I'm wanting to run power from a 20amp breaker from my house to a small storage building 100ft away. All I want to run on this line is a fluorescent light and a small hand drill or small shop vac. What gauge wire do I use? 12 or 10ga?

10 gauge because of the long run. Voltage drop . Sometime you may be up in the shed running a skill saw and will have loss of power better to do it now and besides the cost of wire is down now.

Royboy is right.

THe little more you spend for 10ga at this distance is well worth it.

Steve

Always consult with the national electrical code before beginning any electrical projects, because the code establishes the min. standards for supplying electricity to any structure.

First, determine the total load that will be used on this circuit. Secondly, determine the voltage drop that will occur on the conductors. You are only allowed 5% total voltage drop. Next, you must apply the 75% rule for contineous usage on this circuit(75% of 20 amps).

Voltage drop will will help you to determine what size conductors to use as well as distance. Size 10AWG is a good place to start your calculations. Determine type of cable you will use such as NM-B or UF-B. also you must use cable that can be used for the purpose you wish to accomplish, such as wet or dry location. Above all, consult with the national electrical code book to properly and safely provide electricity to your storage building. I have given you only the highlights. There is more.

did you notice that this post was a few "years" old? Try to stay with posts less that a month old...
(isn't continuous load taken at 125% + non-continuous load = total load? 75% appears to be incorrect)

Ampere, this question is is 4 years old.

Please refrain from answering questions older than a month or so.

Now a few corrections:

#1. 5% vD is not mandatory, only a recommended value by IEEE, for general purposes.

#2. 75% is incorrect derating value for a continuous load. 80% is the correct derating value.

#3. Be careful suggesting to laypeople to consult the National Electric Code. The NEC is difficult enough for trained qualified personel to learn and understand, laypeople will be completely confused and do possible harm by not understanding the Code.

Anyone considering having electrical work done should be consulting with a qualifed electrician.