My 2c, check the sheathing for a voltage rating, most is rated up to 600 volts. Many cheaper extension cords found in the home, "zip cord" style are only 14 gauge stranded 2 conductor. Using 2 conductors eliminates the ground which, depending on who you ask is not a good thing to do.
Back in the day, i used to build some pretty high end mobile DJ rigs pushing some serious power and the speaker wire of choice was high quality SJOW or SO 4 conductor wire you would commonly use for AC power but worked equally well on DC speaker level applications.
My point is the cable doesent usually care whether its AC or DC, every cable has a Voltage rating usually based on the "sheathing" and its ability to contain the rated voltage. With only 2 conductors you run the risk of reduced safety with the loss of the ground, that being said there are plenty of cords still sold today in that format, its all in what you are willing to consider "safe" in your own home.
Tim