Electricity to detached garage questions

[Yetti37]

I am having a detached garage built on my property and I need to run electricity to it from my house. I am very comfortable installing the wiring inside the garage as I have done similar projects to this is the past. My questions are on the sub panel and getting this connected to my main panel in my house as I have never done anything like this in the past.

I have looked through a few threads on this site and I think I figured out some basic items with this but I have some questions and want to verify my plans will work. My plans are below and I would appreciate any advice on better ways to do this or if there will be any issues with code (I live in Vienna, VA).

•There is an empty/unused 50 amp breaker on the main panel in my house. I am planning on using this to provide power to the garage.
•Bury wire underground from the unused 50 amp breaker in the main panel to the sub panel in the garage
•The sub panel in the garage will be approximately 100 feet away from the main panel of the house. The wire will be running through my side and backyard with only walking traffic above it (no driveway, cars, etc). I plan on digging the trench and laying the wire in it and backfilling with dirt.
•This is where my main questions come in. I believe I need 3-conductor wire with a ground wire (and not a 2-condutor wire with ground). I was looking at Southwire 6/3 UF-B for this as according to their website, this wire can handle 55 amps for lengths over 75 feet.
•It appears that this wire can be direct buried or buried in conduit. I was thinking of using PVC conduit as I believe the advantage of this is that I would not need to bury the wire as deep with conduit. It looks like people typically bury wire 12 inches or 18 inches and I am not sure what would apply in my situation.
•Install a 100 amp sub panel in the garage. I know this is more than wire that will be feeding it but I am pretty sure that I read that the sub panel should be able to handle more amps than the wires coming into it.
•Install a 20 amp breaker and two 15 amp breakers in the sub panel. The 20 amp circuit will be used for interior outlets installed around the walls of the garage, one 15 amp circuit will be for the lights and the two garage door openers, and the second 15 amp circuit will be for exterior outlets.
•I believe I need to install GFCI outlets for both the interior and exterior locations.

I would appreciate any advice you all can give me regarding the type of wire I should bury and whether or not I should run it through conduit. Thanks in advance for your help.

 

[pattenp]

Everything you're thinking is correct except the wire in conduit needs to have 18" of cover, 24" if direct buried.

Edit: Put the door openers on their own 20A circuit. And make all outlet circuits 20A. Also depending on how many lights you have you may need a 20A circuit.





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[captain14]

Think about laying a second line in case you want to upgrade in future? It's there but not wired in to the panels.

 

[JoeFin]

Other then a disconnecting means at the subpanel I think you have it pretty well covered

 

[pattenp]

Yep.. use a main breaker panel, not a main lug panel.

Other then a disconnecting means at the subpanel I think you have it pretty well covered

 

[UpstateNY]

Install a second pipe for other future pulls.

 

[Aceman]

Don't forget the ground rods and remember to float the neutral. You may have to buy a separate ground bar depending on the loadcenter you buy.

Also, I would rather pull individual wires, than one 6-3UF cable.

 

[2ManyProjects]

I am having a detached garage built on my property and I need to run electricity to it from my house. I am very comfortable installing the wiring inside the garage as I have done similar projects to this is the past. My questions are on the sub panel and getting this connected to my main panel in my house as I have never done anything like this in the past.

I have looked through a few threads on this site and I think I figured out some basic items with this but I have some questions and want to verify my plans will work. My plans are below and I would appreciate any advice on better ways to do this or if there will be any issues with code (I live in Vienna, VA).

•There is an empty/unused 50 amp breaker on the main panel in my house. I am planning on using this to provide power to the garage.

Are you ABSOLUTELY CERTAIN that 50 amps will be adequate for EVERYTHING you might (eventually) put in that garage? If it's just a glorified car-parking facility, which occasionally gets used for some very light-duty projects, that is surely adequate. But if you start getting into things like semi-serious air compressors, welders, table saws, band saws, surface planers, etc. -- then you could EASILY exceed that 50A supply.

