Two-Family Dwelling Load Calculator
Size a duplex feeder or service under NEC 220.85. Enter one dwelling unit's loads and the calculator runs both calculations the section requires, the Article 220 Part III standard method on the two real units and the 220.84 optional method on three identical units, then reports the lesser of the two in amperes with the standard service rating that covers it.
How to use this calculator
- Enter the floor area of one dwelling unit and its branch circuits. Both units are treated as identical, which is what 220.85's optional side requires.
- Enter the range in kW and the dryer in VA, one of each per unit. These two loads are where the methods disagree most, so the calculator reports each side's treatment of them separately.
- Enter the other fastened-in-place appliances and how many there are. The count drives the 220.53 seventy-five percent factor on the standard side; the optional side takes the same nameplate at 100 percent.
- Enter air conditioning and electric heat. Only the larger counts on either side. Add the largest motor on the service: that field is the basis for the 220.50 twenty-five percent adder on the standard side, not a load, so the motor's own nameplate belongs under air conditioning or the other fixed appliances as well.
- Leave the three 220.84(A) conditions checked if the building meets them. Uncheck any one and the optional side is marked unavailable, so the standard result governs even when it is the larger number. The calculator also checks two of the three against what you entered, so a checked cooking condition with no range still marks the optional side unavailable.
NEC reference
This calculator implements NEC 2020 220.85 (Two Dwelling Units), which permits the lesser of two calculated loads where two dwelling units are supplied by a single feeder. The first is the standard calculation of Article 220 Part III applied to the two units: 3 VA/ft² general lighting per Table 220.12, 1500 VA per small-appliance and laundry branch circuit per 220.52, the Table 220.42 lighting demand factors, 220.53 for fastened-in-place appliances, 220.54 for dryers, Table 220.55 Column C for household cooking appliances, 220.60 for noncoincident heat and air conditioning, and 220.50 for the largest motor. The second is the optional multifamily calculation of 220.84 applied to three identical units, with every load at 100 percent nameplate per 220.84(C), the 45 percent Table 220.84 factor for three to five units, and any house load added after the factor per 220.84(B). Service disconnect minimum per 230.79(D) and standard ratings per 240.6(A). Section 220.85 and Table 220.84 are unchanged in the 2017, 2020 and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Worked example: all-electric duplex, 1200 ft² units
Two identical 1200 ft² units on one 120/240 V service, each with a 12 kW range, a 5.5 kW dryer, a 4.5 kW water heater, a 1.2 kW dishwasher, 1.1 kW of other fixed appliances, a 5 kW air conditioner and an 18 kW electric furnace. The largest motor on the service is 5 kW. These are the calculator's default inputs, so you can check the tool against the arithmetic directly.
Side one: Article 220 Part III, the two real units
| Line (both units) | VA |
|---|---|
| General lighting: 2400 ft² × 3 VA, plus four small-appliance and two laundry circuits at 1500 VA | 16,200 |
| Table 220.42: first 3000 VA at 100%, remainder at 35% | 7,620 |
| Fastened-in-place appliances: 13,600 VA, eight of them, so 220.53 takes 75% | 10,200 |
| Two dryers at 5500 VA (220.54, above the 5000 VA minimum) | 11,000 |
| Two 12 kW ranges: Table 220.55 Column C, two appliances | 11,000 |
| Heat governs over A/C (220.60): 2 × 18 kW | 36,000 |
| Largest motor, 25% of 5000 VA (220.50) | 1,250 |
| Standard calculated load | 77,070 |
77,070 VA ÷ 240 V = 321.1 A, which needs a 350 A service.
