100a/125a service to workshop

[Eric Mims]

My garage workshop currently has 60a coming via a 6/3 line from the house on a 60a breaker. The subpanel is too small (eight 1/2" slots). I am getting a large cnc router this month and am bumping the power up out there. I only have 1 220v circuit right now and have wired in 3 more (window AC unit and 2 more dedicated 220 circuits for tools), plus 1 220v 30amp for the cnc...will make 5 separate 240v circuits total. I also added 2 more 110/20amp dedicated outlets to run the computer and controller box off of.

Anyways, the 6/3 (grounded at the garage, no ground running back to house) is direct buried as I found out today. Am I right to think that a 125 breaker is the highest you can go to branch off the main service to feed an out building? I am trying to decide between 100 and 125. There's a decent 125 box at the borg with plenty of slots that should work well.

Any definite word on wire size? For 100a I have read/heard that 3awg copper is fine (buried 18" in 1.25" pvc), but some places I read you need bigger...

I'm thinking to get that 125amp subpanel regardless and either get 3/3 or 2/3 copper and size my breaker to match whatever wire I end up getting.

Any recommendations?

[Tom Godley]

You need to look at a couple of things.

First in my area - you would need to have four conductors to isolate the ground back to the panel if any changes are made to the sub-panel. Sounds like you are going to change this.

I was just speaking to an inspector on Tuesday and depending on the distance between the buildings you may need a ground rod at the out building - this is easy.

The panel can be oversized in order to provide more spaces -- the breaker at the main panel must be sized for the lessor of the wire or panel. I like to use conduit and I usually oversize it because I have had trouble when working alone on projects - so more space in the conduit makes it easier for me.

I have run into situations when an inspector will not allow a main breaker in a sub-panel - but I forget why :(


You need to check into the size requirements in your area -- You need to make sure it is done correctly. I once had to up size a wire size on an outbuilding. This is one area that it just not worth the hassles of getting it wrong. Also, 100-amp is a lot of power -- actually 60 amps is also.

How many things are going to run at one time?


I understand that telling someone to get a pro or consult with the local inspector is not much of an answer...............But...

[Rod Sheridan]

Hi Eric, the maximum size of branch circuit breaker for your main panel will be determined by the panel manufacturer. You'll have to check your particular panel.

The wire gauge for your feeder to the garage will be determined by the feeder breaker size, and the distance (voltage drop).

I sugest that you seek out the reccomendations of a qualified individual in your area........Regards, Rod.

[Matt Meiser]

I have 100A service in my shop fed from the 200A panel in the house. I wish I had more spaces in the panel, but I've never wished for more power. I can run a huge window AC, 2HP dust collector, lights, computer, stereo and any one tool in the shop and there's no noticeable sag when anything starts up. I've probably even had my large compressor kick in during those times. I also weld with a decent size wire-feed welder--I don't run the AC (because I'm running a fan for ventilation) or dust collector (because that would just be crazy unsafe) when doing that. Plus at any time a number of loads in the house including the stove, hot tub, and/or AC could be on.

[Mike Langford]

I'm planning on running a 100 amp breaker from my shop (200 amp service) to my wife's craft barn with a 100 amp sub panel using AWG2 aluminum direct bury wire a distance of 58 ft.

Copper is going to be very expensive.....besides aluminum is what the power companies run from the pole to your house/shop!

[Bruce Wrenn]

I'm planning on running a 100 amp breaker from my shop (200 amp service) to my wife's craft barn with a 100 amp sub panel using AWG2 aluminum direct bury wire a distance of 58 ft.

Copper is going to be very expensive.....besides aluminum is what the power companies run from the pole to your house/shop!I would consider putting this wire in PVC if permitted. Makes it easier to replace / up grade, as hole in the ground is already there.

[Eric Mims]

thanks everyone.. I checked back at the Depot and noticed that GE didn't sell any higher than 125a breakers for this purpose (as in to use in your 200a main to feed a circuit). Doesn't matter though, as I'll probably just go 100a breaker and either some 3awg copper (it will be grounded at the garage as it is right now in the existing setup..no reason to waste money running ground wire back to the house) or maybe some 2-2-2aluminum. I'll check with electrician on that..

