Electronic safety with watts/volts/amps


There are typically 3 things when connecting a device to power:

1. wall outlet

2. extension cord/phone charger

3. device being charged or powered

Each has watts/volts/amps labeled. With W/V/A, how do I determine if a system is safe or unsafe?

In: Physics

If it is less than 50 volts, it is generally safe. If it is less than 10 milliamps is generally safe, though may produce a shock with high enough voltage.
Watts is just volts times current.

A wall outlet is ~120 Volts (240 in Europe) and can carry up to 15 amps of current (sometimes 20). An extension cord is just a wall outlet extension, exactly the same. a phone charger has an inverter in it which will drop the voltage down to less than 10 volts, a safe level. The device being charged by it is the same. The amperage going through the phone charger can be pretty high, but the voltage is low enough that it won’t be a danger to you unless the inverter at the plug fails. I’ve heard of cases where people have got an electrocuted by dropping their phone in a bathtub while it was charging.

So, the safest thing is to assume that any current carrying conductors is not safe and to treat it as such.

Edit: fixed breaker amprage

Engineer checking in… If the device, cord, and outlet are all in good working condition and there is no exposed wiring then you should be fine. However, if you’re unsure then get someone trained, like a licenced electrician, to inspect it for you.

If you’re trying to design your own circuit for some DIY project then I’m going to suggest that you dont unless you get some real training somewhere. There is no one number for what a “safe” voltage or current is.

What makes these circuits safe is if they are properly wired, insulted, grounded, and protected. No loose outlets, no loose face plates, no loose connections, no frayed insulation, no exposed wires, no ground plugs cut off or subverted. Circuits near water should be part of a GFCI. Plugs should be shielded from water and devices no rated to be used with or near water should not be done so.

Regarding volts and amps, what makes them safe is if their circuits are compliant with code and operation – what makes them safe is that you don’t make yourself a part of the circuit. You’ve got guys who work on high tension wires from helicopters, or walk them, while they’re energized, and those things carry currents that simply boggle the minds of electrical engineers – many, many orders of magnitude more power than what they use in an electric chair, and those electricians are generally safe.

It only takes a few millivolts, a few milliamps across your heart to put it into atithmeia and kill you – some household batteries can do that, but that’s the trick, isn’t it? For do-it-yourselfers, they teach you to work on electric circuits with one hand, so that you don’t make a path across your arms, past your heart. I’ve been shocked by 120v 20A by accident, and it’s not pleasant. It’s no joke, that’s way more than necessary to kill you, but since the circuit was local only to my hand, it was just a bad experience I wouldn’t recommend to anybody.

For the most part, we as a society agree that properly used home electric and connectors are safe for the naive public, and home wiring can be learned without any certification. Beyond that – if you don’t know, if you’re not sure, it’s not safe FOR YOU.

I was looking for the similar info yesterday. The following link imo has really good explanation. Hope it helps


Safe for your person? Then unless they look completely broken, burnt etc most consumer devices are generally safe to use if there is no abuse. (ie don’t stick a screwdriver in a power outlet etc). It isn’t recommended that you open things up and try your hand at rewiring things unless you know what you’re doing.

Safe in the sense that it won’t damage equipment. First, know your region’s standard power – there are (very broadly) two standards for consumer power outlets ~100V and ~240V. There is also multiphase power but you should probably not encounter them too much (and they come with a different looking plug)

Second, look at the device that plugs directly into the outlet. There should be a label and there should be something that looks like “INPUT” or “Input voltage”. If this matches your region’s power, then the device can be plugged in and be expected to work with no damage. Most phone, tablet and computer chargers are universal – expect to see “INPUT 100V~240V” or similar. These can be plugged into any consumer outlet without issue.

Extension cords are convenience devices – they don’t change the power and voltages. The general rule is that if the plug fits the outlet, it is probably safe to use (check for wiring damage if you are concerned) It is recommended that you use these extensions for mostly low powered items and never plug one extension cord into another. Things that are generally “high powered” – electric kettles, microwave ovens, ovens, electric heaters, electric cookers, refrigerators, room heaters (see the trend!) These devices should have their own dedicated plugs, if possible, or not used simultaneously for long periods if sharing an outlet (this is ultra safe).

Power is generally not an issue (see above). Devices draw as much power as they need and won’t “overdraw” power. The higher power the device, then it will require more current (power = current x voltage). An outlet with a limit of 13 A (common in many places) can provide power for a total of ~1500 W (in 110V regions) or around ~3000W in 240V regions. If you don’t stack extension cords or plug in multiple high power devices, this is very much more than needed for things like device chargers which are usually < 100W or perhaps a large laptop possibly 150W or so.

There is a lot more nuance to this but it is not necessary if you’re not going to poke inside the electrical wiring.