Tool trailers that plug into the grid need to have an extension cord that connects them to an outlet.
Normally, this is facilitated simply by having a shore power outlet and uncoiling a cord every time you need to plug in.
A faster way to do this is to mount a cord reel outside your trailer, next to the shore power outlet. A cord reel is a faster way to manage your extension cord and saves you from needing to enter your trailer just to get a cord.
The cord reel might get a bit more weathered than if it was kept inside, but this timesaver is worth it.
