My husband and I just bought a new treadmill. I used it yesterday evening with all of the lights in the room on, as well as a stereo plugged into the same outlet and it worked fine. Today, my husband tried to use it with no lights on, and it kept popping the breaker. He unplugged everything else in the room and it is still popping the breaker. Why would it do that today and not yesterday?

Females traditionally weigh less. Less load. Less current. Or it could be the setting of the treadmill and the occupant combined.

Kiss, good answer!

Sometimes the belt may be too tight, and combined with the weight of the rider will cause the motor to draw more currentthan the circuit can handle.

Or can be a loose connection at the breaker, which causes heat at the connection and this heat causes the breaker to trip.

If the belt is fine, then checking the breaker may be called for. If your not knowledgeable about working in a live panel, you may need to call in a qualified electrician to troubleshoot the problem.

Your treadmill is likely overcoming the home's ability to provide the power required. Most new treadmills require a dedicated 20 amp line (which is usually only found to run the biggest appliances in the home)
As the new treadmill breaks in, it will draw less amperage but the long and the short of it is to check the owner's manual for the power requirements and then check the breaker that it's on.
The likelihood is high that the Tmill wants 20 amps and your breaker is a 12 or 15.
You can elevate the treadmill somewhat as this changes the user's gait and the result is a lowern amp draw but you will need to either:
1. Provide the treadmill with the power required
2. Get used to using at an elevation (harder workouts but faster results)

Nick (24 year fitness equipment technician)