How electricians size a service: the load calculation
The National Electrical Code has a standard method for sizing a home's electrical service. It adds up the general lighting and receptacle load (3 volt-amps per square foot of living area), the small-appliance and laundry circuits, and the big fixed appliances, then applies demand factors that recognize not everything runs at once. The total, divided by 240 volts, gives the required service current in amps.
You do not need to run the full code version to get the picture. The simplified version below is close enough to tell you whether you are in 100-amp territory or 200-amp territory, which is the decision that matters.
Worked example: a 2,000 square foot home
Start with the general load: 2,000 square feet times 3 volt-amps is 6,000 VA. Add two small-appliance branch circuits at 1,500 VA each (3,000 VA) and the laundry circuit at 1,500 VA. That is 10,500 VA of general load. The code demand factor takes the first 10,000 VA at 100 percent and the rest at 40 percent: 10,000 plus 40 percent of 500 is 10,200 VA.
Now the big appliances: an electric dryer at 5,000 VA and central air at 6,000 VA. Add them to the 10,200 VA for a total of 21,200 VA. Divide by 240 volts: 88.3 amps. A home like this, with no EV charger, fits comfortably on 100-amp service with room to spare.
Add a 32-amp EV charger, which contributes about 7,680 VA. The total becomes 28,880 VA, and 28,880 divided by 240 is 120.3 amps. Now 100-amp service cannot carry the calculated load, and 200-amp service is the answer. This is the single most common reason 100-amp homes get upgraded today: the car.
The three signs your service is too small
First, the main breaker trips. Individual circuit breakers trip for local overloads; the main breaker tripping means the whole house is drawing more than the service can deliver. Second, the panel has no spare slots. If there is no room for the circuit you want to add, the service size conversation is already overdue. Third, the lights dim when the AC or dryer kicks on, or the panel feels warm. Dimming means the voltage is sagging under load, and warmth at the panel means connections are working harder than they should.
Any one of these is worth a load calculation by a licensed electrician. Two of them together is a strong signal to budget the upgrade (see our panel upgrade cost guide for the numbers).
The 60-to-100 trap: do not upgrade halfway
If you are on 60-amp service with a fuse box, upgrading to 100 amps costs roughly $1,000 to $2,000, which sounds like the thrifty move. It usually is not. The labor, the permit, the utility coordination, and the service entrance work are most of the bill, and they cost nearly the same whether the new panel is 100 or 200 amps. Going straight to 200 amps costs about $1,200 to $3,500 and you never pay for the upgrade a second time. Nobody has ever regretted having too much panel capacity.
When 400 amps enters the conversation
400-amp service, usually delivered as two 200-amp panels, costs $2,000 to $6,000 and is for genuinely large electrical footprints: homes over about 3,000 square feet, multiple HVAC systems, a workshop with 240-volt equipment, pool and spa gear, or more than one EV charger. The vast majority of homes never need it. But if your electrician is already opening up the service for a 200-amp upgrade, asking what the incremental cost of 400-amp-capable infrastructure would be costs nothing, and the answer is sometimes surprisingly small.