NEC Load Calculator — Service Size (Optional Method)
Who this is for: For electricians quoting panel upgrades and homeowners asking 'will 100 A carry a heat pump and an EV?' — the numbers to bring to the permit counter.
Enter square footage and major appliances — get the demand load in VA and the service size (100/125/150/200 A) the numbers support.
Quick answer: NEC 220.83 optional method: general lighting 3 VA/sqft + 1,500 VA × (small-appliance + laundry circuits); the first 8 kVA counts at 100% and the remainder at 40%; fixed appliances, HVAC and EV charging add at 100%. Divide total VA by 240 V and round up to the standard service — a typical 2,000 sq ft all-electric house lands near 35,500 VA → 150 A.
sqft
Excludes open porches/garages
×
20 A circuits, min 2
×
min 1
W
0 if gas
W
0 if gas
W
0 if gas
W
They don't add
W
Counts at 100%
W
Spa, shop tools…
Total demand
35,500 VA
after demand factors
Design current
148 A
÷ 240 V
Minimum service
150 A
next standard size
How it added up
General lighting (3 VA/sqft)6,000 VA
Appliance circuits (2×1500 + 1×1500)4,500 VA
After demand (8 kVA @ 100% + remainder @ 40%)9,000 VA
Fixed loads @ 100%26,500 VA
Total35,500 VA
EV charging always counts at 100% — it’s the one load the demand factors don’t forgive. If this total pushed you a service size up, an energy management system (EMS) load-shedding the EV may legally keep the smaller panel; check 220.87 and your AHJ.
Simplified NEC 220.83 optional method: first 8 kVA of lighting + appliance circuits at 100%, remainder at 40%; fixed loads added at 100%. Verify against the full article for permit submittals.
Core facts
Lighting
3 VA / sqft (Table 220.42 basis)
Circuit allowances
1,500 VA each: small-appliance (min 2) + laundry
Demand factor
First 8 kVA @ 100%, remainder @ 40%
Service sizes
100 · 125 · 150 · 200 · 225 · 400 A
Example
2,000 sq ft electric-heavy home → 35,500 VA → 150 A
Status
Figures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code.
How does the optional method work?
NEC 220.83 (the optional method for dwellings) trims the raw numbers: general lighting at 3 VA per square foot, plus 1,500 VA for each 20 A small-appliance circuit (minimum two) and the laundry circuit. The first 8 kVA of that subtotal counts at 100%, everything above at 40% — that demand factor is why a 2,000 sq ft house doesn't calculate at full face value. Fixed appliances (range, water heater, dryer), the larger of heating vs cooling, EV charging and any other hard-wired loads then add at 100% in this simplified view. Total VA ÷ 240 V gives the design current, which lands on the next standard service: 100, 125, 150, 200, 225 or 400 A. Example: 2,000 sq ft with a 12 kW range, 4.5 kW water heater, 5 kW dryer and 5 kW HVAC calculates to about 35,500 VA → 148 A → a 150 A service.
Common uses
Deciding 100 A vs 200 A before a panel replacement quote
Checking headroom for an EV charger or heat-pump conversion
Preparing the load calc sheet inspectors ask for with a service change
Comparing two appliance packages (gas vs electric range and dryer)
Frequently Asked Questions
Optional method vs standard method?
Article 220 offers both. The optional method (220.82 for one dwellings, 220.83 for existing) applies demand factors and is faster; the standard method (220.14 etc.) itemizes every load. Inspectors accept either when applied correctly; the optional method usually gives a smaller service.
Why only 40% above 8 kVA?
Diversity. General lighting and receptacle circuits never all run at once, and decades of load studies back the demand factors. The 8 kVA/40% split in 220.83 encodes that reality so a dwelling isn't billed a service sized for a scenario that never happens.
Do heating and cooling add together?
No — take the larger of the two (they don't run simultaneously). Enter only the bigger of your AC tonnage vs heat strips/furnace in the HVAC field. EV charging, by contrast, always counts at 100%.
Is this valid for an ADU or shop subpanel?
A dwelling ADU can use 220.82/220.83 for its own calc. A detached shop is not a dwelling — its feeder follows the general method in Article 220 Part II, and the numbers come out differently.
My calc says 148 A. Do I need 150 A?
You round up to the next standard service size: 150 A. You never round down — 148 A of calculated demand on a 125 A service is an automatic fail at inspection, and it runs the main breaker hot.