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I have a 200 amp main panel I want to run a 100 amp service from the 200 to my garage. There's nothing on the 200 amp panel other than security cameras and lights. Do I need to install a 100 amp breaker in the 200 amp maln to do this ?

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    Does the panel have thru lugs? I.e. an extra set of lugs at the bottom / opposite end of the busbar from the main breaker? Oct 24, 2020 at 14:37
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    Do you mean a 100A feeder to your garage? Oct 24, 2020 at 14:57
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2 Answers 2

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Some initial notes

You can run any amperage you please to the garage.

That thing you're running to the garage is on your side of the meter, so it's feeder not a service (that's why 310.15(B)(7) says "service and feeder").

If the garage is a separate building (i.e. if the meter is NOT mounted on the garage building), then you need a local disconnect at the garage. It does not need to be outdoors. The cheapest way to get a local disconnect is to select a breaker panel that has a main breaker, the size of the main breaker does not matter. Literally anything will do, so if you like a panel with a lot of spaces and those only come in 200A, that's fine.

The wire has to be actually rated for the specific known load that you are driving, or the load calculation you have done. But if you don't have a specific load in mind or just want to exceed those minimums, you can run any size you want.

You must protect the wire with a circuit breaker, but you get to "round up" in breaker sizes. If the wire is rated for 115A, you get to use a 125A breaker. There's one exception: your feeder never has to be bigger than your service wire has to be (as per 310.15(B)(7)).

Feeder in these ~100A sizes should be aluminum - it's what any electrician would use because it's 1/3 the price of copper. The panel lugs are aluminum anyway. (aluminum lugs are the "universal donor", they play nice with both copper and aluminum wire). So I'll be talking aluminum sizes.

Ready?

You don't need a breaker in the main panel if you simply bring over the full 200A to the garage. Why would you do that? Well, what else would you do with it? You don't have anything to power at the main panel if the only things on it are cameras and lights.

However the trick would be attaching 200A wires on the load side of the main breaker. For this you'd need a main panel with "through-lugs", or a "Subfeed lug" that clips in like a regular breaker, or an actual regular breaker that is 200A. They must be made for that panel; you can't use off-brand devices (except for things UL-Classified for your panel, but nobody makes what you'd need).

But if you can do that, then you can carry 200A from the main panel to the sub on 4/0 aluminum wires. This is allowed because 310.15(B)(7) allows 4/0 for the entire service, and you don't need any bigger than that.

There's no need to enlarge the wire for voltage-drop reasons; it isn't far enough to matter.

If you want to carry less than 200A, e.g. to save money on wire or because there's no way to get lugs of 200A breaker... then simply use a breaker appropriate for the wire size (rounding up to the next available breaker): #2 aluminum = 90A, #1 aluminum = 100A, 1/0 aluminum = 125A.

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The simplest approach is install a 100A breaker into your existing 200A panel. Leave the 200A main panel breaker. The 100A is just another branch circuit within that panel. Connect the garage feeder to the 100A breaker.

On the garage side, if you have 6 or fewer branch circuits, you can avoid having a disconnect or main breaker for that building; but realistically, you shouldn't. You don't need another 100A circuit-breaker since there's already one upstream; but you must have a disconnecting means of 6-or-fewer switch throws. You can use a dumb switch to satisfy this and use it to supply a panel of any size inside to supply your garage loads.

            Existing Service               New Garage           New Panel
                 Panel                  Outdoor Disconnect    Inside Garage

             +-----------+
             | 200A MAIN |                   +-----+            +-----+
             +-----+-----+                   | ON  |------------|  20 |
LIGHTS ----- |  15 | 100 | --- to garage --- | OFF |            |  20 |
CAMERAS ---- |  20 | 100 |                   +-----+            | ...
             |     |     |
             | ...

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    I'm not seeing where an outdoor disconnect is required. Do you have a Code cite? Why not just choose a main breaker panel and use the main breaker as the disconnect? Oct 24, 2020 at 14:38
  • Although a circuit breaker isn't required in the garage it may be easier than a separate disconnect. A large panel with lots of breaker spaces that has a main breaker larger than 100A could be used because the feeder and panel is protected by the 100A breaker in the service panel. Oct 24, 2020 at 14:41

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