Underground wiring to garage

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  #1  
Old 04-12-11, 12:53 PM
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Underground wiring to garage

Hi everyone,

I'm in the process of revamping my detached garage and I finally want to get electricity out there. I only want to put a 20A and 15A circuits out there, just enough for a few recessed lights and a few outlets. I will not be running any power equipment through this garage.

My question is what options do I have to get the power out there. In my mind I can come up with two conclusions, 1. run power from the mains to a subpanel in the garage underground, or 2. just run 12 and 15 guage wiring from the main house service panel out to the garage and wire everything from those line( so breakers would be in the main house)

I want to go underground, I'm thinking about doing uf cable from the service panel in the house through conduit out to the garage. Im not sure of the depth but I think it's supposed to be 18 inches (under grass no concrete). The lines will probably be 90-125 ft. Will i have to worry about derating i think its called?

Thanks in advance
 
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  #2  
Old 04-12-11, 01:10 PM
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Your situation sounds appropriate for a 20A multi-wire circuit. From a 20A double-pole breaker in your main panel, run a #12-3g UF-B cable to the garage. The buried portion should be 24" deep, conduit is only needed for the entrance and exit to the ground. Once inside the garage, you'll need a simple double-pole disconnect switch and two GFCI receptacles.

The incoming 12-3g cable splits here, the black goes to one GFCI line terminal, the red goes to the other GFCI line terminal, the white is split to go to each neutral line terminal on the GFCIs. From the LOAD side of each GFCI receptacle, run a 12-2g NM-B cable. At this point the two 12-2 cables can be treated like separate 20A circuits.
 
  #3  
Old 04-12-11, 01:25 PM
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Thanks ibpooks,

Would there be any reason I couldn't just use one 15A and one 20A breaker from my main panel opposed to a double pole breaker? I have room to spare in the panel and I have access to plenty of UF-B. And actually I have a spare GFCI 20A breaker that I could use for the 20A circuit.

Also, the ground here is very hard to dig into. If I used conduit for the entire lenth underground would this decrease the depth requirement?

Thanks for the info!
 
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Old 04-12-11, 02:08 PM
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For a Multi-Wire Branch Circuit (MWBC) as described by ibpooks you must use a two-pole (240 volt) circuit breaker. You would gain nothing by using a single 20 and single 15 ampere circuit breaker other than using parts you already have.

If all you want is a few lights and a convenience receptacle then a single 20 ampere circuit is probably more than sufficient. If you go this route and already have the single-pole GFCI circuit breaker you can get away with burying the type UF cable at only 12 inches deep. If you use a non-GFCI circuit breaker the minimum depth is 18 inches.

Running conduit would be expensive as to get away from the above stated minimums you would have to use rigid metallic conduit (RMC) with threaded fittings. The rigid conduit (no EMT or IMC) would need to be buried a minimum of 6 inches deep or have a minimum of 4 inches (I think) concrete over it.
 
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Old 04-12-11, 02:22 PM
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Originally Posted by Furd View Post
If you use a non-GFCI circuit breaker the minimum depth is 18 inches.
18" for plastic conduit, 24" for direct burial cable. Although if you went with conduit it would make much more sense to use THWN instead of UF-B. With a GFCI breaker, 12" depth for conduit or cable.

I have access to plenty of UF-B. And actually I have a spare GFCI 20A breaker that I could use for the 20A circuit.
If you already have all the parts, then a single 20A circuit using 12-2g UF-B from a GFCI breaker would probably be adequate. The code allows only one circuit to an outbuilding, so you can do a single circuit, a multiwire circuit, or a full subpanel feeder; but you can't do multiple separate runs of 12-2, 14-2, etc.
 
  #6  
Old 04-12-11, 02:44 PM
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I would never run less then a large enough wire for 60 amp. 4 breaker panel to a garage. Why pull two wires when one large one will do the job. Twice this year we had to go back and redo someone else work were some home owner ran to small a wire.
You would install one double pole breaker in the house to tap 220 volt off of to supply the panel.
The breakers need to be in the garage not the house. Every time one trips or there's an emegancy where the powers got to shut off you would have to run all the way to the house.
125 ft. way to far for that small a wire your suggesting using.
It's also going to have two of it's own ground rods 4' apart.
 
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Old 04-12-11, 02:48 PM
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A single general-purpose circuit is probably 25% the cost of even a small panel, so if the homeowner doesn't need power for a workshop why would he install it?
 
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Old 04-12-11, 04:38 PM
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I suggest checking with local code enforcement about burial depth. About ten years ago I had a similar project (240V /50 amp service) and I was required to bury the cable 36".
 
