Replies

1. 
   tms | Mar 18, 2005 08:00pm | #1

   Hey tony,

   First of all, let me say up front that I'm not an lic. electrician, but I do quite a bit of wiring at work, under the supervision of one. And I've wired my own shop, a 200A 3Ø panal. So, with my credentials, or lack of them, out of the way, I can give you the benefit of my experience.

   1. You don't say how far your wire run is from your 200 A service to your new panal. That will determine your voltage drop at a given wire gauge. In any case, I don't think you will want to run NM between them, even if your could get it at the gauge you need. For example, at 100A, you would need to use #1 gauge wire, in NM, which would be huge stuff, expensive and a pain to work with, while TW, the most common single conductor for household wiring would do the job in #4 gauge. Determine your run length first, then go from there.

   2. You need a grounding stake for each sub panal.

   3. Unless you have an unusualy wet, or corrosive environment, I prefer metal conduit because it gives added ground conduction, and it's less suseptable to breakage from impact.

   4. You don't need, or probably even want, a GFCI breaker on your main panal feeding your sub panal. Use GFCI breakers, or outlets, on circuits that terminate withing 10 feet of water, or where you might experience bodily contact with a grounded surface (for instance, lying on the concrete floor, under your car, with a drop light in hand).

   5. For large induction loads (motors), away from ground fault hazards, I would avoid using GFCI breakers, as they will probably trip unnecessarily and the increased duty cycle will shorten their life. It's an unecessary expense.

   6. Use metal boxes AND GROUND THEM. Taking shortcuts with grounding is the worst form of economy. Ground your switches to your boxes, and ground the boxes to your ground conductor. You are most likely to shake hands with the devil at either a switch box, or an outlet box. GROUND YOUR BOXES.

   I hope this helps. If you figure out your run length, I'll do a calculation of voltage drop for you. Better yet, get a copy of the NEC (National Electrical Code) for yourself, or borrow it from the library. Better still, get a licensed electrician to help you, or at least guide you, and look over your work.

   Good luck,
   Tom

1. 
   woodworker418 | Mar 18, 2005 08:51pm | #2

   Hey Tom,

   Thanks much for the very helpful reply.  My run from the main service panel to the new subpanel is about 50feet, but lets say 60 to be safe.  So, if I use separate individual wires instead of NM cable, I would still use 3 wires (2 hot, 1  neutral), plus ground, correct?

   Sounds like I don't need the gfci anywhere, although i will be standing on a basement concrete slab floor if that makes any difference.

   I think that's about it, I can't think of any additional questions.  I would love to do it under the supervision of an electrician, but I've had difficulty finding anyone willing to help on that level, everyone seems to want $1,000 to do the job and I feel confident I can do it safely on my own.

   Thanks Much, tony.

1. 
   tms | Mar 18, 2005 09:29pm | #3

   Hey Tony,I did some more checking, and consulted with my electrician cohort to be sure.Between your panals I would:Use #2 gauge wire, three conductors, and no separate ground (float the neutral). If you can buy the wire by the foot, buy black, red and white. If you have to buy a spool, buy black, and lable the conductors with colored tape.Ground the panal with it's own grounding stake and #2 conductor.Use THHN, or better, grade insulation, and 1 1/2" conduit.Tom

1. 
   BarryO | Mar 18, 2005 11:22pm | #7

   I don't know if your electrician friend is licensed or not, but there is some bad advice here.

   Table 310-15(b)(6) listed conductor types and sizes for 120/240-V services and feeders.  For a 100A feeder, it lists a conductor size of #4 copper, or #2 aluminum.  These are for the current-carrying conductors.  For the equipment grounding conductor ("ground"), it need only be #8 copper, or #6 aluminum.

   They do make #4-3 w. ground Type NM cable (Romex), but it's not easy to find.  Barring that, you'll need individual conductors in conduit.  I don't think #4 wire is available in anything other than black.  If this is the case, the grounded conductor (colloquially called the "neutral") must be marked at each end with white tape.  The #8 equipment grounding conductor must be either green or bare.

