By Alexandr Godonoaga, owner, Cob Services LLC. Illinois electrical contractor license #26-00032356.
The call almost always starts the same way. Somebody wants to know if we can just run a wire out to the garage.
We can. The question is what you plan to do out there, because the answer changes the job by an order of magnitude. A light and a couple of receptacles is one project. A woodshop with a dust collector, a compressor, a mini split for winter, and a car charging in the corner is a different one, and the trench looks identical from the outside either way.
What follows is how we scope these, what the City of Naperville actually requires, and the mistakes that cost people the most money.
Start with the load, not the wire
Everyone wants to talk about wire size first. Wire size is an output, not an input.
Write down what is going to live in that building. Lights and a few general receptacles might pull twenty amps on a busy day. Add a 240 volt table saw and a two horsepower dust collector running together and you are into a different conversation. A compressor has a starting current that dwarfs its running current. A welder has a duty cycle that makes nameplate amps misleading. A mini split heat pump is modest in cooling and much less modest when it is heating a slab floored garage in February.
Then there is the car. If a Level 2 charger is anywhere in your five year plan, size for it now. A 48 amp charger on a continuous load basis wants a 60 amp circuit all by itself.
Here is the part I tell every customer: the trench costs the same whether we pull small conductors or large ones. Excavation, restoration, permit, inspection, labor. Those numbers barely move. The conductors are a modest slice of the total. Undersizing the feeder to save a few hundred dollars, then digging the same trench again in four years, is the single most common regret we hear on this type of job.
Most of the detached garage feeders we install in Naperville land at 60 or 100 amps. I have run 30 amp feeders where the customer genuinely only wanted lights and a garage door opener, and I have run 125 amps to a workshop. What I try not to do is run exactly what today needs and nothing more.
Feeder sizing and the thing that gets skipped
Distance changes everything.
Voltage drop is the reason. Push current down a long conductor and voltage falls along the way. The code treats a three percent drop on a feeder as the target, and while that number sits in an informational note rather than an enforceable rule, any installer worth hiring treats it as one. Motors are the reason. A saw or compressor running on sagging voltage draws more current, runs hotter, and dies earlier than it should.
Practically, a 60 amp feeder that would call for a certain conductor size on a twenty foot run frequently needs to go up a size or two at 150 feet. We calculate it rather than guess, because the guess is always low.
For longer runs we usually pull individual conductors rated for wet locations through PVC conduit, and on the bigger feeders aluminum is often the sensible choice. Aluminum has a reputation problem in this area, and it is deserved when we are talking about 1960s branch circuit wiring in your walls. Modern aluminum feeder conductors, properly sized and terminated with the right lugs and antioxidant compound, are a completely different animal and are what most of the utility grid is built from. On a long run the cost difference is real.
One more note on conduit. Every underground run we install goes in conduit even where direct burial cable would be permitted, and we oversize the conduit. Conduit means the next change is a pull rather than a dig.
Trenching, depth, and the call you are legally required to make
Before anything else: contact JULIE.
Illinois law requires a locate request before any excavation, at any depth, for any project. It is free. JULIE asks for at least three days notice before you dig, and not more than ten days ahead, with weekends and holidays not counting as days. We handle this on our jobs, but if you are doing your own trenching to save money, the responsibility for the locate request falls on whoever is actually digging.
This is not a formality. Naperville lots frequently have gas, water, communications, irrigation and low voltage landscape lighting running through the same side yard we want to trench. The utility marks tell us where we cannot use a machine.
Burial depth comes from a table in the code and depends on what you are burying. Direct burial cable goes deeper than conduit. PVC conduit has its own minimum. Runs under a driveway have a different requirement again. We default to two feet on residential feeders regardless, because the difference in labor is trivial and it keeps the run below anything a future fence post or tree planting is going to reach. Warning tape goes in above the conduit on the way back up.
Two things about Naperville soil specifically. It is heavy clay through most of the city, which trenches cleanly but is miserable to hand dig, and it holds water. That matters for the fittings and boxes at both ends, which need to be rated for a wet location whether or not they look like they will ever get wet. And frost depth is not a factor for the conductors themselves, but it is for anything structural at either end of the run.
