Subpanel for an EV Charger

A subpanel is not an upgrade and it does not add capacity. It is a distribution decision: instead of running one long branch circuit from the main panel to the charger, you run one feeder to a small load centre near the charger and take the branch circuits from there. On a short run that is pointless expense. On a 90 foot run to a detached garage it is usually both cheaper and better, and past a certain distance it stops being optional.

It also brings in a set of rules that a simple branch circuit never touches. A feeder to a separate structure needs four wires rather than three, the neutral has to stay isolated from the equipment grounding system inside the subpanel, and the outbuilding needs a grounding electrode system of its own. None of those are optional refinements. They are the parts inspectors fail.

When does a subpanel beat a home run?

Four situations, and the first is by far the most common.

A detached building. Once you are trenching or running overhead to a separate structure, the marginal cost of a feeder over a branch circuit is small and the marginal benefit is large. A garage with a subpanel gets lighting, receptacles, a freezer circuit and the charger. A garage with one home run gets a charger and nothing else, forever, unless somebody digs the trench again.

A long run inside the house. Past roughly 80 to 100 feet, a 60 amp branch circuit starts needing a conductor size above what ampacity alone requires, purely to keep voltage drop inside the 3 percent design target. At that point the conductor cost climbs steeply and a feeder to a subpanel, with short branch circuits at the far end, can be the cheaper set of copper.

A main panel with no space. A subpanel gives you positions. Moving a handful of 120 volt circuits into a small load centre frees adjacent full-height slots in the main panel for the two-pole breaker the charger needs, which is often cheaper than replacing the main panel outright.

More than one future circuit. Two cars, a workshop, a heat pump water heater in the garage, a solar inverter. If any of that is plausible within five years, put in the feeder now while the wall is open and the trench is dug.

How is a subpanel feeder sized?

The feeder is sized for the subpanel and for the load it will carry, not for the charger alone. In practice residential subpanels for this purpose are 60 or 100 amps, and 100 is the common choice because the incremental conductor cost is modest and it leaves room to grow.

Conductor selection starts with the ampacity tables and then gets adjusted. The number you are allowed to use is governed by the lowest-rated component in the circuit, and almost all residential breakers and lugs are rated 75 degrees Celsius, so the 75 degree column is the honest one. The table below shows every copper size in the reference table, the three temperature columns, and the largest standard overcurrent device that conductor can sit behind at the 75 degree rating.

Read the 3 AWG row for a 100 amp feeder and the 6 AWG row for a 60 amp feeder. Note the small conductor rule capping 14, 12 and 10 AWG at 15, 20 and 30 amps regardless of what the temperature columns say, and note that the 90 degree column is used for derating calculations rather than as a number you get to use at the terminals.

Aluminium is entirely legitimate for a feeder of this size and saves real money on a long run, at the cost of one or two sizes larger for the same ampacity.

A 100 amp aluminium feeder is 1/0, against 3 AWG in copper. Aluminium building wire such as 6 AWG XHHW-2 aluminium costs a fraction of the copper equivalent per foot, which on a 120 foot trench is the difference between a few hundred dollars and well over a thousand. It needs lugs rated AL/CU, an anti-oxidant compound such as Ideal Noalox at the terminations, and correct torque. The full argument sits in aluminium or copper wiring.

Why does distance change the feeder before ampacity does?

Because ampacity is about heat in the conductor and voltage drop is about the voltage that arrives at the other end. A conductor can be thermally comfortable and electrically poor at the same time. The 3 percent branch circuit target is a design recommendation rather than a hard code requirement, but it is the number every electrician works to, and on a feeder it matters twice over because the branch circuits at the far end start from whatever voltage the feeder delivered.

Here is a 100 foot one-way feeder carrying 60 amps at 240 volts.

A 6 AWG copper conductor is thermally fine for 60 amps and lands close to the target at 100 feet. Push the same run to 150 feet and it is comfortably outside it. That is the calculation that decides real feeders, and you can run your own distance through the wire gauge calculator before anyone buys wire.