   Within a structure, Only a single grounding electrode system shall be used.  A separate ground electrode for a feeder is NEVER used (if a feeder is in a separate building, there are exceptions in the code that requirement and permit a second ground electrode, but that's outside of the scope of the discussion here).

   You need 4 wires;  2 hot, 1 "neutral" (properly called the "grounded conductor" in the Code), and one "ground" (called the equipment Grounding Conductor in the Code).  In the feeder panel, the "neutral" and "ground" conductors are connected to separate bus bars and not bonded together (this is very important).

   If you go with NM cable, you don't need to run it in conduit, unless it will be exposed to physical damage.  You can route ist along the building wall surface and secure it well. 

   If you use individual wires, either PVC conduit (not water pipe) or metal conduit is OK.  If you use metal conduit, you can use the conduit itself as the grounding conductor, if you are very careful to ensure electrical continuity (but alot of electricians tend to frown on doing this).

   No need to protect the feeder with a GFCI.

   According to 210-8 of the code, 120V circuits in unfinished basements must have GFCI.

   Plastic boxes are safe, just not a durable as metal ones.  The grounding wire in the branch circuit wiring must be connectede to the ground terminals on switches and receptacles in this case.

   As others have said, it sounds like you're not entirely comfortable doing this, and you may want to seek out someone who could help.

    

2. 
   wilkesland | Mar 18, 2005 10:17pm | #4

   I'm not a licensed electrician either - would suggest you post your questions over at break time where you may be more likely to get the opinion of a licensed electrician.

   Not to be disagreeable, a grounding electrode (stake) is not needed for the subpanel located in the same building as your main service. The 2002 National Electric Code requires that there be no connection between the grounded conductor(neutral) and the equipment ground (ground) other than in the service disconnecting means. You need four wires - two ungrounded conductors (hot), a grounded conductor ( neutral) and and equipment grounding conductor ( ground). I would not "float" the neutral. NEC 250.142 specifically prohibits the gounded circuit conductor(neutral) from being used as a ground on the load side of a system except for ranges, ovens and dryers, or if  you have a separate building or a separately derived system. As described, you do not fit any of the exceptions.

   I think what the other poster just said used to be a common practice and apparently is still in some places. I would recommend you check with an electrician in your area or your local code official. The local code official has the final say.

1. 
   tms | Mar 18, 2005 11:01pm | #6

   Oops!You're right, of course. For some reason, I had it in my mind that his 100A panal was in a separate shop; that's why I recommended a separate grounding stake.I guess it just points out the value of having a qualified person on the ground, at the project.Tom

3. 
   Stuart | Mar 18, 2005 10:25pm | #5

   I'm not an electrician either but I am a professional electrical engineer who designs this kind of stuff for a living.

   You said the main panel is in the garage and the subpanel will be in the basement; I assume this means the garage is attached to the house. Since they are both inside the same structure, you need to keep the neutral and ground separated. Run two hots, the neutral and a separate ground wire. The subpanel will need to have separate bus bars inside for the neutral and ground as well. The grounds and neutrals are only connected together back at the main service disconnect. You don't need a separate ground rod at the subpanel.

   You may or may not be able to use nonmetallic cable, depending on how it gets run to the basement and also depending on local codes (I understand you want to run it in conduit through the garage, I assume that is because the walls are open in there.) I am not entirely sure if you'll be able to readily find type NM cable in a large enough size for a 100 amp branch circuit (that's getting into the neighborhood of service entrance cable), so you may end up with individual THHN/THWN conductors in a conduit anyway. If the conduit is in a place where it can get knocked around or someone could back the car into it, metal conduit would be best but PVC conduit can be used as well.

   The wire size will depend on the rating of the specific wire you choose and the temperature rating of its insulation. The distance you are going will not create enough voltage drop for you to worry about upsizing the wire. Typically I specify three #3 awg THHN/THWN wires for the hots and neutral, with 75 degree C insulation, for 100 amp circuits. The ground wire can be #6 awg. For these wires, the conduit can be 1 1/4", but 1 1/2" will make it easier to pull the wire and is probably more readily available at the Home Depot.

