By Alexandru Goncear, Owner and CEO, ElectON Enterprise. Washington electrical contractor license ELECTEL765MN.
This question usually arrives with a deadline attached. The heat pump is booked, the car is on order, the induction range is sitting in a shopping cart, and somebody wants to know whether the 100-amp panel in the basement can take all of it.
Sometimes it can. A 100-amp service is more capacity than most people think, and a lot more than a house with gas heat actually uses. But the answer turns on a few specific things, and one of them, the backup heat built into many heat pumps, catches people off guard every winter.
Below is the math, simplified but done the way the code does it for an existing house, with numbers you can follow. It will not replace a load calculation on your own house. It will tell you whether you are likely fine, close or over.
What 100 amps actually means
A 100-amp service at 240 volts can deliver 24,000 volt-amps, roughly 24 kilowatts, at any one moment. Every calculation below is measured against that number.
Check what you actually have before you do any math, because the panel label is not always the answer. A panel rated for 125 or 200 amps can sit behind a 100-amp main breaker and 100-amp service wires, and the smallest piece in that chain sets the limit. The main breaker usually tells you. City Light can also confirm it with a service letter about your service size, and its capacity, load and service page explains where else to look. City Light is also clear that it does not do load studies for customers. That part is your electrician’s job.
How the code counts load in an existing house
The National Electrical Code has a method written for exactly this situation, an existing house that is adding load, and Seattle enforces it through the Seattle Electrical Code. In plain terms it works like this:
- Add up the house’s everyday loads: 3 volt-amps per square foot for lighting and receptacles, 1,500 for each kitchen countertop circuit and the laundry circuit, and the nameplate rating of every appliance that is fastened in place or has its own circuit, such as the range, dryer, dishwasher and water heater.
- Count the first 8,000 volt-amps of that at full value and everything above it at 40 percent. The 40 percent reflects the fact that nobody runs everything at once.
- If you are adding heating or cooling, add the larger of the two at full value on top.
- Divide the total by 240 to get amps, and compare it with your service.
EV chargers are where people read the rules differently. The code says to count a charger at 7,200 volt-amps or its nameplate rating, whichever is larger. Some electricians fold it into the 40 percent group. I count it at full value on top, because a charger can run flat out for hours on a cold night while the heat is on. That is the conservative way, and it is how the examples below are done. We wrote more about how the 2023 code treats EV chargers if you want the detail.
A worked example: a 1,600-square-foot Seattle house
Take a fairly typical Seattle house: 1,600 square feet on a 100-amp service, with an electric dryer, a dishwasher and disposal, and a new 30-inch induction range. The figures are approximate nameplate values. Yours will be different, which is the whole reason to run your own.
| Everyday load | How it is counted | Volt-amps |
|---|---|---|
| Lighting and receptacles, 1,600 sq ft | 3 per square foot | 4,800 |
| Two kitchen countertop circuits and one laundry circuit | 1,500 each | 4,500 |
| Dishwasher and disposal | Nameplate | 2,000 |
| Electric dryer | Nameplate | 5,000 |
| Induction range | Nameplate, typical 30-inch | 11,500 |
| Total | 27,800 | |
| After the code’s demand factors | First 8,000 at full value, the rest at 40 percent | 15,920 |
Notice what the induction range actually does. Its 11,500 volt-amps land in the 40 percent group, so it adds about 4,600 to the result, roughly 19 amps. An induction range on its own rarely breaks a 100-amp service. It is what comes after it that does.
Now add heating and an EV charger at full value, the conservative way:
| Scenario | Heating added | EV charger added | Total volt-amps | Amps at 240 volts | Fits 100 amps? |
|---|---|---|---|---|---|
| Gas furnace stays, no EV | 0 | 0 | 15,920 | 66 | Yes |
| Gas furnace stays, 32-amp charger | 0 | 7,680 | 23,600 | 98 | Just barely |
| Gas furnace stays, 48-amp charger | 0 | 11,520 | 27,440 | 114 | No |
| Ducted heat pump with 10 kW backup strips, no EV | 14,500 | 0 | 30,420 | 127 | No |
| Heat pump with no backup strips, no EV | 4,500 | 0 | 20,420 | 85 | Yes |
| Heat pump with no backup strips, 32-amp charger | 4,500 | 7,680 | 28,100 | 117 | Only with load management |
Two things jump out. A 32-amp charger fits in this house as long as the heat stays on gas, and a 48-amp one does not. And the heat pump with 10 kilowatts of backup strips goes well past 100 amps before an EV is even in the picture, while the same house with a heat pump that has no strips fits with room to spare.
The backup heat is the part to ask about
Many ducted heat pumps are installed with electric resistance backup, called heat strips, for the coldest days. In this calculation heating counts at full value, so a 10-kilowatt strip weighs more than the induction range and a 32-amp charger put together. Seattle does not get many hard freezes, but heat pumps here are sized and wired for the cold snaps we do get.
