Replies

1. 
   DanH | Oct 26, 2014 07:57pm | #1

   You will need to consult the latest NEC book.  As I recall, a detached building can either be wired as a "stand-alone" with it's own grounding system (including it's own grounding electrodes -- ground rods), or as a regular subpanel off the main (requiring a 4-wire feed for 240V service.

   But the devil is in the details -- you need to check the code and/or discuss it with someone intimately familiar with the code.  And understand that the rules here changed about 10 years back so the "someone" needs to be familiar with CURRENT code.

   If you can do it as a "stand-alone" you should be able to use the 3-wire service and the panel you bought (but you likely will have to install some ground rods or some such).  If you must do it as a regular subpanel you'll need to install a new 4-wire feed and install a ground bus kit in the subpanel.

2. 
   gfretwell | Oct 27, 2014 10:31am | #2

   There is an exception in the current code that allows an existiing 3 wire feeder to be used as long as there are no other ground paths to the garage and you have a ground electrode system at the garage (like 2 rods, 6 feet apart)

   Use the bonding jumper, screw or strap and set it up like a service panel.

1. 
   prout35 | Oct 27, 2014 12:52pm | #3

   Another question

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   Thanks for the help. We live in very rural Montana where there are no building inspections. But I want the wiring done safely.

    So am I understanding correctly? I put the large green screw that came with the sub panel box into the single bus in the box. Then I attach a bare copper wire to the box (or does that attach to the bus bar?) and run the copper wire to grounding rods in the ground. Then both the white wire and the copper wire from the outgoing circuit cables attach to the single bus bar.

1. 
   gfretwell | Oct 27, 2014 04:34pm | #4

   Yes that is what you do. You are establishing a new ground reference point right there and then it will be separated as you go out into your branch circuits. The intent is that the neutrral can reflect voltage drop without affecting the grounding conductor that is connected to the case of a tool you may be using on a grounded surface.  

   Personally I think that makes more sense than extending the neutral and grounding conductor from another building in a 4 wire feeder but they never asked me. ;)

2. 
   DanH | Oct 27, 2014 06:42pm | #5

   Probably best to run TWO ground wires to the two separate ground rods, for redundancy.  Also, if there is any water pipe or something like a steel frame for the garage, connect to that.  And protect the wires from damage -- ground wires are easily damaged since they're so close to the ground and usually only a single bare copper wire.

   (Note that there's no harm is overdoing the ground wiring.)

3. 
   gfretwell | Oct 27, 2014 07:47pm | #6

   Dan makes a good point. You need to sleeve that ground electrode conductor in conduit. The gray PVC is the easiest solution. If you use metal you will need a city hub on the ground rod and connect the conduit to the rod.

   Usually they just run a single conductor and loop it through the clamp on the first rod, unbroken to the second rod and bury that part of the wire. The rods should be driven at or below grade.