Detached Garage Electrical Questions

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Old 07-11-14, 12:00 PM
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Detached Garage Electrical Questions

Hi all,

I have some questions regarding installing a sub panel and wiring in a detached garage. I would like to say flat out that I'm an electrical newb, but I'm an engineer and I'm here to learn and understand, not to hack this project. If I have any doubts whatsoever for any part of this, I will consult a professional. But, things like gathering materials and doing the labor work I know I can handle. Plus I really want to understand this stuff. So, here goes! I'm located in south east Michigan.

Last fall I ran empty 1.5" conduit (2 separate runs, 1 for low voltage) from the house to the garage (permitted and inspected) before having a concrete patio poured over top of it. 16' from the house to the garage, approximately 35' panel to panel. So now I'm set to wire up the garage. Loads in the garage will be pretty standard for a decently handy homeowner: 5x4' double bulb florescent lights, 1/2hp garage door opener, basic power tools like circular saw, fridge (or maybe mini-fridge), 4-5 outlets, 1 exterior outlet, and one exterior light. Possibly a small air compressor or a table saw or something down the road. I'd also like the capability for 220V to be installed at a later date if I ever need it or even by the next homeowner.

My main panel is 100 amps. My plan is to feed 50 amps (I'm thinking that's enough for those loads?) to the garage sub panel, which is also a (smaller) 100amp panel with 8 spaces and a main breaker shutoff. I bought #6 THHN (2 hots, neutral) and #8 THHN (ground) wire to run from the main panel to the sub panel. This will be done all inside pvc conduit (panel to panel). I'm pretty sure, based on my research, that everything up until this point makes sense as far as planned loads/power/equipment, but I'd love someone to confirm!
Now for my main questions/concerns.

1. Main panel feed breaker. Do I need a 2-pole 50 amp breaker or a single pole for the sub panel feed? I have 2 open spaces in my main panel so either will fit. Is a double pole needed to allow for 220V to be installed in the garage at some point? Or do I need a single pole breaker for the feed regardless, and then later on a 2-pole breaker could be installed in the sub panel for a specific 220V circuit? I just want to make sure I do this so that the capability is there down the road and I'm not sure which is required.

2. Grounding. Because I'm doing a 4 wire feed, the sub panel will be grounded back to the main panel (if I'm not mistaken). So I think I'm set there. My question is regarding a ground rod. I've read 2 grounding rods daisy chained 6' apart, I've read 2 ground rods 6' apart both back to the sub panel, I've read 1 ground rod, I've read no grounding rod needed because it's grounded back to the panel, etc. Ahhh! Which is correct here? First of all, I'm wondering if it's needed because the garage it's only 16' from the house. Seems unnecessary. But if I do need one, I'll get it done. The only other issue is, where the box will be mounted is above concrete patio, which I'm not drilling in to, so if I need a rod (or 2 or whatever) can I install them behind the garage? This would be approximately a 20' run. So...I'd really appreciate some correct info on ground rods!

3. Sub panel breakers and circuits. I'd like some advice on how to break up the circuits when the wiring is done, and also what size breakers to use. From what I read either 15 or 20 amp breakers are fine (adjusting wire size accordingly) but I'm not sure the advantages/disadvantages or which ones to go with. A mixture? Circuit wise, I'm thinking of doing something like dedicated circuits for the garage door opener, the lights, the fridge, and maybe 2 for the outlets? My thought was a dedicated circuit for the outlets near my work bench that will be used primarily for tools and stuff, and then the other outlets for TV, radio, misc stuff on another circuit. That would leave me with 3 open spaces left and hopefully not too much load on any 1 circuit. Maybe 15 amps for the lights, fridge (non GFCI for this one outlet only), and garage door opener, and 20 amps for each outlet circuit? Would something like that make sense?

4. Wiring/conduit/finishing. If I'm not mistaken, if I decide not to finish the garage, all of the wiring must be ran in conduit. First of all, is that right? Regardless, I'm thinking that doing a quick finish (no need for insulation) and hanging plywood or OSB and then painting would look clean and bright, hide the wiring, possibly negate the need for conduit throughout, etc. So I may go that route regardless. But my main question here so I don't get screwed when it gets inspected, is about the conduit. Either run everything in conduit, or cover the walls so the wiring isn't exposed. Right?

Sorry for the long post, and thank you all in advance for your time and especially to those of you that offer advice!
 
