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I'm planning to install a 100 amp sub panel in a new detached workshop in my backyard. I will run power from my home main 200 amp service panel which I have a dedicated 120/240v double pole 100 amp breaker already set up. Consumers in my workshop will be

  • 6X linear fluorescent T5 light fixtures 11 amps
  • 2x exterior wall pack lights 10 amps
  • Garage door opener 15 amps
  • AC/DC stick welding 50 amps
  • Air comp. 20 amps
  • Table saw 15 amps
  • Shop fan 15 amps
  • Bench grinder 15 amps
  • misc small AC power tools.

I anticipate approx 55-65 amps will be the most continuous current at any given time. The cable run distance from the 200 amp main service panel to the sub panel is 185ft. 110ft will be under ground and 75ft will be run inside my home through the attic. I would like to make this pull all in one with no splices.

My questions are

  1. What type and size wire is suitable and meets Electric Metro code to run inside my home and also bury 24" in conduit?

  2. What size and type conduit (PVC or galvanize) should I used for under ground and what is the NEC code for conduit above and below ground.

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    First thing to check is if 24" is deep enough in your town. Ask the Local Authority Having Jurisdiction, since it has to pass your local inspectors. They could also tell you whether there are other requirements such as having it surveyed/filed by the Dig Safe folks, whether safety tape a foot above the wire is required or just an extremely good idea, how it's wired up at both ends, etc. .. and, yes, whether you should use conduit or ground-certified nmc or something else to make them happy. – keshlam Jan 2 '15 at 4:04
  • 24" is the deepest he'd have to go in pretty much any case. Most installations will allow 18". – Speedy Petey Jan 2 '15 at 14:41
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I don't know what the "Electro Metro" code is, nor do you mention your location, so keep in mind my answers are based on a US installation under the NEC.

PVC conduit is fine for the whole run, and the easiest BY FAR. You can use Schedule 40 for any and all parts, buried or exposed but not subject to damage. Areas like where it emerges from the ground outside you'd need to use Sch80. Again, 18" deep (to the top of the pipe) is fine for any residential installations. Table 300.5 is the NEC section for this.

Personally I'd go with #2AL for cost savings and 1-1/4" pipe. Since you'll have a mix of 120V and 240V loads you can run one #2, one #3 or 4 neutral, and a #6 ground. They do sell 100A MHF (mobile home feeder) cable that has this combination already grouped together. Remember, this type of wire/cable cannot be run exposed. You'd have to run the conduit completely from point to point. You could also run the conduit to a big box in the house then change over to #2SER cable, but this takes some skill and knowledge to make the change over and splice.

I'd put this on an 80 or 90A feeder breaker in the main panel.

  • Thanks, I will contact my Local Utilities Systems for clarification on conduit depth and safety tape. Also I live in Lafayette, La. Speedy if you're referring to amp numbers, I got most of them off the tags. Can 100A MHF be run inside my home as long as it's in conduit? – TBNG244 Jan 2 '15 at 17:38
  • Yes, with conduit you can run that cable right to the panel. I was asking Eric about the numbers he came up with. – Speedy Petey Jan 2 '15 at 17:53
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Using the calculator here, you either need #1 copper or #2/0 aluminum to carry the current you want to carry.

I highly recommend that you find whoever does electrical inspections for your area (known as the "local permitting authority") and make an appointment to talk about what you want to do; this is a highly technical project and there are variations in code from area to area.

  • WHERE did you get those numbers??? They are way off. – Speedy Petey Jan 2 '15 at 14:43

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