   Your feeder breaker does not need to be a GFCI, nor do the breakers in the subpanel. However, since your workshop is in the basement you'll need GFCI convenience outlets. You can use the feed-through feature of a single GFCI to supply power to regular convenience outlets.

   I don't see any reason why you couldn't use plastic boxes, as long as you properly ground all receptacles, switches, lights, etc.

   Having said all that, installing a 100 amp panel is pretty serious business. There were a lot of questions asked here...if you're not 100% comfortable doing this kind of work you really should talk to an electrician (and also make sure local codes will permit you to do that kind of work.) I assume you'll need to pull a permit and get it inspected, too. You may save a few bucks up front, but if the house burns down it's not much of a bargain.

4. 
   NateMeredith | Mar 19, 2005 02:12pm | #8

   Tony,
   I do this for a living, and it's simple. Go to an electrical supply house, and buy 100A subfeed cable. It has all the wiwres in it you need, in the correct sizes for your length of run. it you want to run it like NM cable, get the stuff with the grey jacket on it. That way you can run it through walls, or fish it as needed. Get some help, as it is fairly stiff and bulky to manage.

   At the main panel, (assuming no exterior disconnect switch) connect the neutral and equipment ground to the neutral/ground bar. Both hots to your double pole 100A breaker. In your shop, connect hot leads to panel hot lugs, neutral to the neutral lug, and purchase a ground bar kit for your subpanel foe the ground. It goes into a pretapped hole somewhere on the interior of the body of the panel. Do not use the green grounding screw that is shipped with the panel to bont the neutral/ground.

   When using aluminum conductors for the feeder, use some Noalox on the wires and lugs. All of your branch circuits will be wired in the standard fashion, with the exception of the ground - it now goes to the ground bar you installed in the back of the subpanel. As for plastic or metal, it's up to you. Plastic is alot cheaper, but gan break with high impact. Jus be sure that your devices are grounded, and you'll be good to go. Your 120V outlet circuits should be GFCI protected, using gfi outlets, not a breaker. Your 240v and hardwired devices do not need to be GFI protected. It's simple, just be careful, and don't touch any of the shiny parts in the back of the panels once they are hot.

   Regards,
   Nate

   The only thing I practice in moderation is my excess.
1. 
   GHR | Mar 19, 2005 04:54pm | #9

   I like the suggestion to go to the supply store and ask for the proper cable.Industrial suppliers are really nice. They tend to know what inspectors want (might not be code required). That helps everyone get along.To the original poster ---If you have to ask these questions, you should not be doing this work. Pushing #4 (or whatever) wires in a breaker box is harder and thus more dangerous than pushing #12 wires.

1. 
   woodworker418 | Mar 24, 2005 01:57pm | #10

   thanks all for the very useful advise on this topic.  one last question, as i mentioned, i want to ensure that there is no loss of voltage running from the main service pannel to the subpanel.  in this regard, assuming that i use the correct gauge, is there any advantage in using copper over aluminum??  obviously, i'd rather go with aluminum as its much cheaper, but if this risks more voltage drop through the feeder wire, then i would stick with the cooper.  any other potential drawbacks with the aluminum feeder wire?

   thanks much, tony.

1. 
   Kent_F | Mar 24, 2005 06:33pm | #11

   Ensuring "no loss of voltage" is unattainable. There will always be some voltage drop, unless you use superconducting materials (unlikely to be in stock at the local electrical supplier). But it practical terms, it will be negligible.

   By my math, 120 ft (60 ft round trip) of 4 ga copper will drop 3 V at 100 A. That's a 1.25% drop at 240 V. Most of the time, you'll be pulling much less that 100 A,  so the drop will only be a volt or so.

    

2. 
   BarryO | Mar 24, 2005 10:03pm | #12

   any other potential drawbacks with the aluminum feeder wire?

   You have to be more careful with it.  Only hook it up to connectors specifically approved for aluminum.  Always use anti-oxidation goop in the connection.  And using a small torque wrench to ensure the specified torque is acheived ain't a bad idea, either.

   I've had to repair aluminum connections gone bad, in a 1970's era installation (the connectors weren't very well designed for aluminum back then).  What happens is that resistance builds up such that the connections gets hot, which melts the insulation on the wire.  Not fun.