Side two: NEC 220.84, three identical units
| Line (per dwelling unit, 100% nameplate) | VA |
|---|---|
| General lighting: 1200 ft² × 3 VA | 3,600 |
| Two small-appliance circuits and one laundry circuit at 1500 VA | 4,500 |
| Range 12 kW + dryer 5.5 kW + water heater 4.5 kW + dishwasher 1.2 kW + other 1.1 kW | 24,300 |
| Electric heat, larger than the 5 kW A/C (220.84(C)(5)) | 18,000 |
| Connected load per unit | 50,400 |
| × 3 identical units | 151,200 |
| Table 220.84 demand factor, 3 to 5 units: 45% | × 0.45 |
| Optional calculated load | 68,040 |
68,040 VA ÷ 240 V = 283.5 A, which a 300 A service covers. That is the lesser of the two, so NEC 220.85 permits it: 9,030 VA and one full standard service size below the Part III answer. Note where the two sides diverge. Cooking costs 11,000 VA on the standard side and 36,000 VA of nameplate on the optional side, and the optional side still wins, because the 45 percent factor lands on everything at once while Table 220.42 only discounts general lighting.
Why the answer is not always the optional method
Take a small duplex where cooking is most of the load: 600 ft² units, two small-appliance circuits and a laundry circuit each, a 12 kW range, no dryer, no other fixed appliances, and a 3 kW air conditioner. The standard side comes to 6,360 VA of lighting after Table 220.42, plus 11,000 VA of cooking from Column C, plus 6,000 VA of air conditioning, for 23,360 VA, or 97.3 A on a 100 A service. The optional side carries three 12 kW ranges at full nameplate, so its per-unit connected load is 21,300 VA, its three-unit total is 63,900 VA, and 45 percent of that is 28,755 VA, or 119.8 A on a 125 A service.
The standard method is the lesser one here, and 220.85 offers nothing. Reaching for the 67.5 percent shortcut without running the Part III side would have bought a service size that is not needed. The rule is a comparison for a reason.
Frequently asked questions
Where does the 67.5 percent figure come from?
Table 220.84 gives 45 percent for three to five dwelling units. Applying it to three identical units and then comparing against the two units actually present is the same as taking 67.5 percent of the two-unit connected load, since 0.45 times 3 divided by 2 is 0.675. The calculator reports that effective factor whenever the optional side governs. It is a useful sanity check, not a substitute for the comparison, because the two sides count the underlying loads differently in the first place.
What if one unit has gas cooking?
Then 220.84(A)(2) is not met, the optional side is not available, and the Part III standard calculation governs. Uncheck the electric cooking condition and the calculator marks the optional figure unavailable while still showing it, so you can see what the comparison would have produced. The 220.84(A)(2) exception, which compares a building without electric cooking against the same load plus 8 kW of cooking per unit, needs a full Part III calculation of the alternative and is not performed here.
Does this size the feeder to each unit?
No. 220.85 sizes the single feeder or service that supplies both units. The feeder to one apartment is an ordinary single dwelling unit calculation, so use the 220.82 optional method or the Part III standard method for that. Once you have the calculated load here, the service conductors, disconnect and grounding electrode conductor come from the service sizing tool below.
Why is Column C used for every range rating?
Table 220.55 Notes 2 and 3 permit other treatments: Note 2 for ranges of unequal rating, Note 3 for the Columns A and B option on appliances rated under 8.75 kW. This calculator uses Column C at every rating, which is the conservative choice and never undersizes, and applies Note 1 above 12 kW. If your two ranges differ in rating, run Note 2 by hand.
Does the calculation work on 208Y/120?
Not in this tool. The comparison has to run both sides on one system voltage, and the Part III engine behind the standard side is 120/240 V single-phase, so this calculator is 240 V. A duplex fed from a 208Y/120 network service is calculated the same way in VA; divide the governing VA figure by 208 rather than 240 to get the amperes.
Related calculators
- Multifamily Dwelling Load Calculator: the NEC 220.84 optional method for three or more units, which is the second half of this comparison.
- Residential Electrical Load Calculator: the Article 220 Part III standard method, which is the first half.
- Electrical Load Calculator: the NEC 220.82 optional method for a single dwelling unit feeder or service.
- Service Size Calculator: turn the calculated load into a service disconnect, conductors, and grounding electrode conductor.
- Neutral Load Calculator: the feeder or service neutral load per NEC 220.61.
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