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Old 07-11-14, 02:16 PM
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#6 THHN (2 hots, neutral) and #8 THHN
You must use THWN*. You only need #10 for ground but nothing wrong with #8. You will need #6 for the GEC to the ground rod..
1. Main panel feed breaker. Do I need a 2-pole 50 amp breaker or a single pole for the sub panel feed?
Yes a 2 pole 50 amp breaker.
Is a double pole needed to allow for 220V to be installed in the garage at some point?
Yes, to allow for 240 volts at a future date. (Nominal voltage is 240 not 220). There is no reason to feed 120 volts only to the panel.
I've read 2 grounding rods daisy chained 6' apart, I've read 2 ground rods 6' apart both back to the sub panel, I've read 1 ground rod,
It is based on the conductivity of the soil and what the AHJ requires. Usually one ground rod.
I've read no grounding rod needed because it's grounded back to the panel, etc.
That would always be wrong. The EGC from the house is to provide a low resistance path for clearing faults like shorts to a metal case. The GEC is for atmospheric electrical charges.

Usually 120 volt general purpose receptacle circuits are 20amps. They must have GFCI protection. Lighting can be 15a or 20a and does not require GFCI circuits. Breaker size of dedicated circuits depends on the device requirements. Most areas require protection such as conduit sleeves for cable ran lower than 6'7". You can run a complete conduit system using THHN* if you wish.

You didn't mention it but you will need a bonded ground bar and isolated neutral bar in the subpanel. The ground bar almost always is sold separate and has to be added by you to the panel.

*THHN is for use in dry locations. Conduit outside no matter how well sealed is a wet location. Usually a moot point because most wire is dual rates THHN/THWN.
 

Last edited by ray2047; 07-11-14 at 06:39 PM.
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Old 07-11-14, 06:07 PM
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Good job Ray!

The only thing I would change is the breaker in the main panel. Since the OP is using #6 THWN wire, I'd go ahead and bump the breaker up to 60 amps.
 
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Old 07-11-14, 07:23 PM
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I agree, a 60 ampere CB in the service panel. I would also suggest two, 15 ampere lighting circuits. This is in case you are moving some long pieces of material and smash into the lighting and that subsequently trips the CB. With two circuits you won't then be left in the dark.
 
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Old 07-11-14, 09:24 PM
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Thanks so much for the detailed info Ray, and also the others that chimed in!

Yes, the wire I'm using between the panels is all dual rated THHN/THWN. I should've noted that before.

Regarding 50 vs 60 amps...I thought I read that #6 wire was good to 55 amps...so I went with 50 to be safe. If 60 is safe, then I'd obviously rather go with that. However I also remember reading somewhere something about it being against code to "send" more than 1/2 the amperage of the main panel to a sub panel. I'm not sure if I'm wording that correctly, or if I read it correctly for that matter. Is the 60 amp feed to the sub panel being larger than 1/2 the main panel a cause for concern? If not then it's a no brainer to go with 60 amps. And I will use the 2-pole breaker for the feed either way.

I will make sure to install a ground rod. I really can't imagine that my case would require 2 ground rods...after all there is only 1 for the whole house. However I'm still wondering about the distance from the panel to the rod. Would running it ~20' so I can put it behind the garage be unsafe in any way or be against code?

To make things simple and because I already have a good size roll of #12 (yellow) romex, I will go ahead and use 20 amp breakers for all the circuits. I don't see a downside to this. Regarding GFCI, would it be more prudent to install a GFCI breaker for the receptacle circuits or just use GFCI receptacles? Are there upsides or downsides to each method?

Regarding the conduit for the inside wiring...I decided today that I'm going to finish the walls with simple 7/16" OSB and paint it a light color, so all the wiring to the receptacles/switches/etc will be concealed and therefore won't require conduit (even though it's below 6'7"), correct? But, I am also going to be hanging lights from my rafters, and I'm not planning on finishing the ceiling with the OSB. This exposed wiring will definitely be higher than 6'7", so it also won't require conduit, correct? I want to make sure I get this part right!

Lastly regarding the ground and neutral bars in the sub panel...yes I will need those! I actually read up on the sticky for sub panel wiring diagrams...very helpful and informative. But also while I'm on the topic of the guts of the panel, I have a dumb question: Do I need to install the mounting rails or whatever they're called that the breakers attach to in the panel? Granted I didn't spend a ton of time on this, but from a quick look at the directions and the guts of the panel I don't really see how these things are supposed to snap into place! I'm pretty sure I got the correct breakers for the panel type (Homeline)...

Thanks again for the help everyone.
 