Before you sign with an HVAC contractor, ask three questions. How many kilowatts of backup heat are you planning? Would a cold-climate model let us use less, or none? Can the backup be staged or locked out so it does not run at the same time as everything else? Those answers change the electrical scope more than anything else in the house. The heat pump itself goes on a dedicated heat pump circuit, sized for whatever the HVAC contractor actually installs.
Seattle’s Clean Heat Program has moved a lot of homes off oil heat and onto heat pumps. An oil furnace uses almost no electricity, so in those houses the heat pump is all new load, and this math is worth doing before the equipment is ordered.
Houses that heat with electric baseboards start from the other end. Those heaters already count as heating load, and they are often the reason a 100-amp house has no room left. Replacing them with ductless heat pumps can free up capacity, because a heat pump delivers the same heat for a fraction of the electricity.
Measuring instead of calculating
The code has a second method that uses what your house actually draws. If a year of peak demand data is available, or if we put a recording meter on the panel for at least 30 days while the house is lived in, the code lets us take the highest 15-minute demand, multiply it by 125 percent, and add the new load to that.
In older Seattle houses with gas heat, this often shows more room than the paper calculation does. Say the highest 15-minute demand in a year was 9 kilowatts. At 125 percent that is 11,250 volt-amps, about 47 amps, which leaves around 53 amps for new equipment on a 100-amp service. The catch is timing. A 30-day recording taken in August says nothing about January, so it has to cover the heating season or account for the heating some other way.
When the math comes out over 100 amps
You have more options than an upgrade, and some of them cost very little.
- Turn the charger down. A 32-amp charger on a 40-amp circuit still adds roughly 25 miles of range an hour, which covers most commutes overnight. Our five-minute self-check for an EV charger walks through it.
- Add load management. A device that throttles or pauses the charger when the rest of the house is drawing hard costs roughly $300 to $800, and the code accepts it as an alternative to more service capacity. We broke down what load management costs next to a service upgrade.
- Choose the heat pump with the electrical service in mind: smaller backup strips or none, or controls that keep the backup from running alongside everything else.
- Swap a resistance appliance for a heat pump version. A heat pump dryer or a 120-volt heat pump water heater uses a fraction of what the old ones did.
- Consider a battery-equipped induction range. It costs several thousand dollars more than a standard range, but it plugs into an ordinary 120-volt outlet.
- Upgrade the service. A straightforward 100 to 200 amp upgrade in Seattle runs very roughly $5,500 to $9,000 before sales tax, plus whatever City Light charges for its part of the connection. City Light allows four to six weeks just to process the application. If you are planning a heat pump next year and an EV after that, doing the service once is usually cheaper than solving the same problem twice. Our panel replacement and upgrade page covers the work itself.
It is not only amps: spaces and the panel itself
Even when the math works, the panel needs room. A heat pump, an induction range and an EV charger each need a two-pole breaker, which takes two spaces apiece, and plenty of older panels are already full. If you are adding a charger, our EV charger installation page explains how we check space as well as capacity.
And if the panel says Federal Pacific or Zinsco on the door, capacity is the second question. Both brands have a long record of breakers that can fail to trip on an overload, and they should come out before anything new goes in.
Common questions about 100-amp panels in Seattle
Can a 100-amp panel handle an EV charger?
Often, if the house heats with gas and the charger is set to 32 amps or less. A house that already has a heat pump with backup strips, or electric baseboard heat, usually needs load management or a service upgrade first.
Can a 100-amp panel handle a heat pump?
It depends mostly on the backup heat. In the example house above, a heat pump with no backup strips came out at about 85 amps, and the same house with 10 kilowatts of strips came out near 127.
Does an induction range need a 200-amp service?
Rarely on its own. In the code’s calculation for an existing house, most of the range’s rating lands in the group counted at 40 percent, so a typical range adds about 19 amps.
How do I find out my service size?
Look at the main breaker, or ask City Light for a service letter about your service size. The panel label alone can be misleading.
Who does the load calculation?
Your electrician. City Light does not do load studies for homeowners. If you are comparing residential electricians in Seattle, WA, ask each one whether they run this calculation before they quote you an upgrade.
Getting the answer for your house
We come out, look at the panel and the service, run the calculation with your actual appliances and the equipment you are planning, and tell you whether you are fine, close or over. The estimate is free and in writing, with no trip charge. If load management solves it, we say so instead of selling you an upgrade. You can read what we usually find in Seattle houses by neighborhood and build year on our Seattle page. Request an estimate or call (253) 508-7529.
About the author
Alexandru Goncear is the owner and CEO of ElectON Enterprise and a Washington-licensed electrician working on residential systems across King, Pierce and Snohomish County. He writes every estimate himself and stays the point of contact through permitting, the work and the final inspection.
ElectON Enterprise. Kent office, closest to Seattle: 1609 Central Ave S Suite E, Kent, WA 98032. Headquarters: 36007 9th Ct SW, Federal Way, WA 98023. Phone: (253) 508-7529. Monday to Friday, 8:00 AM to 5:00 PM, with emergency service 24 hours. Washington electrical contractor license ELECTEL765MN.