A 100-amp subpanel for a garage and a charger: when that rating is right, and how to check

“100 amp subpanel installation” is searched about 1,300 times a month in the United States, at a competition index of 0.03: almost nobody advertises against it. Like the 200-amp panel question, it names the answer before asking the question. A 100 A subpanel is a common and often sensible choice for a garage that will hold a charger. It is also a rating that has to fit inside the main service, and that is where it is checked.

Why 100 amps is the number people name

It is a round figure, a standard breaker size, and large enough for a Level 2 charger with room left for lights, a door opener, a compressor or a second charger later. That makes it a good default for a detached garage or a workshop. A default is not a calculation, though, and the rating of a subpanel says how much it is allowed to draw, not how much the house can give it.

The check that comes first

A 100 A feeder breaker in a main panel does not create 100 A of supply. Whatever the subpanel draws comes out of the same main service as everything else in the house. The load calculation therefore counts the house as it is, adds what the garage will actually run, the charger counted as a continuous load at its set output, and compares the total with the service rating. A house on a 100 A service can feed a 100 A subpanel, because the rating is a limit and not a draw, but only if the loads added up say so. The method is described under panel capacity check.

When a smaller subpanel is the better answer

A garage that will only ever hold one charger and a few lights may be better served by a smaller feeder: a lighter conductor, a cheaper trench, a smaller bite out of the main panel. Sizing up for the future is reasonable when the future is planned, a second vehicle, a workshop, an outdoor kitchen. When it is not, the extra rating mostly buys a heavier cable.

When it is not enough

A garage planned for two chargers at high output, plus a heat pump or welding equipment, can exceed what a 100 A feeder carries. In that case the choice is between a larger feeder, load management between the two chargers, or both chargers set to share. Most two-vehicle households charge overnight, and a shared setting usually covers them.

The run to the garage

A detached garage puts a trench, a conduit and a re-entry between the panel and the subpanel. The burial depth, the conduit type and the conductor size are set by the code edition your jurisdiction enforces and by the length of the run, which may call for a larger conductor to hold the voltage at the far end. The trench is often the largest part of the work, which is why the route is chosen before the rating. The same logic, applied to a single charging circuit, is in installing a home charging station.

What the garage end needs

Separate neutral and ground bars with no bond between them, a four-wire feeder, and for a detached building its own grounding electrode. A disconnecting means at the building, which the subpanel’s main breaker usually provides. And slots: a 100 A subpanel with too few spaces fills up as fast as the main panel did.

Permit, inspection and the charger in the same visit

The feeder, the subpanel and the charging circuit are one project and belong on one permit. Done in that order, the inspector sees the trench open, the panel wired and the charger circuit landed, rather than three visits for three pieces of the same work. Where a panel upgrade turns out to be needed first, the sequence is described under panel and capacity.