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I have a newly constructed shed that I want to run service to. My current panel is full and already has a 100amp sub-panel tied to it. The sub panel is not full. I want to run 100 amp service to the new building but am not sure of the size of the wire due to the distance. I've read some of the codes, but I'm overloaded with information. I've been told I could use a tandem 100 amp breaker from the main to run the second panel - and this is where my confusion starts. If I run 3 3 3 5, at 18" depth, is this sufficient? I've already purchased 1" conduit (plastic) for the burial, but after what I've read, don't think it's big enough. What do I want to run in the new building? Nothing heavy, probably 1/2 dozen plugs, overhead lights and enough to run a compressor (110) and a welder I haven't acquired yet - but have looked at 225V. Additionally, there will be an old table saw that I'll want to run periodically. Any insight would be appreciated. Or can I just run 10/3 from the house to the building new panel?

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    You need to determine that size of the equipment to size it right or pull enough to be sure. Still, You're over doing it sounds like. Regardless of whether the panel is full isn't as important as what it's current amperage. I'd turn everything on in both panels and read the ampacity at both panels. As far as the 10/3 goes - I seriously doubt it. – ChiefTwoPencils Feb 6 '15 at 4:42
  • BTW, what is 3 3 3 5? – ChiefTwoPencils Feb 6 '15 at 4:51
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    You want a "double pole" breaker, not a tandem breaker. A tandem breaker gives you two 120V circuits, while a double pole gives you a 120/240V circuit. – Tester101 Feb 6 '15 at 13:53
  • @ChiefTwoPencils 3-3-3-5 cable is a cable with three 3 AWG conductors and a 5 AWG conductor. – Tester101 Feb 6 '15 at 13:59
  • I read about a maxed out panel. I see a question if running 100A 150' on 10/3 is ok. Hire an Electrician to look at what's there. This is not something for which safe advice can be accurately provided over the internet. – ben rudgers Feb 6 '15 at 23:07
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Per the calculator here[http://www.electrician2.com/calculators/vd_calculator.html], 150 feet at 100 amps on 10Ga copper would be 15.5% drop - not the least surprising given that 10Ga is 30 amps in normal use and that's a long run.

  • 3AWG copper is 3.1% drop - not quite kosher, but pretty close.
  • 2AWG copper gets it down to 2.4%
  • 3AWG aluminum is 5% drop.
  • Aluminum has to be 1/0 (0) gauge to get below 3% drop (2.5%).

1" schedule 40 Conduit is not adequate to run a service with any of the sizes that will actually carry that load. I'd suggest 2" schedule 80 so that you could run it on aluminum wires, since they will cost a good deal less than copper, even at the larger size, in general. Schedule 80 only costs a little more than schedule 40, and is thicker/more robust. Since you already have it, run the 1" conduit in the same trench and you can use it for network, telephone, intercom or the like.

In a quick look at my local prices from vendors that publish prices...(there are often better prices from other vendors, but the relationship between Cu & Al is less variable.)

  • adequate 2Ga Cu $1.79/ft
  • not quite adequate 3Ga Cu $1.39/ft
  • overkill 2/0 Al $0.98/ft
  • and adequate 1/0 Al $0.82/ft

If you buy a separate ground wire, and upsize the conduit to fit it, there's also a 4/0 4/0 2/0 aluminum wire (twisted together, but not jacketed) for $2.09/ft, or less than 3 individual 1/0 wires. A better supplier might have a similar product with all 4 wires at a similarly attractive price. Depending on insulation type, that will still fit in a 2" conduit (or need 2-1/2 for other insulation types.)

None of this assesses whether your service into the building will carry the additional load, either. You may need to upgrade that before proceeding.

Both compressors (moreso) and tablesaws are hard to start - and both benefit from being run as 240V loads if possible, since they'll draw less current that way.

  • I'm getting 1.25 Sched. 40 for 3 3s and a 6. I used 6 because 5 isn't standard and if it's a cable assembly the sheathing is adding volume. – ChiefTwoPencils Feb 6 '15 at 21:21
  • Pulling at maximum conduit fill is No Fun At All. If it's a cable, you actually need a conduit sized for one-conductor fill by the cable (which is significantly larger than 3+ conductor fill for the wires making up the cable - I don't suggest using cables in conduit, just wires) In any case, 3's are not quite up to snuff for the desired load. – Ecnerwal Feb 6 '15 at 22:30
  • ...And the cost of copper .vs. aluminum well exceeds slightly larger conduit cost to fit the aluminum. Less than half the cost per foot for 'adequate' sized conductors. – Ecnerwal Feb 6 '15 at 22:37
  • That's true - but "we" never over-sized pipe for that reason even 750+. I didn't know you had to do 1-cond. sized (it's the same either way, 1.25 is good for 4/2 as well but I digress) but that makes a lot of sense. I wouldn't pull assemblies in pipe either, probably why I didn't know. – ChiefTwoPencils Feb 7 '15 at 0:16
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    All - thank you for the insightfull/informative responses. I have measured my run - and I can probably eliminate 25-30' by going in a straight line from my service box to the building - but it's still a long run. I do have a friend who is a Master Electrician that I will have over for coffee and have him take a look. But the answers here are very insightful and I thank each and every one of you. – Bobby Feb 8 '15 at 4:49

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