100, 150 or 200 amp service: what size does a house need?

Gas-heated vs all-electric: two service load calculations

Both houses below are 2,400 ft² with 2 kitchen circuits, 1 laundry circuit, a dishwasher, a disposal and a 4-ton air conditioner, entered at 1,600 VA per ton (6,400 VA). The gas house heats, cooks, dries clothes and heats water with gas. The all-electric house swaps in a 12,000 VA range, a 5,000 VA dryer, a 4,500 VA water heater and a heat pump with 10 kW of backup strips.

The gas house lands well under 100 A, so the code minimum sets its size. The all-electric house needs 62.9 A more, which carries it past the 100 and 125 A panel sizes to a 150 A service. The electric range, dryer and water heater account for 35.8 A of that after the 40% demand factor. The heat pump's strips add the other 27.1 A, counted at 65% on top of the compressor with no discount.

What pushes a house from 100 to 150 to 200 amps

Start with the gas house and switch one load at a time to electric. Highlighted rows are where the common panel size steps up.

The electric range alone keeps the house under 100 A. The dryer tips the calculation just past it, and the next standard rating, 110 A, is not a common panel size, so the house goes to 125 A. Electric heat is the biggest single step. The EV charger then carries the load past 150 A, and the next standard rating, 175 A, puts the house on a 200 A panel. Common panel sizes run 100, 125, 150, 200, 225, 400 A.

Why heat counts more than a range or an EV charger

The optional method treats two groups of load differently. Lighting, receptacle circuits and every nameplate appliance go into one general load. The first 10,000 VA counts in full and the rest at 40%, because those loads rarely all run at once. Heating and cooling are added after that discount, at 100% for an air conditioner or heat-pump compressor and 65% for electric heat.

• Range: 12,000 VA nameplate × 40% = 4,800 VA, or 20.0 A.
• EV charger: 48 A × 240 V = 11,520 VA. NEC 220.57 counts at least 7,200 VA. At 40% it adds 4,608 VA, or 19.2 A.
• Heat-pump strips: 10,000 VA × 65% = 6,500 VA, or 27.1 A, with no further discount.
• Air conditioner or heat-pump compressor: 6,400 VA at 100%, or 26.7 A, in both houses.

So a larger heat pump, bigger strips or electric baseboard heat moves the service size faster than another cooking or laundry appliance. To use the watt rating on an appliance label, convert it with the watts to amps calculator or enter the watts as VA.

Choosing between 150 and 200 amp service

The calculation gives the minimum. A service sized right at the calculated load leaves little room for the next appliance. The all-electric house above has 12.4 A of calculated room on a 150 A service, and one EV charger uses all of it. If a charger, a hot tub, a shop or a switch from gas is likely, run those loads through the calculator now and size for the result.

The service rating also sets the service conductors, the meter base and the grounding electrode conductor. For a dwelling service, NEC 310.12 lets the service conductors be sized at 83% of the service rating; the wire ampacity chart has the conductor ampacities. The utility has its own rules for the meter and the drop, and a house built under 220.82 is calculated differently from loads added to an existing house (220.83).

A service change needs a permit and, in most places, a licensed electrician. Use these numbers to plan the upgrade and check the electrician's load calculation, then follow the code edition your town has adopted.