100a/125a service to workshop

[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.

[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...

[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.