The Optional Method Load Calculation, Worked on One House

Why this matters

The most common service question a residential shop gets now is a stacked one: the customer is replacing a gas furnace with a heat pump and wants an EV charger, on a 150 A service. Run the standard method and the answer is often a service upgrade. Run the 220.82 optional method, which the code offers for exactly this kind of dwelling, and the answer is sometimes "fits, if the charger is set right." The difference to the customer is a service change or not. This card works one complete optional-method sheet so the method, and the place each number comes from, are on one page.

What the optional method is, and when you may use it

NEC 220.82, 2023 edition (2020 where the section moved), as adopted by your jurisdiction, is an alternative calculation for a single dwelling unit served by a 120/240 V three-wire (or 208Y/120 V three-wire) service or feeder with an ampacity of 100 A or more. It lumps almost everything into one "general load," gives the first 10 kVA of it full weight and the rest 40 percent, and then adds the heating or cooling load by its own rule. It calculates the ungrounded conductors; the neutral is still sized under 220.61.

It does not require new construction. An existing single-family house qualifies on the same terms. Article 220 also offers 220.83, a version written for existing dwellings, and 220.87, which works from a year of measured demand; both are compared at the end of this card.

The general-load pieces it starts from (3 VA per square foot, 1500 VA per small-appliance and laundry circuit) are defined on the standard-method card: see related 'Standard Method General Load Pieces for a Dwelling'. What the optional method changes is what it adds to them and how it discounts the total.

The house

An existing single-story house, 2,400 sq ft on the outside walls. 150 A service with 2/0 copper service-entrance conductors. Gas furnace being removed. Being added: a heat pump with 10 kW of electric strip heat in the air handler, and a 48 A EV charger. Existing electric loads: an 11.0 kW range, a 5,500 W dryer, a 4,500 W water heater, a 1,200 VA dishwasher, a disposal rated 8.0 A at 120 V, and a 1,500 VA over-range microwave. Two small-appliance circuits and one laundry circuit.

The heat pump's outdoor unit nameplate gives compressor RLA 16.0 A and fan FLA 1.2 A at 240 V. The air handler's blower is rated 4.3 A at 240 V.

The sheet

220.82 OPTIONAL METHOD - EXISTING 150 A SERVICE
NEC 2023 AS ADOPTED

(B) GENERAL LOAD
  lighting 2,400 sq ft x 3 VA                7,200
  small-appliance 2 x 1,500                  3,000
  laundry 1 x 1,500                          1,500
  range, nameplate                          11,000
  dryer, nameplate                           5,500
  water heater, nameplate                    4,500
  dishwasher                                 1,200
  disposal 8.0 A x 120 V                       960
  microwave-hood                             1,500
  EV charger 48 A x 240 V                   11,520
  general load total                        47,880
  first 10,000 at 100 pct                   10,000
  remaining 37,880 at 40 pct                15,152
  general load after demand                 25,152

(C) HEATING OR COOLING, LARGEST OF
  cooling: compressor 16.0 + fan 1.2 = 17.2 A
    x 240 V = 4,128 + blower 1,032          5,160
  heat pump + 65 pct of strips:
    5,160 + 10,000 x 0.65 = 5,160 + 6,500  11,660
  largest                                   11,660

TOTAL 25,152 + 11,660 = 36,812 VA
36,812 / 240 V = 153.4 A  against 150 A  OVER 3.4 A

Reading each block

The general load takes nameplates, not demand values. Under 220.82(B), the range goes in at its 11,000 W nameplate, not at the 8,000 VA a single range gets under the standard method's Table 220.55. The dryer goes in at 5,500 W with no 5,000 W floor to argue about. Every appliance fastened in place, permanently connected or on a specific circuit enters at its nameplate rating, and the EV charger is an appliance on a specific circuit, so its 11,520 VA (48 A times 240 V) goes in here. The 220.57 rule that counts EV equipment at 7,200 W or nameplate belongs to the standard method's feeder and service part, not to this sheet.

Then one demand factor for all of it. The first 10 kVA at 100 percent, everything above at 40 percent: 47,880 minus 10,000 is 37,880, times 0.40 is 15,152, plus 10,000 gives 25,152 VA. This single factor is why the method favors houses with many large appliances; it treats them as one population and assumes they do not all run together.

Heating and cooling are taken separately, at the largest of the listed options. 220.82(C) lists six cases in substance (check the current edition for the exact wording): 100 percent of air conditioning and cooling; 100 percent of a heat pump used with no supplemental electric heat; 100 percent of electric thermal storage or other heating expected to run continuously at full nameplate; 100 percent of the heat pump compressor plus 65 percent of the supplemental electric heat for a central system; 65 percent of electric space heating with fewer than four separately controlled units; and 40 percent with four or more. You compute every case that applies to the house and keep the largest.

