I have my home electricity wires, and I want to attach directly a set of wires that will be plugged into a controller box with power supply. that power supply was a phone charger that i adapted into my controller, and i took only its circuit board. I soldered screw terminals to the charger's board in order to have easy plug for the home electricity. The only question I have is if I need to take into consideration the wire's width. the screw terminals holes arent the largest, and i wish to know if I can use a smaller diameter wires with it. The home electricity I already added terminals to split it, both to its regular destination and to my project power supply. So from that "junction", can I use a smaller set of wires? How can I know how small? Its just a phone charger, its not taking alot of current, so I think its possible. I just dont know what the limits are.


closed as off topic by Dave Tweed, Leon Heller, Martin, Nick Alexeev, Brian Carlton Feb 15 '13 at 18:47

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    \$\begingroup\$ Hope that you know what you are doing. Projects involving 220VAC mains current, should be undertaken knowing the significant risks of electrocution / fire-hazards, thorough understanding of how to avoid those and with the right tools. In any case, your question isn't clear. What do you mean by "controller box" ? What is it controlling ? Why do you want to wire it directly to mains ? Why is using standard electrical plugs, not an option ? And what is the relation to phone-charger ? \$\endgroup\$ – icarus74 Feb 15 '13 at 9:37
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    \$\begingroup\$ If you aren't familiar with wire gauge, current, and power, don't start with mains voltages. Start small and work your way up. What you are doing is dangerous. \$\endgroup\$ – JYelton Feb 15 '13 at 9:37
  • \$\begingroup\$ Its an arduino and a relay to control 220vac.everything is working OK. in order to finish it, i need to use a power supply. this entire "box" will be located far from a plug, so I took a phone charger that i can (and its working for me great) power-up my "box", and took its board. that board now (after some work) contains screw terminal. I want to wire it to the 220vac wires. but those regular "high diam." wires are very hard so I wanted to know if i can use some smaller diameter wires. \$\endgroup\$ – e-r-a-n Feb 15 '13 at 9:44
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    \$\begingroup\$ Why not install a wall outlet socket and do it the safe way instead of jeopardizing your home/fire insurance coverage? \$\endgroup\$ – jippie Feb 15 '13 at 10:23
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    \$\begingroup\$ 'but what difference does it make if it uses the 220vac this way or the other..its the same "risk"' Check that with your insurance company. If they don't cancel your insurance, it is OK. This is highly depending on the country you live and local laws. \$\endgroup\$ – jippie Feb 15 '13 at 11:04

What you are doing is dangerous, and it doesn't sound like you understand that. If your house is supplied with 220V, and it has a 15A circuit breaker, it can supply as much as 3.3kW, or 3.3 thousand joules of energy per second. A bullet fired from a rifle has about 2 thousand joules of kinetic energy. So, roughly speaking, the mains voltage in your house has the potential to shoot you with a rifle once per second.

This is why we have so many safety codes. Through about 150 years of people being killed by modern electrical distribution, some people have engineered very good ways to minimize the hazard presented to ordinary people while still reaping the benefits of this very convenient, very concentrated energy source. You don't sound like one of those people, and I'd advise you to give them due deference. The codes don't exist to annoy you. They exist to protect your life.

Here's just one way what you are doing might kill you: you say you have soldered screw terminals to your device. Most electrical codes forbid solder on unfused mains connections. This is because when something goes wrong with your device, a large current may flow. This large current makes a lot of heat, and solder melts at a relatively low temperature. The screw terminals may fall out, and if the neutral line falls out but the live line doesn't, there's no reason the electricity won't use your body as a path instead of the neutral line. This isn't a hypothetical failure mode. People have been killed this way, and that's why all the wiring in the electrical boxes are done with wire nuts which are rated to withstand the temperatures that can exist in a short circuit condition.

This is just one of thousands of hazards you must keep in mind. I've been working with electronics for a long time, and even I avoid building projects that connect directly to mains. Power supplies are cheap enough, and I value my life.

Since you are just going to ask your question elsewhere if we don't answer it here, I might as well. There are two obvious ways a wire can fail through too much voltage or too much current.

If there is too much voltage, the insulation on the wire isn't sufficient to withstand that voltage, and it breaks down. You should use wire rated for at least 220V AC to avoid this.

If there is too much current, the wire overheats, melts, melts its insulation, starts a fire, or otherwise does bad things. The degree of heating per unit of current is a function of the diameter of the wire. There are many tables on the internet which can help you calculate this. So in theory, you could use a small wire for 220V supply to a device that requires little current.

In practice, people don't use small wires for this, because they break easily and kill people. I'm sure there's something in your local electrical code about this. Again, what you are doing is a bad idea. If you absolutely feel compelled to continue down this path, don't blame me when you burn down your house, kill your pets, or kill yourself.


What you are looking for is the current rating of the wire. There are places to get easy access to a wire gauge chart. You just need to ensure the wire you are using is rated to a higher current. AWG is designed to be logarithmic, that means you can double your current every time you subtract 3 from the gauge.

You can use a smaller wire if your current requirement is lower then that required by the rest of your house, if it is a manufactured product the fact it does not fit a larger wire gauge is probably a sign it does not pull that much current. However you do have to ensure you are not violating building codes, if you have your project go awry you might be very liable.


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