Powering and Detached Garage

[kmacht]

I know this has been covered in the past but all the posts I saw had different lenght runs, different breaker sizes, different size shops, etc. I figured I would post what I have and what I need and hope someone can point me in the right direction. I am having a 24x24 garage with a 2nd story delivered in 2 weeks. It will be detached from the house but only about 5 feet away. There is concrete between the house and the the garage so I would have to run a line out of the box, past the slab, and then back up into the garage. Total run would be about 50' from the box in teh house to the subpanel. My main panel in the house is 100 amps. I would like the garage to have enough power for lights, 3 110 circuits downstairs, 2 circuits upstairs (all with multiple outlets) and at least one 220 plug for future use (maybe small welder or 220v machinery). What will I need to run from the house to do this? Can it all be run off of one feeder line or do I need a seperate one for the 220? What size wire and how many conductors do I need to run from the house to the subpanel? Recomendations for the main breaker size for the subpanel? What about grounding? I have rebar in the slab but no way to tie into it. I think I remember reading about the need for 2 ground rods 6' apart. Do I still run ground and neutral from the house to the sub panel then? If someone has a good website that shows how to wire a subpanel for a detached structure that would be helpful. If not, any advice you have is appreciated.

Keith

 

[mrb]

you need 4 wires from the house to the garage.

you will need the two ground rods if (hopefully) your inspector doesnt make you expose rebar and bond to it. Code is if you have a Ufer Ground (rebar) you HAVE to use it, not optional.

 

[spongerich]

My $.02 is that it sounds like a lot of circuits. I'd look at one 20 amp for lighting, one for convenience outlets, 2 for machinery and a 240/30 for a welder etc.

Think about what you'd be running concurrently. If it's just you, you're not likely to have a table saw going upstairs while you're welding downstairs (unless you've got really long arms)

You can put a bunch of receptacles on a single circuit, what really matters is how many you'll have active at once.

I just rewired my 3000 sq ft barn. I pulled in a 240v 50 amp circuit to a 8 place sub panel. I'm probably going to end up with 4 120v/20a and 1 240/30a total.

 

[mrb]

dont forget you need a disconnect for the detached structure (main breaker in your subpanel)

 

[kmacht]

I did some reading through the archives after posting this and have some followup questions. Lets make sure I have this right first.

1) I need 4 wires coming from the main panel in the house (2 hot, 1 neutral, and 1 ground)
2) The neutral has to go to an isolated neutral bar in the subpanel
3) The ground has to go to a bar in the subpanel that is connected to the subpanel frame
4) I have to have 2 ground bars 6' apart and connected to the subpanel ground block. These must be connected with a continuous piece of wire and buried flush or slightly underground

Now for some questions

1) Why must I have a main breaker in the subpanel? Everything else I read said no, just a 2 lug subpanel is appropriate and a main breaker is only used if it is connected directly to the service entrance (meter).

2) What size wire to feed the sub panel with a 50' run from the house panel. What size copper and what size aluminum for 50 and 60 amps. Do all 4 conductors have to be the same size?

3) What size and type of wire do I have to run to the 2 (6 foot seperation) ground bars

4) Any problems running other things in the same trench as the sub panel wire as long as it is not in the same conduit? I want to run an air line and a phone line out there and would prefer to not dig a 2nd trench.

I know it sounds like alot of circuits but I figure two circuits downstairs and 2 circuits upstairs will allow me to run two power hungry items on each floor at the same time. If I put an air compressor downstairs I want to be able to plug someting else in at the same time downstairs without tripping a circuit. Since I am doing flouresents I can probably get away with one circuit for both upstairs and downstairs. That plus at least one 220 outlet already puts me at 5 circuits. I figure I will go with an 8 circuit panel in case I want to run more later.

Keith

 

[kmacht]

I forgot to mention. My rebar situation isn't really normal. I added on to an existing slab. The old slab was 18x20 and I made it 25x30. As far as I can tell there was never any rebar in that old slab (poured in the 50's.). When I added onto it I tied into it with rebar (drilled and expoxied) and then used wire mesh for the rest. So I have some rebar in there but not much. Most of it either has no rebar or a little bit around the inside edge.