On this house two cases apply. Cooling is the outdoor unit (16.0 plus 1.2 is 17.2 A, times 240 V is 4,128 VA) plus the air-handler blower (4.3 A times 240 V is 1,032 VA), for 5,160 VA. Heating is the same heat pump at 100 percent plus 65 percent of the 10 kW strips, 5,160 plus 6,500, for 11,660 VA. The blower is carried in both cases because it runs in both modes. Heating wins.

The total. 25,152 plus 11,660 is 36,812 VA. Divided by 240 V that is 153.4 A, against a 150 A service: over by 3.4 A.

What brings it under 150 A

Each of these changes a different input; each is a real option to put in front of the customer, and each is a design decision that belongs on the permit drawing.

• Set the charger to 32 A. Most chargers have a configured maximum, set by the installer and restricted from the user, which 625.42, as adopted, recognizes in substance as the rating for load calculation purposes. At 32 A the charger enters at 7,680 VA (32 times 240). The general total drops by 3,840 VA to 44,040; the remainder above 10,000 becomes 34,040, at 40 percent 13,616; general after demand 23,616. Plus 11,660 for heating gives 35,276 VA, which is 147.0 A. Under 150 A with 3.0 A to spare. Note the 3,840 VA cut in the charger saved only 1,536 VA in the total (36,812 minus 35,276), because the cut came off the 40 percent portion. A charger setting is a weak lever under this method; it worked here because the house was only 3.4 A over.
• Specify 5 kW strips instead of 10. 65 percent of 5,000 is 3,250, so heating becomes 5,160 plus 3,250, or 8,410 VA, still larger than the 5,160 VA cooling. With the charger back at 48 A, the total is 25,152 plus 8,410, or 33,562 VA, which is 139.8 A. This is a heating-design question for the HVAC contractor and the customer; in a cold climate the smaller strip package may not carry the house below the heat pump's balance point.
• Lock out the compressor when the strips run. 220.82(C), in substance, lets you leave the compressor out of the heating case where the controls prevent it from running at the same time as the supplementary heat. Most heat pumps run strips and compressor together in second-stage heat, so this applies only where the equipment is actually configured that way and the installation instructions confirm it. On this house it would make heating the blower plus 65 percent of the strips, 1,032 plus 6,500, or 7,532 VA, and the total 32,684 VA, or 136.2 A.

What does not bring it under: dropping the dishwasher off the sheet because "it hardly runs," or using the 8,000 VA standard-method range value. The optional method takes nameplates, and the 40 percent factor already accounts for how seldom things run.

The other two existing-dwelling paths

220.83, written for existing dwellings. In substance (check the current edition): where heating or air conditioning is being installed, it takes that equipment at 100 percent of nameplate and the rest of the load at 100 percent for the first 8 kVA and 40 percent above that. On this house the other load is the same 47,880 VA: 8,000 plus 39,880 times 0.40 (15,952) is 23,952 VA. Heating at full nameplate is the 5,160 VA heat pump and blower plus the full 10,000 VA of strips, 15,160 VA. The total is 39,112 VA, or 163.0 A. For an all-electric heat addition with strip heat, 220.83 usually comes out higher than 220.82, because it gives the strips no 65 percent factor. Either method is permitted where it applies; run both and file the one you can defend.

220.87, measured demand. Where the utility can supply a year of maximum-demand data for the service, or a 30-day recording is made under the conditions that section sets, the existing load is the measured maximum demand times 125 percent, and the new load is added to it. On a house that has run gas heat until now, the measured year contains none of the new heating load, which is fine, because the new load is added on top at its calculated value. Data quality governs; the AHJ will want to see the source.

Where this sheet goes wrong

• The 100 A and three-wire gates. A house on a 60 A service, or fed at 208Y/120 V by a four-wire feeder in a condo building, falls outside 220.82(A) as worded. Check the gate before you run the sheet.
• Nameplate amps read off the wrong label. The outdoor unit's MCA is a wire-sizing figure that already includes 25 percent on the compressor; using MCA in the heating line overstates it. Use RLA and FLA as this sheet did. See related 'Reading an Appliance Nameplate for Circuit Sizing'.
• Double counting the blower. The blower belongs in the heating and cooling block, not also in the general load.
• Forgetting the range's real nameplate. On older ranges the label sits behind the storage drawer or under the cooktop; a guessed 12 kW where the plate says 11.0 adds 1,000 VA to the general load, and because that load already sits above the first 10 kVA, 400 VA to the total (1,000 times 0.40). On a house 3.4 A over the line, 400 VA is another 1.7 A. Read the plate.

The EV choice itself, charger setting against load management against a service upgrade, is not this card's job. The sheet tells you the number each option has to beat.

References

• NFPA 70, National Electrical Code, 2023 edition as adopted by your jurisdiction: 220.57, 220.61, 220.82, 220.83, 220.87, 625.42
• See related: Standard Method General Load Pieces for a Dwelling
• See related: Electrical Load Calculation Reference; EV Charger Load Calculation and Service Sizing
• See related: Reading an Appliance Nameplate for Circuit Sizing