We had a client once comment: “I have been reading about plugging extension cords. I want to make sure when I go on one of our sites and say, ‘You can’t plug one cord into another’ that I can give a reason why.
In a letter dated September 28, 2015, OSHA references 1926.403(b)(2) – Installation and Use. Part of the letter’s answer clearly states that “OSHA will continue to enforce the listing, labeling, or certification requirements as outlined in 29 CFR 1926.403(b)(2) – Installation and use. That provision requires that listed, labeled, or certified (i.e., approved) equipment be used in accordance with the instructions included in the listing, labeling, or certification.”
A second letter, dated November 18, 2002 references 1910.303(b)(2): “Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling…For example, the UL Directory contains instructions that require UL-listed RPTs to be directly connected to a permanently installed branch circuit receptacle; they are not to be series-connected to other RPTs or connected to extension cords.”
Another source addresses misuses of power strips. This article goes into a little more detail of what “listing or labeling” means.
The bottom line is this: follow the guidelines that are given by the manufacturer.
Always use a Ground Fault Circuit Interrupter (GFCI) with extension cords. The GFCI is plugged closest to the source and furthest from the load!
Extension cords are designed to carry a certain amount of current over a fixed distance (length of the cord). Daisy chaining cords (plugging cords together) increases this distance and thus increases the extension cord’s resistance.
Without getting into the mathematics and physics of it, essentially this results in greater heating of the cord and possible damage to the insulation – this may be associated with a risk of fire and electrical shock. (commonly known as I2R or Joule heating)
Secondly, you have the chances of the interconnection of plugs coming loose or pulling apart, resulting in a loss of power.
Thirdly, there is less current available to operate the load (although this is marginal and negligible). A fault furthest away from the source, in a daisy-chained arrangement, may result in the breaker failing to trip.
Finally, if the interconnection comes loose gradually, it could likely create a spark and that is once again a risk of fire (just through a different failure mechanism).
Also see another one of our blog articles, Electrical Safety: 4 Common Mistakes Using Extension Cords.