Obviously, NOW is the time to make sure you have your bases covered, particularly in terms of choosing the feeder cable which runs between the two panels. You can presumably swap out the breaker later, if need be; but you want to run that cable ONCE.

•Bury wire underground from the unused 50 amp breaker in the main panel to the sub panel in the garage
•The sub panel in the garage will be approximately 100 feet away from the main panel of the house. The wire will be running through my side and backyard with only walking traffic above it (no driveway, cars, etc). I plan on digging the trench and laying the wire in it and backfilling with dirt.

So far, so good, at least in principle. But as noted later, there are some important details involved here which need to be considered.

•This is where my main questions come in. I believe I need 3-conductor wire with a ground wire (and not a 2-condutor wire with ground). I was looking at Southwire 6/3 UF-B for this as according to their website, this wire can handle 55 amps for lengths over 75 feet.

If that 6/3 w/g is copper (as I am assuming it is), it will indeed support your 50-Amp feed. And if indeed you limit the current to 50A, voltage drop won't become a problem until the run exceeds about 140 feet. HOWEVER... It's also going to cost you an arm and a leg, figuratively speaking. For significantly LESS cost, you could use aluminum 2-2-2-4 MHF, which is also a direct-burial-rated cable, and be good for up to 90 Amps.

•It appears that this wire can be direct buried or buried in conduit. I was thinking of using PVC conduit as I believe the advantage of this is that I would not need to bury the wire as deep with conduit.

That's true, as far as it goes; but it is not the (primary) reason to use conduit.

It looks like people typically bury wire 12 inches or 18 inches and I am not sure what would apply in my situation.

As "pattenp" noted, that's not enough.

I think the figures he quoted are the code minimums, per the NEC; but you should check your LOCAL codes to be sure they don't have a more strict requirement. Personally, I prefer to go down below the frost line whenever possible; but I would not argue that it is necessary to do this. Also, DO bury some marker tape about a foot above the wire/conduit, through the entire distance of the run.

•Install a 100 amp sub panel in the garage. I know this is more than wire that will be feeding it but I am pretty sure that I read that the sub panel should be able to handle more amps than the wires coming into it.

Well... Sort of.

Obviously, the sub-panel itself MUST be rated for at least the maximum current that could possibly flow through it (as determined by the upstream breaker, as well as its own "Main" breaker -- which isn't REALLY such, but we'll get to that in a moment). Beyond that, you WANT to have plenty of spare branch breaker slots, to cover future contingencies; and typically, large slot counts are mostly/only available in higher-rated panels anyway.

In addition, if there will (or can ever) be more than six branch breakers in the sub-panel, it MUST be equipped with a "Local Disconnect". This is most easily accomplished by using a panel with a "Main" breaker rated for AT LEAST as much current as the breaker actually feeding the panel (which will be located back at the main service panel). Alternately, you can "back-feed" a standard dual-pole common-trip breaker (also rated for "at least" the same capacity as the supply breaker) with the incoming feeder cable, and use that to power the panel's buss bars. Note that in NEITHER case are these breakers actually used for overload protection; that job is reserved solely for the breaker in the main service panel, whose job it is to protect the feeder cable. Hence, this "Local Disconnect" breaker can be rated significantly higher, to no ill end.

•Install a 20 amp breaker and two 15 amp breakers in the sub panel. The 20 amp circuit will be used for interior outlets installed around the walls of the garage, one 15 amp circuit will be for the lights and the two garage door openers, and the second 15 amp circuit will be for exterior outlets.

Again, unless this is just a glorified car-parking facility, make that at least TWO 20A breakers for the general-purpose 120V outlets, and TWO 15A breakers for the lighting. There are good reasons for both of these recommendations; peruse some of my past posts for the gory details. Also, dedicate at least one 20A circuit for the garage door opener(s); and hard-wire it/them if at all possible.

•I believe I need to install GFCI outlets for both the interior and exterior locations.

Yes -- which is yet another reason to hard-wire both the lighting and the GDOs.

I would appreciate any advice you all can give me regarding the type of wire I should bury and whether or not I should run it through conduit. Thanks in advance for your help.

