On this page 5 sections
A subpanel adds more circuits where the main panel has run out of room
A subpanel is a second distribution point fed from the main panel. It is what you add when part of the house or property needs more circuits and the main panel is out of breaker spaces. The service can be perfectly healthy — the panel simply has no room left in it.
The main panel is full — every space taken, or the only spaces left are the wrong type.
A room, a wing, or a shop needs its own group of circuits closer to where they are used.
A remodel or a new load adds circuits the main panel physically cannot hold.
You want a local shutoff for a workshop or an attached garage, without walking back to the main.
Get the service measured first. Whether you need a subpanel or more capacity is a number, not a guess — and that whether-decision lives on the panel upgrade page.
Subpanel installation: a full panel does not mean you need a bigger service
This is the distinction worth getting right before anyone quotes you. A panel out of breaker spaces and a service out of capacity are two different findings — and only one of them involves the utility.
Out of spaces
If the service has headroom and the panel is simply full, a subpanel gives you the circuits without touching the utility, the meter, or the service size. It is the smaller job, and often the right one.
Out of capacity
If the house genuinely draws more than the service is rated for, more spaces do not help. Whether you need more capacity is the panel upgrade question — the answer can be no. If that need is already established, the increase is a 200-amp service upgrade.
The main panel itself is the problem
Heat, damage, or equipment nobody makes breakers for is a panel replacement. Adding a subpanel downstream of a failing main does not fix the main.
A subpanel feeder keeps neutral and ground separate
A subpanel is not a smaller copy of the main panel wired the same way. The neutral and the ground are bonded together once, at the main service — and they must be kept separate everywhere downstream of it. Get that wrong and the equipment ground can carry current it should never carry.
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Four-wire feeder
A subpanel is fed with two hots, a neutral, and a separate equipment ground — four wires, not three. The old three-wire method is not how a new subpanel is fed.
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Isolated neutral bar
The neutral bar floats on its own, insulated from the enclosure, and the main bonding jumper — the screw or strap tying neutral to ground — is removed, so the two stay separated (2025 California Electrical Code / NEC 2023, 250.24(A)(5)). Leaving that screw in is a bonding fault, not a style choice.
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Sized to the load, not to what fits
The feeder and the subpanel are sized to the circuits they will carry and the distance to them, calculated per the code’s load rules (NEC Article 220) — not picked because the breaker was on the shelf.
Subpanel location, size, and whether the service is enough
A subpanel has a few real choices, and they are worth making on purpose.
Where it lands
In the same building, the feeder runs from the main panel to wherever the circuits are needed. A detached garage or outbuilding is a different job — its own grounding electrode, its own disconnect — and it lives on the garage subpanel page.
How big
The amp rating follows the circuits it will feed, with room to grow — not a round number. Oversizing the feeder for load that never arrives is money spent on nothing.
Subpanel or service change
If the service has capacity, a subpanel answers it. If it does not, that is a panel upgrade decision, and the increase after the need is established is a 200-amp service upgrade — not a subpanel dressed up as one.
Subpanel installation: what changes the cost
A subpanel is priced from the feeder run and what it feeds, not from the box on the wall. These are what move the scope once we see the route and the main.
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Distance and route
The run from the main panel to the subpanel is the biggest variable — a short run in unfinished space is quick; fishing a feeder through finished walls or across the building is not. A detached structure is not this run; that is the garage subpanel.
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Feeder size
A larger subpanel needs a heavier feeder and larger conduit, so the amp rating drives the material.
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Whether the main can feed it
The main panel needs a spare space and the capacity for the feeder breaker. If the main itself is full or failing, making that space can become a panel replacement rather than squeezing a subpanel onto equipment that cannot feed it.
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Permit and inspection
A new subpanel and feeder are permitted and inspected. The authority for the address is confirmed before scheduling.
Not sure this is the right job?
Tell us what you are seeing — what is tripping, what changed, or what you are planning — and we will tell you what the work actually is before anyone drives out.
Subpanel questions
Do I need a bigger service to add a subpanel?
Usually not. A subpanel exists so a service with spare capacity can gain more circuits without an upgrade. Whether you need more capacity at all is the panel upgrade question. If that need is already established, the increase is a 200-amp service upgrade — which we measure before proposing.
Why do the neutral and ground have to be separated?
Because they are only bonded together once, at the main service. Downstream of that, keeping them separate means the equipment ground never carries normal current. A subpanel wired like a main panel — with the bonding screw left in — is a bonding fault, and one of the most common things we find and correct.
Can a handyman add a subpanel?
The feeder sizing, the neutral-ground separation, and the permit are exactly where a cheap install goes wrong, and the mistakes are not visible until something faults. It is permitted work for a reason.
How many circuits can a subpanel hold?
That depends on the subpanel you choose and, more importantly, on the feeder and the service behind it. The number of spaces is easy; sizing the feeder to actually carry them is the real question.
Is a subpanel the same as a panel upgrade?
No. A subpanel adds a second distribution point. A panel upgrade asks whether the house needs more capacity — the answer can be no. Short on spaces with capacity to spare is this job; short on capacity is not.
Is a garage subpanel the same job?
An attached garage is this job — a feeder to another wall of the same building. A detached garage needs its own grounding electrode and disconnect, which is the garage subpanel.
Can I install a subpanel myself?
A subpanel is a second panel, fed by a cable from the main, with its own shutoff and grounding. We size it from the main and pull the permit.
What is the 80% rule for sub panels?
We size the cable from the main so the calculated load stays inside the breaker's listing. People call that the 80 percent rule. The number comes from the load calculation.
How big of a subpanel can I install off a 100 amp service?
The breaker that feeds the subpanel, and the calculated load, have to fit what is left on the 100 amp service. We start at the main.
Still not sure it is the right job? Call (279) 972-9272 — available 24 hours.Want the detail? ADU electrical
Serving 19 communities across the Sacramento area
Litel Electric serves homeowners throughout all 19 communities in our service area — from Davis and Woodland to Roseville, Folsom, and Elk Grove.
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Tell us what is happening, or what you are planning. We read the details, tell you what else we need to know, and explain the next step before anything is scheduled.
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You send the problem or the plan, with the address if you have it.
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We reply with what it involves and what has to be seen in person.
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Nothing is scheduled, and nothing is priced, until the equipment has been looked at.
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