Existing Dwelling Load Calculator
Check whether an existing dwelling service can carry an added load, per the NEC 220.83 optional method. Enter the existing and new loads, pick whether you are adding air conditioning or heat, and get the calculated load in amperes, the recommended service size, and the headroom against your existing service, with every line of the demand calculation shown.
How to use this calculator
- Enter the floor area (3 VA/ft²), the number of small-appliance and laundry circuits, and the existing service rating you are checking against. Pick 120/240 V or 208Y/120 V for the amp divisor.
- Enter nameplate VA (or watts) for every dedicated-circuit appliance, existing and new: range, dryer, water heater, dishwasher, EVSE, and so on. Fold small gas-appliance electrical loads into "other."
- Enter the air-conditioning and electric-heat nameplates. Only the larger is counted (NEC 220.60 noncoincident) and the tool shows which governed.
- Choose whether AC or heat is being added. "No" runs 220.83(A) (all loads through the 8 kVA/40% split); "Yes" runs 220.83(B) (HVAC at 100% separately).
- Read the calculated amps, the recommended standard service size, and whether the load fits the existing service. The breakdown documents the demand split for permit paperwork.
NEC reference
This calculator implements NEC 2020 220.83 (Optional Calculation for Additional Loads to an Existing Dwelling Unit) for a dwelling served by a 120/240 V or 208Y/120 V, 3-wire service. The connected load follows the 220.82(B) items: 3 VA/ft² general lighting, 1500 VA per small-appliance and laundry circuit (minimums two and one per 210.11(C)), and appliance nameplates. Under 220.83(A) all loads are taken at 100% of the first 8 kVA plus 40% of the remainder; under 220.83(B) the governing air-conditioning or space-heating load is taken at 100% and every other load at 100% of the first 8 kVA plus 40% of the remainder. Air conditioning and electric heat are treated as noncoincident loads per 220.60 (larger only). Recommended service ratings are the NEC 240.6(A) standard sizes.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Worked example: 1500 ft² home, adding a load (220.83(A))
A 1500 ft² dwelling with a 12 kW range, 5.5 kW dryer, 4.5 kW water heater, 1.2 kW dishwasher, and a 4 kW air conditioner (gas heat), checking a new load against an existing 100 A service. No AC or heat is being added, so 220.83(A) applies and every load rides through the 8 kVA/40% split.
| Line | VA |
|---|---|
| General lighting: 1500 ft² × 3 VA | 4,500 |
| Two small-appliance circuits × 1500 VA | 3,000 |
| Laundry circuit | 1,500 |
| Range 12 kW + dryer 5.5 kW + water heater 4.5 kW + dishwasher 1.2 kW | 23,200 |
| Air conditioning (larger of AC 4 kW and heat 0) | 4,000 |
| Combined load | 36,200 |
| First 8 kVA @ 100% + 28,200 @ 40% (11,280) | 19,280 |
| Calculated load | 19,280 |
19,280 VA ÷ 240 V = 80.3 A → 90 A minimum standard rating. Against the existing 100 A service that fits, with 19.7 A of headroom. These are the numbers the calculator produces with its default inputs, so you can check the tool against the worked example directly. Switch "adding AC or heat" to Yes and the 4 kW AC jumps out of the 40% pool to a full 100%, raising the calculated load.
Frequently asked questions
Why 8 kVA here but 10 kVA in the 220.82 tool?
The two optional methods use different breakpoints. 220.82 (a whole new dwelling service) takes the first 10 kVA of general load at 100%; 220.83 (an addition to an existing dwelling) takes the first 8 kVA at 100%. Everything above the breakpoint is taken at 40% in both. The 8 kVA figure is the headline difference, and it makes 220.83 slightly more conservative on the general load.
Why is only the larger of AC or heat counted?
Air conditioning and electric space heating are the classic noncoincident pair: a house does not cool and resistance-heat at the same moment. NEC 220.60 permits using only the larger of two loads unlikely to run together, so the smaller of AC and heat is dropped. Counting both would inflate the calculated load and can turn a service that is actually adequate into a phantom upgrade.
Does 220.83 apply a 65% heat factor like 220.82?
No. The 220.82(C) diversity factors (65% central heat, 40% for four or more separately controlled units) do not appear in 220.83. Under 220.83(B) the governing air-conditioning or heating load is taken at a flat 100%. This tool therefore does not offer the 220.82 heat-type factors, and a home with four or more separately controlled heating units in the case (B) scenario should be run by the standard method instead.
Can I use this for a 200 amp panel?
Yes. Enter 200 as the existing service rating. 220.83 is a capacity check against whatever service you already have; the method is the same at 100, 125, 150, or 200 A. The tool floors nothing at 100 A (unlike the 220.82 optional method), since 220.83 is about an existing service of any standard size.
Related calculators
- Electrical Load Calculator: the NEC 220.82 optional method for sizing a whole new dwelling service.
- Residential Electrical Load Calculator: the NEC Article 220 Part III standard method, category by category.
- Service Size Calculator: turn the calculated load into a service disconnect, conductors, and GEC.
- Neutral Load Calculator: the feeder or service neutral load per NEC 220.61.
Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
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For deeper NEC training on load calculation and service sizing, Mike Holt's NEC courses are the industry standard.
Working from the printed code? NEC code books and tab sets are available from Mike Holt.
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