See above. I'd go with the 2-2-2-4 MHF in a heartbeat. If I were doing the job myself, I'd probably direct-bury it, just because it's easier. But if you want to hassle with conduit, it is indeed the "better" way in the long run.

 

[2ManyProjects]

Think about laying a second line in case you want to upgrade in future? It's there but not wired in to the panels.

That makes no sense whatsoever.

First, per code, an detached structure may have one -- and ONLY one -- incoming power feed. So that "spare" feeder would have to be large enough to REPLACE the first one if/when he wanted to upgrade.

Second, THE major expense (and hassle) of this job is the buried feeder cable. It makes no sense to "double up" ("triple-up", by the time the larger cable/conduit is factored in?) on that work/expense unless it's really going to be needed; in which case, it would be better to simply run -- and USE -- the higher-capacity cable in the first place.

Install a second pipe for other future pulls.

If -- and ONLY "if" -- you are referring to such things as low-voltage computer/telephone/intercom lines, fine. Otherwise, see above.

Don't forget the ground rods and remember to float the neutral. You may have to buy a separate ground bar depending on the loadcenter you buy.

Yep.

Also, I would rather pull individual wires, than one 6-3UF cable.

Toe-MAY-tow, Toe-MAH-tow. Neither one is "fun". With an adequately strong pull rope fed in first (and maybe some sort of mechanical "assistant"), I'd probably go with one big pull vs. four smaller ones -- especially given that the fourth wire is going to be fighting an already fairly full pipe, with the three other wires NOT all lying neatly straight & flat inside.

 

[Phil De]

If you are going to spend time in your garage (hobby shop or the like) you may wish to include a TV cable in your run. I did and enjoy having the cable TV available. My wife and I also use hand held communicators between the house and detached garage/shop.

 

[Yetti37]

Thanks for all of the responses.
- For the depth of the trench, you were correct. I found table 300.5 of the NEC code and 18 inches with conduit or 24 inches direct bury is required.
- Direct burial may be an easier option after seeing responses and thinking about it. I am going to rent a trencher and the two options I have are a 12-inch one and a 24 inch one. Since I have to get the 24 inch one I might as well make a 24 inch trench and save some money on conduit.
- I will make sure I add the grounding rods and will look into the grounding bar depending on the panel I get.
- I am going to use a main breaker panel as suggested.
- I measured yesterday and the total run will be 105ft so I am not worried about the voltage drop mentioned at 140ft.
- The suggestion on adding another conduit for different runs (telephone, internet, TV) will not apply for this. We don't have a landline phone and my wireless internet has a good signal in the garage. I can stream TV into there is well over the internet.
- Using 2-2-2-4 MHF was mentioned. It looks like the cost would be similar. I found 125ft of 6/3 for $190 and 2-2-2-4 is $1.99 per foot.

I should have given so more info on the uses of the garage. It is a 24X28 single-story garage (largest building I was allowed to have based on my town's lot coverage codes and height restrictions). It will be a glorified parking structure with the work being done on cars and small projects for the house. The tools that will be running in there are a miter saw, table saw, small air compressor, and wet saw but they will not be used simultaneously . I have already run all of these items off of exterior outlets of my house (at separate times) which are on 15-amp lines without any issues. I have no plans of welding, getting a surface planer, band saw or anything like that (space in the garage will already be limited with two cars and the tools I already have).


But now there is another issue. I thought I had a spare 50-amp breaker in the main panel in my house. In testing the breakers yesterday to be sure I found out that this breaker is actually for my A/C condenser. I didn't realize this before because the A/C hasn't been on for a couple of months. So now I need to figure out a new way to hook up wires to the house main panel. My main panel is a Square D HOM30M150C. Here is a picture of it showing that I do have some spare slots. I am going to look into some other threads on this site to get more info on what my options are now.

 

[2ManyProjects]

Thanks for all of the responses.
- For the depth of the trench, you were correct. I found table 300.5 of the NEC code and 18 inches with conduit or 24 inches direct bury is required.

