Is there a "Watts" rating on the treadmill? Unless we know this, we might bbe making a mountain out of a molehill. Shorter is better, but sometimes it can be hard to find larger gauge (12) in shorter lengths. You kill the advantage of a larger gauge conductor by having excessive length. Sometimes I buy larger gauge cords, but them and install my own "ends". (Just be sure to wire them properly!)

Probably not.

The 12 gauge cord is preferable.

Yes. Your problem will be that short, heavy duty cords are hard to find.

The best short heavy duty, 15amp 12ga extension cords I found are the one's sold for us on a window airconditioner. Usually 4-6 foot long.

You can always purchase a longer cord and cut it and install new ends to make two extension cords.

The price of cordage is often so high (plus the ends) I purchase extension cords and cut off the female end to make power cords for shop equipment.

You can then put a single male end on any leftover piece and still have an extension cord.

The watts are not shown on the machine. Thanks very much for your help and suggestions. Off to look for a cord...

"The watts are not shown on the machine. "

Either a current requirement (amps) or a power requirement (watts) is on the nameplate of every UL listed piece of equipment.

The OP said 15amps.

The OP wrote that 15 amp dedicated circuit is required. That is different from the nameplate power or current consumption.

I think what many places call "appliance cords" are about the heaviest cords you can get in a short length. I would check one of these out. Walmart has them so that will save you from going on an extended search. Not that I am a fan of walmart.. any home depot or lowes or hardware store will also.

If you read the OP correctly, it says
"Its manual states that the treadmill must be powered by a dedicated 15 amp outlet" and also "The label on the machine reads "120V, 50/60Hz, 15 Amp."

RTFOP

If the machine requires 15 amps it is not allowed on a 15 amp circuit.

No equipment should pull more than 80% of the rated current.

Something is VERY wrong.

As in it does not sound UL listed, a real warning sign.

"If the machine requires 15 amps it is not allowed on a 15 amp circuit.
No equipment should pull more than 80% of the rated current.
Something is VERY wrong.

As in it does not sound UL listed, a real warning sign."

Not true. NEC requirements and UL requirements are not the same. In fact, for loads that operate for less than 3 hours under UL a standard breaker can be loaded to 100%. If it shall be more than 3 hours continuous you need a 100% rated breaker.

There is no inherent time limit on the treadmill.

Time limits normally only apply to things like elevators that simply cannot operate continuously.

I never said UL and NEC were the same, just that the device appears to have a problem in its labeling and might NOT be UL listed.

1. No, you will not damage the machine by using an appropriate extension cord.
2. Use a cord rated at least 15A. You can always use larger gauge wire.
3. Yes, always use the shortest possible cord. A 6-foot appliance extension cord should suffice.
That said, the treadmill manual says not to use an extension cord because extension cords are intended for temporary use only. An example of temporary use would be to run power to a battery charger to get your car started. You use it for an hour or 2 and put it away.
The proper solution is to have another receptacle installed near the treadmill.

"There is no inherent time limit on the treadmill."

How do you know? You don't know anything about this treadmill. Saying things like:
"Something is VERY wrong.

As in it does not sound UL listed, a real warning sign."

Is just scaring people for no reason. Stop it with the FUD.