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?
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.