Electrical · Service

Subpanel or panel upgrade — which do you actually need?

A subpanel divides the capacity you already have. It does not create any. That single fact answers most of the questions people arrive here with.

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In short

What is subpanel installation?

A subpanel is a smaller distribution panel fed from your main panel, placed where circuits are needed — a garage, workshop, addition or ADU — so branch circuits run a short distance instead of all the way back to the house panel. It is the right answer when you need breaker spaces and local distribution, and the wrong answer when you need capacity: a subpanel divides what your service already delivers, it does not add to it.

The job

How subpanel installation actually works

A load calculation, first

Everything depends on whether your main service has headroom. If it does, a subpanel is usually the cheaper and less disruptive answer. If it does not, feeding a subpanel simply relocates a shortage, and the conversation is really about a panel or service upgrade.

Watch for: A subpanel proposed as a way of avoiding a panel upgrade on a service that is already full. It is not a workaround for capacity; it is a way to be short of power in two places.
Sizing the feeder and the panel

The feeder is sized to the load the subpanel will actually serve, with an allowance for distance — a long run to a detached building loses voltage along the way and often needs larger conductors than the amperage alone would suggest. The panel itself is chosen for the number of spaces as much as for its rating.

Watch for: A feeder sized from the panel's label rather than from the load and the distance. Runs to detached buildings are where this goes wrong most often, and it is not something you can correct later without redoing the trench.
Detached buildings and what they add

Feeding a separate structure brings its own requirements: a means of disconnect at the building, a grounding electrode system at that building, and a burial depth and protection method for the run. This is why a garage subpanel costs several times what a basement one does.

Watch for: A trench quoted without anyone establishing what is already buried on the route. Water, gas and communications lines all live in the same yard.
Permit, inspection and the panel directory

Subpanel installation is permitted work in most jurisdictions, and the inspection covers the feeder, the disconnect and the grounding arrangement. When it is finished, both panels need accurate directories — the main panel should say which breaker feeds the subpanel.

Watch for: A finished job where the main panel does not identify the subpanel feed. It is the first thing the next electrician will look for.
Torque, labeling and final inspection readiness

Lugs are torqued to spec, grounds and neutrals are separated correctly in the subpanel, circuits are labeled, and the installation is left ready for whatever inspection the jurisdiction requires.

Watch for: Neutrals and grounds landed on the same bar in a subpanel fed by a four-wire feeder. That is a common and dangerous shortcut; the final walkthrough should prove the separation.

Money

What does subpanel installation cost?

A subpanel inside the same building, close to the main panel, is a modest job. Feeding a detached garage or shop is several times that once the trench, the burial protection, the separate grounding and the disconnect are included. Distance and the route are the variables that matter.

See the full subpanel installation cost guide — by material, size and region →

Straight answer

When subpanel installation is the wrong call

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

Your main service is already at capacity. The most important line on this page. A subpanel splits existing capacity between more places; it creates none. If a load calculation says the service is full, the job is a panel or service upgrade.
You need one or two circuits in a nearby room. Running those circuits directly from the main panel is simpler and cheaper than a subpanel, provided there are spaces for them.
The main panel has spare spaces and the location is close. Subpanels earn their cost through distance and circuit count. Neither applies in a basement ten feet from the main panel.
You are planning a full panel upgrade soon anyway. Do them together. A subpanel installed now and re-fed six months later means paying twice for the same feeder work.
It is being used to dodge a permit. Adding distribution without inspection is a problem at sale, at claim time, and for whoever works on the building next.

Your options

Types of subpanel installation

Panel upgrade instead

The answer when capacity, not spaces, is the constraint.

What a subpanel is

The component, its feeder and its disconnect.

Panel upgrade vs full rewire

Where distribution work sits in a bigger plan.

Compare electrical options side by side →

Straight answer

Can I do this myself?

A subpanel feeder is live at full service current, and the conductors feeding it are not protected by the subpanel's own breakers. Neutral and ground must be kept separate in a subpanel — bonding them, as you would in a main panel, puts current onto grounding conductors under normal operation. It is a common error, it passes a casual glance, and it is exactly the kind of fault that only becomes visible when something goes wrong.

Reasonable to DIY

Working out what you want to power and where, before anyone quotes it
Clearing access to the main panel and the intended subpanel location
Digging an approved trench yourself where the electrician agrees the route and depth in advance and the utilities have been located and marked
Labelling the finished panel directory accurately once circuits are live

Call a licensed pro

The whole installation. Feeder sizing, overcurrent protection, disconnect requirements and the grounding arrangement are code judgements, and the feeder is live from the main panel
Any work inside the main panel, where the main lugs remain energised even with the main breaker off
Neutral and ground separation in the subpanel, and the grounding electrode system at a detached building
Anything buried, where depth, conduit type and protection are prescribed and an inspection covers them
The permit and the inspection, which should be the contractor's responsibility, not yours

Before you sign

What to check when hiring for subpanel installation

Does my main service have headroom for this, and what does the load calculation say?

If the answer is no, you are being sold the wrong job. This is the only question that really matters here.
What size feeder are you running, and how was the distance accounted for?

Long runs to detached buildings frequently need larger conductors than the amperage alone would suggest.
For a detached building — disconnect, grounding electrodes and burial method?

These are the requirements that make a garage subpanel much more expensive than an indoor one, and the ones that get quietly omitted.
Will neutrals and grounds be separated in the subpanel?

A basic requirement, a common error, and something you can ask about without having to pretend you are an electrician yourself.
Who pulls the permit, and will the main panel directory be updated?

Both are the contractor's job, and both are what a future inspection or sale will look for.

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Local pros

Subpanel Installation near you

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Related work

Often done at the same time

Panel Upgrade

When capacity is the real problem

Circuit Installation

A dedicated circuit may be enough

200 Amp Service Upgrade

If the service itself is the limit

Outlet Installation

What the subpanel usually feeds

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FAQ

Subpanel Installation — questions people ask

Still have a question? Talk to our team →

Will a subpanel give me more power?

No. This is the most common misunderstanding about subpanels. A subpanel takes a portion of the capacity your main service already provides and distributes it somewhere more convenient. If your service is 100 amps and a load calculation says you are near that limit, adding a subpanel does not change the limit — it just moves where you run out. More capacity comes from a panel or service upgrade, not from another panel downstream of the same service.

Do I need a subpanel or a panel upgrade for my garage or shop?

A subpanel, if your main service has headroom and the garage needs several circuits. That is the normal case for a workshop with tools, lighting, heating and receptacles: one feeder out to the building and short branch circuits from there beats running six long circuits from the house panel. If the load calculation shows the service is already committed, you need the upgrade first, and possibly both.

Why does a detached garage subpanel cost so much more?

Because of everything around the panel rather than the panel itself. A feed to a separate structure typically needs a trench at a prescribed depth with an approved conduit or cable type, a disconnecting means at the building, and its own grounding electrode system. Add the yard restoration and the fact that nobody knows what else is buried on the route until they dig, and the outdoor version of the same panel becomes a substantially bigger job.

Do I need a permit for a subpanel?

In most jurisdictions, yes, with an inspection covering the feeder, the overcurrent protection, the disconnect and the grounding arrangement — and for a buried feed, usually an inspection of the trench before it is backfilled. Requirements vary between cities and counties, so confirm locally. The trench inspection in particular is one you do not want to discover after the ground is closed.