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Old 04-12-11, 06:35 PM
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ibpooks,

That's exactly what I'm looking to avoid, paying a good chunk of cash to install a sub panel in the garage. I had to have the main box replaced in the house, and it wasn't cheap haha.

Joecaption,

I have no need for more than a few receptacles and some lights, which will probably be the CFL's variety so low amperage draw. I have a workshop at a different location so there wont be any need for large amperage draws.

ibpooks,

How many amps would the circuits in the garage have access to if I used the multi-wire branch circuit approach. If I have this correct it would be two circuits connecting back to the main service panel at a 2 pole breaker, right? So would that be two circuits in the garage sharing 20A, or two 20A circuits? I more or less just completed a re-wire of the house, so I'm familiar with wiring the receptacles and lights and switches, but have not had to use a 2 pole breaker yet.

thanks everyone, i appreciate the info!
 
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Old 04-12-11, 07:12 PM
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Two 20a 120v circuits.

............
 
  #11  
Old 04-15-11, 12:02 PM
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Sorry to ask again,

But someone at work was confusing me about the amount of amps going to the garage, although I think he was confused himself.

So If I run a multiwire branch circuit from the service panel to the garage, I would get two 20A circuits. Would this mean I would have 40A capacity to the garage? Or just 20A between the two? I think what im trying to ask is if the 2 pole breaker allows 40A or 20A to go out from the multiwire.

Sorry about all the misunderstanding on my part,
 
  #12  
Old 04-15-11, 12:42 PM
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A 20A multiwire circuit is essentially equivalent to two 20A 120V circuits. It provides a total of 4800 watts, which is 40A at 120V or 20A at 240V or some mixture of the two. The caveat is that the 40A load must be roughly balanced into two parts that each do not exceed 20A. For example you could power two 15A loads for a total of 30A, but you could not power a 5A and a 25A load. In practice this is rarely a problem with residential sized tools and equipment.
 
  #13  
Old 04-15-11, 01:00 PM
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thanks ibpooks,

That is a perfect solution. While I don't need a full 40A, it will be nice to be able to run the lights on a 15A circuit, then have a full 20A circuit for outlets and such.

Thanks again guys!
 
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Old 04-15-11, 01:18 PM
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You wouldn't have a 15A and 20A circuit, you'd have two 20A circuits. There's no need to put lights on a 15A, they will be fine on the 20A.
 
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Old 04-15-11, 01:46 PM
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That's what I meant. I know I would have two 20A circuits. Just used to saying lights go with 15A circuits. My point was more about being able to split the lights and recepticles onto two different circuits. Im going to run this all by the electrician when he comes over to do some work in my service panel, i need an upgrade.

On a side note: Could I use a non GFCI two pole breaker(240V) in the service panel as opposed to a GFCI two pole breaker? Those are expensive and as Im probably only going to be putting 4 recepticles in the garage. Do the GFCI recepticles in the garage(first ones after the split) provide protection to the recepticles "downstream" from them on both circuits? It makes sense to me that they would.

Thanks everyone
 
  #16  
Old 04-15-11, 02:11 PM
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Originally Posted by InNeedofHeat View Post
Could I use a non GFCI two pole breaker(240V) in the service panel as opposed to a GFCI two pole breaker?
Yep, but you can't do a 12" burial on the cable. Plain breaker would require 18" depth for plastic conduit or 24" for naked cable.

Do the GFCI recepticles in the garage(first ones after the split) provide protection to the recepticles "downstream" from them on both circuits? It makes sense to me that they would.
As long as you wire the LINE and LOAD terminals to do so. The GFCI receptacles must be after the shared neutral split. GFCI receptacles cannot provide downstream protection on the section of the circuit that shares the neutral.
 
  #17  
Old 04-16-11, 10:16 AM
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Originally Posted by ibpooks View Post
Yep, but you can't do a 12" burial on the cable. Plain breaker would require 18" depth for plastic conduit or 24" for naked cable.
This is where you realize a significant cost advantage to putting in a single (120V) circuit, because then you can have a regular 20A breaker in the load center, and locate a regular GFCI recep (or dead front) upstream from the underground run.

You can't do this with a multiwire because once you split out the neutrals on the load side of the GFCI, you have lost your multiwire.
 
  #18  
Old 04-18-11, 09:43 AM
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The amount of cost savings depends a lot on the panel brand. For one like Siemens where the GFCI breakers are reasonably priced the difference between breaker and receptacle price is not that great. If we're talking Square D QO, then the breaker costs a fortune so any option with GFCI receptacles is going to be significant savings.
 
