Overview
Your electrical service is the total amperage your main panel and the utility feed can deliver to the house at once. On residential jobs you see two sizes far more than any other: 100-amp and 200-amp. There are others (150A is common, 400A shows up on large homes), but the real decision for most houses comes down to 100 versus 200. The number is not about any single outlet or appliance. It is about total load, meaning how much the whole house can pull simultaneously without tripping the main breaker.
What is driving this decision today is the shift toward all-electric homes. A house with a gas furnace, gas water heater, and gas range sips electricity. Start adding an EV charger, a heat pump, an induction range, and an electric water heater, and the demand climbs fast. That trend pushes new and remodeled homes toward 200A. One thing to keep straight: this is the service and main-breaker size, fed by the utility transformer through your meter. It is a different question from how you wire an individual circuit, which is where a guide like romex vs conduit lives. Here we are sizing the whole feed.
What Service Size Means
Two limits matter inside your panel. First, the main breaker rating (100A or 200A) caps how much current the whole house can draw at the same instant. Pull more than the rating and the main trips. Second, the panel has a finite number of slots, or spaces, for branch-circuit breakers. A small 100A panel can run out of room long before it runs out of amperage, which is its own reason people upgrade.
100A was the standard for decades and it is genuinely fine for a smaller, gas-heated home with a modest electric load. 200A is the modern default for most new construction and any larger or all-electric house. A service upgrade is not a quick swap. It means a new panel, usually a new meter, coordination with the utility, and sometimes a heavier service drop or underground feed.
The part that gets skipped: a pro does not eyeball this. They run a load calculation under NEC Article 220, adding up the connected loads with the code's demand factors to land on a real number. That calculation is what tells you whether 100A still has headroom or whether you have outgrown it. A guess is how you end up redoing the work.
100-Amp Service
100A service is enough for a smaller home, roughly under 2,000 square feet, that heats with gas and runs a modest electric load. Picture a gas furnace, gas water heater, gas range, standard central AC, lighting, and ordinary outlets. That house lives comfortably inside 100A, and millions of them do. It is cheaper to install, and it is the right size for plenty of older and smaller houses. There is nothing wrong with 100A when the load actually fits.
The problem is headroom. 100A leaves little room to grow. Add a Level 2 EV charger, swap the gas furnace for a heat pump, put in an electric range or an electric water heater, or drop in a hot tub, and you can max out a 100A service in a hurry. When that happens you are looking at either a full upgrade or a load-management device to keep the peak draw in check. The panel slots are the other pinch point: smaller 100A panels fill up, and then you have no room for new circuits even if the amperage would technically allow them. Think of 100A as adequate for a modest gas home, not as a foundation for going electric.
200-Amp Service
200A is the modern standard, and for good reason. It is what most new construction gets, what larger homes need, and what any house going electric should have. The headroom is the whole point. A 200A service comfortably carries a Level 2 EV charger, a heat pump for heating and cooling, an induction range, and an electric water heater, with room left over. The panel also comes with more slots, so you can add circuits for a shop, an addition, or a future load without running out of space.
That future-proofing matters more every year. The direction housing is moving is electric, and a 200A service means you are not tearing out the panel and re-coordinating with the utility five years after you finish the house. You do it once.
The downsides are real but modest. A 200A panel and the gear around it cost more upfront than a 100A setup. If you are retrofitting an older home, the upgrade itself adds cost: new panel, new meter, permit, inspection, and sometimes a heavier service entrance. For new work the incremental cost over 100A is small relative to what the headroom buys you. 200A is the all-electric default.
When You Need to Upgrade
A few clear signs tell you that you have outgrown 100A. You are adding a Level 2 EV charger. You are converting to a heat pump or any electric heat. You are putting in an electric range, an electric water heater, or a hot tub. Your main breaker trips when several big loads run together. Your panel is full with no open slots. Or you have an old panel that is unsafe on its own merits.
The load calculation decides it, not a hunch. A pro runs the NEC Article 220 numbers on your existing and planned loads and tells you whether 100A still has room. Sometimes the answer is that you do not need a full upgrade at all: a load-management device or circuit-sharing setup (common with EV chargers) can let you add a big load without going to 200A. That can save real money when the calculation is close.
When an upgrade is the right call, scope it fully. It is a new panel, almost always a new meter, sometimes a new service drop or underground feed, coordination with the utility, and a permit with inspection. If your panel is a Federal Pacific or a Zinsco, treat replacement as urgent regardless of amperage, for reasons covered in the take below.
FAQ
Is 100-amp service safe? Yes, when the load fits. A 100A service in a smaller gas-heated home with a modest electric load is perfectly safe and code-compliant. Safety problems come from overloading a service or from a defective panel (like Federal Pacific or Zinsco), not from the 100A rating itself.
Do I need 200-amp service for an EV charger? Not always. A Level 2 charger is a big load, but a load calculation may show your 100A service has room, or you can use a load-management device that throttles the charger when the house demand peaks. Have a pro run the NEC Article 220 numbers before assuming you need a full upgrade.
Can I upgrade from 100 to 200 amps myself? No. A service upgrade ties into the utility's equipment, requires a permit, and must pass inspection. It involves the meter and often the service drop or underground feed. This is licensed-electrician work, and an unpermitted service change is both unsafe and a problem at resale.
How do I know what size service I have now? Look at the main breaker in your panel; the handle is usually stamped with its rating (100, 150, 200). If there is no clear main, or you are unsure, an electrician can confirm it during a load calculation rather than leaving you to guess.