What I've documented here is how I wired conduit in my garage shop. I'm comfortable with this level of wiring. Code in your area may differ in many ways than what's allowed here. If you still want to do it yourself, read about code in your area, consider having a professional do it or at least review your work. Mess up and you might just pop a breaker or you might feel 200A "finding ground" through your body. It's easy, but pay attention, double-check, and save the beers for after it all works ;)In advance: sorry for the length of this entry; lots to cover! Here's a table of contents, however:
Planning the Subpanel
Selecting Wire Size
Selecting Ground Size
Selecting Conduit Size
Main Panel Tour
Connecting Main Panel to Subpanel
Wiring the Subpanel
Wiring Plan for Shop
What is GFCI anyway?
Wiring Pigtails for Ganged Boxes
Making Custom Covers
Ugly's Electrical Reference as it has all the necessary NEC charts and a whole lot more. Best $9 you can spend on this project.
I'll be adding a 100A subpanel in the shop. It is fed from a 60A breaker from the main service panel. The main service panel is the breaker box on your house; the subpanel is just a smaller version that will be located in the shop. The subpanel is rated for 100A meaning the current paths in the box are capable of handling 100A continuous-duty. I only need 60A brought into the shop so the feed from the main panel will have a 60A breaker.
Question #1: which size wire between the main service panel and the subpanel? I'm running THHN wire inside of rigid plastic schedule 40 conduit (say that 5 times fast...). If you've seen insulated wires going to your wall plugs or switches, that's THHN in a sleeve (sleeve called NM wire or Romex).
I will need 3 conductors (2 hots, 1 neutral) and ground. I sometimes see forum postings implying that neutral isn't a conductor. Oh, it certainly is. Current coming from the 'hot' going through your toaster returns to the service panel on the neutral. It is called 'neutral' because that's the name of the tap on a transformer where neutral is connected in upstream circuits; if you rather, call it 'return'.
Ugly's reprints an NEC chart for allowable ampacities of conductors. I've highlighted the important parts (sorry, you'll likely need to open that in another window). I am using THHN with copper conductor so that selects the fourth column. I need 60A. Smallest wire that can carry that is 6ga. Actually, it looks like I could put a 75A breaker for this sub panel. Not. Look further below for temperature corrections. I'm in Arizona and we get hot. Let's use 110ºF as a maximum service temperature in the shop. I've highlighted the correction line. 75A * 0.87 gives a maximum amperes of 65A. Perfect. Note, too, that this chart assumes at most 3 conductors in a raceway (conduit) which also fits my scenario.
Question #2: which size conductor for the ground? Though the main panel ties ground and neutral together, you absolutely cannot in the subpanel otherwise you'll likely have current flowing on ground. For a 60A breaker ("automatic overcurrent device"), I need 10ga copper.
Question #3: which size conduit from the main service panel to the subpanel? So at this point, we have three 6ga conductors and one 10ga ground between the panels. This chart shows that for 6ga THHN wire, I can put 4 in a 3/4" rigid PVC conduit.
...and you thought figuring out where the drops go was tough... :)
Quick tour of a service panel. This picture has the cover removed. At the top is the main breaker; flip this and your whole house goes dark. You'll want to flip this off when working in the panel. NOTE! I circled the two hot lugs that come from the power company. They are on the input-side of the main breaker, which means they are still hot when the breaker is flipped. Be conscious of those lugs. Down below I circled the hole in the panel for the feeders/runners distributing power throughout the house. This hole goes into a stud cavity in my garage wall.
You see two columns of breakers. The power company drop has 2 conductors bringing 220V to the panel. One conductor is connected to the left "rail", the other the "right" rail. (Rails are also called buses.) These are the "hots" mentioned in this entry. Neutrals are attached to a bus to the right of the orange circle in the picture. In the main service panel only, these are bonded to ground to effectively split the difference between the hots (an over simplification) so you can create a 110V circuit with 1 hot (either right or left) and 1 neutral. A 220V circuit uses both hots and no neutral. I explain this more in a previous posting about special wiring I did for my SawStop and router table.
My main service panel is on the other side of a finished garage wall. When connecting a breaker in the service panel, the wires enter the house through the wall cavity behind the panel. For me it was easier to locate where the access hole would be located, mark the stud cavities, and where I wanted to cut out the drywall.
I cutout a pretty large piece of drywall to make it easier to work inside the cavity; a clean square is easy to replace, mud, texture, and forget :) Word of caution: that square of drywall is near the access hole behind the service panel so there are lots of wires very near that hole; cut carefully and make shallow cuts to avoid hitting anything. Consider turning off the main breaker to the service panel just in case.
stripper from Green-Lee for the same price.
(it won't be in the discussion further). The two hots for the subpanel go to the two lugs for the 220V breaker, the neutral to the neutral bus (white wires below) and the taped-green ground wire goes to the ground bus also with the white wires because in the main panel, ground and neutral are bonded on the same bus.
At this point, I have runners wired into the main service panel and run through conduit to the subpanel. That's where we go next...
While we're on this picture, notice the breakers. They take 2 slots to form 220V, but they are composed of half-size breakers so the outside edges have a 20A breaker separately. I'll use the outside 20A breakers to form 20A 110V circuits and the two barred half-size breakers in the middle to form 20A 220V circuits. The two reds with blue tape are each a separate 20A 110V circuit along with a white neutral each (see top bus). The two reds in the middle form a 20A 220V circuit.
At this point, I'll divulge my wiring plan for the shop. Reading the breakers in the subpanel from left to right, I have:
|20A @ 110V||Beer fridge :)|
|20A @ 220V||"Tools" Circuit|
|20A @ 110V||Bandsaw DC|
|20A @ 110V||Miter Saw|
|20A @ 220V||A/C|
|20A @ 110V||MFT|
|20A @ 110V||open|
|20A @ 220V||Dust Collector|
|20A @ 110V||"Rockler" circuit|
The "Rockler" circuit is so-named because of a Rockler extension reel (buy one, now... oh wait, they are waaay expensive now). I generally connect my Festool CT-22 dust extractor to it or other ad-hoc tools.
The A/C is for a portable A/C unit I use in summer.
The dust collector circuit is also interesting in that I ran 4 wires to the DC: 2 hots, neutral, and ground. Right now, I ignore one hot and made a 110V circuit of it for my current DC. If I upgrade later to a 220V DC, all I have to do is connect the hot and disconnect the neutral. Plan ahead :)
All of these circuits are just run-wire-connect-repeat. I did do some interesting wiring on the "tools" circuit for the SawStop and router table as mentioned earlier. With all 4 wires, the "tools" circuit also provides a few other 110V circuits for 110V tools like the drum sander. This same idea (running 4 wires instead of 3) was repeated for the DC circuit so I can later upgrade to a 220V DC. With this scheme, my tools and DC are always on a separate circuit.
Addendum: I followed up this posting with another that discusses GFCI sockets in more detail (including how they are chained) and the meaning of the socket blade configurations here.