Home Improvement → Re: 150' Extension Cord

beeron and fireflier have basically nailed it. The voltage drop is twice that of the length of the cord, and has some reactive impedance drop, as well. The impedance is relatively small for resistive loads over SO-type rubber cords - a lot more for wiring in conduits and reactive loads.

For a 20 amp load at 120V, single phase; 50 feet is about the maximum you can run on 12 AWG without exceeding the NECs maximum (3%) on branch circuits. And you have yet to include the voltage drop within the branch circuit wiring within the building (which could already be up to or over 3%), and any drop on the feeders. So, before you even plug in the extension cord, you could already be down a total of 5% from nominal voltage.

The main point to remember is that to size the extension cord's gauge properly, you have to consider the current draw (and possibly power factor) of the load(s). I would recommend 10 AWG minimum to be a decent 150' long extension cord that can handle a variety of power tools.

I have a different rule that I use for sizing extention cords...if you can carry it around conveniently, it just aiin't sufficient.