Panel / subpanel question


Old 03-11-14, 11:01 AM
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Panel / subpanel question

Still very new to this and trying to learn about my current setup.

In the basement of my house there is this panel:

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The main breaker is 100A and the breaker at the bottom left is a 40A which feeds a sub panel in my outbuilding (a barn without animals). Here's a closeup:

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The wire which runs from the main panel to the sub panel is 6 gauge and I would guess runs somewhere around 75 feet.

Here's a picture of the subpanel:

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And here's the inside of its cover:

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So from what I understand, I have a 100A main service and a 40A sub panel with 6 available spots. I'm having the barn rewired to add a bunch of lights and outlets as there are currently almost none. Probably 3 15A light circuits and 3 20A receptacle circuits. I'll need additional space for a bathroom that is upstairs (lights, a heater, and a water heater). Here are my questions:

1. Can I simply increase the size of the box in the barn in order to give me more available circuits? I was looking to swap it for something like this. I don't think the draw will ever be above 40A at any one time, but I don't want to overload individual circuits and would like to have more than 6. I'm trying to avoid tandems for the short term cost of breakers and long term I might actually want to increase the ampacity of the subpanel when more power is needed.

2. If I wanted to bring more power to the sub panel, what would you recommend doing?

Thanks so much!
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Old 03-11-14, 11:14 AM
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A larger panel can be installed. You will still be limited to the 40 amps that is being supplied now. Once you get over 6 breaker handles you will need some means of disconnect either in the panel or ahead of the panel.

To being more power to the shed you are going to need to replace the feeder between the building with a larger one.
Old 03-11-14, 11:22 AM
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  • Is the cable to the barn NM-b (Romex). If so it can not be used for that purpose.
  • If UF-b cable is it 3-conductor (white, red, black, bare)? If not it may be grandfathered but if major changes are made it must be brought up to current code which requires three wires plus ground.
  • If conduit do you have four wires?
  • If it is 6-3 is there a ground bar in the subpanel? If so is it bonded and the neutral bar isolated?
Can I simply increase the size of the box in the barn in order to give me more available circuits?
Yes. You could use a 12 space or larger 100 amp panel. The panel will require a disconnect either by using a main breaker panel with the main breaker used as a disconnect switch or a main lug panel with a back fed breaker to act as disconnect.
If I wanted to bring more power to the sub panel, what would you recommend doing
#6 at that distance can be used on a 60 amp breaker.
Old 03-11-14, 11:30 AM
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Could you explain (or refer me to some reading material on) why you need a shut off when you have more than 6 breakers and how to wire that?

Would the box that I linked to be adequate?

Is it really as simple as adding a larger gauge wire for the larger ampacity and swapping in a larger breaker?


I think I might have said outbuilding when it is in fact attached via an ell. So the wire is always inside. It is labeled Essex 6-3G Non-metallic.

Would it be safe to change the breaker at the main panel to 60A?

Thank you again!
Old 03-11-14, 01:03 PM
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As far as the more than 6 breakers, there is an NEC rule [230.71 (a)] that requires all electricity to a building must be able to be disconnected with no more than six movements of the hand (and no, it does not take into consideration that most people can hit two or three breakers in one shot! ). So if you have a panel with 6 breakers or less, a 'main' disconnect is not required. Once you have more than 6, you need a main to comply with this rule.

Note that this does not apply to subpanels within the same single-occupancy building as the main panel. So if this is in fact an 'attached' structure, it will not need a main. However I am not sure if your AHJ would still consider this 'barn' an outbuilding for code purposes regardless of the shared wall, or if it would be classified as a garage.. Is there access from one building to the other through the shared wall or do you have to go outside to go from one to the other? If you have to go outside, you can pretty much bet a main will be required.

On that same note, given the shared wall, yes provided that cable contains a separate bare or green ground wire in addition to the red/black/white it is adequate and can safely be upgraded to a 60A breaker.

The subpanel will require all grounds and neutrals to be on separate bus bars. They can not be combined under any circumstances.
Old 03-11-14, 05:33 PM
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Thanks Matt!

The house is connected to the barn via an ell. So you can walk to the barn without ever going outside. Not sure if that changes things. But it sounds like a disconnect would make sure I'm covered.

So just to clarify (sorry for being so redundant), if I'm trying to upgrade the power and wiring in the barn, can it be as simple as:

1. Change the 40A breaker on the main panel to a 60A breaker.
2. Install a new box, like this one, in the barn and make sure there is a disconnect.

And now I would have a 60A sub panel to work with in the barn?

I really appreciate all of this help, and sorry again for the redundancy, I'm just trying make sure that I understand everything.
Old 03-11-14, 07:55 PM
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If the cable is 6-3 yes. One detail the 100 amp main breaker in the subpanel may not be listed for wire as small as #6. If it isn't pigtail the #6 to the smallest wire that the lugs are rated for. A big blue or gray wire nut should be good for the pigtail connection. The connection would be made in the subpanel.

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