Why does a separate building need four wires?

Older installations to a detached garage ran three wires, two hots and a neutral, and re-bonded the neutral to the equipment grounding system at the outbuilding, treating it like a second service. That is no longer permitted for new work. A feeder to a separate structure now runs four conductors: two ungrounded conductors, an insulated grounded conductor, and an equipment grounding conductor.

The reason is straightforward once you see it. With three wires and a re-bond, every return path between the two buildings is in parallel: the neutral, the equipment grounding system, any metal water line, any gas line, any cable television shield. Normal load current then divides between them, which means current on conduits, on ground rods and on pipework in ordinary operation. Adding an equipment grounding conductor and keeping the neutral isolated puts return current where it belongs and leaves the grounding system carrying nothing until there is a fault.

If your detached garage already has a three-wire feeder from decades ago, adding an EV charger is the moment that gets revisited. Sometimes the existing feeder can be reused with a fourth conductor pulled in alongside it, if it is in conduit with room. Sometimes it cannot. That assessment is part of the quote, and it is one of the reasons detached garage numbers vary as much as they do.

Why are neutral and ground separated at a subpanel?

The grounded conductor and the equipment grounding system are bonded together at exactly one place in the whole installation: the service disconnect. Everywhere downstream of that point they stay separate. In a subpanel that means three things in the box.

The main bonding jumper, usually a green screw or a bonding strap fitted at the factory, is removed. The neutral bar is left floating on its insulated standoffs and lands only the white conductors. A separate equipment grounding bar is added and bonded to the enclosure, and it lands the bare or green conductors including the one that came with the feeder.

Get this wrong and nothing appears to be wrong. The lights work, the charger charges, and current flows on grounding conductors and metal parts every time a 120 volt load runs. It is a genuine hazard and it is invisible without opening the panel, which is precisely why the inspection exists.

What grounding does a detached building need?

A separate structure supplied by a feeder needs a grounding electrode system at that structure. In residential work this is normally a pair of eight foot copper-bonded rods spaced at least six feet apart, connected with a grounding electrode conductor to the equipment grounding bar in the subpanel. Where the building has a concrete-encased electrode, or metallic water piping that qualifies, those get bonded in as well.

Two clarifications that trip people up. First, this electrode system is in addition to the equipment grounding conductor run with the feeder, never a substitute for it. Rods are not a fault-clearing path; they cannot carry enough current to trip a breaker. Their job is lightning and voltage stabilisation. Second, the rods never connect to the neutral bar. Hardware such as an eight foot copper bonded ground rod with clamp is inexpensive, but where and how it is driven and terminated is inspected work.

Does the outbuilding need its own disconnect?

Yes. A separate structure needs a means of disconnecting all ungrounded conductors supplying it, located at a readily accessible point either outside the building or immediately inside where the conductors enter. In practice this decides which load centre you buy, because a panel with a main breaker satisfies the requirement by itself while a main lug panel does not.

That is why a main breaker load centre such as the Siemens 100 amp 20-space load centre or the outdoor-rated Square D Homeline 100 amp outdoor main breaker panel is the usual choice for a detached garage, while a main lug box such as the Square D QO 100 amp 6-space main lug load centre is the right answer inside an attached garage where the feeder breaker in the main panel is the disconnect and the whole thing is one building.

Which load centre should go in the garage?

Prices were accurate when researched and change without notice. The pattern across all four is worth noting: the load centre is the cheapest significant component in the entire project. Buying eight spaces instead of six, or a main breaker instead of main lugs, is a rounding error against the trench. Buy the bigger one.

Inside the box you will also need a two-pole branch breaker listed for that panel, such as a Square D QO260 for a QO panel or an Eaton BR260 for a BR panel. Breaker lines are not interchangeable between manufacturers, and a breaker that physically fits a bus it is not listed for is a defect regardless of how well it seats.

Does the economics actually work?

Take a detached garage 90 feet from the house. The researched range for that job is 2,500 to 6,500 dollars, and the note attached to that figure in our cost data says the quiet part out loud: trenching dominates. The trench costs the same whether you pull one feeder or three branch circuits through it.