While the trench is open, put in a second empty conduit. It costs very little at that moment. Data, a security camera, a future subpanel feed, a gate operator, low voltage lighting, whatever comes later goes in without a shovel. Of all the advice in this article, that is the one people thank me for years afterward.
Sub-panel sizing and the four details DIY installs miss
A detached building with its own feeder needs a disconnecting means at the building, readily accessible, at or near the point where the feeder enters. That means the panel you install out there is a main breaker panel, not a main lug panel. If a fire crew needs to kill power to that structure, they need to do it at that structure, not by walking into your basement.
Then there are four things we correct constantly on work somebody else did:
The feeder has to be four wire. Two hot conductors, a neutral, and a separate equipment grounding conductor. The old three wire method, where the neutral did double duty, has not been permitted for detached structures for a long time, and we still find it because somebody copied a diagram from an old book.
Neutral and ground get separated inside the sub-panel. The bonding screw or strap that ties the neutral bar to the enclosure has to come out, and the grounds land on their own bar. Bonding them together at the sub-panel puts current on the equipment grounding conductor and on anything metal it touches. This is the single most common error we find in owner installed garage panels.
The detached building needs its own grounding electrode system. Ground rods, driven at the garage, bonded back to the panel. Two rods spaced apart is the practical standard unless someone is testing resistance, which nobody does on residential work. Skipping the rods entirely is the second most common error, and inspectors here look for it.
Receptacles in the garage need GFCI protection. That is not optional and it has not been for years.
The code also requires a receptacle outlet in a detached garage that has power, plus wall switch controlled lighting. If you are running a feeder anyway, you will exceed that easily, but it is worth knowing the floor exists.
Size the panel with more spaces than circuits. A 12 space panel with 12 circuits in it is a panel you will be unhappy with. Spare spaces are the cheapest thing in the entire project.
The Naperville part, which is not what most articles tell you
Two things about doing this work inside Naperville city limits that get written incorrectly on a regular basis.
First, permits. Naperville requires a permit before any electrical work begins, and the work has to be performed by a state licensed electrical contractor who is also registered with the City of Naperville specifically. Being licensed in Illinois is not sufficient on its own. The city maintains its own contractor registration, and a contractor who is not on that list cannot pull your permit. When you are collecting quotes, that is a fair question to ask on the phone, and the answer should come instantly.
Naperville currently enforces the 2023 National Electrical Code. Some of what you will find online about garage circuits and GFCI requirements is written against older editions, and the GFCI rules in particular have expanded meaningfully across recent cycles.
Second, and this is the one I want to flag hardest, Naperville is not a ComEd town. The city has owned and operated its own municipal electric utility since 1899, and it serves more than sixty three thousand customers. If your project needs anything from the utility, a service upgrade, a meter change, a temporary disconnect, you are calling the Naperville Electric Utility at (630) 420-6059 for new service, not ComEd. ComEd serves plenty of the surrounding suburbs, and if you live in an unincorporated pocket outside the city boundary your provider may well be ComEd, but for an address inside Naperville it is the city.
I mention this because I have watched homeowners lose a week calling the wrong utility, and because a contractor who tells you they will handle the ComEd coordination on your Naperville job has told you something useful about how well they know this town.
The inspection that catches people
There are two inspections on a job like this, and the first one is the one that costs money if you get it wrong.
The trench gets inspected before it is backfilled. Conduit in, before dirt goes on top of it. If you or your landscaper fills that trench because the weather looked bad or the hole was a hazard, it comes back open. I have seen it happen and it is a genuinely bad day.
The final inspection covers the panel, the grounding electrodes, the bonding, the circuits and the devices. Standard stuff if the work was done right.
Plan the schedule so the trench does not sit open over a weekend waiting on an inspector, and do not let anyone with a skid steer near it in the meantime.
When your house cannot support the garage yet
Sometimes the honest answer is that the feeder is the second half of the project.
