Saturday, January 29, 2011

Wiring the Shop for 220V

Awhile ago, I decided to run a sub-panel into my garage shop to hopefully forever end problems with extension cords stapled to the wall or needing to run a long extension into the house to run the drum sander! This picture is of the extensions I pulled from everywhere when I was done :)  (Actually there were 2 more in use awaiting the special wiring I did for the SawStop and router table!)  This also lets me stop hijacking the hot-water heater's 220V outlet :)
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

Okay, now let's get started with the planning preliminaries... and it helps to have a copy of 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.
With the hole cut, I position the 90º conduit.  You can barely see the access hole behind the panel inside the wall.  I want to conveniently feed wires from here and patch drywall later.
I drilled a hole for the 90º leg of the conduit box then feed all my conductors through.  NOTE: I have so far described the three 6ga and one 8ga wire I'll use to get to the subpanel, but for just the vertical part here, I'm running a separate three 12ga wires for a freezer nearby.  That said, the vertical part (here to wall-ceiling junction) is 1" PVC to handle the additional conductors.
Here they are popping out of the main service panel ready to attach.
These are the breakers I'm adding to the main panel.  The small one only connects to one hot bus and therefore makes a 110V circuit with a separate neutral line (this is the freezer I mentioned).  The large one connects to both hot buses and therefore provides a 220V circuit on its own (no neutral).  Every other breaker slot attaches to the same hot bus.  Two in a row, therefore, would attach to one and the other bus.  That's why the large one looks like two small ones glued together because they very nearly are.  Each individually grabs a different hot.  The key is that the breaker switch is barred: when the current through either hot exceeds 60A, the "popping" breaker will also shut off the other breaker thus breaking current in both sides of the circuit.
The breakers have a slot that pinches the tab for the hot bus it connects to.  The ball of snot in the slot is actually flux and should be there; when you push the breaker on locking it in place, that flux ensures a good connection.  It is also how the big boxes know when you're trying to return used breakers :)
The side of the breaker includes a stripping guide.  No, this has nothing to do with brass poles.  It's the length of insulator you need to remove for a good connection to the breaker.
I use these "automatic strippers" from Lee-Valley.  Very very nice.  But they don't seem to be offered there anymore.  Here's a similar stripper from Green-Lee for the same price.
Here are the runners for the subpanel stripped.  A note about colored tape: my local dealer was out of most colors of wire.  Black and red are, by default, considered hot.  The white here is neutral and neutral is by code white.  By code, ground is green, but they were out of green.  I'm allowed to tape it green an inch above the stripped end.  I did that here plus I put another couple marks of green tape further up the wire.  While I didn't need to label a black as hot with red tape, I did just so I knew which hot it was in the main service panel; this red tape wasn't required.
I put the wires through the 90º conduit then pushed them through the rest of the conduit (so, up to the ceiling, across to a corner, turn the corner, then across to the subpanel).  I taped the wires together every couple feet to keep them from tangling up.  Also, liberally apply KY... er, wire lubricant before pushing them through; this is a night-and-day difference.  Pretty sure it's just a coincidence that it looks like KY...
Back inside the service panel, route and connect the wires to the breakers.  The subpanel breaker is on the right above the "single" breaker for the freezer (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...

This is my subpanel photographed poorly :)  The breakers for this box install vertically so you see the two hot buses run horizontally.  You can barely make out a dotted line of brass screws above the buses.  Those are for the neutral bus.  Where's ground?  That goes on the grounding bus in my hand.  It installs by screwing into the subpanel (so, it is grounded).  Why wasn't it part of the box?  Because you might have purchased the box as a main panel, which uses a common bus for neutral and ground.  As a subpanel, you need to buy this...
...and attach it here.
This picture shows a number of things; it was taken after a couple other shop circuits were wired.  The main runners from the main service panel arrive from the leftmost conduit on top.  The neutral attaches to the neutral bus lug to the right of the neutral bus screws.  If you look carefully, one hot from the main panel attaches to a lug to the left, the other (with red tape) to a lug on the right.  These lugs feed the hot buses mentioned earlier.  Ground (green-taped black) attaches to the ground bus in the lower right corner.
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 @ 110VBeer fridge :)
20A @ 220V"Tools" Circuit
20A @ 110VBandsaw DC

20A @ 110VMiter Saw
20A @ 220VA/C
20A @ 110VMFT

20A @ 110Vopen
20A @ 220VDust Collector
20A @ 110V"Rockler" circuit
The "tools" circuit goes everywhere.  Since I run one tool at a time, it works well.  It goes to two 220V tools: SawStop PCS and a 14" bandsaw and I break it down to 110V (explained later) for the drum sander and air compressor.
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.