One more question. What do I use for a grounding rod and how deep do I have to pound it?

Keith

 

[mrb]

1: detached structure requires a disconnect (unless you have 6 or less handles in your subpanel), cheapest way ($12 or whatever for a 60a 2 pole to use as a backfed main) to accomplish this is with a main breaker in your subpanel.

2: #6 CU or #4 AL ground can be #10 cu or #8 AL for 60 amps.

 

[mrb]

I forgot to mention. My rebar situation isn't really normal. I added on to an existing slab. The old slab was 18x20 and I made it 25x30. As far as I can tell there was never any rebar in that old slab (poured in the 50's.). When I added onto it I tied into it with rebar (drilled and expoxied) and then used wire mesh for the rest. So I have some rebar in there but not much. Most of it either has no rebar or a little bit around the inside edge.

One more question. What do I use for a grounding rod and how deep do I have to pound it?

Keith

ah ok, so you dont have a Ufer Ground then.

As far as what to use for a grounding rod, you use a ground rod. Theyre 8 feet and you need all 8 feet in the ground.

 

[Charles (in GA)]

You can find the ground rods and approved clamps for the ground wire at the big box stores. Have two people and a ladder and a sledge hammer. I have also seen a hammer dirll used to drive ground rods and it works rather well. I prefer a heavy solid wire for the ground, its a durability issue, since you are not going any real distance buy a short roll of #6 solid bare copper, it works well for this.

If you buy a small panel, say 12 or 16 spaces (remember each 240v circuit takes up two spaces) as mrb said, use a 60 amp breaker as a main disconnect. Mount it all the way at the top or bottom, or where ever, but be sure to get the clip that is designed to make removal of this breaker difficult, its required to have a special retainer for a breaker used as a main breaker. Don't forget to properly label it as the MAIN DISCONNECT. You will find them on the pegs with the ground bars and other odd lugs and such, next to the panels in the big box stores. They are brand specific, so get one for the same brand of panel (GE clip, GE panel, etc).

Get a bigger panel than you think you will need. They are cheap in the big scheme of things, and the bigger, the more room you have to route wires and such inside it. I would not get less than a 16 space for this application. (but thats my personal opinion, others may disagree, but whats $5 or $10 more, one time for something like this?)

Charles

 

[kmacht]

Great. Thanks for the replies and all the advice. I think I have it sorted out enough to start putting together my list of supplies I'll need.

Keith

 

[osu69]

If I may interrupt with a related question...

The neutral and the ground from the main panel are tied to a common bus in the outbuilding's sub-panel, right?

Then the #6 bare copper from the outbuilding's ground rods should be connected to just the subpanel box and kept isolated from the feed neutral and ground as well as the neutral and ground wires in the building's individual circuits, right?

I've been confused about sub-panel ground connections for years...so I hope someone can straighten me out.

 

[mrb]

If I may interrupt with a related question...

The neutral and the ground from the main panel are tied to a common bus in the outbuilding's sub-panel, right?

Then the #6 bare copper from the outbuilding's ground rods should be connected to just the subpanel box and kept isolated from the feed neutral and ground as well as the neutral and ground wires in the building's individual circuits, right?

I've been confused about sub-panel ground connections for years...so I hope someone can straighten me out.

no that is very wrong. You can only have one connection between neutral and ground. This is called the main bonding jumper and is at your service, either at the meter can, main disconnect, or main panel depending on how your service is laid out. Downstream from this point the neutral and ground have to be kept seperate.

At your subpanel, you terminate the neutral on the neutral bus bar and either leave out or remove the screw that bonds it to the can. Then you install (if there isnt one there already) a ground bar. The grounding conductor from your feed terminates on the ground bar as does the grounding electrode conductor (the wire from your ground rods).

Edit:
the grounding conductor from the supply to the outbuilding is very important. Many people think that if you have a ground rod thats good enough. The main purpose of the grounding conductor is to provide a path to clear fault currents, the ground rod is for lightning and other stray currents. Without a solid ground (wire, not dirt) between the outbuildings panel and whatever is supplying it you have a very dangerous situation.