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JAYRAN 10-01-2007 01:34 PM

Sub feed house from new garage
I recently tore down our old garage & built new. Due to the location of the existing overhead services, I want to run a new 200A service to the garage & then sub feed the house with 100A which is the houses current size service. I want to run the wire through the garage approximately 50', then to the house in a duct for another 20'. All together the distance will be about 90'.
What size wire should I run, what type of duct & how deep does it need to be?

jogr 10-01-2007 02:07 PM

Your current service entrance panel will now become a subpanel if I understand you correctly.

It would be best to ask your local building inspector what size is ok. There is sometimes disagreement about whether the wire size listed as ok in the NEC for service entrance cable is also ok to feed subpanels. The wire size will dictate the conduit size (duct work is for air, conduit is for wires). Around here the conduit must be at least 24" deep but you should check with your building inspector.

Since your old panel will become a subpanel you will need to run 4 wire to the subpanel and be sure your neutrals and grounds in your old panel (new subpanel) are on separate bars that are not bonded together.

I am not an electrician so hopefully a real electrician will jump in here and correct my advice!:)

J. V. 10-01-2007 03:02 PM

This is an interesting question. I am a Master Electrician in SC.
Why do this job backwards. The service is at the house. Upgrade the house panel to 200 amp and set the 100 amp sub panel in the garage.

100 amp sub panel installation requirements:
Get a homeowners permit first. Inspector can tell you what he wants you to do. He can help you too.

1) 3 number #3 THHW conductors. 2 Hots and 1 neutral marked with white tape at terminations
2) 1 number #8 Green (EGC)
3) Minimum 1" sch. 40 pvc buried at 18" deep. I would use 1.25"
4) Seperate grounds and neutrals in the sub panel. (you may need a ground terminal bar), if it is not provided with the panel.
5) 90' is not enough to worry about unless you have big electrical loads in the garage.

Run the conduit unabated from one end to the other. You can buy many pvc condulets to assist in entering both structures.
I can't explain everything in this forum. Call your local AHJ. You might get the inspectors email address like I do in my area.

Stubbie 10-01-2007 04:13 PM

Most likely will require approval by your local power company and codes department...I'm bettin they will not allow it.

The dwelling will require the main panel/meter and not be allowed to be fed as a sub-feed. You can always try.

Farms are a different story as power many times comes to a pole mounted meter and then is ran to the dwelling and barn...garages...etc..

Every juristiction has its specific requirements and they differ as night and day.


frenchelectrican 10-01-2007 05:20 PM

I did see that kind of arrangement of this setup.

few place will allowed to this set up.

but a heed a warning watchout with the exsting main breaker box when you change to the subfeed there are some thing you have to keep in your mind that the netrual / ground wires have to be separted and the bonding screw tied to the netral bussbar that have to be removed.

my best advise is get the inspector and POCO involved on this set up some have issue with the garage set up.

Merci , Marc

sluggermike 10-02-2007 02:57 AM

Why do you have to separate the neutral and the ground? All the residential service panels I've seen have one buss bar for the neutral and ground. Is there a different requirement for a sub panel? I'm curious because I have been thinking about putting a sub panel in my garage.

J. V. 10-02-2007 11:01 AM

Slugger.....It's an NEC requirement. The neutral and ECG on subpanels must be seperate. The main service panel is where they are bonded.
I will check the NEC article, but someone here will know it without looking it up.

Andy in ATL 10-02-2007 04:16 PM

It actually isn't the main service is at the first means of disconnect. If my "main panel" is within 6ft.(or is it ten stubbie? my code book's in the dang van) of my meter then my disconnect can be in that panel and I would bond my neutral and ground in that panel. If, however, my main panel is in my basement 75ft. away from my meter then my disconnect has to be outside at the meter and my neutral and ground are bonded in this meter/disconnect combo. The grounds and neutrals would then be seperate in my "main panel 75ft. away in the basement.

Stubbie 10-02-2007 04:31 PM


The nec requires that no bonding of the equipment ground and neutral can occur load side of the main disconnect enclosure. There is, however, one exception for detached garages that have no other metallic paths ran to them outside the feeder to a sub-panel. In this case you may run a 3 wire feeder and bond the neutral and ground together as at the main disconnect. In order to do this you need to not have any water pipe, phone or data lines , gas lines etc out to the garage. Other wise a 4 wire feeder is required with neutral and ground seperated. NEC 250.32(b)(1).

If you look at the 1st image below you will see the neutral and ground bars isolated from one another as required with a 4 wire feeder. This prevents neutral current from being on the equipment ground. I'll try to explain. Take the green wire and move it over to the neutral bar with the feeder neutral coming from the main disconnect enclosure. We now have them bonded. Kirchoffs law says that current will take all paths back to the source. Most will flow on the path with the least resistance but it will take all paths. With the ground and neutral bonded neutral current returning from all your branch circuits out of that sub-Panel will split with current traveling on the feeder neutral and the ground wire back to the main panel. You do not want objectionable current on that ground wire! It is not to carry current unless there is a ground fault and then only briefly till the breaker trips. Therefore they are seperated at sub-panels load side of the main disconnect enclosure. The only place the bond between the two is to take place is the main disconnect panel at the "service equipment". This is because you only have 3 wires (H-H-N) supplied by the poco to the mains of the single family dwelling and the only path back to the source (transformer) for neutral current and fault current (so a breaker can trip) is the service neutral, so we must bond the neutral to the ground at the service equipment and no where else outside of the few exceptions mentioned.

Note: As of the 2008 code cycle the 3 wire feed to a detached building will no longer be allowed.

See images below.......

1st image for reference

2nd image shows the proper set up

The 3rd image is for clarification if using metal conduit for your fault path instead of a equipment grounding wire. This is a main disconnect and main panel that is wired like a sub-panel with 4 wire feeder because the main panel is load side of the main disconnect enclosure . The principle is the same however for a detached building.


The below image is where they are using a metal raceway as the ground fault return path but it could be a ground wire instead of a metal conduit it is just a little more dangerous due to the ease of contact by a person. In the case of a open neutral as shown all neutral current will show up on the metal conduit. This is because they were bonded together improperly and now the equipment ground has become the neutral and only path.

Stubbie 10-02-2007 04:49 PM


Yes you have it right. I'm not aware of any set distance for the main disconnect from the meter. I believe that is poco and local code determined. but that is generally very close and 10 feet or less is a good bet.

The 3rd image is a set up like you are describing in your post. This is a main disconnect after the meter and the main panel is wired as a sub-panel in this case. The first means of disconnect is also a correct way of looking at it. I find more people are able to understand it as "the main disconnect/(s)" enclosure.


Andy in ATL 10-02-2007 06:08 PM

Found the code on-line...This is a case where my local addendum(in GA) gets swapped around in my little brain to mean the NEC. The NEC code that applies is 230.70(A)(1) READILY ACCESSIBLE LOCATION: The service disconnecting means shall be installed at a readily accessible location either outside of a building or structure or inside nearest the point of entrance of the service conductors.

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