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Old 07-11-14, 09:54 PM
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I also remember reading somewhere something about it being against code to "send" more than 1/2 the amperage of the main panel to a sub panel.
What is the make and model of your panel? That refers to the largest size branch circuit breaker can use. Look on the label inside the box. I doubt it is less than 60 amps.
Regarding GFCI, would it be more prudent to install a GFCI breaker for the receptacle circuits or just use GFCI receptacles?
Usually cheaper to just make the first receptacle GFCI and run non GFCI receptacles from the load side of the first.
I'm going to finish the walls with simple 7/16" OSB
Then cable in the attic shouldn't need protection.
Do I need to install the mounting rails or whatever they're called that the breakers attach to in the panel?
No, everything you need is there. If you bought a main breaker service panel kit (usually cheapest) it should have come with an assortment of breakers that fit. If you bought a panel only verify the breakers you bought are listed on the label in the panel. One thing to check is the smallest wire that fits the main breaker. If it is larger than #6 you will need to pigtail larger wires to the two hot wires to the main breakers. Neutral and ground are not a problem.

Just to verify you did buy white for neutral and green for ground?
 
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Old 07-12-14, 04:52 AM
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I assume this is permitted and inspected work?

To make things simple and because I already have a good size roll of #12 (yellow) romex, I will go ahead and use 20 amp breakers for all the circuits.
You might want to ask your AHJ if romex is allowed in a garages.


I will make sure to install a ground rod. I really can't imagine that my case would require 2 ground rods...after all there is only 1 for the whole house.
You'll need to obtain the properly calibrated test equipment to perform a resistance test. Or you may want to avoid that and just drive 2.

Some of your questions are better suited for the AHJ, afterall it is him you will need to satisfy.
 
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Old 07-12-14, 03:59 PM
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I checked inside the main panel and it says the largest 2-pole breaker acceptable is 100 amps...so I think then I'm good with 60 for the sub panel feed.

I will make the first outlet in each receptacle circuit GFCI at a minimum, so that'll easily take care of that.

Just to clarify, the "attic" in the garage is all open rafters (no regular ceiling). But because it's above the 6'7" mark it can still be open...?

After looking at it a little harder for 5 seconds I figured out how to snap the breakers in place in the sub panel...no problem there. How do I check what the minimum size wire is that the main breaker can accept? Is it labeled somewhere or do I physically investigate the connector?

Yes I got white for neutral and green for ground. They were out of #6 black, so both hots will be red (assuming that's ok, I'll tape the ends maybe 1' back or something with electrical tape to keep track of what's what).

Also another question about the bonded ground in the sub panel: I see both buses are connected together at the top. Can I disconnect that tie and then "bond" one of the buses to the box with the big green grounding screw that came with the panel? Seems like that would work, but I'm not sure if that's what that's for. Or will I need to buy a specific bonded grounding bus for this purpose?

I guess I can just drive 2 ground rods to be safe. I'm not finding anything from looking into a minimum distance from the rod to the panel requirement, so it at least appears I'll be OK on putting the rod(s) behind the garage.

If romex isn't allowed in garages, will I have to run all individual THHN wires to each recepticle/etc? That seems like a big PITA. Or could I use some kind of outdoor rated 3-wire stuff?

Although I'm more concerned with doing it right that actually getting a permit and inspection and everything, yes I will likely play by the rules.
 
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Old 07-12-14, 04:23 PM
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because it's above the 6'7" mark it can still be open...?
Usually but your AHJ has the last word.
How do I check what the minimum size wire is that the main breaker can accept?
Not sure. Wait for a pro to answer.
I'll tape the ends maybe 1' back or something [of the hots] with electrical tape to keep track of what's what).
No need, they are interchangeable.
Also another question about the bonded ground in the sub panel: I see both buses are connected together at the top. Can I disconnect that tie and then "bond" one of the buses to the box with the big green grounding screw that came with the panel?
Sometimes they are, sometimes they aren't.

Ask the AHJ about how many ground rods and acceptable distance. Never heard of Romex not being allowed in a garage. Again ask the AHJ.
 
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Old 07-13-14, 01:43 AM
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All codes are local. Model codes, such as the National Electrical Code have no power of enforcement UNTIL enacted into law by a local, regional or state legislative body. The enabling legislation has the power to add to or delete from the model code. This is why you need to check with the local Authority Having Jurisdiction (AHJ) BEFORE doing any work.

