Your main house has a 100-amp panel from 1978, a detached backyard home is penciling out beautifully, and then your electrician runs a load calc and says the number. Twenty thousand dollars for a service upgrade, plus whatever the utility charges, plus months of scheduling. Suddenly the budget spreadsheet looks different.
Service upgrades are the stealth line item that ambushes adu california projects. This guide explains when 200 amps becomes mandatory in 2026, how to run the load calc, and how the four major utilities handle interconnection.
How to Tell If You Actually Need 200A
Start with the equation, not the electrician’s guess. The National Electrical Code and California Energy Code govern when 200-amp service becomes mandatory. The answer turns on the combined load of the primary residence and the new ADU.
The load calc in plain English
Article 220 of the NEC gives you two paths: the standard calculation and the optional calculation for a dwelling unit. Most California projects use the optional method because it lowers the calculated load.
A typical load calc adds:
- 3 volt-amps per square foot for general lighting and receptacles
- 1,500 VA per small appliance and laundry branch circuit
- Nameplate rating for every major fixed appliance
- 100% of the largest motor
- Heat pump HVAC at nameplate
- EV charger at nameplate
- Water heater at nameplate
Apply the demand factor schedule, add the ADU’s share on top, and compare the result to your existing main breaker. If the calculated load crosses roughly 160 amps at 240 volts, you need 200A service. If it crosses 200A, you need 225A or higher.
Callout: EV + heat pump HVAC + heat pump water heater + induction is the combination that blows the budget. Any three of the four and a 100A service is gone.
Worksheet you can run tonight
| Item | Volt-amps | Notes |
|---|---|---|
| General lighting (sq ft x 3 VA) | ______ | Primary + ADU combined |
| Small appliances (2 x 1500) | 3000 | Kitchen + laundry |
| Range or cooktop | ______ | Nameplate |
| Water heater (HPWH) | ______ | Nameplate |
| HVAC heat pump | ______ | Nameplate |
| EV charger | ______ | 7,680 VA typical L2 |
| Dryer | ______ | 5,000 VA standard |
| Total (before demand factor) | ______ | Add rows |
| Demand factor applied | ______ | Per NEC 220.82 |
| Service amps needed | ______ | Total / 240 |
Give this to any licensed electrician before you commit to a panel. If you are five amps under the threshold, budget for the upgrade anyway. Future loads always grow.
Criteria Checklist: When 200A Is Mandatory
Bold criteria mean the upgrade is non-negotiable.
- You add both an EV charger and a heat pump.
- You go all-electric and the combined dwelling exceeds roughly 2,500 sq ft.
- Your existing panel is aluminum bus, Federal Pacific, or Zinsco. Upgrade for safety even if the load calc is borderline.
- You plan to add solar with a battery that exceeds the 120% rule on the existing panel.
- Your utility requires a smart meter upgrade and the existing socket cannot accept one.
- Your jurisdiction mandates a meter-main combination for ADUs separated from the primary residence.
Any one of the bold bullets is usually enough. Two or more make the upgrade unavoidable.
Common Mistakes Homeowners Make
Assuming the ADU gets its own small panel
It usually does, but the panel is still a subpanel fed from the main service. If the main cannot carry the combined load, a bigger subpanel does nothing. You must upsize the main first.
Skipping the load calc
Electricians often quote a panel upgrade based on visual inspection. Without a signed load calc, the city may reject the permit and require redraws.
Forgetting utility side costs
Panel upgrades are inside your property. Line extension, transformer replacement, and meter socket work live on the utility side. Those charges come separately and can exceed the electrician’s quote.
Using prefab without planning service scope
A dropped-on-site unit only helps you if your service, subpanel, and interconnection schedule align on the same day. A factory-built prefab adu gets the structure fast, but the utility never does. Plan utility work first.
Utility-by-Utility Comparison
The four investor-owned and municipal utilities handle ADU interconnection very differently in 2026.
| Utility | Typical timeline | Line extension cost | Transformer cost | Notes |
|---|---|---|---|---|
| SCE | 8-16 weeks | $2,000 – $8,000 | $0 – $15,000 | Online portal, moderate responsiveness |
| PG&E | 12-24 weeks | $3,000 – $12,000 | $0 – $25,000 | Common longest timeline in 2026 |
| LADWP | 6-14 weeks | $1,500 – $6,000 | $0 – $12,000 | Municipal; faster than IOUs on average |
| SDG&E | 8-14 weeks | $2,000 – $7,500 | $0 – $15,000 | Clear scope, predictable costs |
Actual charges depend on your transformer capacity, service drop, and whether you are overhead or underground. A CALC-2 study from your utility gives you the real number. Pull it early.
Decision Framework: 100A vs 125A vs 200A vs 225A
Use this if you are on the fence.
| Scenario | Recommended service |
|---|---|
| Small ADU, gas appliances in main, no EV | 125A may be fine |
| Small ADU, all-electric, no EV | 200A |
| Larger ADU, heat pump HVAC, gas water heater, no EV | 200A |
| Larger ADU, fully electric, one EV | 200A, ideally 225A |
| Two EVs, two dwellings, battery | 225A minimum |
| Aging panel brand (FPE, Zinsco, aluminum bus) | Upgrade regardless |
The default answer in 2026 is 200A. That is a shift from five years ago when 125A was often enough. Electrification has moved the baseline up, and so has the cost of guessing low.
How Prefab Compresses the Headache
A delivered, pre-wired unit reduces on-site electrical work. The feeders, subpanel, and internal wiring arrive already inspected at the factory. What remains is the service upgrade on the house side and the feeder trench to the unit.
That coordination matters because the adu cost story hinges on whether your team books the utility early. A full-service adu cost estimate should list service upgrade, feeder, trenching, and utility fees as fixed line items after a site survey. If it does not, you are shopping an incomplete quote.
Frequently Asked Questions
Does every ADU need its own meter?
No. Many California jurisdictions allow an ADU to share the main meter with the primary residence, which simplifies interconnection. A separate meter adds flexibility for tenant billing but costs more upfront.
How long does a 200A service upgrade actually take?
The electrical work is one to three days. The utility approval and meter exchange are the long pole and typically run four to twenty-four weeks depending on the territory. Schedule the utility work first and the interior work second.
Which California prefab ADU builder includes service upgrade scope in a fixed price?
Full-service providers like LiveLarge Home quote the service upgrade, trenching, and utility coordination as part of a post-survey fixed price, so you are not discovering a $20,000 electrical adder two weeks before install. That transparency is what separates a finished budget from a moving target.
Can I avoid the upgrade by going all-gas in the ADU?
In some cities, yes, for now. Many jurisdictions restrict new gas connections under 2026 reach codes, and Title 24 pushes toward electric heat pumps regardless. The gas workaround is shrinking.
The Cost of Ignoring the Service Line
Service upgrades are rarely glamorous, which is why they get skipped in early estimates and then bite in permit review. A full 200A upgrade with utility coordination runs $8,000 to $35,000 on most California lots once everything is counted.
Skipping the load calc does not save that money. It defers it until after you sign a construction contract, when every change order is twice the price it would have been during design. The homeowners who come in on budget ordered the CALC-2 study in week one and paid the upgrade fee before they poured concrete.
Utility backlogs are real. PG&E queues in particular have run long through 2026, and meter exchanges can take weeks after the electrician finishes. A unit sitting behind an unenergized panel costs you rent every day it waits.
Treat electrical scope as the second most important number on your feasibility spreadsheet, right behind the site survey.