The Ask Marilyn column in the April 8 Parade magazine says the reason that one is not supposed to link three or more outdoor extension cords in series is that doing so would create a fire hazard. That doesn't make sense to me.
The heat generated per unit length in an extension cord is I squared time R where I is the current and R is the resistance per unit length. The cord should be able to safely dispose of that heat for normal current levels. By adding more extension cords in series, one increases the resistance of the total circuit, thereby reducing the current. It reduces the heat generated per unit length. Of course if one piled more and more extension cord into one confined space, one would have more heat to get rid of from that space, and that might be the reason for the warning. But stretching out several extension cords in a long line (for the same final load) should not increase the fire hazard.
Extension cords come in various sizes, and a heavy duty cord will generate less heat for a given load. A longer cord should, in theory, be heavier gauge, generating less heat per unit length, and also dropping less voltage per unit length.
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