Can I add an additional subpanel between my main panel and existing sub panel?

Question

My main panel in on the side of my garage. It currently has 3 breakers:

1. Main breaker
2. A/C breaker
3. Sump pump breaker.

The main panel goes to a subpanel on the back interior wall of the garage which feeds the house. This panel has no breaker spots left.

I am remodeling my garage and adding lights and various outlets. My question is, how best should I handle these circuits?

1. Can I add a new panel between the existing main and sub panel - i.e. Sending the main power to this panel first and then on to the second existing panel?
2. Should I try and use an existing circuit -- there is a switch for an outdoor light already in the garage
3. Update existing panel -- The panel is not in an ideal spot for wiring the garage when it comes to routing the wires. I have shear walls etc and I don't really want the cables to be visible.

I feel like 1 is the best option here.

Main Panel:

Main Panel Inside:

A/C & Sump Pump Conduit:

Sub-panel:

Answer 1

Looking at your meter/main panel, it has two huge strikes against it. First, that's a Zinsco combo panel. They are known fire starters. Second, it's a "Rule of Six" panel which has no proper main breaker. The only overload protection is the NEC Article 220 Load Calculation. You can't add loads to that without redoing the Load Calculation to account for the new loads.

Though it's a big job, I cannot do less than recommend replacement with an appropriate and modern meter+main.

If I were you, I would then come off the meter-main to the new subpanel you have planned, and then onward to the original house's panel.

Alternately you could select a "meter + main + some breakers" panel and have it there at the meter. One favorite type is a "Ranch Panel" which has all that and also "thru lugs" allowing full panel power to go onward to a panel.

I would wild-guess that your present service is 125A or 150A - the power company would know, or your service wire size may tell.

Answer 2

I would highly recommend replacing the subpanel with a much larger panel - minimum 30 spaces. You currently have a 12-space/"24 circuit" panel with "20 circuits" in use and all spaces filled. At the same time, you have a small meter-main with only a few spaces (as is normal for that type of box) and I am a little concerned by the Connecticut Electric breaker as to what type of panel it really is. Connecticut Electric is a replacement brand often used in some problematic panels (e.g., Zinsco). So there are really 3 issues here:

• Make sure the meter-main is safe. If there is any identification - typically a label inside the front cover - that would help. If this is a known-bad panel type then replacement is a priority for safety. If it is an OK panel type but just some questionable breakers then the breakers can be replaced. And it might turn out to be just fine, but it is definitely worth checking. In any case, I would only consider adding more breakers to this panel as a last resort because space is so limited and replacement is not simple because of the meter.
• Subpanel size (spaces/circuits/breakers). The ideal thing to do is to replace this panel with a larger one. Minimum 30 space - and larger is even better. Assuming your existing breakers are the correct ones for this panel type (I can't tell - more detail, label from front cover, etc. needed) then getting the same brand/line of panel but in a much larger size makes sense as you would then be able to transfer most (possibly all) of the breakers.
• Subpanel main breaker. Using a main panel as the new subpanel (even though it will still technically be a subpanel) makes a lot of sense. Typical is to get a large panel with a main breaker included - typically 200A. They normally come with a bunch of "free" breakers, plus if you plan it right you can transfer your existing breakers. The only extra you may need is to add a separate ground bar (depending on the panel) as a main panel can have grounds on the neutral bar but a subpanel must have grounds and neutrals separated. This gives you a disconnect here which helps for future work and sets you up for...
• Heavy up! You have 100A service. As you add more loads, you may need more power! If you have nearly everything (really everything except AC and Sump Pump) in a bigger, 200A rated, main-as-subpanel, then a future upgrade will only require replacing/upgrading the meter-main (which you may have to do anyway, see above...) and running big feeder cable from the meter-main to this main-as-subpanel.