Allowable ampacity through cable
This is a question of resistive heating, so it's about RMS, not average current. You need to maintain an RMS current of 12.5 amps. That depends on the shape of the pulses and the duty cycle. If we assume your pulses are perfectly rectangular:

$$
I_{RMS}=I_{PK}\sqrt{D}
$$

Plug in 12.5A RMS and 25A peak, and your duty cycle is .25.

Now, this is only really useful if your pulses are short and close together. If you're on for ten minutes, off for thirty, you may have a 12.5A RMS, but it's an RMS over a much longer period than the thermal time constant of the wire.

The thermal time constant of the wire will be on the order of 30 minutes to a few hours.  Welders use a 10 minute cycle for "short enough to use RMS".