100 Amp vs 200 Amp Service: Which Does Your Home Need?

If your house has a 100 amp service, sooner or later someone will tell you it needs 200. Sometimes that is true. Often it is not, at least not yet. The answer comes from arithmetic rather than from the number printed on the main breaker, and in Ontario that arithmetic is set out in the Ontario Electrical Safety Code (OESC). This guide explains the calculation, what tips a house over, the alternatives, and what an upgrade involves.

What the amp rating actually means

The service rating is the most current your house can draw through the main breaker and the service conductors at one time. At 240 volts, a 100 amp service can carry about 24 kilowatts, and a 200 amp service about twice that. It is a ceiling, not a measure of how much electricity you use, and most houses spend nearly all their time far below it.

The real question is whether the big loads could run together and push past that ceiling. The code answers it with a load calculation.

The load calculation, in plain terms

For a single house, the OESC uses Rule 8-200. The Electrical Safety Authority (ESA) publishes a worked example in Bulletin 8-3-16, and it shows how the pieces add up:

• A basic load for the living area: 5,000 watts for the first 90 square metres, plus more for the area beyond. ESA’s example, a 140 m² (about 1,500 sq. ft.) house, adds 1,000 watts for the extra space.
• Heating or cooling: air conditioning counts at its full rating and electric space heating follows its own demand rules. Where an interlock stops the two running together, only the larger one counts.
• An electric range: one rated up to 12 kilowatts counts as 6,000 watts.
• EV charging equipment at 100 percent: a Level 2 charger drawing 32 amps at 240 volts counts as 7,680 watts.
• Other large appliances at a fraction: in ESA’s example, the dryer and a tank water heater count at 25 percent.

Add it up, divide by 240 volts, and you have the minimum ampacity the service needs. ESA’s example house, with central air, an electric range, a dryer, an electric water heater and a 32 amp charger, comes to about 26,000 watts, or 108.6 amps, and the bulletin rounds that up to a 125 amp service. Remove the charger and the same house calculates to about 77 amps.

The same bulletin notes that a typical 2,500 sq. ft. house without electric heat, with a range, an air conditioner and a dryer, calculates to about 85 amps, and that the real demand in houses like that tends to be lower than the calculation.

When 100 amps is enough

A 100 amp service is usually fine for a house that heats with gas, has central air, a dryer and an ordinary range, and isn’t adding anything large. Trouble starts when a big new load arrives.

Even then, there is a second route that often saves an upgrade. Rule 8-106 8) lets the calculation use the home’s demonstrated load: the highest hourly reading your utility recorded over the past 12 months. ESA’s method takes that peak kilowatt-hour reading, adds 25 percent for diversity, converts it to amps, and adds the full nameplate rating of the new equipment. If the total is at or below the rating of the service conductors or the main breaker, whichever is lower, the existing service can stay. ESA’s own example shows a 100 amp house peaking at about 52 amps, leaving room for something like a hot tub.

One catch: the service conductors can rate lower than the breaker. ESA’s example treats a 100 amp service with No. 2 aluminum conductors as a 90 amp service.

In Kitchener, Waterloo, Wilmot, Woolwich and Wellesley, Enova Power account holders can ask for 12 or 24 months of demand data through Enova’s usage request form, which lists hot tubs, EV chargers and panel upgrades as typical reasons. When we price an addition to a 100 amp house, that data is worth having.

What pushes a house to 200 amps

• An EV charger. At 100 percent of its rating, a 40 or 48 amp charger takes a large share of a 100 amp service. A lower setting or load management can help; our guide to hardwired vs plug-in chargers covers the options.
• A heat pump. ESA’s bulletin says a heat pump without electric backup counts at 100 percent of its nameplate, like an air conditioner. If electric backup heat can run at the same time, that counts too. Moving from a gas furnace to a heat pump moves your heating onto the electrical service.
• An induction or electric range replacing gas. It adds 6,000 watts or more to a calculation that had none for cooking.
• A hot tub. Using the demonstrated-load method, the hot tub counts at its full nameplate rating. See hot tub wiring.
• A basement suite. A second unit brings a second kitchen, often a second laundry, and its own heating or cooling, and the calculation has to cover both units.

Load management: the alternative for EV charging

The OESC allows an energy management system (EMS) to watch the load and automatically turn EV charging down or off when the house is busy. ESA Bulletin 86-1-7 explains the effect. When an EMS controls the charger, its demand counts only up to the maximum the EMS allows. Where the EMS monitors the service conductors themselves, the charger doesn’t have to be counted in the calculated load at all. Rule 86-300 also lets a charger share a branch circuit with other loads when an EMS is installed.

That is how many 100 amp houses get a Level 2 charger without a new service. The trade-off is slower charging at busy times, which matters little if the car charges overnight. The rules ESA explains in that bulletin are written around EV charging, so if a heat pump or induction range is also on the list, a full upgrade is often the cleaner answer.

What a 200 amp upgrade involves

1. A site visit and a calculation. We look at the existing service entrance, meter base, panel and grounding, and confirm what the house needs now and what it is likely to add.
2. Utility paperwork. The utility has to agree to the new service. Enova Power asks for a service request for any new or upgraded service. GrandBridge Energy, which serves Cambridge and North Dumfries, requires a service layout for an increase in amperage and says layouts take about three weeks, depending on scope.
3. An ESA notification. The licensed electrical contractor files it before work starts. Our guide to ESA permits and inspections explains the process.
4. Disconnect, replace, inspect. The utility disconnects the power, we replace the service equipment, and ESA says a service upgrade must be inspected before power is restored. Plan the day around being without power for the swap.
5. Reconnect. Once ESA authorizes it, the utility reconnects. GrandBridge, for example, requires written connection authorization before it reconnects a service.
6. Keep the certificate. ESA issues a Certificate of Acceptance to whoever filed the notification. Ask your contractor for a copy.

So which do you need?

If you are adding one moderate load to a gas-heated house, start with the demonstrated load or, for a car charger, load management. If you are electrifying your heating, adding a suite, or stacking a charger, a hot tub and an induction range, 200 amps is usually the right size, and doing the service once beats doing it twice.

For typical prices, see the panel upgrade cost guide or our panel upgrades page. To have us run the numbers on your house, request a free quote or call 519-591-1442.