I feel I need to go to 100a since I have a 240v table saw, bandsaw, and jointer. A 240v AC/heat unit that pulls quite a bit when in heating mode. The cnc router has a 7.5 hp spindle which requires a 30a breaker as well as 2 side ganged hitachi routers on the gantry, 3 stepper motors running, gecko drive controllers, computer, monitor, dust collection, possible future vacuum hold down setup.. and so on. Considering the nature of CNC, I fully plan on using my other equipment while the cnc is cutting..so I did the math and I felt 60a wasn't enough (not worth the risk for the 60a breaker to trip in the middle of a 6 hour cut because my compressor kicks on...)

[Mike Heidrick]

Where is a good place to buy awg2?

[Mike Langford]

Where is a good place to buy awg2?

I got mine at Lowes'......

[Josh Mountain]

Where is a good place to buy awg2?

Niether my Home Depot or Lowe's carried AWG 2 but I've read here many do across the country. I ordered aluminum SER mail order for about $1.60 per foot (4 conductor as my sub is same building) -- cut to length.

[Rollie Meyers]

I'm planning on running a 100 amp breaker from my shop (200 amp service) to my wife's craft barn with a 100 amp sub panel using AWG2 aluminum direct bury wire a distance of 58 ft.

Copper is going to be very expensive.....besides aluminum is what the power companies run from the pole to your house/shop!


100A is too large a circuit breaker for #2 AL, people use the NEC table ( 310.15(B)(6) ) for residental services and that exception does not apply to your case which means a 90 A circuit breaker is max see table 310.16 2002 NEC.

[Eric Mims]

thanks Rollie, I'm gonna triple check the right gauge wire for sure before buying it for sure... it's easy to change breakers out... not so much to pull wire out of the ground!

[Wayne Cannon]

Eric,

I have a 60 A 240 V service to my shop. I've always had plenty of juice for simultaneous operation of 3 HP dust collector, 3 HP cabinet saw, lighting, and heat-pump. Having multiple people running big tools simultaneously is the only reason I can think I would need more. I do use 240 V for tools when possible which halves their 120 V requirements.

I recently rewired my shop and wanted numerous circuits/breakers. I got a 100 A panel but used a 60 A breaker for the main, since I couldn't find any 60 A panels with enough slots.

I did learn that the code does put a maximum limit on the number of breaker slots in a panel for a given feed capacity (to prevent the temptation to exceed the feed capacity by too much, I guess), so I had to use a relatively small panel (for a 100 A panel) to avoid exceeding that limit.

--Wayne

[Jack Briggs]

I recently wired a sub in my shop from a 100A breaker in the 200A main panel. I used about 35' of 2/2/2/4 Aluminum wire from HD. The wire was about $.90/ft. HD does not carry 2G. copper. The feed is not buried. The panel is a 100A GE Homeline with room for 12 breakers (I have 3 2-pole and 6 single-pole breakers in there). It all passed; the electrician even commented on my tidy wiring! (I did do panel wiring as my first full-time job when I was 16).



Cheers,

[Rollie Meyers]

Eric,

I have a 60 A 240 V service to my shop. I've always had plenty of juice for simultaneous operation of 3 HP dust collector, 3 HP cabinet saw, lighting, and heat-pump. Having multiple people running big tools simultaneously is the only reason I can think I would need more. I do use 240 V for tools when possible which halves their 120 V requirements.

I recently rewired my shop and wanted numerous circuits/breakers. I got a 100 A panel but used a 60 A breaker for the main, since I couldn't find any 60 A panels with enough slots.

I did learn that the code does put a maximum limit on the number of breaker slots in a panel for a given feed capacity (to prevent the temptation to exceed the feed capacity by too much, I guess), so I had to use a relatively small panel (for a 100 A panel) to avoid exceeding that limit.

--Wayne

There is no limit other then the 42 circuit max in a panel. (This rule was removed in the 2008 NEC*) A 100A main breaker loadcenter is avail w/ 30 spaces & a 40 or 42 space main lug panel could have a main breaker back fed if a code required "retainer clip" is used,this clip is used to hold down the breaker in place.

