Can I use an extension cord with a dehumidifier or sump pump?

Don’t use an extension cord with a dehumidifier or sump pump unless you have absolutely no other option. If you must, use a short, heavy-duty 12-gauge cord rated for the full amperage and rated for outdoor use. Standard 16-gauge cords can overheat, cause voltage drop, and start a fire, especially with sump pumps that sit in water and run unattended. The safest bet is to plug directly into a GFCI outlet.