3. 
   lwj2 | Mar 25, 2005 04:43am | #13

   Check with your local electrical inspector.Many jurisdictions will not allow aluminium wiring. Local code takes precedence over national in this case.That said, I'd use copper instead of aluminium. It makes insurance folks a lot happier.Regards,Leon Jester

4. 
   UncleDunc | Mar 25, 2005 08:03am | #15

   An aluminum wire has more resistance, and therefore causes more voltage drop, than a copper wire of the same size. That's why the code specifies a larger aluminum wire for a given load. If you use the specified aluminum wire, you won't get any more voltage drop than if you use the specified copper wire.Ditto the previous poster on using the correct connectors and technique if you do go with the aluminum.

1. 
   woodworker418 | Mar 25, 2005 04:55pm | #16

   thanks everyone for your help!  i very much appreciate it.  best, tony.

5. 
   PlaneWood | Mar 25, 2005 05:25am | #14

   My shop is not connected to the house.  I ran grey PVC electrical conduit underground about 50' and used #4 stranded copper wire, 3 wire with ground.  It was black, so I color coded the ends of the wire.

   At the house meter, I dropped to a 100 amp disconnect, then ran the #4 wire from the disconnect to a 100 amp sub panel in the shop.  Doing this keeps the shop load off of the main breaker in the house breaker box.  You may have to call the electric company to get the meter disconnected.  If you live in a city limits, the electric company will probably only do that for a licensed electrician.  I live in a rural area and that was not necessary.  I did have to call them to get it resealed.  If you have the disconnect wired and ready to go, many electricians will do just the wiring from the meter to the disconnect. 

   My house has two air conditioners.  That, plus all the other stuff, puts a heavy load on the main breaker.  I didn't want to load it down any more.

   I did use a 10' copper ground rod for the shop breaker box.  Since corrosion is common here, I silver soldered the copper wire to the rod.

   Oh yeah, figure out how many 110 outlets you need then double it!

   PlaneWood by Mike_in_Katy (maker of fine sawdust!)
   PlaneWood

1. 
   BarryO | Mar 26, 2005 12:27am | #19

   Since corrosion is common here, I silver soldered the copper wire to the rod.

   I hoped you used a clamp as well.  Soldering by itself is not a code-compliant method for connecting a grounding electrode to a grounding electrode conductor.

   The reason?  Solder melts at a fairly low temperature.  The connection needs to be done with one of the clamps sold for the purpose (which should give a gas-tight connection if installed correctly), or via welding.

1. 
   PlaneWood | Mar 26, 2005 08:54pm | #20

   Barry  -

   Yes, I used a clamp, then silver soldered the whole thang.

    

   PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood

6. 
   Terre | Mar 25, 2005 08:09pm | #17

   Good Morning,

   Here is an idea or two for you to ponder.  We recently had a new house built with a three car garage. Wood shop gets 1 1 /2 bays and the minivan has the other side.

   One thing we did was put a 220 @ 40 amps outlet in the ceiling.  Good location for tools on a mobile base.  We also had another outlet of the same type placed on a wall near a corner.

   To do it over I would have put the wall 220 in the ceiling also.  Same area of the garage.  Why?  We have an eighteen month old in the house.  We are afraid to let her roam in  the garage. 

   We also installed  outlets of 110 @ 15 amps and 110 @ 20 amps.  We  had even thought of a 110 @ 25 amps outlet but decided against it for dollar reasons.

   Hope this helps.

   Terre

1. 
   BarryO | Mar 25, 2005 08:24pm | #18

   We have an eighteen month old in the house.  We are afraid to let her roam in  the garage. 

   In my shop, all power tool and outlet power (except for refrigerator and charger outlets) goes through a lockable main switch.  When I'm not in there, the switch is locked.  When the switch is on, a red light bulb is illuminated, to remind me that power is active.  It helps protects the little ones, and is an easy way to disable power to a tool for blade changes and the like.

1. 
   Terre | Mar 28, 2005 05:43am | #21

   What a better ideaa.  Where were you last May?  Hahahhaha