So let me describe one plug setup.  The down-rod on the subpanel goes to this set of sockets.  The left is the 220V for the A/C.  This is a trivial connection described in the other article.
The right duplex socket actually has each socket on a different circuit.  How?  I wanted to do this as the long-term intention was to get a shop-vac on one plug for the miter saw on the other (slept through that Black Friday sale...)
A non-GFCI duplex socket has a cross-bar that connects the two sockets together.  In this way, you wire one and the other comes for free.  The pencil points to this cross-bar.  It has a slot for a screwdriver so you can wiggle it off.  Only wiggle it off on the "brass" side (other is aluminum).  The "brass" side gets the hot so now you can connect two different hots, one to each brass screw.  Just one neutral is necessary to the "white" side, that being aluminum.  Now, each socket is on its own circuit.
This case doesn't use a GFCI socket like I use everywhere else.  I may replace this one duplex socket with two GFCI duplex sockets.
Speaking of GFCI, I also routinely see on forums people explaining how some tool started sucking current fiercely before failing and "it didn't trip the GFCI before the breaker".  GFCI is Ground-Fault Circuit Interrupter.  It has nothing to do with over-current protection.  It watches the current entering and exiting the socket.  If they are equal, no fault.  Once they are unbalanced, it means some current is escaping via ground and it will trip.  Two totally different failures.  So, don't count on GFCI to protect for over-current.
A last tip on wiring ganged boxes (ones with more than one socket).  If you look closely at this picture, you'll see a green wire and bare copper wire tied with a red cap.  This ganged box has one ground wire going to two sockets.  Each socket has a small bit of bare copper wire attached to its grounding screw.  I then tie the incoming green ground wire and two bare copper wires together with a wire connector; it's a plastic cap with a spring inside that cinches the wires together.  This type of connection is much easier than connecting the sockets together then adding in the incoming green ground wire as ground screws are freaking tiny.  I also use this pigtail method with neutrals and/or hots where appropriate.
That last picture shows a 2-ganged box that is next to impossible to find a cover for.  The 220V is what makes it difficult to find.  My solution was to take a cover with the 2 holes for the right-most socket pair and a small hole for a flip-switch on the left.  I put a single cover for a 220V socket on the other cover and used the screws to screw the covers together.  I then used the 220V cover's hole as a drilling guide to use a 35mm Forstner bit to make the hole.  A little filing and it worked perfectly.
Finally, if your mother is overly organized, borrow her label maker to label the subpanel circuits and the circuits around the shop.

Again, sorry for the length of this posting, but there's a lot of material to cover.  As this is a recurring topic on forums, bookmark it and send people here to give them an overview.

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.


  • James said...

    Awesome documentation! This will probably be very useful to several people. I have seen a lot of posts lately about how to wire your shop properly.

  • daiku said...

    Thanks so much for this. Electrical projects are always a bit intimidating. This gives me the confidence to finish a couple circuits in my own shop. Thanks!

  • mdhills said...

    What did you do with the unused hot from the DC circuit in the sub-panel? (tied to neutral for the time-being?

    And how did you site the sub panel? Seems like garage shops have lots of demand on space that would challenge pretty much any clearance requirements.


  • Paul-Marcel St-Onge said...

    Hi, Matt,

    In the subpanel, the DC circuit is a 220V breaker giving me 2 hots. Separately, I have a white wire from the neutral bus going to the DC socket. At the socket box, I used one white and one red for the 110V socket and simply taped over the other red so it wouldn't suddenly energize something. I could have left it disconnected back in the subpanel, but was concerned I'd lose track of it.

    My subpanel is located between the two garage doors; perfect fit on a basically useless wall. I put it fairly high up so there's still room for leaning brooms n boards underneath it with no concern. The feeder from the main panel was a relatively short run and from that spot, it was easy to fan in the two directions for all the receptacles.

    I'm glad I wired the DC circuit like that because I'm already waiting for the Laguna cyclone to come on sale at Woodcraft to upgrade my DC.

  • Anonymous said...

    This is a great article.



  • dorald said...

    Great article. Thanks for taking the time to detail and post. I just purchased a SawStop 220v unit and was debating on hiring an electrician or DIY. After reading your post, I think I'll try it.

  • Paul-Marcel St-Onge said...

    Thanks, dorald (and Jeff! didn't see that comment before!)

