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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?

Niall C.
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Ryan
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    This answer might be helpful. – Tester101 Jul 28 '14 at 13:56
  • Yes, I read that post. That is for 60 ft run, does the 300 ft in my case make a difference? – Ryan Jul 28 '14 at 14:57
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    Only if you care about power quality. 300' of #3 uncoated copper @ 100 amperes, will have a voltage drop of 14.7 volts. So if the panel is fully loaded (drawing 100 amperes), and you have 240 volts at the service equipment panel, you'll end up with 225.3 volts at the panel. And that's at the panel. You'll loose even more along the branch circuits. If the panel is only drawing 50% of the max (50 amperes), you'll only loose 7.35 volts. This is why load calculations are important, they allow you to figure out what wiring is required. – Tester101 Jul 28 '14 at 16:40
  • Without knowing more about the equipment that will be connected to the panel, it's difficult to give an accurate answer. – Tester101 Jul 28 '14 at 16:41
  • I used this link to figure out the capacity of the panel. Is this what you mean about load calculation? The highest single amp draw i have is a welder 240v / 30 amps. Simultaneous high draw tools is 23 amps (table saw + shop vac). Continuous draw amps is 15 (lights). The equation in the link gives me 76 amps and I bumped it up to 100. – Ryan Jul 28 '14 at 16:57
  • If this is a simple one man shop, it's not likely you'll be welding while you cut things on the table saw. The lights will always (or usually) be on while you're working, so you'll have to figure for that. However, for the most part, you won't be drawing anywhere near the 100 ampere capacity. Which means you'll probably never have a problem using #3 conductors. If you often have helpers, and you'll be using the welder and table saw simultaneously. You might want to bump up the wire size, to reduce the voltage drop across the long wire run. – Tester101 Jul 28 '14 at 17:43
  • NEC recommends (but does not require) a maximum of 5% voltage drop from service to consumer. So you'll probably want to try and design the system to reduce voltage drop as much as possible. – Tester101 Jul 28 '14 at 17:45
  • ok, so you are saying the minimum I can use is #3 copper. I'm confused by this because on table 310.15 from this answer I have to use the temp rating of the conducter which is 60C for copper which says #3 is 85 amp max. – Ryan Jul 28 '14 at 18:09
  • Ryan, don't forget, you CANNOT simply add up 240V and 120V amperages. This is a completely ambiguous number. – Speedy Petey Jul 28 '14 at 19:49
  • @Ryan you said the terminals are 60/75, so you can use the 75 column. – Tester101 Jul 28 '14 at 20:37
  • thank you for your help. For the voltage drop, I used southwest's calculator which told me I need a 1awg copper line. – Ryan Jul 29 '14 at 11:21

2 Answers2

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Old question, but it deserves a proper answer...

TL;DR Aluminum 1 AWG or possibly 1/0 if you expect consistent high usage, in 1-1/2" rigid metal conduit (6" burial depth) or 1-1/2" PVC schedule 40 (18" burial depth).

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.

manassehkatz-Moving 2 Codidact
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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.

Mike
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