How do I know what size wire to buy?

When buying wire, one of the first questions that is always asked is "What size do I need"? There are a few considerations. The first being the location of the cable.  Will it be underground? Overhead? In conduit? Cable tray?  The answer to this question can narrow the category of cable that can be used and reduce the choices to manageable levels.  Once that decision is made, one must determine how far the cable run will be.  In other words, how long a cable is needed to get from point A to point B.

Length is an important consideration because all conductors lose a bit of power over distance,  This is referred to as voltage drop.  Copper wire has excellent conductivity, whereas Aluminum is good, but not nearly as efficient as copper. Efficiency improves as you increase the gauge, number of conductors, and stranding of a wire.  In fact, one way to reduce power costs over time is to use higher gauge wire than you might otherwise need.  There will be less voltage drop, and therefore greater efficiency in the power used.  The end result is a lower power bill.

The last decision ties in with length and that is determining what the power needs are for the project.  What voltage is necessary at the end of the run?  The main rule is you do not want to be under supplied.  When it comes to selecting the gauge, it is always better to be safe than sorry.  In short, the cable selected can always be larger than is actually needed (and may provide a benefit as explained above), but can not be smaller than needed.  If a cable is rated for 600v, but the project requires only 400v, then it is fine.  For a power requirement of 300v, using only an SJOOW cable rated for 300v would not be advised as it may cut it too close.  In this instance, it would be wise to step up to the SOOW 600v cord.  You can always go up.