The Large 4 Pin Ones have 12 & 5 V DC as does the small 4 Pin type which is for the Floppy and similar drives as well as Some High End Video Cards which require a Power Connector.
The Large one is for the Optical Drive, HDD of the IDE Type and some SCSI HDD's.
The third type is normally a Black plug with 5 Wires going to it and this is for Optical and HDD of the Serial ATA Type.
The Plugs are interchangeable and are not specific to the device that they plug into but they are Polarized so they all only fit one way and it is not possible to fit them incorrectly without using Brute Force or filing down the Plug which should never be done.
Modern M'Boards have either a 20 or 24 Pin ATX Plug as the main Power Connector and either a 4 or 6 Pin Auxiliary 12 V Power Plug that plugs into the M'Board generally somewhere near the CPU. Again it is not possible to incorrectly fit any of these plugs without using Brute Force or making other modifications tot he plugs so they too are Polarized Plugs which can only be fitted one way.
There are no Ratings for the actual Plugs other than the 12 & 5 Volt Measurements which can not be altered in the current that they supply they will only provide enough for the device that they are supplying power for and provided that the polarity is correct which requires either Brute Force or other Modifications to not get right they simply plug in as you have been told previously.
You can read the voltage ratings for Computer Power Supplies here as far as the actual Voltages produced
http://tinyurl.com/ybdjlj
And the Standard pin Outs are listed here
http://tinyurl.com/5dfgr6
Some Dell systems do not use a Standard Pin Out on the 20 pin ATX Connector to the M'Board but the Pin Outs for the Drives are always the same and are made to a Standard Type for that plug type. So they are interchangeable for the type of Plug that they are where they get fitted doesn't matter provided that they are correctly Fitted to the Socket on the Drive or other device that they are being plugged into. Like The Chas I plug in the Drives and other items in the neatest way that the provided Cabling supports.
Col