*The 2008 has only been adopted in limited areas & older panels will not be allowed to jammed w/ more breakers then the manufacturer has allowed, instructions will state the max allowed....

[Ben Franz]

I'm just now setting up an attached 2 car garage as a shop. I decided to install a 100A sub-panel, fed from 200A main service. Home Depot sells a 100A Square-D Homeline package with 5 20A breakers for around $60. This has enough room for everything I need including 6 240V circuits, 4 20A runs plus extras. I used 2/2/2 aluminum service cable (w/ground) at less than $1.50/foot. I bought cable at local elec. supply house and my electrician's discount was applied. I'm sure there is an equivalent direct burial cable but I would opt for conduit - no matter how deep you dig, someone (or some animal?) will find a way to damage a buried cable - my bias.

[Eric Mims]

Well, I think I've decided to go with 2-2-2-4 aluminum XHHW-2 wire in some 1-1/4 PVC conduit. It's around 1.50 here and that's about 1/3 the price of copper equiv.

Anyone know if there's a code violation in backfeeding a 100a breaker in the subpanel? As in, you have a 100a breaker at the main panel feeding your shop, but the subpanel you put in the garage doesn't have a main breaker...so can you insert another 100a in the first slot and just wire in the hots to it feed the rest of the bus? The box I have right now (my 60a panel) doesn't have a main breaker, but it would be nice to have a main breaker in the garage.

other than that, it looks I need to be 18" down with the pvc, and anythign over 50' run of pvc needs a sliding expansion adapter.

I'll have a ground running back to the 200a main panel, so does one still need a ground rod at the detached building? Isolate the neutral too I believe..

[Rollie Meyers]

#2 aluminum is good for 90 amperes max the rule allowing it to be used for 100 A does not apply to your shop feeder cable.

An answer to the question about backfeeding breakers yes it can be done if a hold down bracket is to fasten it into place see 408.16(F) 2002 NEC.

[Eric Mims]

yea, I saw those brackets at the store today. GE sells one that'll fit perfectly into the box I got. Well, still a bit confused on the 90a 100a bit...seems to be alot of confusion or shall I say 'disagreement' on the different electrician forums I was reading on about that issue.. some guys swearing you can only do 90a with that set up and saying inspectors make change it, and other guys saying they do it all the time and it passes inspection every time (nobody ever says where in the US they live, where this passes or doesn't pass) based on how one interprets the code...

[Rollie Meyers]

yea, I saw those brackets at the store today. GE sells one that'll fit perfectly into the box I got. Well, still a bit confused on the 90a 100a bit...seems to be alot of confusion or shall I say 'disagreement' on the different electrician forums I was reading on about that issue.. some guys swearing you can only do 90a with that set up and saying inspectors make change it, and other guys saying they do it all the time and it passes inspection every time (nobody ever says where in the US they live, where this passes or doesn't pass) based on how one interprets the code...

The reason for the confusion is that for a residental 100A service #2 AL is OK, but for the subfeed to your shop it's not OK, see 310.15(B)(6) 2002 NEC,da rules are da rules.:D

[Eric Mims]

yea, it's strange why some accept it. Well, not totally strange, since the wire is the wire and breakers are breakers... maybe they make the difference because they assume you'll have more things running full-time in such a detached shop?? Or maybe I can put a bed out there in the shop, er...I mean, my detached living area, yea that's the ticket... heck my CNC router can double as a bed. I'll check prices on the 1/0 stuff... :D

[Rollie Meyers]

If it is accepted the inspector is not following code, # 1 AL is good for 100A & only 1 size up.

[Josh Mountain]

Hmmm. . . Unfortunately, I don't remember all the hard details I learned about 2005 NEC but six months back when I did the 100A sub, I learned on multiple forums that 2/2/2/4 aluminum is commonly used for this purpose. I seem to remember one section of the codebook saying it was good and another not.

Anyways, back then I went to the inspectors and asked (when I got my plan approved and they said it 2/2/2/4 aluminum is just fine. Got inspected a month later and no issues.

