EV charger in a detached garage: what the trench actually costs

A single home-run circuit is one 240-volt branch circuit from the house panel to the charger. It is the cheaper thing to install today and it does exactly one job forever.A feeder to a small subpanel in the garage costs more today — a panel, a disconnect, larger conductors — and then gives you lights, receptacles, a second charger later, and a place to land anything else the building ever needs. If the garage currently has no power at all, or has a 1970s lighting circuit, this is usually the better buy, because the trench is the expensive part and you only want to open it once.There is a code reason to think about it now rather than later. Under NEC 225.30 a separate structure is generally supplied by a single feeder or branch circuit, with limited exceptions. So you do not get to run one circuit today and a second one next year and treat that as normal. Decide the building's total electrical future in one conversation.Two more items belong to the outbuilding and show up on a correct quote: a disconnecting means at the structure (NEC 225.31 and 225.32) and a grounding electrode at the separate structure (NEC 250.32), fed by a four-wire feeder with the neutral and the equipment ground kept separate at the subpanel. If none of those words appear anywhere in a quote for a detached building, the bidder has not scoped it.

Burial depth is not one number. NEC Table 300.5 sets a different minimum cover for each wiring method, because the protection comes from what is around the conductors, not from the dirt alone. Deeper trench means more excavation, more spoil to handle and more surface to put back, so this choice has a direct price consequence.The values below are the ones for the table's general row — locations not otherwise specified. A run that crosses a driveway, a parking area or a public street falls into its own row with its own deeper minimum, and the row for a public street is deeper than the row for a driveway serving the house. Ask your electrician which row applies to your route.

The twelve-inch column in Table 300.5 is for residential branch circuits rated 120 volts or less, 20 amps or less, with ground-fault protection. It exists for landscape and outbuilding lighting. It is also the number that gets repeated on forums by people who never read the column heading.An EV charger circuit is 240 volts and 40 to 60 amps. It gets the 24, 6 or 18-inch answer depending on the method. If a quote is priced against a twelve-inch trench, either the bidder has the wrong number or the run is not what you think it is.

This is the variable that separates a $2,500 job from a $5,500 job at the same distance. Digging a trench through lawn is cheap and the repair is a rake, some topsoil and seed. Digging one under a poured concrete driveway means saw-cutting, breaking out, hauling away, forming and pouring — a concrete job that happens to be attached to an electrical job, priced by a concrete crew, and it will very likely not match the colour of the slab beside it.Between those two extremes sit pavers, which come up and go back down reasonably well; asphalt, which patches visibly; mature tree roots, which are slow and occasionally fatal to the route; and rock, which is the one nobody finds until the machine is on site.There is a way around a hard surface: boring under it rather than cutting it. Directional boring or a simple push rod avoids the restoration entirely and is frequently cheaper than cutting and replacing concrete. Ask whether the bidder can bore. Not all of them can, and the ones who can often do not mention it unless asked.The other honest reason to ask about route length rather than straight-line distance: a route that keeps to grass and skirts the driveway may be forty feet longer and several thousand dollars cheaper.

Set against the attached-garage baseline so you can see what the outbuilding is really costing you.

Before anyone prices a trench, find out the size of the existing feed and how it was run. A 20-amp lighting circuit will not carry a charger and cannot be upgraded in place. A 60-amp feeder to a small subpanel might carry a 32-amp charger comfortably, which turns a $4,000 job into a much smaller one.Reusing an old trench only helps if the existing conduit is large enough for the new conductors and is in good enough condition to pull through. Direct-buried cable cannot be reused as a raceway at all — there is nothing to pull through. Have the electrician look before you plan around it.

Call 811 first. It is the free national one-call service that gets your buried utilities located and marked, it is required in every US state before excavation, and hitting a gas line or a fibre drop is a much worse day than the trench you were digging. The locate is free; the notice period is usually a couple of business days, which is worth putting on the calendar before you book the electrician.One scheduling detail that catches homeowners who dig their own trench: the inspection generally happens on the open trench. If you backfill before the inspector sees it, you may be digging it out again.