Extension Cord Calculator

Last Updated on by Anthony Brown

Need a new extension cord but not sure what size – use our handy extension cord calculator and drop voltage chart to help choose the best cord for your pump, power tools or other outdoor use!

Extension cords can only carry so much power at a time. This term is call Ampacity and can be found on the label and head of most extension cords. This is important because some power tools like leaf blowers need a cord with a much higher Ampacity than a small fan!

Online Cord Calculator

  • Please enter a number from 0 to 50.

Voltage Drop and Extension Cords

Any wire that carries power (including extension cords) have resistance. Longer wires mean more resistance and less power getting through to your pump or power tool.

In some cases this is not a big deal and you won’t even notice anything is wrong. In other cases the motor may short out, randomly die, or over heat and burn out!

Specialized equipment – like sump pumps – are designed to work at certain voltage levels. While they can withstand some voltage drop it is important to give them the right voltage level by using the right size wire and cord.

Most power tools can handle 10% voltage drops without experiencing any problems but this number varies from tool to tool and manufacturer to manufacturer. Even the power circuit into your home from the street experiences a 5% fluctuation in voltage.

How to Size an Extension Cord

Sizing an extension cord is easy! All you need is the cord as well as a general idea of the types of appliances you will be using.

Choosing the right wire size is important because some tools like saws use much more power than small hand drill and you don’t want to damage your electric motors! Can’t find the manufactures guide? Check the handle for the amp and recommended cord size!

Step by Step Instructions

  1. Determine the length of cord
  2. Locate the amp of your tools
  3. Check the wire gauge, letters and cord length

Extension Cord Letter Guide

Not quite sure what those letters on your extension cord mean? Use our handy table to make sure you are using the right cord. The numbers are usually near the head or size of the wire. If the cord is old the letters may have faded in which cases it is time fora new cord!

Letter on CordMeaning
SGeneral use
OOil resistant
WWeather resistant
J300v insulation
PParallel wires
TVinyl jacket
RTemperature resistant

Wire Gauge Sizing Guide

Different gauge (thickness) wires can transport different amounts of electricity. The thicker the wire – and lower gauge – the more power it can carry!

Most Common Gauge Sizes

18 – Best for saws
16 – Best for chainsaws
14 – Best for lawn mowers
12 – Best for vacuum cleaners
10 – Best for fans