Voltage Drop Calculator
Quick explanation
Use this basic voltage drop calculator for quick checks on run length impact. If you are specifically choosing a temporary cord, try the Extension Cord Size Calculator. For a practical long-run example, see 100 ft Extension Cord Voltage Drop. For broader power checks, use the amps to watts calculator. New to voltage and current concepts? Electricity Basics covers the core relationships.
Estimated voltage drop: 6 V
Drop percent: 5%
What is voltage drop?
Voltage drop is the reduction in available voltage at the load because conductors have resistance.
It happens when current flows through that resistance, so more current and longer wire runs produce more drop. Lower delivered voltage can lead to dim lights, weak heaters, and motors or electronics that perform poorly or run hotter.
- Wire length
- Wire gauge (thickness)
- Current
- Conductor material
- Temperature
- Connection quality
Longer runs and higher current increase drop; thicker wire reduces it.
Real-world examples of voltage drop
If you plug a 1500W space heater into an outlet at the far end of a long garage circuit, it can draw about 12.5A at 120V. Over a long run, that current can cause the voltage at the heater to drop. The heater may feel a bit weaker, and the wiring may run warmer, especially if other loads are on the same circuit.
Why this matters: high current + long wire length = more drop.
A long, thin extension cord has more resistance than a short, heavy one. If you run a power tool or shop vac on a long extension cord, you might notice dimmer lights, weaker tool performance, or the tool struggling to start. That is often voltage drop: more current is needed under load, and the cord resistance takes some voltage before it reaches the tool.
Quick takeaway: use a shorter or heavier-gauge extension cord for high-draw tools.
Assumptions
- Simplified estimate for early planning only.
- Conductor material, temperature, and installation method are simplified.
- Verify final sizing against applicable code and site conditions.
Worked example
Example: Longer runs with the same current produce more voltage drop.
FAQ
Why does voltage drop matter?
Excess drop can reduce equipment performance and increase heating risk.
Is this enough for final design?
No. Use it for preliminary estimates and verify with code-compliant methods.
What increases voltage drop?
Longer runs, smaller conductors, and higher current all increase drop.
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⚠️ Sanity Check Only
This tool provides informational estimates only. It is not professional engineering advice. Electrical work is dangerous and governed by strict local codes.
Before you start:
- Verify these results with a licensed electrician.
- Cross-reference with the latest Electrical Code (NEC/CEC).
- Never work on live circuits.