I am trying to mount a strip of three-prong outlets and a strip of USB charging outlets on an old wooden desk. I can handle the carpentry aspect, but I am struggling with the electrical aspect.

I need the basics: are there preferred types of three-prong / USB outlets for this, what type of wire should I use, safety, etc.

Thanks in advance.


closed as off topic by Passerby, Leon Heller, Dave Tweed, Kaz, Anindo Ghosh May 3 '13 at 7:03

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  • \$\begingroup\$ At best, this belongs on DIY.stackexchange.com \$\endgroup\$ – Passerby May 3 '13 at 1:15
  • \$\begingroup\$ @Passerby: Admittedly I'm a newbie here, but this looks on topic to me: the FAQ says this site is for "electrical and electronics engineering topics", not just electronics. Safely installing a mains outlet sounds like "electrical engineering" to me. \$\endgroup\$ – Ilmari Karonen May 3 '13 at 16:16
  • \$\begingroup\$ @IlmariKaronen there is not a single electrical wiring question on here. Look at DIY.stackechange.com for those questions, which this is. Electrical engineering is design and control of high voltage sources. If OP was designing a electrical transformer sure. Simple house wiring is Home Improvement. \$\endgroup\$ – Passerby May 3 '13 at 18:50

AC Outlets

Since it sounds like you aren't that familiar with electricity, I would recommend taking a standard power strip and embedding it into the desk. If you wanted, you could remove the top plastic shell of the strip and cut out your own from wood to match the desk. I only suggest that since you mentioned your carpentry skills. But you have to ensure all of the internal contact and master switches will still work properly and cannot possibly short.

If you wanted to try and wire your own electrical outlets, you could do so using the standard outlets you find in a hardware store or even Walmart. The wire gauge specified by the National Electric Code (NEC) is 14 gauge for a 15A breaker, 12 gauge for a 20A breaker, and 10 gauge for a 30A breaker. I think most power strips use 14 gauge internal wiring. Also note, that since these desk outlets will be powered from one regular wall outlet, the current through the circuit of that wall outlet will include all of the current used through any of the desk outlets meaning you shouldn't try to power anything too "big" from these desk outlets. Also, remember to never plug one type of spliiter (power strip, 3x1 or 6x2 outlet splitter, etc) into another for the aforementioned reason.

Outlet Wiring

Outlet Wiring

  • Ground Wire (Green or Pure Copper) connects to the little screw in one of the corners.
  • Neutral (White Wire) goes to the silver terminal - should be the longer of the two outlet slots.
  • Hot (Black Wire) goes to the gold terminal - should be the shorter of the two outlet slots.
  • Leave the removal plate on side terminals in place to keep the two outlets in parallel.

USB Ports

USB operates at 5V DC. Typical ports are nothing more than voltage rectifiers/converters to give a steady output at the desired level. It is up to the device being plugged into determine how to regulate the current flow from the outlet. While there are many commercial outlets available which have USB built in (Like this, for example) they greatly increase the cost of the outlet.

USB OUtlet

On top of the added cost, they waste power. The port is regulating the voltage regardless of a device being plugged in or not - similar to the waste from leaving a standard phone charger plugged in all the time. In addition, they can make it more difficult to use an outlet for any other purpose unless small plugs are used. I would recommend you get a common powered USB Hub like this:

Powered USB Hub

The thing to note about using USB is the available amount of available current. From a computer, the default is 100mA. Once a device has gone through the "enumeration process," much more current can be drawn. This process is talked about in this relevant question. However, since you are using an external, powered USB hub rather than the ports on a computer, this can usually be bypassed to allow charging (or powering) of multiple devices from the hub. In fact, you will not need to connect the USB hub to a computer at all, just power it from one of the electrical outlets. Although, some hubs may still require enumeration before sourcing larger currents - I'm not entirely certain on that. You might want to wire an outlet under the desk for this purpose to avoid extra cables on top. You could also get a switched outlet so you can shut off the USB ports when you aren't using them to avoid wasted power.

  • \$\begingroup\$ @bitpshr Thank you. I would mention that the outlet shown in Amoch's answer is a good choice as well. Also, you might want to "unaccept my answer" for a day or two to see if anyone else provides a better one. Many people will ignore this question if it has an accepted answer. Just be sure to check back in a few days and accept whatever answer is best! \$\endgroup\$ – Kurt E. Clothier May 3 '13 at 1:14
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    \$\begingroup\$ Downvotes without a comment are very unproductive... \$\endgroup\$ – Kurt E. Clothier May 3 '13 at 1:42

Depending on your location there are various rules and regulations about whether you are legally permitted to perform electrical mains wiring without the appropriate qualifications. These rules are in place because of the health and safety risks of working with mains power.

I would suggest purchasing an off the shelf power board with a mains plug and build that into your table rather than wiring up the same item yourself.

  • \$\begingroup\$ Thanks very much for the info! Sorry for such a basic question, but when you say "off the shelf power board," I assume you don't mean a regular power strip? Is there something specific you had in mind? \$\endgroup\$ – bitpshr May 3 '13 at 0:09
  • \$\begingroup\$ I do mean a regular power strip. They come with USB ports built in. Something like this \$\endgroup\$ – Amoch May 3 '13 at 0:52
  • \$\begingroup\$ Those rules are often for the health and safety of others, not the person doing the work. Like plumb laws. It's illegal to plumb other people's property (or rental) without a license. You can still do your own plumb work (NY State, and NYC). \$\endgroup\$ – Passerby May 3 '13 at 1:20
  • \$\begingroup\$ Like I said, "depending on your location". In some countries (Australia for example) it is illegal to perform your own electrical wiring if you do not hold the correct qualifications. And additionally, an insurance company won't pay out a claim (short circuit causing fire for example) if it was caused by illegal work. \$\endgroup\$ – Amoch May 3 '13 at 2:42
  • \$\begingroup\$ @Amoch that just reeks of nanny state and/or extortion for fees and money. \$\endgroup\$ – Passerby May 3 '13 at 3:42

Since you say you are experienced with carpentry but not electrical work, I suggest that you buy a pre-made unit designed to fit into a desk, which will connect with cables on the underside rather than any hard-wiring. Try searching with the keywords “desk grommet” or “in-desk” combined with “power” or “USB” (I'm not linking to any particular brand or store).

A basic desk grommet is just a plastic cap for a circular hole in a desk, but there are devices in the same form factor which have AC power and/or USB sockets. They are generally sold for use in offices.

Note that in the case of USB there are three different ways such a device could function; make sure you know what you're getting, and if you're buying a combined AC/USB device whether the USB is powered from the AC or not.

  • Full USB hub — carries power and data. May require being connected to a computer to function for charging.
  • USB power only.
  • Just USB sockets, which you have to connect on the back-side to a computer or hub to give them any function.

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