I have a detached 24'x30' garage, approximately 40' from the house. The house has a 200 amp main panel. Currently there are 9 110v outlets powered from the main panel into the garage through a buried 1.25" conduit, running lights and outlets. I would like to run (7) 220v outlets into the garage to power my welder, compressor, mill, lathe, metal band saw, Cerakote oven and plasma cutter. (Not all machines running at the same time, at best compressor and plasma cutter at the same time) I have a 125 amp sub panel to supply all the 220v outlets from. Can I use the following; 2" PVC conduit buried 24" deep, running (3) 2/0 thhn copper feed cables (double lugged from the 200 amp main service panel) and (1) #6 solid bare copper ground wire into the 125 amp sub panel.The sub panel will have it's own earth grounding rod also. Each circuit running a 50 amp breaker with #6 wire to each outlet. Will this work? Thank you in advance for your help and recommendations.
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1Panel size does not matter. The size of the breaker feeding the sub panel matters, just make sure it 125 amps or smaller. You also need a load calculation to see how big a breaker you can use if any. 200 amps is the total/everything. 200-125 leaves 75 amps for the house, not good unless you use gas for all heating devices(stoves, dryer, house heat).– crip659Commented Jul 30 at 19:26
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do not switch to aluminum conductors it is not worth the trouble– DIY75Commented Jul 31 at 1:27
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1@Traveler -- the secret isn't copper, its an inch-pound torque wrench. alloy alluminum stuff with proper torque is fine in anything short of coastal salt air. (have you ever seen a utility service run in copper?)– ThreePhaseEelCommented Jul 31 at 2:33
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3@Traveler You're wrong. The CPSC was referring to small 15-20A branch circuits. We are talking about heavy feeders 50A or more, and aluminum is proven safe and reliable there. Lugs are made of aluminum (why ADD a dissimilar metal) and people have always torqued them to spec.– Harper - Reinstate MonicaCommented Jul 31 at 4:45
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1@Traveler -- the key words there are "branch circuit" -- aluminum branch circuit wiring was terminated by being hooked onto steel screws, not by being stuffed into box lugs, and the old stuff was made from the less-forgiving AA-1350 "EC" utility grade aluminum, not modern AA-8000 series alloys– ThreePhaseEelCommented Jul 31 at 11:33
1 Answer
First of all, it has been 120V and 240V for a long time. But that doesn't change anything about the panels, etc.
Do NOT use 2/0 copper. That is a waste of money for a couple of reasons:
- It is far more capacity than you actually need. 2/0 copper = 175A, 1/0 copper = 150A, 1 AWG copper = 130A. So with copper you don't need more than 1 AWG for a 125A panel.
- Aluminum will save you a lot of money. 2/0 aluminum is good for up to 135A.
Values are from the Southwire ampacity chart.
Actually, there is a catch. You would need to power this from a 125A breaker instead of from the lugs. That breaker (quick search on Home Depot, may do better elsewhere) typically costs between $ 75 and $ 200 depending on panel type. Also note that if you really want to do this via lugs to save the cost of a breaker, you need to make sure the panel actually supports doing that.
For the ground, you can use 6 AWG copper or 4 AWG aluminum.
But while you are at it, go ahead and move the existing 120V circuit to a breaker on the new panel. Hmmm...now you don't need that 1.25" conduit any more. And that means you can reuse it for your feeder! The catch though is that the details really matter a lot with 1.25". Assuming the existing conduit is mostly PVC 40 but at least some PVC 80, you have to stick to the PVC 80 fill. Which means that 2/0 aluminum won't fit. And 1/0 aluminum won't fit (I think it will fit with PVC 40). But if you can back down just a bit to 100A instead of 125A then you can:
- Use 3 aluminum 1 AWG wires for hot/hot/neutral
- Use either 8 AWG copper or 6 AWG aluminum for the ground wire
- Use the existing conduit - no cost for conduit or for trenching
That will all more than pay for the extra 100A breaker instead of using the lugs.
But wait, there's more!
You MUST do a Load Calculation. Actually, two of them. One for the existing main feed to determine how much room you have in the 200A. And another for your new shop to determine how much you need. You may find you have far less than 100A available, in which case the whole plan may not work because you may not have enough for what you want to do. Or you may have a lot more than 100A available, but provided the shop can make do with 100A, I would go with my 100A/existing conduit plan.
As far as the actual circuits to install:
Each circuit running a 50 amp breaker with #6 wire to each outlet.
Check the specs on each piece of equipment. I wouldn't be surprised if some of them actually specify 20A or 30A circuits. I highly recommend using each piece of equipment on an appropriately sized circuit with a matching receptacle. For example, if you rewire a 20A piece of equipment for a 50A plug then you could have a huge overcurrent situation (e.g., a stuck motor) that will never trip the breaker.
