Here’s a conversation we have most weeks in Mount Prospect and Arlington Heights. A homeowner wants a heat pump, or an EV charger, or an induction range — sometimes all three over a few years — and the first contractor through the door says “you’ll need a 200-amp service” before measuring anything.
Sometimes that’s true. Often it isn’t. There is an actual calculation behind the answer, it’s in the National Electrical Code, and a homeowner can follow the logic. Knowing it is worth thousands of dollars, because the panel upgrade is usually the largest single line item in an electrification project.
Two Ways to Size a Service
The standard calculation (NEC Article 220, Part III)
This is the method used on new construction and the one most electricians default to. You add up the general lighting and receptacle load based on square footage, the small-appliance and laundry circuits, and then every fixed appliance — range, dryer, water heater, HVAC, EV charger — applying the code’s demand factors. It’s conservative by design, because it has to work for a house nobody lives in yet.
On an older 1,600 sq ft ranch, the standard calculation frequently says you need more than 100 amps even before the heat pump, which is why it produces so many “you need 200 amps” answers.
The existing-dwelling calculation (NEC 220.87)
This is the one homeowners rarely hear about. For a home with at least a year of data, an electrician can use the measured maximum demand instead of the assumed load. Take the highest 15-minute demand over the past 12 months, multiply it by 125%, and add the new load you’re about to install. If that total fits inside your existing service, the service is adequate — by code, not by opinion.
In practice we get that data two ways: from utility interval data where available, or by installing a recording meter on the service for 30 days during a season with real load (mid-summer with the AC running, or mid-winter). The difference between assumed and measured is often dramatic. A house the standard method says is at 110 amps frequently measures a peak of 40–55 amps, because nobody runs the dryer, the oven, the AC, and the water heater simultaneously for 15 minutes.
What Actually Adds Load
| Addition | Typical added load | Notes |
|---|---|---|
| Cold-climate heat pump, 3 ton, dual fuel | ~15 – 22 A at 240 V | Gas furnace carries the deep cold; no electric strip heat |
| Same heat pump, all-electric with 10 kW strip backup | ~42 – 60 A | The strip heat, not the heat pump, is what forces the upgrade |
| Ductless mini-split, 1–2 heads | ~10 – 20 A | Often fits with no drama |
| Level 2 EV charger, 40 A circuit | 32 A continuous | Can be load-managed or dialed down to 24 A / 16 A |
| Heat pump water heater | ~12 – 15 A | Usually replaces a gas unit; also dehumidifies the basement |
| Induction range | ~30 – 40 A | Replaces a gas range; real added load |
| Electric dryer replacing gas | ~24 A | Heat pump dryers run on 120 V and add far less |
Look at the second row. Electric resistance backup heat is the single biggest driver of residential panel upgrades in this area. Choosing dual fuel — heat pump plus the existing gas furnace as backup — typically cuts the added load by two thirds and very often eliminates the panel upgrade entirely. That’s also the configuration that performs best in Chicago winters, so it isn’t a compromise.
Four Ways to Avoid the Upgrade
- Dual fuel instead of all-electric. As above. The biggest lever by far.
- Load management devices. UL 916 / NEC 750-listed energy management systems, DCC devices, and smart splitters let two large loads share one circuit by never allowing them to run at once — EV charger and dryer, or EV charger and range. Code explicitly permits this. Installed cost is usually $650–$1,800, against $2,400–$8,000 for a service upgrade.
- A lower-amperage EV circuit. A 24-amp charger adds roughly 18 miles of range per hour. Almost nobody needs more than that overnight.
- Retiring a load you no longer use. Old electric ranges, hot tubs that are gone, a shop circuit that feeds nothing. Removing dead load from the calculation is free.
When You Genuinely Do Need the Upgrade
Be skeptical of contractors who say you always need it — and equally skeptical of anyone who says you never do. Clear cases for a service upgrade:
- Measured peak plus new load exceeds the existing service, even after the mitigations above.
- 60-amp service, or a fuse panel. These are at end of life regardless of your electrification plans.
- A known-defective panel — certain Federal Pacific Stab-Lok and Zinsco/Sylvania panels have documented breaker failure-to-trip issues. Replace on safety grounds alone.
- No physical space left and no room for tandems or a sub-panel, with a bus that’s already fully loaded.
- Evidence of overheating: scorched bus, discolored breakers, a burning smell, breakers that are warm at rest.
- You’re planning all-electric conversion across the whole house on a defined schedule. Then do it once, properly.
2026 Costs, North and Northwest Suburbs
| Work | Installed cost |
|---|---|
| Documented load calculation with 30-day recorded demand | $350 – $750 (credited toward the work in most cases) |
| Panel replacement, same service size (100A → 100A) | $1,900 – $3,400 |
| Service upgrade to 200 A, overhead service | $2,400 – $5,500 |
| Service upgrade to 200 A, underground or long/complex run | $4,500 – $9,500 |
| Load management device (EV or appliance sharing) | $650 – $1,800 |
| Sub-panel to add circuit space | $1,200 – $3,200 |
| Level 2 EV charger circuit, short run | $900 – $2,200 |
Ranges reflect mast condition, meter relocation, village permit and inspection, and whether ComEd has to disconnect and reconnect. Mount Prospect and Arlington Heights both permit and inspect this work — anyone proposing to skip the permit is telling you something about the rest of their work.
What to Ask the Next Contractor Who Says “You Need 200 Amps”
- “Can you show me the load calculation?” A real answer is a sheet of paper with numbers, not a glance at the panel.
- “Did you use the standard method or the existing-dwelling method in 220.87?”
- “If we go dual fuel instead of all-electric, does the answer change?”
- “Would a load management device solve this for less?”
- “Is the panel itself a safety concern independent of capacity?”
If those questions get a shrug, get another quote. If they get clear answers, you’re probably dealing with someone who’ll do the install properly too.
Frequently Asked Questions
How do I know what service I have?
Look at the main breaker — the number stamped on it (100, 125, 150, 200) is the service size. In fuse panels, or where the main is stamped differently from the bus rating, you need an electrician to sort it out.
Is 100-amp service ever enough for a modern house?
Frequently, yes, in a gas-heated home with gas water heating and cooking. Add a dual-fuel heat pump and a load-managed EV charger and many 100-amp homes still measure well under capacity. Go all-electric and the answer flips.
Will the panel upgrade improve anything besides capacity?
Yes: modern breakers with AFCI/GFCI protection where required, proper grounding and bonding, labeled circuits, and a clean platform for whole-house surge protection. Homes with 1960s–80s panels usually get real safety improvements, not just amps.
Do you do the HVAC and the electrical, or do I need two companies?
Both, under one roof. That matters on heat pump projects specifically, because the load calculation and the equipment selection are the same decision. See our electrical services.
How long does a service upgrade take?
One day of work in most homes, plus utility coordination and inspection. Power is out for several hours. Permits typically add a few days to a couple of weeks depending on the village.
Serving Mount Prospect, Arlington Heights, and Nearby
MMDS handles load calculations, panel and service upgrades, EV circuits, and heat pump electrical across Mount Prospect, Arlington Heights, Palatine, Des Plaines, Wheeling, and Schaumburg.
Talk to MMDS
MMDS handles HVAC, electrical, and plumbing under one roof across the north and northwest suburbs. Call (847) 221-6280 or request a visit here. Financing is available through Sunbit and PowerPay, so a repair or upgrade can be spread into monthly payments instead of one hit to the bank account.