Why does long wound cable draw more current? 1

[CraigD]

Ever since I was a young lad I was always told to unwind the extension cable fully before plugging it in because it will draw too much current. Now I'd like someone to give me a technical reason for why this is so. I know that a looped cable is effectively an inductor but can't work out why that would make a difference.

 

[jmarko]

Hello, CraigD!

Wires in cable have beside inductance also capacitance; closer wounds are stacked and more wounds, greater the capacitance. This capacitance leaks some AC current, even if no load is connected. But if length of cable is in range of few meters, it can be ignored.

Best regards, Jmarko

 

[jghrist]

If the coiling increased the inductance, the result would be less current, not more. Because both the wires are in the loop, however, any magnetic flux lines from one wire are counteracted by those of the other. I don't think the overall inductance would be changed by looping.

If the cord is heavily loaded, it may overheat if looped because of the heating of adjacent loops of cord. I think you were given good advice for the wrong reason.

 

[TheBlacksmith]

Most likely, at household voltage/current levels, it is heating. This is usually lumped with the same warning not to run cords under rugs, which beside the potential for physical damage also decreases the cord's ability to shed heat.

Blacksmith

 

[CraigD]

Thankyou all. That makes alot of sense.

 

[busbar]

The long coiled-cord/voltage-drop problem just about drove one electrician mad. He got a call from a homeowner that complained there had to be a problem with bathroom receptacles in two places. Often, when the husband and wife were using either of the outlets, their shaver or hair dryer would slow down for ~10 seconds, then speed up for a couple of minutes and slow down again. It happened in both bathrooms as far as they could tell, and the occupants were convinced the house was possessed. The overhead lights would perceptibly dim in the process.

The electrician’s hair was starting to fall out in clumps tracing wiring and checking voltages. It took several trips to figure that one gfci seemed to feed it all. {They were sort of new at that time—so one was used for the whole house—two bathrooms, one outdoor and a few non-dedicated garage receptacles.}

Here’s what he finally found. In a dark corner of the garage, an older refrigerator had been “temporarily” powered from the end-of-the-loop receptacle, but because its six-foot cord was a bit short, the homeowner grabbed his nice new 100-foot 16/3 SJT extension cord still in its cardboard sleeve—purchased for his hedge trimmer—to make up the 2-foot gap. {They thought it would be nice to have a refrigerator for iced tea and beer, but had never checked it for operation—just plugged it in “for later.”} The fridge’s hermetic compressor had never started. It would cycle in locked rotor for a bit, then thermal out for a while—again and again for weeks, while the electrician was put through hell trying to find it.

Heat accumulation in the coiled 16/3 cord had developed to the point of jacket deformation…a fire may not have been far off.