NEC 120.83 and 120.84 for the Journeyman Exam: Existing Dwelling Additions and the Multifamily Optional Method

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.

Where this fits

The standard method and the single dwelling optional method are on the service and feeder load calculations page. This page covers the other optional calculations in the same part of Article 120:

Section	Use it when
120.83	You add load to one existing dwelling unit and need to know if the service or feeder can carry it
120.84	Three or more dwelling units share one service or feeder
120.85	Two dwelling units share one feeder
120.86	A school building service or feeder
120.87	The existing load can come from measured demand instead of a calculation
120.88	A new restaurant service or feeder

On exam day, the hard part is choosing the method. Read the question for “existing”, “three or more units”, “school” or “restaurant”, then go to that section.

120.83: adding load to an existing dwelling

The method is for an existing dwelling unit served by a 120/240 V or 208Y/120 V, 3-wire service or feeder. Start with the existing connected load: general lighting and general-use receptacles at 2 VA per square foot, 1,500 VA for each 20 ampere small appliance and laundry circuit, and the nameplate rating of the fixed appliances, ranges, ovens, dryers and water heaters.

Load	Percent in Table 120.83
First 8 kVA of existing and new load	100 percent
Existing load, remainder	40 percent
New EVSE	80 percent
New central electric resistance space heating	80 percent
All other new loads, including heat pumps	50 percent

The 2023 edition split the method by whether the new load was air conditioning or space heating and counted that load at 100 percent. The 2026 table replaces that split, so older study guides give a different answer. In the examples below the existing load alone is more than 8 kVA, so the first 8 kVA is covered by existing load and each new load is counted at its own row’s percentage. Add the applicable loads and convert the total to amperes. Compare the result with the existing service or feeder rating, then check the conductor and overcurrent protection requirements before concluding that the addition can be supplied. Check the exact rows and notes of Table 120.83 in your code book.

Worked example: an EV charger or an electric furnace on a 100 A service

A 1,800 square foot house has a 100 A, 120/240 V service, gas heat and no air conditioning.

Existing load.

1. Lighting. 1,800 × 2 = 3,600 VA.
2. Small appliance and laundry circuits. 3 × 1,500 = 4,500 VA.
3. Appliances. 12 kW range + 5 kW dryer + 4.5 kW water heater = 21,500 VA.
4. Existing total. 3,600 + 4,500 + 21,500 = 29,600 VA.
5. Table 120.83. 8,000 + (29,600 – 8,000) × 0.40 = 8,000 + 8,640 = 16,640 VA.

Option A: a new 7,200 VA EVSE (30 A at 240 V).

1. 7,200 × 0.80 = 5,760 VA.
2. 16,640 + 5,760 = 22,400 VA, and 22,400 ÷ 240 = 93.3 A. The calculated load is below the 100 A service rating, so the addition is supported by this load calculation. The installation still has to meet the conductor, overcurrent protection and EVSE requirements.

Option B: a new 10 kW central electric furnace instead.

1. 10,000 × 0.80 = 8,000 VA.
2. 16,640 + 8,000 = 24,640 VA, and 24,640 ÷ 240 = 102.7 A. The calculated load exceeds the 100 A service rating, so this furnace cannot be supported under the example’s assumptions without reducing the calculated load or increasing the service capacity. A different heating system needs its own calculation.

120.84: the multifamily optional method

The method can be used when all of these are true:

• Three or more dwelling units are on the same service or feeder.
• No dwelling unit is supplied by more than one feeder.
• Each unit has electric cooking equipment, and electric space heating or air conditioning or both. For units without electric cooking, an exception lets you compare the standard calculation with this method using an assumed 8 kW of cooking per unit and use the lesser result.

For each unit, include general lighting and general-use receptacles at 2 VA per square foot, 1,500 VA for each required small appliance and laundry circuit, the nameplate loads of appliances and cooking equipment, clothes dryers as 120.84 specifies, permanently connected motor loads not otherwise included, and the larger of the air conditioning or fixed electric space heating load. Then add all units together and apply one demand factor from the multifamily table.

Number of dwelling units	Demand factor
3 to 5	45 percent
6 and 7	44 percent
8 to 10	43 percent
12 and 13	41 percent
24 and 25	35 percent
62 and over	23 percent

This is part of the table. The full table uses ranges of unit counts, with some counts listed on their own rows. Find the row that covers the number of units in the question and use that row’s factor. House loads such as hallway lighting, laundry rooms and pumps are calculated separately with the standard method and added after the demand factor.

Worked example: a 10 unit building

Ten identical units, each 1,000 square feet, with a 12 kW range, a 5 kW dryer, a 4.5 kW water heater, a 1.2 kW dishwasher, a 7 kW electric heater and a 3.6 kVA air conditioner. The service is 120/240 V single phase. There are no house loads in this example.

1. Lighting. 1,000 × 2 = 2,000 VA.
2. Small appliance and laundry circuits. 3 × 1,500 = 4,500 VA.
3. Appliances. 12,000 + 5,000 + 4,500 + 1,200 = 22,700 VA.
4. Heating or cooling, larger value. 7,000 VA heat is larger than 3,600 VA air conditioning: 7,000 VA.
5. Connected load per unit. 2,000 + 4,500 + 22,700 + 7,000 = 36,200 VA.
6. All units. 36,200 × 10 = 362,000 VA.
7. Demand factor for 10 units. 43 percent: 362,000 × 0.43 = 155,660 VA.
8. Amperes. 155,660 ÷ 240 = 648.6 A calculated load.

The neutral is calculated separately under the standard method rules; this method sizes the ungrounded conductors.

120.85 to 120.87 in one line each

• Two dwelling units (120.85). If the standard calculation for two units comes out higher than the optional calculation for three identical units, the lesser of the two may be used (NEC 120.85).
• Schools (120.86). An optional method based on load per square foot with stepped demand factors.
• Existing loads from measured demand (120.87). Use the highest demand recorded over one year, or a continuous recording over at least 30 days under the stated conditions, at 125 percent, then add the new load. The result still has to fit the conductors and the overcurrent protection.

Check yourself

1. Which Article holds the dwelling load calculations in the 2026 NEC? Article 120.
2. In Table 120.83, what percent applies to a new EVSE? 80 percent. The first 8 kVA of existing and new load is 100 percent, and the rest of the existing load is 40 percent.
3. What is the minimum number of units for the multifamily optional method? Three.
4. In the multifamily method, are house loads part of the demand factor? No. They are calculated separately and added.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Article 120 Part IV