How do I check what the minimum size wire is that the main breaker can accept?
Every circuit breaker I have seen has the range of wire sizes allowed printed on the CB body. You might also find the range of a main circuit breaker wire sizes listed on the panel inside the enclosure door.

Also another question about the bonded ground in the sub panel: I see both buses are connected together at the top. Can I disconnect that tie and then "bond" one of the buses to the box with the big green grounding screw that came with the panel?
Well, maybe but it is usually far easier to simply buy the correct equipment grounding bus and attach it to the holes already in the enclosure. The grounding bus is generally around five dollars.
 
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Old 07-14-14, 07:18 AM
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How do I check what the minimum size wire is that the main breaker can accept?
Every circuit breaker I have seen has the range of wire sizes allowed printed on the CB body. You might also find the range of a main circuit breaker wire sizes listed on the panel inside the enclosure door.
I agree that the minimum and maximum wire sizes a breaker will accept are many times on the breaker's label in the fine print, but I have also found that this isn't always true. I just looked at a new 15 amp C-H BR series breaker and the info isn't there. I have found it easier to consult the manufacturer's catalog for this information and to not rely on the information being in the fine print.
 
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Old 07-14-14, 10:31 AM
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Thanks again for all the help guys.

I will look further into the minimum acceptable wire size for the panel.

I also went ahead and bought a grounding bus and installed into the panel...very simple. So that part is ready to go.

I also have a couple questions regarding the ground rods. I'm going to go ahead and prepare to drive 2 just in case. I bought the rods, direct burial clamps, and #6 bare stranded copper to tie everything together. I'm planning on driving them so they sit a few inches below the surface and then attaching the connecting wire a few inches down from the tops of the rods and digging a shallow trench to bury the wire. That way everything will be below grade and well protected and hidden. But my question is, what about the bare copper coming through the garage wall and going down to the first ground rod? Obviously I'll come out through the wall near the bottom, but there will still be a few inches of exposed wire. Does that have to be protected in conduit? On my house ground rod it isn't, but I'm not sure if that's up to code or not...?
 
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Old 07-14-14, 11:13 AM
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Obviously I'll come out through the wall near the bottom, but there will still be a few inches of exposed wire. Does that have to be protected in conduit? On my house ground rod it isn't, but I'm not sure if that's up to code or not...?
That sounds like a pretty typical GEC installation. I'd say it is fine.
 
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Old 07-14-14, 02:15 PM
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Thanks, that's what I was hoping.

I thought of another question also: when running my wiring across the attic rafters (which will remain exposed for now anyway), should I run across the 2x4 rafters for the perpendicular runs or drill and run through them? My thought is going through will allow someone to have the capability to finish the ceiling if they so choose, but is drilling through an acceptable practice? Or should I just go on top with staples?
 
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Old 07-14-14, 02:33 PM
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Drilling if there may be a ceiling in the future. Drilling is standard practice.
 
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Old 07-14-14, 02:48 PM
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Although I'm more concerned with doing it right that actually getting a permit and inspection and everything, yes I will likely play by the rules.
Doing it right means that you must get a permit before you start any work. Then you need to follow the codes as set by the AHJ. Seriously, this is NOT a "learn as you go" type of job.

Also keep in mind that some (or all) of the work you're doing may very well require that you posses an electrician's license (especially since you're wiring a detached building).
 
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Old 07-14-14, 03:19 PM
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You want to keep the conductor to the rod as short and as straight as possible.
 
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Old 07-14-14, 03:32 PM
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Originally Posted by FMB42 View Post
Doing it right means that you must get a permit before you start any work. Then you need to follow the codes as set by the AHJ. Seriously, this is NOT a "learn as you go" type of job.

Also keep in mind that some (or all) of the work you're doing may very well require that you posses an electrician's license (especially since you're wiring a detached building).
Thanks. I know my city building inspector and electrical inspector. They are cool guys and have permitted and inspected multiple projects for me over the years. I have some empty conduit connected up to the empty conduit run I had inspected last fall. That's pretty much it. I'm not stupid and I know this isn't a learn as you go job. One would have to be stupid to think it was. It's not like I'm coming on here after running a bunch of wire and guessing how to hook it up and when it doesn't work I'm asking for help. Like I said in my first post, I'm gathering materials and PLANNING. When I call my city and get the permit and discuss/draw up the plan I want to know what I'm talking about.

And homeowners are permitted to do electrical work in my city as long as they are the owner and resident of the home.

You really shouldn't talk down to everyone like they are just going to hack their job and not get anything inspected. :-)
 
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