    Take your time with it all and test as you go and you'll be just fine. Many people have emailed me directly saying they read the article a couple times until everything was in their head then got it all first try (so, no pressure :)

    You might be interested to look at my Electrical label and SawStop label for other modifications you might like for example the single power cable to the cabinet that gets split into a 220V for the SawStop and 110V for an extension-wing router table and light. Then there's the over-arm dust collection mod that is really nice...

  • Anonymous said...

    Thank you for the lovely article. I intend to do the same for my shop soon.
    I do have one comment. I am not an electrician and you may have wired it correctly anyway, but there might be a problem with the way in which the duplex 110 volt outlet was wired. The two hot leads must come from different buses since they share a common neutral-if the wires are from the same bus the breakers would allow 20 amps each to pass through and the neutral would be carrying 40 amps resulting in a very hot wire. If the hots are from different buses this problem is avoided.


  • Paul-Marcel St-Onge said...

    Thanks, Dale... All the duplex sockets for 110V have one hot and one neutral to make the circuit. That hot is limited to 20A so the return current on the neutral is also limited to 20A.

    If you mean the special wiring I did for my SawStop that uses 4 wires, for that, one 110V duplex uses one hot and the neutral while the other uses the other hot and neutral as you mentioned.

    All the 110V-only circuits have a home-run neutral rather than trying to keep track of which hot is from which bus cuz I'd screw that up in a heartbeat (plus not too nice to leave for someone else later).

    But a good point; thanks for bringing up the reminder! This entry, though very lightly commented, is by far the most popular on my site.

  • Anonymous said...


    I might be confused still. Here is the part of the article I referred to:

    A non-GFCI duplex socket has a cross-bar that connects the two sockets together. In this way, you wire one and the other comes for free. The pencil points to this cross-bar. It has a slot for a screwdriver so you can wiggle it off. Only wiggle it off on the "brass" side (other is aluminum). The "brass" side gets the hot so now you can connect two different hots, one to each brass screw. Just one neutral is necessary to the "white" side, that being aluminum. Now, each socket is on its own circuit.

    The neutral referred to above will have to handle the current from both hot wires, unless the hots are from different buses.


  • Paul-Marcel St-Onge said...

    Thanks for posting the confusing part, Dale.

    In the quote, it says "you can connect two different hots, one to each brass screw". So the top socket is on one bus, the bottom socket on the other bus, and a common neutral.

    The current in the two buses is opposite; assuming each socket described above had a 15A load running, the effect of the opposite current directions of the two buses means the net current in the shared neutral would be 0A. If one had a 15A load and the other a 10A load, the neutral would carry 5A back. So long as the two hots using a common neutral are from the different buses, the neutral will never carry more current than either hot individually.

    Now, if you just took two arbitrary hot wires you pulled through the conduit and wired this up, you'd run the risk that the two hots used were going to separate breakers on the same bus. In that case, the neutral return would carry the sum of the two currents. In the 15A/15A example above, it would be 30A and a load of trouble.

    The electrical code allows for a connection like the one described. A neutral return is allowed to be shared between two circuits that originate on different buses.

    If I still made no sense, let me know and I'll try finding the code that describes it.

  • Anonymous said...

    aha, yes, understood.

    thank you


  • Erik Benson said...

    I don't think you're allowed by code to have more than one circuit in an electrical switch box. The reason is if you turn of the breaker but still have two hot circuits it's not going to be very safe for someone who doesn't know what you wired up.

  • Paul-Marcel St-Onge said...

    Hmm, I could see that being a code rule, Erik. Honestly don't know if it is or not so someone would want to verify before doing their own thing. For me, I suppose I'd have to change the wall-mounted box from a dual bay to two single bay boxes, which wouldn't be difficult.

    My parents' house was built in the late 70s and I know their bathroom switches are in a dual bay box with the switch and a duplex of sockets. The switch and the sockets are on different circuits. Certainly code has changed since then.

    Thanks for pointing it out!

  • Robert Allen said...

    My generator is good example of separating two 120V outlets. The top one is connected to one leg of the 240V and the bottom to the other. They have a cord available that plugs into both of the 120V outlets and has a 240V plug on the other. I'm not sure why they did it that way because it also has a standard 240V twist type outlet.

    For residential use of separating the two outlets, it is mainly done where one outlet is switched and the other not. Say for Xmas tree in front of a window with a switch to the side on the wall. In this case they are both fed from the same 120V breaker so minimal confusion and only one breaker to kill the outlet.

    Here's the real thing. Always wire per standards and code. It not only keeps you safe, but if someone else comes along (perhaps purchases the home when you move due to your big hit on the lottery) all of your work will be useful to the next person and they will not feel the need to rip it all out. Panels are relatively cheap. Use the right one with some expansion left:)