This anecdote doesn't do much for you, but the preponderance of posts I researched found the same conclusion. May be worth your while to check with those guys.

[Eric Mims]

yea, I meant #1, not 1/0.. 1-1-1-4 is the stuff. Hopefully I can find it.. shouldn't be too much more expensive and there will be no questions later (and no forcing me to buy some expensive 90a breaker..which would negate any savings of the #2 stuff). And I don't know what I was smoking.. I shouldn't need an expansion coupling since it's underground and there isn't any appreciable temperature change.

though, there was a family of squirrels living in my shop last year, surely that's a single family dwelling..

and for future reference sake, the 2005 NEC shows the back-fed hold down requirement in 408.36(F).. those brackets only cost like 1.50 at the borg.

[Rollie Meyers]

The breaker hold down req. 1st showed up in the 1990 NEC , and it applies only to plug-in type breakers not bolt-on but except for the now defunct I-T-E Pushmatic line, bolt-on breakers are unlikely to be found in residental panels.

Also be careful when "upsizing conductors'' if they are increased in size due to voltage drop considerations the equipment grounding conductor (if used) needs to be upsized as well. 2002 NEC 250.122(B)

[Rollie Meyers]

Hmmm. . . Unfortunately, I don't remember all the hard details I learned about 2005 NEC but six months back when I did the 100A sub, I learned on multiple forums that 2/2/2/4 aluminum is commonly used for this purpose. I seem to remember one section of the codebook saying it was good and another not.

Anyways, back then I went to the inspectors and asked (when I got my plan approved and they said it 2/2/2/4 aluminum is just fine. Got inspected a month later and no issues.

This anecdote doesn't do much for you, but the preponderance of posts I researched found the same conclusion. May be worth your while to check with those guys.

The table is 310.15(B)(6) is for dwelling 120/240 Volt 3-wire single phase services & feeders, and they do not apply to other structures see 310.15(B)(6), the deal breaker is references to "Dwelling unit" & no meantion of outbuildings or accessory structures being allowed to use that exception.-

[Wayne Cannon]

Thanks, Rollile. That's good to know. It was an issue for me, because I was using a 100 amp panel with a 60 amp main breaker. Some of the 100 amp panels allowed more breakers than the code at that time (2003?) allowed for a 60 amp service.

[Eric Mims]

just picked up some 1-1-1-3 for 1.99/ft from local electrical supply house.. there weren't too many places that kept this stuff in stock.. most jumped from #2 straight to 1/0.

oh, and I don't know where I saw 1-1-1-4.. I thought I saw it on a wiring manufacturers site, but went back and checked and of course it's all 1-1-1-3..

[Chase Gregory]

FWIW,

Inspectors not following code is not all that unusual and never a defense for doing the same. The code is a 'minimum' set of requirements so anyone who falls short of that minimum - no matter who approves it - is taking on liability.

Working in 28 states we've had a lot of inspectors 'let' an owner/contractor do something less than code requires and NOT per our documents. We document our disagreement but we never change our documents to reflect what an inspector 'allows' if we cannot also corroborate it by code.

They can require us to exceed the code, but not fall short.

Chaser

[Eric Mims]

so I spent the weekend doing some insane amount of work. I trenched for my conduit about 20" deep through old bricks, old pipe, and other garbage the builders left in the ground 30 years ago. Then yesterday I use a fence pipe and water hose to horizontally bore under my driveway (11' wide). BRUTAL! 9 hours yesterday and another 9 today and I now have the conduit buried and the cable in it. Super hard clay is a bear to get through.. plus it ended up angling downward even though I did my best for it not... so it ended up being over 2' deep on the far side.

couple more questions.

What all circumstances do you use NOalOx? is it fine to just use it on all connections including the breakers and ground bar when using Alu. wire, or just in certain connections?

does anyone cement the PVC box adapter bushing?

My workshop is 80' from the box and the wire I laid has a ground running out there. Do I understand correctly that I need to keep the neutral bar on my new workshop panel unbonded (and panel screw out), keep the ground attached to the panel with a screw, and do I need a separate ground rod at the shop to also attach to the ground bar in this setup?

I'm almost done. :D