  #19  
Old 04-19-11, 12:41 PM
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I'm interested in what the experts think about the distance. How far can you run #12 before it can no longer be used with a 20-amp breaker? I think he said his run was about 125 feet.

Also, it sounds like he's got 12-2g at hand, not 12-3g. Even if the 12-2g is free, am I right in thinking he could not legally run a second cable to carry a multiwire branch circuit?

If that's right, it sounds like doing even a multiwire branch could kick the price up a lot, since he might have to pay to get the 12-3g. Assuming the 12-2g UF is very low cost to him, and given that he describes such small needs, I would be tempted to think about burying one run of 12-2g to power only circuit, BUT bury it alongside a run of conduit to save having to redo the digging when (not if ) someone comes along and decides they want more power (or cable TV, or ethernet, ...) in the Garage.

Just some thoughts to consider.

-- Rich
 
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Old 04-19-11, 01:34 PM
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Rich: You're correct, I do not have the 12-3 on hand. However, I would be able to buy it at a decent cost. The 125ft might be a little stretch. I would say the distance from house service panel in the house to the garage is 85-110ft, and probably on the lower side of that estimation.

Also, I have a few three way lights I have to rewire so I could probably use some 12-3 anyway.
 
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Old 04-19-11, 01:43 PM
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Originally Posted by RichA View Post
I'm interested in what the experts think about the distance. How far can you run #12 before it can no longer be used with a 20-amp breaker? I think he said his run was about 125 feet.
By code, as far as you like because voltage drop is only a recommended consideration. If you're looking to do the best possible design, then it depends on the load you're powering. If you're doing light hand tools, basic lighting and so forth, moderate voltage drop is acceptable -- maybe 150' - 200' for up to 10% voltage drop. If you're doing some type of hard starting motor like a compressor or pump, you would probably want to keep it under 100' to maintain a <5% voltage drop.

am I right in thinking he could not legally run a second cable to carry a multiwire branch circuit?
He could not do that.

I would be tempted to think about burying one run of 12-2g to power only circuit, BUT bury it alongside a run of conduit to save having to redo the digging when (not if ) someone comes along and decides they want more power
That would be a reasonable thing to do, or even just pull the UF through the conduit. Most pros wouldn't do it, but it is legal to put UF in conduit.
 
  #22  
Old 04-19-11, 02:02 PM
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IBpooks,

Would it matter if I have two service panels in the basement? The house is a top bottom duplex, each apartment having its own 100A box, with their own meter and mains. Could I run a 20A, 12-2 UF line from each panel to the garage? Or does the code say that only one line can supply power to a detached building?
 
  #23  
Old 04-19-11, 02:15 PM
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One circuit per detached building.
 
  #24  
Old 09-01-12, 09:22 AM
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Just popping in to say thank you to IBPooks. Just wired an outbuilding and used a 20 amp, 240 breaker in the house, a 2 pole disconnect rated at 60 amps in the outbuilding (about 10 bucks), and two GFCI boxes. In other words, just what IBP suggested. Thank you very much for your clear and effective wiring advice. The whole job is going to end up costing less than a couple hundred bucks.

One question, if anybody is still reading this thread.
The GFCI I happen to already have on hand is rated at 20 amp pass through and 15 amp receptacle. Obviously, the 20 amp pass through works well, and the 20 amp breaker in the house protects all the down line wiring (12 gauge).

My question is about the actual receptacle on the GFCI, which is only rated for 15 amp. Theoretically, no device should plug into the GFCI that draws more than 15 amps, because the 15 amp receptacle has the 15 amp arrangement of prongs (Standard), and not the 20 amp arrangement with one prong sideways.

BUT, if a device pulls more than 15 amps but less than 20, for whatever reason, the breaker in the house won't trip but the receptacle will be over its rating. Issue?

Obviously, I could just install a 20 amp GFCI and be done with it.

It's 15 bucks for the outlet, so I'll probably end up doing it, but it begs the theoretical question: why offer a 20 amp pass through if the receptacle can only handle 15 amps?
 
  #25  
Old 09-01-12, 09:30 AM
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The code allows a 15 amp receptacle on a 20 amp circuit if there are two or more places to plug into the circuit. You are fine. The internals are the same. The difference is the slot configuration.
 
  #26  
Old 09-02-12, 12:31 PM
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Thank you very much - good to know, and I just connected to the panel and I am hot in the outbuilding! After digging trenches, running line, backfilling, wiring in the outbuilding to the disconnect - it is a really good feeling to turn the breaker on, walk to the building, and see the GFCI circuit light come on. Sweet!
Thanks forum! Thanks guys.
 
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