Just remember, that is a MINIMUM. It would be prudent to go still deeper, if reasonably feasible.

- Direct burial may be an easier option after seeing responses and thinking about it. I am going to rent a trencher and the two options I have are a 12-inch one and a 24 inch one. Since I have to get the 24 inch one I might as well make a 24 inch trench and save some money on conduit.

Any chance you can find a trencher capable of, say, 36 inches? (Just a thought. )

- I will make sure I add the grounding rods and will look into the grounding bar depending on the panel I get.
- I am going to use a main breaker panel as suggested.
- I measured yesterday and the total run will be 105ft so I am not worried about the voltage drop mentioned at 140ft.

Just for sake of clarity, please understand that even at 105 feet, you will still have SOME voltage drop. The ~140 mark is where that drop can be expected to exceed the commonly accepted 3% limit.

- The suggestion on adding another conduit for different runs (telephone, internet, TV) will not apply for this. We don't have a landline phone and my wireless internet has a good signal in the garage. I can stream TV into there is well over the internet.

While you may indeed have these secondary applications "covered" for now, given "current technology", you can't really be certain that will be the case in the future. So IF you run conduit, it might behoove you to oversize it enough to be able to make some additional pulls later on (and, of course, leave a robust double-length pull rope in the conduit, forever). I'll invite the code experts chime in on this point; but I think it's OK to mix power feeds and low-voltage lines in the same conduit, as long as this does not lead to over-filling the conduit. OTOH, if you are going to direct-bury, the whole point is moot.

- Using 2-2-2-4 MHF was mentioned. It looks like the cost would be similar. I found 125ft of 6/3 for $190 and 2-2-2-4 is $1.99 per foot.

That's an exceptionally good price for 6-3 w/g; are you CERTAIN that it is really solid copper (not just "copper clad"). If it is aluminum (with or without the copper cladding), it won't cut it even at 50 Amps. But even if it is really copper, I'd probably still use the aluminum 2-2-2-4 MHF anyway. For less than another $0.50/ft., the additional 40A of capacity becomes the overwhelming factor -- it's just too cheap an upgrade to NOT take advantage of.

I should have given so more info on the uses of the garage. It is a 24X28 single-story garage (largest building I was allowed to have based on my town's lot coverage codes and height restrictions). It will be a glorified parking structure with the work being done on cars and small projects for the house. The tools that will be running in there are a miter saw, table saw, small air compressor, and wet saw but they will not be used simultaneously . I have already run all of these items off of exterior outlets of my house (at separate times) which are on 15-amp lines without any issues. I have no plans of welding, getting a surface planer, band saw or anything like that (space in the garage will already be limited with two cars and the tools I already have).

Even so, I'd still recommend the 90A feeder. Yes, you could likely "get away with" 50 amps without incident, IF you never upgrade your tools, or add any sort of HVAC, or have a second person working with you (and using tools) at some point down the line... But the relative economics just don't support making that large a cut in your potential capabilities to save a relatively trivial amount (maybe $50) of money.

But now there is another issue. I thought I had a spare 50-amp breaker in the main panel in my house. In testing the breakers yesterday to be sure I found out that this breaker is actually for my A/C condenser. I didn't realize this before because the A/C hasn't been on for a couple of months. So now I need to figure out a new way to hook up wires to the house main panel.

You'll need to obtain and install an appropriately rated dual-pole breaker in the panel, to accommodate the new sub-panel feeder.

My main panel is a Square D HOM30M150C.

Here you go:

http://www.homedepot.com/p/Square-D...p-Two-Pole-Circuit-Breaker-HOM290CP/100206837

Here is a picture of it showing that I do have some spare slots.

It would appear so. That panel is listed as "30-space, 30 circuit" (implying that it does NOT support tandem breakers; but that is not an issue here). You seem to have 20 of those spaces filled; so it should be simple to add a new dual-pole breaker near the bottom of the panel.

I am going to look into some other threads on this site to get more info on what my options are now.

