Extension cords enable us to deliver power where needed. However, an extension cord is just a temporary solution, and should not be used as a long-term extension of your household's electrical system.
Extension cords are designed to carry a certain level of current over a fixed distance which is the length of the cord. Once you have two identical extension leads connected in series, one after the other, the extension cord's resistance effectively increases. This results in greater heating of the cord and possible damage to the insulation, which can be the source of fires and electrical shock.
Another problem with interconnecting plug together arises when the cords become loose and pull apart, resulting in loss of power. There is also less current available to the load in this arrangement, so a fault that occurs far away from the source may result in the breaker failing to trip.
As a long-term safety precaution you should never permanently rely on extension cords. If you don't have enough outlets, have an electrician install more. Electrical outlets that are installed professionally by qualified electricians are built in compliance to standards developed over many years which ensures robustness and safety. Attaching two extensions, one after the other, is not a robust solution.
If you must use an extension cord, then you should ensure that the longer it is the greater is the cross sectional area of the conductors and that it is protected by a fuse to prevent excess current flow.
Using extension cords properly is critical to your safety. The Electrical Safety Foundation International (ESFI) offers the following tips related to the use of extension cords for staying safe from electric shock and electrical fires:
Assuming each extension cord is correctly rated and protected, once you have two identical extension leads in series, one after the other, the combined lead will experience a voltage drop per unit length and also increases the overall resistance of the two cords
As a result, the load does not draw more than the rated current and also causes the temperature to increase inside the cord.
It is also considered unsafe for other reasons such as trip hazard. That's why in normal practice, we avoid connecting 2 extension cords together.
Always remember that extension cords are not intended for permanent use, however they can be used temporarily for a specific period of time.
There are two main important concerns:
Extension cords are marked with their amperage. As long as you don't exceed that it doesn't matter how many cords you have plugged in. At least theoretically.
Consider an example:
It is possible to ruin a motor with precipitous voltage drop which results from a circuit being too long but if you are only plugging in two extension cords that is not likely to be a problem.
If your motor has trouble starting up, then you might have a problem. If that happens go up a size or two with your extension cords and make sure all the plugs (caps) are in good condition.