Extension Cord Load Calculator
Estimate voltage drop for an extension cord based on length, wire gauge, current draw, and supply voltage, with a safe/caution/not-recommended rating.
Divide the device's watts by the supply voltage if you only know watts.
Result
Voltage Drop
2.52 V (2.1%)
Status
How to Use the Extension Cord Load Calculator
Enter the cord's length, its wire gauge (AWG — the standard sizes found on extension cords are 16, 14, 12, and 10, with a smaller number meaning a thicker wire), the current draw of the device in amps, and the supply voltage (120V by default for standard US household outlets). The calculator uses the standard voltage-drop method: it looks up the wire's resistance per 1,000 feet for the selected gauge, multiplies by the round-trip distance (length out and back), and compares the resulting voltage drop against the supply voltage.
A drop under 3% of supply voltage is generally considered safe, 3-5% is a caution zone where performance may suffer on sensitive equipment, and over 5% means you should use a shorter cord or a thicker gauge.
Example
A 50 ft, 14 AWG extension cord carrying 10 amps at 120V has a resistance of about 2.525 Ω per 1,000 ft, giving a voltage drop of roughly 2.5V — about 2.1% of supply voltage, which falls in the Safe range.
Common Use Cases
- Choosing the right gauge extension cord for a long run to a power tool or space heater.
- Checking whether a cord you already own can safely handle a specific appliance's load.
- Understanding why longer cords need thicker wire to avoid overheating and power loss.
FAQs
- Is this an exact substitute for the cord's rated ampacity? No — this is a general voltage-drop estimate using standard published resistance values per gauge. Always check the actual ampacity rating printed on your specific extension cord and follow your local electrical code; a cord's real-world rated capacity depends on its insulation, construction, and intended use (indoor vs. outdoor), not just voltage drop math.
- Why does cord length matter so much for voltage drop? Voltage drop scales directly with the round-trip length of wire the current has to travel through — doubling the cord length roughly doubles the resistance the current encounters, which doubles the voltage drop for the same current and gauge.
- What happens if the voltage drop is too high? Motors and heating elements can run hotter, less efficiently, or fail to start correctly, and the cord itself can heat up more than expected. Switching to a thicker gauge (a lower AWG number) or using a shorter cord reduces resistance and brings the voltage drop back down.