The Three Service Sizes
Residential electrical service in MA falls into three primary sizes:
- 200-amp service
- Standard for new MA construction since the 1990s. Single 200A main breaker. 30-42 breaker spaces in a typical panel. Service feeder: 4/0 AWG aluminum or 3/0 AWG copper. Sufficient for typical 2,000-3,000 sq ft single-family with full electrification.
- 225-amp service
- Bridges 200A to 400A. Common when calculated load is 180-200A but doesn't justify the 400A premium. Same physical panel as 200A in many manufacturer lines (Square D QO, Eaton CH); only the main breaker is upsized. Service feeder upsizes to 250 MCM aluminum or 4/0 copper.
- 400-amp service
- Two 200A main breakers (or single 400A main + sub-feeds). 600-800 MCM aluminum service feeder. Required for: large homes 3,500+ sq ft, multi-EV households (3+ chargers), homes with ADU on same service, properties with hot tub + sauna + EV combo loads.
The NEC Article 220 Load Calculation
NEC Article 220 (the dwelling unit load calculation) determines required service size. The calculation uses demand factors that reduce nameplate sums to expected actual demand:
| Load Type | Demand Factor | Notes |
|---|---|---|
| General lighting + small appliance | 3 VA/sq ft + 3,000 VA | 40% demand on first 3,000 VA, 35% on remainder up to 120,000 |
| Cooking equipment (range, oven) | 40% of nameplate | Per NEC Table 220.55 |
| Dryer | 5,000 VA or nameplate, whichever greater | Demand factor 100% for first dryer, 75% for second |
| Heat pump heating (continuous) | 100% of nameplate | NEC 625.41 — no demand factor for continuous loads |
| Cooling (AC or heat pump cooling) | 100% of nameplate | Compare heating vs cooling; use larger |
| Water heater (electric tank) | 100% of nameplate | HPWH typically 5A continuous |
| EV charger | 100% of nameplate | Continuous load, no demand factor allowed |
| Other fixed appliances (4+) | 75% of nameplate | Per NEC 220.53 |
Three Worked Examples
The math becomes concrete with example homes:
- Example A: 1,800 sq ft Cape with full electrification
- Heat pump (3 ton): 25A heating / 21A cooling → use 25A · HPWH: 5A · Range (induction): 33A after demand factor · Dryer (electric): 22A after demand factor · EV (40A continuous): 40A · General + small appliance: 24A · Total: 149A → 200A service sufficient.
- Example B: 2,800 sq ft Newton colonial with EV + planning second EV
- Heat pump (4 ton): 32A · HPWH: 5A · Range (induction): 35A · Dryer: 22A · EV #1 (48A): 48A · EV #2 planned (40A): 40A · General + small appliance: 32A · Total: 214A → 225A service required (400A overkill).
- Example C: 4,200 sq ft Lexington home with hot tub + sauna + 2 EVs + ADU
- Heat pump (5 ton primary + 1 ton ADU): 38A + 8A · HPWH × 2: 10A · Range (induction): 40A · Dryer: 25A · EV #1 (48A): 48A · EV #2 (40A): 40A · Hot tub: 50A · Sauna: 30A · ADU mini-split: 12A · ADU range/dryer: 22A · General + small appliance: 48A · Total: 371A → 400A service required.
Cost Differences Among the Three Sizes
The cost spread among 200A, 225A, and 400A is smaller than most homeowners assume:
| Service Size | Panel Upgrade Cost | Net After $4K Mass Save |
|---|---|---|
| 200A standard | $2,400-$3,800 | $0-$0 (rebate exceeds or matches) |
| 225A | $2,800-$4,200 | $0-$200 |
| 400A (single 400 main) | $5,800-$8,400 | $1,800-$4,400 |
| 400A (split into two 200A panels) | $5,400-$7,800 | $1,400-$3,800 |
The 200A vs 225A delta is small ($200-$400 net), and 225A buys substantial future-proofing for the cost. The 400A jump is much more significant — typically only justified by load calc, not by 'future-proofing' speculation.