A house on 100 amp service, already running central air and a dryer and an electric range, does not automatically have 60 spare amps to send to a workshop. The code allows a calculation using demand factors rather than simple addition, which is why a house that looks maxed out on paper often has more room than you would expect. But it has to be calculated, and calculated honestly.
We run that load calculation before we quote the feeder, because finding out afterward is how a garage project turns into a garage project plus a service upgrade with no warning. If the number comes back short, your options are a service upgrade first or a smaller feeder with load management on the charger, and we will lay both out with real numbers rather than steering you toward the bigger job.
Our electrical panel work and EV charger installation pages cover what those look like on their own.
Overhead feeds and older Naperville properties
Some of the older properties near downtown still have an overhead feed running from the house to the garage, usually installed decades ago and often on conductors nobody would specify today.
Overhead is still permitted. There are clearance requirements above grade, above driveways and above anything walkable, and a span that was fine when it went up may not be after a deck or a patio went in underneath it. What we find more often is that the old overhead run is undersized, weathered, or bonded incorrectly at the garage end.
When we replace one, we go underground. It gets the conductors out of the weather, off the tree limbs, and out of the way of the next storm.
Mistakes we get called out to fix
In rough order of how often we see them: neutral bonded to ground inside the garage sub-panel. No grounding electrodes at the detached structure. Three wire feeders on installs done in the last fifteen years. Romex buried directly in the ground, which is not a wet location wiring method and will fail. Extension cords run through PVC as a permanent feed. A main lug panel where a disconnect was required. Trenches backfilled before inspection. Undersized feeders where the customer got exactly what they asked for and nothing more.
Almost every one of these came from a well intentioned weekend project or a handyman who was confident. None of them are hard to do correctly the first time.
Questions we get
Do I need a permit to run power to my detached garage in Naperville?
Yes. Naperville requires an electrical permit before any electrical work starts, and the contractor performing it must be a state licensed electrical contractor registered with the City of Naperville. The trench is inspected before backfill and the finished work is inspected at completion.
How deep does the trench have to be?
It depends on the wiring method. Direct burial cable requires more cover than conduit, PVC conduit has its own minimum, and runs under a driveway have a separate requirement. We install at two feet on residential feeders as a matter of practice, with warning tape above the conduit.
Do I have to call JULIE if I am hiring an electrician?
Someone does, and by law it is whoever is actually excavating. If we are trenching, we submit the locate request. If you are digging your own trench to save on labor, the request is your responsibility. It is free, and Illinois requires it at any depth for any project.
What size sub-panel should I put in a detached garage?
Size the feeder to your realistic future load rather than today’s, since the trench is the expensive part and re-digging it is the thing people regret. Most residential detached garages we wire in Naperville end up at 60 or 100 amps. Choose a panel with more breaker spaces than you currently need.
Can I use the existing wire that already runs to my garage?
Sometimes, if it is a four wire feeder in good condition and sized for what you want to add. Frequently not, because older runs to detached garages are undersized, direct buried without conduit, or wired with three conductors under rules that no longer apply.
Does Naperville use ComEd for electricity?
Not inside the city. Naperville has operated its own municipal electric utility since 1899, serving over sixty three thousand customers. For a service upgrade or meter work at a Naperville address you contact the Naperville Electric Utility. Addresses in surrounding communities and some unincorporated areas are served by ComEd.
Get a real number before you dig
If you are planning a garage or shop, the useful conversation happens before anyone rents a trencher. Send us the distance from your house panel to the building, a photo of your existing panel with the door open, and a list of what is going in there, including anything you might add later.
We will tell you what feeder that needs, whether your service supports it, and what the permit and inspection sequence looks like. Free estimates, no trip charge, and you will be talking to a licensed electrician rather than a call center.
Call (630) 427-5923.
More on our work: residential electricians in Naperville, IL, dedicated circuits, outlet grounding, and custom lighting installation. If you are still deciding whether a sub-panel is the right approach, our post on what a subpanel is and when you need one covers the basics.
Cob Services LLC, 2020 Calamos Ct Suite 200, Naperville, IL 60563. Licensed, bonded and insured. Illinois license #26-00032356.