Positions and Capacity Are Not the Same Thing
The number of breakers a panel can hold and the amount of current the service can deliver are unrelated numbers. A panel can be physically full while the house draws only a fraction of what the service could provide. A panel can equally have four empty spaces in a house that is already at its limit.
A subpanel answers the first case and not the second. It is a second enclosure, fed from the existing panel by one feeder, that trades a two-pole breaker in the main panel for a dozen new positions somewhere more convenient. Nothing about the service changes. If the house genuinely needed more capacity, it still needs it afterward.
So the order matters. Establish the capacity question first, with a load calculation, and the positions question resolves itself into either a subpanel or a straightforward replacement. Do it the other way around, and you can end up fitting a bigger panel to a house whose problem was never the panel.
Questions About Full Panels
1.My panel is full. Do I need a bigger service?
Not necessarily. The two questions are independent of each other, and a load calculation settles which one you have. Where the house is comfortably inside its service, a subpanel gives you the positions without the day the power is off. Where it is not, the calculation says so, and you have learned something you needed to know either way.
2.Can I just use tandem breakers?
The panel's own label decides that, not what physically fits. Plenty of panels accept tandems in some positions and refuse them in others, and the label names which. Where the label permits them, they are a legitimate answer to a shortage of space. Where it does not, fitting them anyway gives up the tested behavior under fault conditions.
3.Is a subpanel a downgrade compared with a new main panel?
It sounds like one, which is why the question keeps coming up. A subpanel is a standard component of a residential electrical system. Where the service is adequate, it gives you the positions, a local disconnect, and room to grow, and it leaves a working main panel. Where the service is not adequate, we will say so.
4.Where does a subpanel go?
Somewhere with the working clearance in front of it that any panel needs, and somewhere the feeder can reach without an unreasonable run. Garages and utility spaces are the usual answer. Bathrooms and clothes closets are not, and there are places a panel cannot go at all, which is part of what the first visit establishes.
5.How long does it take?
Shorter than a service change and without the utility in it, which is most of the difference. The power is off while the feeder is landed in the main panel, and not for most of a working day. We pull the permit, and an inspector signs the work off. What we will not do is give you a date before we have run the calculation, because the calculation decides which job it is.
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