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ponch37300 11-27-2007 01:40 PM

100 amp sub panel hook up?
I am installing a 100 amp sub panel in my detached garage. the main panel in the basement of the house is a 200 amp. i bought i 100 breaker for the main panel. does the 100 amp sub panel need to be main breaker or can i use a main lug since i have a disconnect in the main panel. If i remember right anything over 6 ft it will need a main disconnect? also can i use aluminum ser cable for the feed in between panels and if so what size? Will 2-2-2-4 sluminum ser cable work for this? Also i bought two ground rods for outside of the garage, do i need these if i run 2-2-2-4 with the 4 ga ground wire? Thanks for any help or advice on this. I used to work for an electrician about 10 years ago so i have good knowledge on electrical, just a little rusty and things have changed and i want to do this right.

NateHanson 11-27-2007 01:46 PM

I'm not sure about the main breaker requirement for the sub, but you do need the ground rods for the sub in a detached building.

ponch37300 11-27-2007 02:02 PM

Thanks Nate. I thought so, that's why i bought them but i wasn't sure. I beleive i need a main breaker panel but once that is confirmed and the wire is confirmed i will be good to go! Thanks again

HouseHelper 11-27-2007 02:03 PM

If the garage is detached, you must have a main disconnect in the subpanel. Ground rods are required. The 2-2-2-4 cable is only good to 90A, which should be more than sufficient in a residential garage.

ponch37300 11-27-2007 02:15 PM

2-2-2-4 wire only 90 amps? I thought i read it was rated for 100 amps. Is this wire ok to use with a 100 amp breaker in the main panel? The main reason for wanting this wire is i can get it for 1.76 a foot. I was going to use number 2 copper in conduit but that is 1.17 a ft and i would need three lines so i'm looking at 3.51 ft plus conduit compared to 1.76 with the 2-2-2-4 alum. Thanks for your help, it is much apppreciated.

HouseHelper 11-27-2007 02:22 PM

Ask your local code official if they allow Table 310-15(b)(6) to be used for the feeder to a detached garage. If they do, then the 100A breaker is OK. If they don't, then Table 310.16 would apply and the limit for #2Al is 90A.

As I said before, 90A should be more than sufficient for a residential garage.

ponch37300 11-27-2007 02:29 PM

Thanks house helper, i will call them. I already have the 100 amp breaker and i'm not sure if they make a 90 amp? Thanks again for your help

Andy in ATL 11-27-2007 03:12 PM

HH, Why would they not allow the table? Local ammendment?


HouseHelper 11-27-2007 04:01 PM


Originally Posted by Andy in ATL (Post 76708)
HH, Why would they not allow the table? Local ammendment?


Some interpret this as: Since a garage is not a dwelling the table doesn't apply.

Stubbie 11-27-2007 08:26 PM

1 Attachment(s)
Table 310.15 (B)(6) is intended for main power feeders to dwellings and services to dwellings. These are conductors that carry the load to the entire dwelling. A detached garage is not a dwelling and its feeder is not a mains feeder. There is some clarification on table 310.15 b 6 coming in the 2008 code cycle to hopefully put an end to the controversy over using it for non-mains feeders.


Can you help me with the main breaker required in that detached garage load center? Is there a code section that makes this clear?

Here is my understanding based on article 225. You are required to have a disconnect at the nearest point of entrance of the feeder conductors. It can be a disconnect on the outside/inside of the garage then a mlo on the load side of it anywhere you want to locate it once that disconnect is installed at the entrance point of the feeder. Or as you say a main breaker in the sub-panel. Or the inevitable 6 disconnect rule can be utilized.

Having said that I personally like the remote disconnect with a load side mlo sub panel in the garage.

Ponch here is a diagram of the requirements for a 4 wire feeder from the main panel to the detached garage

ponch37300 11-27-2007 09:12 PM

Stubbie...Thanks for the drawing, looks like it took some time. In your reply you say 08 clears up the dwelling issue, you say a detached garage can't go by table 310.15 b 6, so you are saying 2-2-2-4 alum wire is a no go for a 100 amp panel? If that wire is no good, I have about a 60-70 ft run, about 20-25 ft of it outdoors, what size and type of wire would you suggest, or do they make a 90 amp breaker like someone else suggested? I am trying to do this on a budget and already bought the 100 amp breaker for the main panel so i am trying to decide if buying bigger wire or a new breaker and smaller wire would be cheaper. Thanks again to everyone for taking the time to help, your advice is trully appreciated. Also, stubbie, your drawing doesn't show a disconnect for the main lug sub panel, from your reply you say i need one right? And if the sub panel is a main lug within 6 ft of the cable entrance that will do for a disconnect right? Thanks again

Stubbie 11-27-2007 09:30 PM

Yes it is not allowed to carry 100 amps in your situation. Yes they make 90 amp breakers.

100 amps is huge for a garage most are done with 60 amp feeders to a 100 amp panel. Are you going to have a large shop out there or just the typical
power tool stuff? Welder? Big air compressor?

I was in the process of adding the disconnect rule to the drawing... it shows up there now....sorry I was in a hurry. You may use the ruling that if you do not install more than 6 swipes of the hand to disconnect all power in that sub-panel this will be acceptable as a disconnect. This for example means 6 single pole breakers, or 5 singles and 1 double ....get the idea? A main breaker works for compliance or a remote disconnect works. Once you add that 7th swipe of the hand you must install a main this making sense to you?

That wire is an excellent price so use it just step down on the breaker size. You might be fine with a 70 or 80.

Stubbie 11-27-2007 09:35 PM

One other thing are you using the ser underground in conduit? It is not listed for direct burial. I read the thread but didn't notice where you said one way or the other.

ponch37300 11-27-2007 09:43 PM

Thanks stubbie you really share great advice. I do have a big welder and compressor, not sure the exact amprage. I understand the 6 rule now thanks to you. I will get that wire and a new breaker and a main breaker sub panel. Yes the wire will be in conduit for the underground part, what size should i use for this, 1.5 inch? One last thing what size does the stranded ground wire have to be from the sub panel to the ground rods, 6 ga? Thanks again

Stubbie 11-27-2007 10:09 PM


I'll have to calculate the conduit size for you it is a little different than individual wires I'll let you know in bit if 1 1/2 is good.....FYI cables are rarely ran underground in conduit however SER is rated for wet location so you can run it in conduit if you like just be freakin sure to be careful pulling it thru the bends and use sweeps whenever possible, you may have to upsize the Lbs that transition your conduit into & out of the structures.

If I may.... see what you can get mobile home feeder for this will really work great if you want that much amperage out there in that garage, you can direct bury it or run it in conduit your choice. If this works for you just direct bury at 24" and use sch.80 pvc where it comes out of the ground for protection with a sweep at the bottom of the trench. Here is what I'm talking about......

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