My 200 amp Square D 40 breaker panel is full. Can I add a second 200 amp Square D panel that I already have as a sub?

No. The branch circuit breaker limitation is most likely 100 or 125A.
You can feed the 200A panel with a 125A breaker but I advise against it. A 40 space panel as a sub might be too tempting to over load.

If I understand, there are two ways to handle this.

Add a sub feed lug kit to the main panel and run wire to a main lug panel (sub panel 125a as you recommended? Or

Pull two breakers from the main panel and replace them with a double power transfer breaker then run wire from the breaker to the sub panel.

Am I correct, off base or need to add something?

I should add that I have already doubled up on two breakers in the main panel and need the sub to eliminate that and add an outdoor fan and outlets to a gazebo. I'm sure more add ons are in the future.

Thanks for your help Stanfortyman.

Have you condidered having a "calculated load" completed to see if your residence is adequatley serviced. You may want to up your service to 300 amp and use a dual meter to feed both the residence's main service and the secondary panelboard.

Have you condidered having a "calculated load" completed to see if your residence is adequatley serviced. You may want to up your service to 300 amp and use a dual meter to feed both the residence's main service and the secondary panelboard.

My average daily usage is 38kwh or about 4 per hour.

200amp service should be adequate for that. Correct?

Have you condidered having a "calculated load" completed to see if your residence is adequatley serviced. You may want to up your service to 300 amp and use a dual meter to feed both the residence's main service and the secondary panelboard.Are you really suggesting a second meter to feed a sub-panel inside a residence?

My average daily usage is 38kwh or about 4 per hour.

200amp service should be adequate for that. Correct?That is not a load calculation, and really is not an indication of demand load.
A real load calculation would be required.

Unless the home is really big, like 3500 sq/ft+, and/or uses electric heat or a whole house on demand water heater, a 200A service should be more than adequate.
Just adding a sub-panel does not add load. It's what you put in it that does.
Why do you need another 200A panel? Just because you have it on hand?

Personally I would not add a sub-feed lug kit to add a 200A panel to a 200A main panel. If you really need that much space then you actually may fit the description above requiring a 320/400A service.

Thanks. I realize that a load calculation is the way to go. I was just trying to show that the demand is not that heavy.
The home is about 2500 sq. ft. with oil heat. 4 beds 3 baths. The larger draws would come from the furnace/ac, through wall heat/ac unit in garage, well pump, normal 220 appliances and 220 shop tools. All the 110 is standard for this size house.
There is also a 45kw whole house automatic backup generator tied to the system.



That is not a load calculation, and really is not an indication of demand load.
A real load calculation would be required.

Unless the home is really big, like 3500 sq/ft+, and/or uses electric heat or a whole house on demand water heater, a 200A service should be more than adequate.
Just adding a sub-panel does not add load. It's what you put in it that does.
Why do you need another 200A panel? Just because you have it on hand?

Personally I would not add a sub-feed lug kit to add a 200A panel to a 200A main panel. If you really need that much space then you actually may fit the description above requiring a 320/400A service.

I should add that I have already doubled up on two breakers in the main panel
What does that mean?

What does that mean?

Two circuits run off one breaker.

Sound to me like all you need to do is replace 4 regular breakers with 4 twin or slim line or piggy back breakers. That would allow you correct the doubled up breakers and give you two additional breakers for the circuits you need.

Sound to me like all you need to do is replace 4 regular breakers with 4 twin or slim line or piggy back breakers. That would allow you correct the doubled up breakers and give you two additional breakers for the circuits you need.

Thanks Harold. That will work for now. I will worry about future expansion when the need arises.