I have just added an attached garage, and now need to add electricity to it. I have a 125 amp subpanel mounted in the basement of the garage, it is approximately 40-45 feet from the main panel. My question is - what size cable do I need to run from panel to panel. I'd prefer to use copper, and does the subpanel need it's own ground cable/ ground rod? I hope this was the correct forum to post this question to ,I apologize if it isn't.
Sub panel wiring?
- Thread starter ELMO67
- Start date
- Views: 3119
Inspector507
Super Member
Attached garage does not need a ground rod, but yes needs a grounding conductor from the main panel. Decide if you want 125A, 100A, 90A....then make your wire selection . (We'll help) /forums/images/graemlins/grin.gif
Thanks for the replies, so far! The garage is attached to the house by way of a 10'x 12' mudroom. The house, mudroom and garage all have basements, which is where the main panel ( house), and subpanel (garage) are located. The subpanel I have mounted in the garage basement is rated at 125 amp capacity; I would like to have that full capacity available. I think that the most I'll be drawing at one time would be 60 amps, but that may not allow for a start up draw.
TWINKLE_TOES
Veteran Member
- Joined
- Sep 3, 2000
- Messages
- 1,366
<font color="blue"> (We'll help) </font>
You just did /forums/images/graemlins/smile.gif I have had the ground rod on my to-do list for 5 years. Amazing how time solves problems, thanks
You just did /forums/images/graemlins/smile.gif I have had the ground rod on my to-do list for 5 years. Amazing how time solves problems, thanks
Inspector507
Super Member
For 125A feed to the garage, you’ll need to keep the following in mind.
Assuming all your terminations are rated at 75°C, you can use #2AWG Cu to feed the sub-panel. The #2 is rated at 115A at 75°C, but then you can “round up” to the next standard fuse size, which is 125A.
Run 3-#2Cu, one of them being marked white in some fashion. AND include a #6 bare or green for the grounding conductor.
The above is all based the premise you want to run Copper, sizes must be changed accordingly if you use Aluminum.
Jerry
Assuming all your terminations are rated at 75°C, you can use #2AWG Cu to feed the sub-panel. The #2 is rated at 115A at 75°C, but then you can “round up” to the next standard fuse size, which is 125A.
Run 3-#2Cu, one of them being marked white in some fashion. AND include a #6 bare or green for the grounding conductor.
The above is all based the premise you want to run Copper, sizes must be changed accordingly if you use Aluminum.
Jerry
bky
Gold Member
Remember to keep the neutral bar isolated from the panel and keep all neutral (white) wires on it and add a ground bar and put all the ground (bare or green) wires on it and bond (connect the ground bar to the metal breaker box
yooperdave
Veteran Member
- Joined
- Nov 28, 2001
- Messages
- 1,174
- Location
- Marinette, WI
- Tractor
- Tool Cat 5600, LS XJ2025H, Branson 4215HC
BKY-
I was told by an electrical engineer and a electrical inspector that you only use the bonding screw to bond the main box and not the sub box.
Please verify.
Thanks
Yooper Dave
I was told by an electrical engineer and a electrical inspector that you only use the bonding screw to bond the main box and not the sub box.
Please verify.
Thanks
Yooper Dave
bky
Gold Member
yes true ion your main pannel. but in the sub pannel you have to bond the ground bar not the nuteral bar
Inspector507
Super Member
bky,
There is always an exception........ /forums/images/graemlins/grin.gif
In a separate building you DO want to use it to bond the neutral bar to the cabinet if you don't pull a ground out, which is permissable.
Jerry
There is always an exception........ /forums/images/graemlins/grin.gif
In a separate building you DO want to use it to bond the neutral bar to the cabinet if you don't pull a ground out, which is permissable.
Jerry
bky
Gold Member
Well if the panel is feed from the main panel from the house and you run to a separate building it is still considered a sub panel and at that point if you don’t take a ground out to it then you have to drive a ground rod and bond the ground wire to the ground bar and bond the ground bar to the panel but the neutral bar still needs to be isolated /forums/images/graemlins/laugh.gif
Inspector507
Super Member
Correction........if you don't run a ground and there are no continuous metallic paths between buildings, you need to drive the ground rod AND connect it to the neutral bar. Neutral bar and ground bar are one in the same in this case.
Read Art. 250-32B2 of the 2002 NEC® /forums/images/graemlins/grin.gif
Jerry
Read Art. 250-32B2 of the 2002 NEC® /forums/images/graemlins/grin.gif
Jerry
bky
Gold Member
ok i will check that artical. now that you mention it ii do rember somthing about that at the last code change seminar . thanks for setting me stright. /forums/images/graemlins/blush.gif
Anonymous Poster
Epic Contributor
- Joined
- Sep 27, 2005
- Messages
- 29,678
As a general rule, you should never have two ground bonds of any kind at different places connected together.
The obvious apparent exception is at the entry panel, where the ground is bonded to both the neutral and the system earth ground. Note that this is done at a single location, and that is significant.