Future-Proofing vs Solving Now
Two patterns to avoid:
- Speculative oversizing: 'I might add a hot tub someday' isn't a load calculation. NEC Article 220 calcs based on actual planned loads. If you're not committed to the hot tub, don't size for it. The Mass Save rebate is the same regardless of service size — there's no upside to oversizing.
- Undersizing to today's load: If full electrification is planned in next 24 months (per the electrification roadmap), include all planned electrification loads in today's panel calc. Don't leave the panel at 100A and plan to upgrade in 18 months — the upgrade cost is essentially the same now or later, but the disruption (one project vs two) is significantly lower if done together.
Right-size for: today's actual loads + planned electrification within 24 months + 15-25% growth allowance. That math typically lands at 200A for most MA homes, 225A for borderline cases, and 400A only when explicitly required.
Frequently Asked Questions
How long does a panel upgrade take in Massachusetts?
200A standard upgrade: 6-8 hours of labor + utility coordination. The home is without power for the install window (typically 4-6 hours) — Eversource or National Grid disconnects service at the meter, the electrician swaps the panel, the utility reconnects. Total project from quote to completion: 3-5 weeks (lead time on electrician scheduling + utility coordination).
Do I need permission from Eversource or National Grid for a service upgrade?
Yes — service feeder size changes require utility approval and meter coordination. The licensed electrician handles the utility application as part of the project. Utility lead time for service upgrade scheduling: 4-8 weeks typical. The panel installation can be completed in advance of the utility visit; the new service goes live on the day the utility installs the upgraded meter.
Can I add a sub-panel instead of upgrading the main panel?
Sometimes. If your existing panel is full (no available breaker spaces) but the service capacity is sufficient, a sub-panel ($800-$1,800) is a low-cost solution. If the service capacity itself is insufficient (calculated load exceeds main breaker rating), a sub-panel doesn't solve the problem — main service upgrade is required. Pro Build's load calc determines which path applies.
Is 100-amp service ever enough for a Massachusetts home?
Rarely in 2026. 100A handles a single-family home with gas heat + gas hot water + gas range + minimal electric loads. Adding any major electric appliance (EV charger, heat pump, induction range) typically pushes the load calc above 100A. Most MA homes still on 100A service are pre-1980 and have not been substantially renovated.
What's the lifespan of a residential electrical panel?
Modern panels (Square D QO, Eaton CH, Siemens) are rated for 30-40+ year service life with normal use. Older panels (Federal Pacific, Zinsco, FPE) have safety issues that warrant replacement regardless of age — these brands had documented breaker failures and are flagged on every MA home inspection.
Can I install a Tesla Powerwall on a 200-amp service?
Yes — Powerwall integrates as a separate sub-panel-fed system, not as a load on the main panel. The Powerwall's continuous discharge rating (5 kW per Powerwall, ~21A at 240V) is handled by its own circuit infrastructure. Multiple Powerwalls (up to 10 in a typical residential install) work the same way. 200A main service is sufficient for most Powerwall installations.
Does my MA panel need surge protection?
Per 2020 NEC adopted in 527 CMR, all new residential service installations require Type 1 or Type 2 surge protection. Surge protector adds $180-$420 to the panel install cost. Existing panels are not retroactively required to add surge protection, but Pro Build recommends adding one during any panel replacement or upgrade.
What happens if I exceed my panel's amperage capacity?
Best case: main breaker trips when total demand exceeds rating. Inconvenient but safe. Worst case: chronic over-current heats the panel bus and breakers, causing premature failure or fire risk. Modern panels with proper main breakers handle the trip-out safely. Old panels (pre-1990 with worn or improper breakers) may not. NEC Article 220 calc + correct service sizing prevents the issue.
References & Sources
- National Electrical Code Article 220 — Branch-Circuit, Feeder, and Service Calculations. https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70
- 527 CMR 12 — Massachusetts Electrical Code Amendments. https://www.mass.gov/regulations/527-CMR-12-massachusetts-electrical-code-amendments
- Mass Save panel upgrade rebate program. https://www.masssave.com/saving/residential-rebates
- Eversource — residential service upgrade procedure. https://www.eversource.com/content/residential/about/our-network
