Detached garage project (electrical service)

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Old 12-18-14, 01:17 PM
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Detached garage project (electrical service)

I am looking to start a detached garage project (residential) this spring coming up and am starting to do some in-depth research about each area, my questions here are related to electrical service to the garage. I intend to also ask this question on a garage forum to cast a wide net. I live in Michigan and my township currently adheres to 2011 NEC.

1. I am leaning to 100 amp service since it feels like from a practical standpoint it is either 60 amp or 100 amp and I have never heard anyone regret going larger. House service is 200 amp. The service will be 220/240V, from research so far it seems like 100 amp may be a order of magnitude more difficult / expensive to do, but I do know that upgrading 60 amp to 100 amp after complete is much, much more difficult. My question is what can I do with 100 amp that I cannot do with 60? I intend to enjoy this garage for many many years and will likely get into welding and upgrade to a two stage air-compressor at some point.

2. My understanding is that the wiring of the subpanel to detached garage (regardless of 100 vs. 60 amp) is the first diagram in this post. Is this correct?

3. My main question is I have not been able to nail down the actual wire that is required and I have many questions here (assuming 100 amp service):

a. My current understanding is that I need to run bundled wire (e.g., Romex style) inside the house (and garage), but outside (underground) I have to run USE-2 style THWN or XHHW wire (e.g., 2-2-2-4) within conduit. I cannot use the USE-2 inside the structures, and I cannot use the inside wire outside underground. In the past I have ran 60 amp service to a detached garage using 6/3 UF-B WG wire for the whole run, that was many years ago and I am not sure if that is still the way 60 amp would be done. This seems to be the main reason why 100 amp service is more difficult / expensive (other than for the obvious larger wire)

b. Copper vs. Aluminum - I believe I have to use at least #2 Aluminum and #3 Copper, maybe more based on length (subpanel will be close to but within 200' of main house panel). Does the ground need to be #6 or #4? Is there a reason to go copper over aluminum, I know the latter is cheaper?

c. THWN vs. XHHW - I believe this is the insulation (and can be mixed and matched with Copper or Aluminum), is there an advantage / disadvantage to either one? A recommendation either way?

d. Converting outside wire to inside wire (at both house and garage). I am led to believe that you mount junction boxes to each structure (outside) and that is where you convert the wire. Is this correct? I would imagine the box mounts to the structure and has the inside wire go through the back and the outside wire within conduit that goes down from the junction box into the ground. Is this correct as well?

e. Any other items I have missed that I should know about?

Thanks in advance, I am eager to learn and make sure I do this right.
 
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Old 12-18-14, 01:46 PM
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outside (underground) I have to run USE-2 style THWN or XHHW wire (e.g., 2-2-2-4) within conduit.
Type USE wire is rated for direct burial, there is no need to pull it into underground conduit. I prefer installing a complete conduit system from the main panel to the garage subpanel and pulling individual conductors the full length, conductors such as XHHW or THHN/THWN (dual rated). The major difference between these two types of wire in the supply stream is that XHHW is commonly found as aluminum at supply houses and THHN/THWN is commonly found as copper at supply houses. Copper XHHW and aluminum THHN/THWN are both available on special order. The XHHW insulation may be a little more robust when used underground, but I doubt you would ever see a difference in the two types in a residential application.
 
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Old 12-18-14, 02:04 PM
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1. Most one-person workshops would do just fine with 60A, but the materials cost is not much higher to do 100A and the labor is the same. If you have multiple machines running at the same time, 100A is the way to go. I would recommend 100A unless you're really squeezing the budget.

2. The first diagram is the correct one.

3. There is no "right answer" with the wire. Lots of types can work.

3a. Some types of cable (bundled wire) can only be used inside, some only outside above ground, and some only underground. Some have ratings for multiple locations; this is why it gets confusing. Individual conductors THWN and XHHW must be in conduit, but can be inside, outside and underground. I would probably recommend XHHW in 2" PVC conduit.

