I have a 200 amp feed through panel about 40 feet from my barn. The 200 amp feed through main panel feeds my house. The house uses a 100 amp main. I want to install a 125 amp sub panel in the barn and put the main breaker in the feed through panel. Can I do this without having a main disconnect in the barn subpanel? I also need to know what size wire to use. I was planning on using 2 hots a common and a insulated ground. I was also going to install a earth ground at the barn panel. I hope this makes sense

Well on paper, NEC article 310.15 (6), and in table 310.15 (B) (6) you should use "UF" or "USE", copper 2/3 cable or three #2 conductors with a ground wire of 4 AWG for safety's sake. For Aluminum cable. You will need 1/0 conductors with a ground wire of #3 AWG.

You will need to run a four wire feed from the main panel. The main panel will house the OCPD (breaker) for this subpanel feeder.

At the subpanel, Neutral has to be isolated from Ground. Use a MLO (Main Lug Only) panel box, you do not need an OCPD on the main feed conductors. It is protected by the breaker in the Main Panel. Black and Red will go to the hot positions. White will go to Neutral. Ground will go to ground bus bar. DO NOT BOND NEUTRAL AND GROUND TOGETHER at the subpanel. The only place the Neutral and Ground may be "Bonded" together is in the main panel box.

You will also need to use an equipment grounding conductor and either attach it to two (2) copper ground rods (set about 8 ft. apart) or to a "UFER" ground system. Ufer is about a 20 ft. ground conductor set in concrete. The concrete block must rest on the ground.

If you choose to use the UFER system, you do not need any additional grounding attachments.

A few corrections needed to Don's advice:

#1. The barn panel must either have a main disconnect, or cannot be more than 6 branch circuit breakers that will act as the main disconnect. Any separate structure needs a service disconnect.

#2. Ground rods can be as close but no closer than 6 feet, to each other.

#3. The equipment ground for a 125 amp residential feeder can be either #6 copper
Or #4 aluminum.


I suspect you will simply drive two ground rods. Only one is required by national code, check local codes as more may be required. If you do decide to use a Ufer ground, which I doubt, check back as there are other requirements.

The grounding conductor from the equipment ground bus in the new panel to the ground rods need to be a min of #6 copper.

Not sure what this means:

The concrete block must rest on the ground.

TK,

1) Even though the subpanel is OCPD from the main panel the requirement for a main cut off or six breakers maximum still stands. Makes perfect sense. Question, if the subpanel feeder passes through a quick disconnect switch prior to entering the subpanel, would that meet the Code requirement?

2) Thank you.

3) I choose to use #4 copper for safety. Since I had no idea as to what loads would be in the barn.

"The concrete block must rest on the ground."

What I meant, Since the UFER is a concrete block , it must make contact with the earth. It cannot rest on top of a plastic sheet, for example. As in 250.52 (3)

Thanks for the informaition. The panel I bought for the barn is a 125 amp subpanel (homelite). The cover has a knock out for a main breaker. I thought I would buy a 125 amp main breaker for the panel and wire it that way. If I run the four wires 2 hots a neutral and ground where do I hook the earth ground in the panel?

You connect the ground from the main panel to the ground busbar in the subpannel. Make sure that the Neutral busbar is isolated from the grounding system at the subpanel.