By all means, feel free to confirm; but I'm confident the above is correct.

 

[Yetti37]

2ManyProjects - thanks for the additional information. I am going to go with the 2-2-2-4 MHF now so that I have some additional capacity for future tools. I went through some other threads (many of which you had responded in) and it looks like people have had success with the MHF.

 

[pattenp]

Check Lowe's for the 2-2-2-4 MHF. It's $1.47 a foot at my local Lowe's. Without reading back I don't remember if any one mentioned that the MHF has to be in conduit where installed inside.

 

[rodm1]

You should rethink using PVC spec it for 100amp. You have a fairly big garage and much room for expansion. you probably will get years out of the 50amp service but if you start adding large loads you could be running short eventually. It would such to retrench the yard just to upgrade to 100amps. If you use pvc you would just have to re pull new wire.

50amp service are kind of outdated if you must direct berry wire I would berry 100amp wire Spec everything else 50amp.

 

[CNGsaves]

+1 to bury 2" sch 40 conduit that you trench in at 30 inches (or more) depth. Once above ground, conduit needs to be sch 80. Recommend you follow PattenP advice and go with mobile home feeder which I think would give you 90 amps in detached garage (thus 90A breaker in main panel to protect wire and then common 100A subpanel in detached garage).

While you are trenching, might as well add another conduit at higher level (at least 12" from main electric after backfilling) so you could have low voltage runs to garage (internet / phone / CATV / security / etc). I'd recommend quantity 3 ethernet CAT 5e and one RG6, along with an extra pull string for future expansion.

"Do it once . . . and Do It Right !"

 

[Ch3No2]

Yep.. use a main breaker panel, not a main lug panel.

Just curious...is this mandated by code to have a disconnect at the sub since there is already one at the main?

 

[2ManyProjects]

Just curious...is this mandated by code to have a disconnect at the sub since there is already one at the main?

Yes, IF there is (or will be) more than six branch-circuit breakers (pedantically, "breaker handles") within the sub-panel. And it is ALWAYS a good idea, regardless of breaker count.

 

[rburke65]

Run the 2" PVC. Ran direct burial for my neighbor and we were doing it over in 8 years because he lost one hot leg...rock cut it or what ever? Had to do it all over again plus the cost of new wire....and labor....and a trencher. Run the 2".

 

[Tripp2012]

Have you looked into seperate service for the garage? I went with seperate 200amp service from my power company. I would have cost me triple to run a feed line from the house. I have a seperate meter billed to the house bill and in my area it averages about $20 bucks a month. It would give you much more room to grow and save you renting a trencher and running the feed line. With my building I pulled a permit wired the shop wired and hung the meter box and the power company did the rest. Cost me $375 bucks for them to run the line and connect it to the meter box.

 

[Falcon67]

Just curious...is this mandated by code to have a disconnect at the sub since there is already one at the main?

Not that I know of but its dang handy to have a full power shutoff right in the shop. One reason why my panel is next to the door. In case of emergency I can kill every thing when leaving. Or vacation.

Also, as the OP may have seen in other threads - I ran 2-2-2-4 and fused it at 70A. I have a lot of stuff, no power problems at all.

 

[Norcal]

Just curious...is this mandated by code to have a disconnect at the sub since there is already one at the main?

If more then 6 breakers, yes. In a lot of areas main breaker panels can be cheaper then main lug because they are so common, use a large a panel as possible since they make changes easier, 6 or 8 space panels ****! Unless they are used where the circuits/needs are limited.

 

[pattenp]

Per the NEC the main disconnect is required if the sub panel is in a detached building, but as said, only if there is more than six breakers to disconnect the power. If the sub panel is in the same building as the main panel then no means of main disconnection is required at the sub panel regardless of the number of breakers in the sub panel.

Just curious...is this mandated by code to have a disconnect at the sub since there is already one at the main?

Yes, IF there is (or will be) more than six branch-circuit breakers (pedantically, "breaker handles") within the sub-panel. And it is ALWAYS a good idea, regardless of breaker count.