More electrical questions...

737Pilot

Ok, here's a few questions I need to know.

Right now, my subpanel in my garage has a Cutler Hammer 125A main LUG load center. What is the difference between that and a 100A main LOAD center that Cutler Hammer sells?

Also, I have found out that the aluminum 2/2/4 URD that is run from my house panel to my sub panel in the garage should have a 90A breaker at the house panel as opposed to the 100A that's there. The distance is about 140 feet. So, what size aluminum URD should be run so that I can get full use of the 100 amps?

Thanks for all the help.

nap

a main LUG (known as an "MLO" "main lug only") does not have a main breaker

as to getting full use of the 100 amp breaker; if you have not tripped the breaker, you are getting all the use you need, apparently. I would have to look but sometimes you are allowed to upsize to the next standard size breaker and your situation may have allowed that. I'll have to look.

737Pilot

Thanks for the reply...anyone else also want to take a stab at the wire size for distance? Anyone....anyone.....!!

J. V.

If your garage is unattached, you need a main breaker panel in it, not a lug panel. There are exceptions, but I must assume you have more than 6 breakers in the sub panel.

Quote: Right now, my subpanel in my garage has a Cutler Hammer 125A main LUG load center. What is the difference between that and a 100A main LOAD center that Cutler Hammer sells?

The difference is one has a main breaker (100A) vs the lug panel (125A) that has no main breaker. You should have a main for a 125 amp panel.

So change the breaker inside the house to 90 amps. Leave the 125 amp panel alone and install a main breaker in it. Some panels allow whats called back feeding. That means you can use a standard 2 pole breaker as your main in the sub panel. You install it just like the 90A in the house. A 125 amp breaker panel in the sub and a 90 amp breaker in the main panel is fine. However, you are only will have 90 amp protection. You could operate a small house easily on 90 amp service.
Note: Check your lug panel and see if it will accept a main breaker, good chance it will.

Another thing I see is you have only 3 wires going to the sub panel. This was compliant before, but it is not now if your area is following the 2008 code cycle. You better have a ground rod installed at the sub panel too.

It looks like distance is not an issue. Someone here can give you the exact formula or correct wire size. I do not have the material handy.

737Pilot

The garage is detached. I do have a grounding rod at the subpanel. Right now there is also a 100 breaker in the subpanel that the URD wires attach to. I am going to be running A/C and 220 compressor/welder in the garage. I originally told the electrician that I wasn't gonna have A/C in the garage...I have since changed my mind. That's why I want to know the correct wire size for 100A. Thanks for all the help.

InPhase277

I wanted to get to you sooner 737, but went of town for a couple days. Anyway, the wire size is what limits you to 90 A, not the distance. But to limit voltage drop, the bigger the wire the better. 90 A is right for your 2-2-4 URD. If you want 100 A, you have to go to the next size wire minimum, and maybe even the next size again to account for voltage drop. For 100 A, you need at least #3 copper or #1 aluminum. But for longer distances, you may need #2 copper and 1/0 aluminum.

737Pilot

Inphase...no problem. Thanks for the answers! I just want to make sure I have it done right.

nap

since you have a 125 amp panel, you can feed it with the proper wire to up the main breaker (that is what the 90 amp breaker is being used for) all the way to a 125 if you want. (and available). Make sure the breaker in the panel in the house, or whatever has the same sized breaker as well as you will not have any more power available than the smaller of the 2 breakers.


as to the 3 wire or 4 wire; there is more than it was legal pre-2008 code. Even then there were situations where you had to use 4 wire even then.

The most notable is; are there any other metallic paths between the garage and the house (or wherever the main panel is)? If so, you must use 4 wire system.

I would suggest running 4 wire simply because it removes a possible problem in the future, even if there isn't one now.