Electrical question

[Troy Faccio]

My main panel is 200 amp max. I'm assuming that there are 100 amp per leg or side if that is true then in theory I could put in another dbl pole 100 amp breaker on the opposite side of the current dbl pole 100 amp breaker, and use the new one to run to my shed. And if this is still true I could then run two #3 wires from my new dbl pole 100 amp breaker in my main to the two legs in my box in the shed, and run everything else internally from there. Sounds too good to be true.

Everything in the picture is not in use except the lower right corner.(100 amp dbl pole breaker which goes to the box in the house)

[Mike Henderson]

A double breaker attaches to both sides of the incoming line - it has to to be able to provide 240V. If you add another double breaker, it doesn't matter on what side of the box you mount it, it will connect to both sides of the incoming line.

There's a calculation on how many "breaker amps" you can have in a box based on the rating of the box and others who are familiar with that will hopefully post here.

Mike

[Art Mann]

It isn't correct to say 100A per leg for a total of 200A. In a 240VAC circuit the exact same current flows through both legs. If your box is loaded to 200A @ 240VAC then both legs are carrying 200A.

[Troy Faccio]

So can I just run 2- 100 amp single pole breakers to the new panel in the shed one to each leg?

As you might be able to tell this is the one using I know nothing about.

[Mike Henderson]

So can I just run 2- 100 amp single pole breakers to the new panel in the shed one to each leg?

As you might be able to tell this is the one using I know nothing about.
No, you shouldn't do that. You need to use a double pole breaker to protect your wire going to your shed. You could get into trouble attempting to use two single pole breakers and it will definitely not meet code.

And if you don't know anything about it, please get professional help.

Mike

[Dan Friedrichs]

Troy, I assume the box you've shown is an outdoor, weather-tight box that is mounted on a pole where your service connection is made, and it feeds different buildings or loads on a farm, etc. Thus, you have one 100A double-pole breaker feeding the house, giving you 100A service at the house. The other, unused, breakers are probably for feeding security lights, outbuildings, etc?

The box you picture is rated 200A, and doesn't have a main disconnect inside it (although there is probably a meter nearby that serves as the service disconnect).

So, yes, you can install another 100A double-pole breaker anywhere in that main box and use it to feed a subpanel in your shed, using #3 copper wire (assuming the shed is not too far away).

Not sure why you think that would be "too easy" :) The total breaker ampacity inside the main does NOT need to be less than the panel's rating, since presumably the loads are not running at 100% all the time, simultaneously. As Mike says, there is a calculation to determine the total "amp breakers" installed in a given panel, but my impression is that it's hard to overload a normal panel. You may even be able to run a 200A double-pole breaker to the shed to provide 200A service, there (but you likely wouldn't come close to needing that, and it would require thicker wire).

[Troy Faccio]

Thanks Dan,

You are correct on your assumptions.

It seem logical but I lack the confidence with electricity since I hear it's unforgiving. Been doing some research and this is what I can up with. Insert a dbl pole 100 amp breaker in main panel run 2-2-4 black stranded Al Stephens urd cable (local home depot) to the main lugs in the panel in the shed, along with hooking up the neutral wire. Install grounding rod near shed and install ground wire to the rod and panel that's in the shed and I also believe I have to bond the grounding strip together. (not sure how to word it correctly)

This would be run about 60 feet.

[Jim Becker]

You need four conductors to the subpanel...two main, one ground, one neutral. Ground and Neutral are only bonded in the main panel.

[roger wiegand]

You need four conductors to the subpanel...two main, one ground, one neutral. Ground and Neutral are only bonded in the main panel.

Best to check with an electrician. My understanding is that most subpanels in remote structures require a separate ground. I believe you may run one branch circuit to a shed that is grounded at the main panel, but this kind of application would want three conductors (mains and neutral) and a grounding rod at the shed's subpanel.

[Dan Friedrichs]

Troy - yes, you are correct on all counts.

Thoughts:
1) #2 aluminum is a pain to work with - thick and stiff. You might want to see if you can find #3 copper cable somewhere, instead (for 60', the cost difference wouldn't be huge).

2) Make sure to shut off power to that main panel before installing the breaker. If there isn't a knife switch or something, the local power company would likely be willing to come and unplug your meter for 10 minutes while you install the breaker.

3) Your subpanel in the shed needs a main disconnect. You'll probably just buy a 100A main service panel for the shed, so just install the 100A main breaker (in the subpanel) and use that as the disconnect.

4) The grounding requirements are highly subject to local conditions and adoption of code. NEC generally requires 2 grounding rods, but some local authorities have amendments only requiring one. Newer versions of NEC require a "4 wire" (hot1, hot2, neutral, and ground) feed from the main panel to the subpanel, as well as 2 ground rods at the subpanel, and the neutral and ground bus bars electrically isolated in the subpanel. Older versions only required a "3 wire" (hot1, hot2, neutral) feed to the subpanel, as well as 2 ground rods at the subpanel, and the neutral and ground bus bars electrically connected. If your installation is subject to this, you should check with local authorities about which versions of NEC they've adopted and/or amended.

4b) If you aren't concerned (or subject to) code-compliance, I'd do exactly as you proposed: 3 wires to the subpanel, a ground rod at the subpanel, and the neutral and ground busses connected together in the subpanel.

[David L Morse]

There's a lot of good information here.

This might be considered a bit picky but I don't think the box shown in the photo is a main panel. It looks like an enclosure with multiple service disconnects for multiple service conductors in accordance with NEC 230.71 as allowed by 230.40:
352853352852
That means that the wires supplying the shed will be service entrance conductors not subpanel feeders. The panel in the shop will be a service entrance and a main panel and should be chosen accordingly. Of course at a home center that's probably your most economical choice anyway.

Note that even if the shed were being supplied from a main panel then it would still be considered to be a service entrance since it's a separate building.

The grounding requirements for outbuildings seem to change with every edition of the code so Dan's advice to determine your local rules first is important.

[Troy Faccio]

Isn't there some sort of database for local codes in my area located online? I can't find anything about local codes, not even on my county website.(ie: grounding rod depth, wire depth)

[Lee Schierer]

Isn't there some sort of database for local codes in my area located online? I can't find anything about local codes, not even on my county website.(ie: grounding rod depth, wire depth)

A quick call to your county government should get you headed in the right direction on what codes you need to meet. The NEC applies nationally, but there can be additional local codes as well.

[Robert Engel]

Based on your questions, I STRONGLY suggest you let a licensed electrician do this job. From experience I know it is worth paying someone and know its right.

BTW, each hot leg carries 120V. If you look carefully, you'll notice the busses alternate inside the service panel every other slot, not one side or the other.
A double pole breaker spans both hot legs.

The amperage rating of the service depends on the type of panel which in turn dictates the wire size. Also, a panel can be loaded up with breakers that total more than the amperage of the service without issues.

I believe it is best practice to ground the service panel to its own ground rod (s) although I don't think this is necessary. The electrician ran a ground wire from main panel to sub and said it was code.