As stated, if you run a ground conductor from the main panel to the sub panel, you must not bond the neutral and ground at the sub panel. Doing so will split the neutral return current between the neutral and ground conductors. Besides causing there to be continuous current on the ground, this arrangement is potentially disastrous /forums/images/graemlins/shocked.gif- if the neutral conductor were disconnected, all of the neutral return current would flow through the ground conductor, which is typically not sized for the this capacity.
Also, if you have two earth grounds at different locations connected together by a ground conductor path, you will see stray currents due to variation in the grounding potential variations between the two earth grounds. This is particularly true if the entry panel earth ground and the sub panel earth grounds are of different types, or installed in different soil types. (I had an amusing experience where the GFI outlets in an outbuilding would trip whenever the automatic sprinklers would come on. The moisture would dramatically the efficiency of the local ground rod, causing enough stray current through an otherwise extremely effective grounding system. I will admit here and now that it was my own doing, /forums/images/graemlins/blush.gif and defend myself only by saying I was much younger, and a different kind of foolish, than I am now. /forums/images/graemlins/tongue.gif )
Again, as stated, if you don’t run a ground conductor from the entry panel to the sub panel, then you have to install a separate system earth ground for the sub panel and ground the neutral to the ground at the sub panel. At that point, the sub panel is now configured in the same manner as an entry panel with respect to grounding and neutral bonding.
Given the apparent close physical proximity of the entry panel and sub panel, and assuming that the outlets, fixtures, and devices fed from the sub panel will be in close proximity with those fed from the entry panel, a ground conductor should be run from the entry panel to the sub panel, and there should be no separate earth ground or neutral bond at the sub panel. This will avoid the possibility that the two grounds, potentially with different potential, will come in contact.
One of the big problems in trying to apply any commercial building techniques, be they electrical or structural, is that residential inspectors are often unfamiliar with anything other than the plain vanilla tract home methods. On the one hand they will reject extremely well thought out, well engineered applications simply because they haven't seen it before. Even worse they cannot be counted on to catch errors, their primary role, if they do not understand what is being done.
If you are going to do anything more than install a breaker and a couple of outlets, I would recommend getting a copy of the NEC or one of the annotated guides on the code. It is far too easy to kill someone or burn down your house with a well-intentioned misconnection. (Or well meaning advice from some guy on the internet. /forums/images/graemlins/cool.gif)
The obvious apparent exception is at the entry panel, where the ground is bonded to both the neutral and the system earth ground. Note that this is done at a single location, and that is significant.
As stated, if you run a ground conductor from the main panel to the sub panel, you must not bond the neutral and ground at the sub panel. Doing so will split the neutral return current between the neutral and ground conductors. Besides causing there to be continuous current on the ground, this arrangement is potentially disastrous /forums/images/graemlins/shocked.gif- if the neutral conductor were disconnected, all of the neutral return current would flow through the ground conductor, which is typically not sized for the this capacity.
Also, if you have two earth grounds at different locations connected together by a ground conductor path, you will see stray currents due to variation in the grounding potential variations between the two earth grounds. This is particularly true if the entry panel earth ground and the sub panel earth grounds are of different types, or installed in different soil types. (I had an amusing experience where the GFI outlets in an outbuilding would trip whenever the automatic sprinklers would come on. The moisture would dramatically the efficiency of the local ground rod, causing enough stray current through an otherwise extremely effective grounding system. I will admit here and now that it was my own doing, /forums/images/graemlins/blush.gif and defend myself only by saying I was much younger, and a different kind of foolish, than I am now. /forums/images/graemlins/tongue.gif )
Again, as stated, if you don’t run a ground conductor from the entry panel to the sub panel, then you have to install a separate system earth ground for the sub panel and ground the neutral to the ground at the sub panel. At that point, the sub panel is now configured in the same manner as an entry panel with respect to grounding and neutral bonding.
Given the apparent close physical proximity of the entry panel and sub panel, and assuming that the outlets, fixtures, and devices fed from the sub panel will be in close proximity with those fed from the entry panel, a ground conductor should be run from the entry panel to the sub panel, and there should be no separate earth ground or neutral bond at the sub panel. This will avoid the possibility that the two grounds, potentially with different potential, will come in contact.
One of the big problems in trying to apply any commercial building techniques, be they electrical or structural, is that residential inspectors are often unfamiliar with anything other than the plain vanilla tract home methods. On the one hand they will reject extremely well thought out, well engineered applications simply because they haven't seen it before. Even worse they cannot be counted on to catch errors, their primary role, if they do not understand what is being done.
If you are going to do anything more than install a breaker and a couple of outlets, I would recommend getting a copy of the NEC or one of the annotated guides on the code. It is far too easy to kill someone or burn down your house with a well-intentioned misconnection. (Or well meaning advice from some guy on the internet. /forums/images/graemlins/cool.gif)
Again, I thank everyone for their advice. I haven't yet decided which route I'll take, most likely it will be running a ground from panel to panel. TractorlessSacto- thanks for the reply, but especially for your last statement... " Or well meaning advice from some guy on the internet". I think that is advice anyone who surfs the Internet should read and remember.