3b. At 200' you will want to upsize the wire a little bit to fight voltage drop. #1/0 aluminum or #2 copper would be a good choice. The ground wire is sized once the hot wire sizes are chosen.

3c. Practically there is not much difference between XHHW and THWN.

3d. Correct if you go that way. If you go with individual conductors the whole run you only need LB fittings instead of junction boxes.

3e. Any kind of low voltage wiring like cable, internet, phone, etc would need to go in a separate conduit at least 12" from the power conduit.
 
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Old 12-19-14, 12:18 PM
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Others have answered the technical side of your questions, I'll just throw in my opinion...

For a one-person shop, I think 60A is more than enough. Most larger shop tools are 15-20A max, and even if you're running a table saw + compressor + dust collector, you won't be even approaching that 60A limit. Sure, bigger is better, but if it were my money, I'd rather spend it on upgrading a tool or two rather than electrical capacity I'd probably never need.

If you're planning multiple workers in the shop, then it may be worth upgrading, or at the least running conduit from end-to-end to make a future upgrade doable.
 
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Old 12-20-14, 07:19 AM
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Is this garage going to be heated/cooled? If so how? You will want to add that into the load calculation of the garage. IMO - I would go 100 amps.
 
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Old 12-21-14, 09:09 PM
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Thanks for the responses so far, very helpful.

Answers to questions asked:

I am planning on heating the garage with a Modine Hot Dawg HD/HDS-125 (info). My understanding is that will run off a 15A 120V, in my last garage (which I had an HD-75 in), I had it on a dedicated 15A circuit. I was happy with my last heater (HD-75) and I don't intend to paint cars (the big drawback to gas fired). I have heard some who like radiant better, admittedly I know little about that style.

As for cooling, I would entertain it, given it is not too expensive. I have not made up my mind on whether cooling is needed, obviously it would not be an issue with 100A.

It sounds like the first very wise thing to do is conduit the whole way with let's say 2" PVC (I have a few questions about that below). Second wise thing may be to just do 100A and be done with it, however if I do the first wise thing (conduit), I can upgrade as need be (with obvious wire size limitation in conduit).

My questions (assuming using 2" PVC conduit the entire run):

1. My understanding is schedule 40 is acceptable the whole run with two exceptions. The exceptions are at house and garage from bottom of LB fittings to 18" underground which need to be schedule 80 (for physical damage resistance). Meaning the entire run inside the house and garage can be sch40 and the underground run once below ground can be sch40. Is this correct?

2. I have a metal breaker box in the house, can I simply have the 2" PVC go into it? What is the proper fitting to do that (I imagine threaded with a metal or PVC nut on it)? Can I place an LB fitting near the box (or anywhere in the house/garage) for tight bends, or is that poor craftsmanship, against code, and/or a bad idea?

3. Is there any restriction to what can be ran within conduit inside a structure? Or if it is good enough to run outside, can it be ran inside? I ask because my understanding is some of those XHHW insulations can burn toxic (god forbid I have a fire or something).

Again thank you all for your time.
 
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Old 12-22-14, 09:02 AM
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The exceptions are at house and garage from bottom of LB fittings to 18" underground which need to be schedule 80 (for physical damage resistance).
Generally sch 80 is only required where there is likelyhood of damage, such as if the pipe comes up right beside the driveway.

can I simply have the 2" PVC go into it? What is the proper fitting to do that
You run right into the box knock out with a threaded male adapter and a lock nut. LBs or other condulets are a great idea as long as they remain accessible (can't be covered by drywall, etc).

Is there any restriction to what can be ran within conduit inside a structure?
Conductors with THWN, XHHW, RHW rating are okay indoors. Cable that is only rated USE or URD is outdoors only, usually required to be in conduit where above ground.
 

Last edited by ibpooks; 12-22-14 at 10:07 AM. Reason: fix formatting
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