I want to put in a 100amp subpanel in my garage. The run will be underground and 300ft long.
I have a Siemens panel with breakers rated at 60/75 deg C.
What is the proper wire size I need? I was originally thinking I needed to put the wire in conduit, but is there a better way to go with a direct burial wire?
Panel size is listed as 100A. Based on comments of a load calculation of 76A, that would be correct. I am dubious about that (I expect it to be much lower), but let's assume it is correct for the moment, and use that as the value for voltage drop calculations. In addition, welder and at least some of the other key tools are 240V rather than 120V, so I'll use 240V for voltage drop calculations.
Note that you can (and generally should) use a 100A or larger panel as a subpanel, even if the feed is much smaller. For example, if you use a 90A feed breaker and 2 AWG aluminum wire, you can feed a 100A (or 200A!) panel. Larger capacity (current) panels are also larger capacity (spaces) panels, and you can always upgrade the feed wire and breaker if you use large enough conduit.
Minimum wire sizes (wires, not cables, 75 C rated) are 3 AWG copper and 1 AWG aluminum. For any significant distance, aluminum is almost always the better choice due to cost.
The Southwire Voltage Drop Calculator gives 4.46% for 3 AWG copper, 300', 240V, 76A and 4.61% for 1 AWG aluminum. Bumping up one size to 2 AWG copper gives 3.46% and to 1/0 aluminum gives 3.96%.
One thing that has changed quite a bit in the 9 years since this question was originally posted is electric vehicle supply equipment or EVSE, used to charge electric vehicles. If this garage is used as a garage and not just as a workshop then adding 20A - 30A @ 240V for electric charging is a common and reasonable thing to do. That could take a workshop garage from a much smaller real-world load - perhaps 40A - up to 60A - 70A or possibly more. Which in my opinion turns the 76A number from a way too high value to something quite reasonable.
Definitely go with conduit. Buried cable, which must be properly burial rated - you can't throw ordinary Romex in a trench - has to be 24" underground. PVC conduit only needs 18" and rigid metal conduit needs only 6", except under driveways. For a short distance this makes rigid metal conduit a very good deal as you can dig the trench easily by hand. For a 300' installation it may not matter so much because it will pay to rent equipment to get the job done in a day. But I would definitely trust PVC conduit in the long term much more than buried cable.
Conduit size will vary. A quick calculation on Southwire's site shows that 1-1/2" PVC 40 or Rigid Metal Conduit will do fine. You may be able to use a slightly smaller size, but a larger size will allow for 1/0 so if you start with 1 AWG and later need to upgrade you can do that.
So for anyone finding this page, none of the answers are incorrect or be qualified electrician. please for any electric services needed find and ask a qualified electrician.
Basically the breaker size protects the wire, So the breaker should not be higher amperage than the wire rating, however can over size the wire to meet voltage drop requirements. load calculations are for what minimum size panel/service needed but not the actual service rating if the breaker from the service side is a 100amp breaker feeding this then wire rated for 100 amp, etc. Most ampacity charts are up to 100 feet and go up one size of additional 100 feet. So, if using a 100amp breaker at the service side, then need to minimally size the wire accordingly and there are different rating for non burial, burial and in conduit Aluminum sizes will be larger than copper. For that service would use URD Aluminum 4 conductor cable (need 2 hot, neutral and should have separate ground to shop, grounds isolated from neutrals in shop panel. for 300 feet for 100 amp rated service I would use Aluminum direct burial 1/0-1/0-1/0-1/0, the forth can be as low as #4 for the ground (but also in conduit, even if in conduit must still be rated underground wire and required by code also) Also note the size wire the breaker can handle, cannot cut strands to make fit, no no no, so check the breaker specs first. So if not really needing full 100 amp load, can go down smaller wires sizes, but use smaller breaker to meet the maximum amperage rating for that wire. so for example could use a popular 2-2-2-4 URD Aluminum direct burial wire, but would only use a 60 amp breaker for that distance, 80 amp if needed but could experience some voltage drop at high load, bad for running motors any length of time. Copper would allow smaller wire size, bigger wire still fit into breaker but is way more expensive. best to google look at a few wire size charts better contact an actual qualified electrician.