So compare what fills it. Three home runs at 90 feet means three sets of conductors sized for their distance, three sets of terminations at the main panel, three breakers, and a conduit sized for the combined fill with derating applied for the number of current-carrying conductors in the raceway. One 100 amp feeder means four conductors, one breaker at each end, a smaller conduit, and branch circuits at the garage end that are ten feet long and therefore cheap. Add the load centre at well under a hundred dollars and the feeder wins clearly.

For comparison, the same charger on a 40 foot hardwired run inside the house is quoted at 1,100 to 2,000 dollars. The gap between those two numbers is almost entirely the trench, which is why the advice on a detached building is always to dig once and put in more than you need. Conduit selection and burial depth are covered in conduit for an EV charger circuit, and a conduit such as 1 inch Schedule 40 PVC is the ordinary choice for a residential burial.

What does the inspector look at?

Six things, and five of them are on this page. The four-conductor feeder with an insulated neutral. The main bonding jumper removed and the neutral bar isolated. A separate equipment grounding bar bonded to the enclosure. The grounding electrode system at the separate structure and its connection. The disconnecting means at the outbuilding. And the sixth is burial depth and conduit protection where the feeder leaves and enters the ground, which is why the trench inspection happens before backfill rather than after.

Two of those, the backfill and any concealed portion of the run, cannot be inspected later without undoing work. Get the sequencing right with your electrician so nobody is digging twice.

Where to go next

If the garage is detached, the trenching, burial and overhead options are worked through in detached garage EV charger. If you are still deciding whether the main panel can carry the feeder at all, start with EV charger on a 200 amp panel. And if the subpanel is part of a larger project, the whole thing is costed line by line in the panel upgrade buildout.

Common questions

Do I need a subpanel for an EV charger?

Not for a charger on the same wall as the main panel. A subpanel earns its keep when the distance is long, when a detached building is involved, or when the main panel has no space. In those cases one feeder plus a small load centre near the charger usually costs less than a long home run in heavy conductor, and it leaves positions free for garage lighting, receptacles and a future second circuit.

What size feeder does a 60 amp charger circuit need?

The feeder is sized for the subpanel, not for the charger. A 100 amp feeder in copper is typically 3 AWG at the 75 degree column, and a 60 amp feeder is 6 AWG. Long runs frequently need a size larger than ampacity requires, because voltage drop rather than heat becomes the governing factor beyond about 100 feet. Your electrician sizes it for both.

Why does a detached garage need four wires?

Because current codes require a separate equipment grounding conductor to be run with the feeder to a separate structure. The older practice of running three wires and re-bonding the neutral to ground at the outbuilding is no longer permitted for new installations. Four wires means two hot conductors, an insulated neutral and an equipment grounding conductor, with the neutral kept isolated from ground at the subpanel.

Why are neutral and ground separated at a subpanel?

The neutral and the equipment grounding system are bonded together at exactly one point, the service disconnect. Bonding them again at a subpanel creates parallel paths, so normal return current flows on grounding conductors, on conduit and on any metal that happens to connect the two buildings. That is a shock and fire concern, and it is one of the most common findings when an inspector opens an outbuilding panel.

Does a detached garage need its own ground rods?

Normally yes. A separate structure supplied by a feeder requires a grounding electrode system at that structure, which in most residential cases means a pair of eight foot rods bonded to the equipment grounding bar in the subpanel. That electrode system is in addition to the equipment grounding conductor run with the feeder, not a substitute for it, and the rods never carry the neutral.

Is a subpanel cheaper than running three separate circuits?

Almost always, once a trench or a long interior run is involved. Three home runs means three sets of conductors over the same distance and three times the fill, while one feeder plus a load centre puts the branch circuits inside the outbuilding where they are short. The trench itself is the same price either way, and trenching is what dominates a detached garage quote.

Getting your own panel and load numbers ready for an electrician? The EV Home Charging Install Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.