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To illustrate, I have the following setup:

  • 15-Amp breaker
  • 100ft 16-gauge extension cord rated at 10-Amp
  • 13-Amp corded electric Lawnmower

When I make the mower work extra hard by trying to mow some thick group of weeds, the breaker in the electrical main trips.

Is my 13-Amp lawn mower simply drawing more than 15-Amp? Or is the thin extension cord the culprit? How? Won't the wire just melt/burn on excessive current?

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Both can be a reason. If you overload an electric motor it will draw more than the rated value causing the breaker to trip. The second reason is excessive length of a small cord can cause a large voltage drop browning out the motor causing higher than normal current. A 15A breaker is only truly rated for 12A with a continuous load (more than 3 hours) or motor loads. You normally would want a cord with at least a 16A rating for the mower since it is a motor, I just plugged in 120V, 13A at 100' this provided a #10 cord to keep the voltage drop under 3%. here is a link to SouthWire® voltage drop calculator.

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    To add a one sentence summary: because of the long 10 amp cord, voltage under load at then end is decreased which directly causes the motor load to be higher than the rated 13 amps. If you don't need 100' get a shorter and heavier cord, if you do need 100' get a very heavy cord.
    – Tyson
    Commented May 24, 2016 at 21:24
  • Can always use one heavy cord and a lighter cord (like 25 feet) at the end for mowing with. Dragging a heavy cord is difficult.
    – user50401
    Commented May 24, 2016 at 21:44
  • @nocomprende yes and no... I've seen cases where there is so much resistance created by the extra male/female connection that all gains are lost are lost in heat at that extra connection. I would recommend a single cord not any longer than it has to be.
    – Tyson
    Commented May 25, 2016 at 0:32
  • I bought a 50' 14-gauge cord, rated 13-Amp.Tried to bog down the mower through some weed patch, the breaker didn't trip. Now I'll see what the situation will be in my front lawn, which is 45' deep
    – Rollo R
    Commented May 29, 2016 at 1:24
  • With a shorter cord the voltage drop will be less and may work fine as you have found.
    – Ed Beal
    Commented May 29, 2016 at 1:52

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