, utility, or safety decision.