Here is an Ohms Law chart for colculating volts and currents.
If you know 2 factors in your cicuit you can use this
chart to find the additional information you need.
Example = You want to add 3 light fixtures to your shop,
each with 1 100 watt bulb and you know you have a 115 volt
service. You would devide 300 (P) by 115 (E) to find (I) which
would be 2.06. So now you know that your new circuit needs
to be capable of handling 2.06 amps.