[floatingatoll]

It matters if it short circuits, yes. (Electricians, I hope I got this right enough to convey the 'why'; I know it won't be perfect.)

The outlet plugs specify the circuit breaker limit. Most consumer electronics in the US use a 15amp plug, which can be plugged into a 15amp outlet or a 20amp outlet — but no higher, due to physical incompatibilities in the outlet design.

The electronics that use the 15amp or 20amp plugs are therefore built not only to draw no more than the amps rated by their plugs, but also to self-destruct relatively safely if they draw the maximum amps available from the circuit breaker backing that plug.

So if a cheap device correctly assumes as part of its "don't explode" protections that it will never receive more than 20amps due to using a 15-or-20 amp plug, and then it short circuits while plugged into a 25 amp circuit using an adapter, it could very well explode, because the basic guarantees of electrical safety were violated. The plug used guaranteed it would never receive more than 20amp, and now it's receiving 25!

This is especially relevant when you're considering how to make use of an idle 30A dryer outlet in a garage. If you just plug an adapter into it, and your device short circuits, it will explode even more violently. Risk of harm increases with outlet power.

There exist fancy "breaker box" adapters that have a 30A plug on one side, a fuse box with a 15A or 20A fuse in the middle, and a 15A outlet on the other side for you to use. It's not really an adapter at that point, but the presence of that 15A/20A fuse provides the missing piece of protection for your 15A/20A device that a plain adapter wouldn't have.

So in summary, the only way to safely use a 15amp device on an outlet that delivers power higher than 15-20A is to somehow inline a 15-20A breaker between the device and the outlet (or, to rewire the outlet and its power feed to 15-20A).

TL;DR: Hire a licensed electrician to tell you what your options are and decide how much you care to spend and whether you want to make permanent modifications to get the job done.

[mohaine]

This isn't really correct though and gives a false sense of security. The house breakers are sized for the wires in the wall, not the device plugged in. Those should have their own internal fuses to protect themselves.

Most lamp cords are only sized 100ish Watts or ~ 1A with the bulb being the only fuse. If these somehow pulled 15A for any amount of time the wire would quickly get smoking hot but you still plug them into a 15A plug.

Devices are supposed to protect themselves. Most 15A devices will cause fires if they actually pull 15A for any amount of time. If you try to use a a small extension cord on your space heater, you will soon be smelling burnt plastic while never getting over 15A draw. That is part of why these are such a fire hazard.

Alos in the US a 20A socket is designed to allow a 15A plug to work in it, But not the opposite way as that would cause issues in the wall.

[jk700]

Right, it's even more complicated. Such breakers also don't trigger at specified current exactly. They have two breakers inside of them, one for overload protection triggering once it heats up, could be an hour for 2x current if starts cold, and one for short circuit protection triggering in less than a second, but on 3x+, 5x+ currents, etc. So a 15A breaker, a 15A outlet and a cable for 15A all could easily see 30A of current for some periods of time and heat up.

[mohaine]

I actually used to calibrate and QC high end breakers for a well know company who's name is a letter and a shape. To pass QC the breakers would need to heat trip when run at 135% Amps between roughly 20-45 minutes. Both too fast and too slow were a failure.

The actual range was a bit different by Ampage which I never quite understood.

[Dylan16807]

Any device that becomes significantly more dangerous when the breaker trips at 25 instead of 20 amps is relying on way too thin of a safety margin and I would consider it a lurking hazard on any circuit.

[catblast]

That is not how safety margins work. Safety margins are meant to give a buffer for unforeseen circumstances, they are not a ticket to just cheat. With this logic... why stop at a 25A breaker (30 in the US)? Why not just plug a 20A device into a 100A breaker, or no breaker at all?

No safety margin can account for purposeful circumvention, which is what connecting a 15/20A outlet to a 30A circuit is.

[Dylan16807]

You see no difference between 20 vs. 25 amps and 20 vs. 100?

I didn't say any difference was unacceptable, but a significant difference for 20/25 should not be accepted.

If the danger gets gradually worse for every 5 amps on the fuse, that's fine. Then the excess danger at 25 or 30 amps is only a tiny fraction of the excess danger at 100 amps. Good work.

If the danger has a sudden sharp increase at a certain amperage, then that amperage threshold needs to be further away than a mere 20/25 difference. Or even 20/30.