SFB-1 — Load-Calculation Workflow

Separate the four decisions

A load calculation answers how much electrical load must be supplied under the applicable 2017 NEC method. It does not by itself finish conductor, overcurrent, or equipment selection. Keep four stages separate:

1. Connected load: the sum of nameplate or assigned loads before permitted demand factors.
2. Calculated load or demand: the result after applying Article 220 rules.
3. Minimum conductor ampacity: the calculated load adjusted under feeder, service, continuous-load, motor, terminal, and other applicable rules.
4. Overcurrent and equipment rating: selected under their own articles and standard-size provisions.

Using the same number at every stage without checking the rule is a common error. So is treating a demand factor as a safety factor. Demand factors recognize expected simultaneous use; a 125 percent continuous-load multiplier addresses conductor or equipment loading under a different requirement.

Choose a method before calculating

Identify occupancy, supply system, new or existing work, and whether the standard method or an optional method is permitted. Article 220 Parts II and III contain branch, feeder, and service calculations; Part IV contains optional calculations with eligibility conditions. Do not start a standard-method calculation and then borrow one attractive percentage from an optional method.

Next list assumptions: voltage, single-phase or three-phase, floor area, number of required circuits, nameplate loads, continuous-load status, motor loads, and whether loads are noncoincident. When a question omits necessary data, state what can and cannot be concluded.

Build the load schedule

Use consistent units, usually volt-amperes. A practical sequence is:

• Calculate general lighting from floor area and Table 220.12 where applicable. For a dwelling, the assigned value is 3 VA/ft².
• Add required small-appliance and laundry loads under 220.52. Each required small-appliance branch circuit contributes 1,500 VA, and each required laundry branch circuit contributes 1,500 VA.
• Add receptacle, sign, show-window, appliance, cooking, dryer, heating, air-conditioning, motor, and other loads under the provisions that apply to the occupancy.
• Apply a demand factor only after its eligible load subtotal is established. Read table headings, ranges, and notes.
• Apply noncoincident treatment under 220.60 only where it is unlikely that both loads operate simultaneously; use the larger applicable load rather than assuming every alternating device is noncoincident.
• Calculate neutral load under 220.61 from maximum unbalance and use only the reductions that the section permits.

For dwelling appliances, 220.53 permits a 75 percent demand factor for four or more appliances fastened in place, but it excludes electric ranges, clothes dryers, space-heating equipment, and air-conditioning equipment. Clothes dryers use 220.54 and Table 220.54, with 5,000 W or nameplate rating for each dryer, whichever is larger. Household ranges use Table 220.55 and its notes. These rules are not interchangeable.

Motor loads are incorporated under 220.50 by reference to Article 430. For multiple motors, the applicable calculation generally includes 125 percent of the highest-rated motor load plus the other motor loads. Identify the motor current source and do not substitute overload-protection percentages into a feeder load.

Worked dwelling general-load example

Assume the 2017 standard method for a 2,000 ft² dwelling, exactly two required small-appliance branch circuits, and one required laundry branch circuit. Exclude fixed appliances, cooking, dryer, HVAC, and motors so this example isolates the general-load subtotal.

General lighting is 2,000 ft² × 3 VA/ft² = 6,000 VA. Small-appliance load is 2 circuits × 1,500 VA/circuit = 3,000 VA. Laundry load is 1 circuit × 1,500 VA/circuit = 1,500 VA. Connected subtotal is 6,000 VA + 3,000 VA + 1,500 VA = 10,500 VA.

Under Table 220.42 for dwelling units, the first 3,000 VA is at 100 percent and the remainder through 120,000 VA is at 35 percent. The remainder is 10,500 VA − 3,000 VA = 7,500 VA. Demand on that remainder is 7,500 VA × 0.35 = 2,625 VA. Calculated general load is 3,000 VA + 2,625 VA = 5,625 VA.

Do not add the range or dryer and call the result “general lighting.” They are separate load categories. Do not apply 35 percent to the entire 10,500 VA. The first block remains at 100 percent.

Carry the result into a feeder step

Assume a separate 240 V single-phase feeder supplies 10,000 VA of noncontinuous load and 4,000 VA of continuous load, with no applicable demand factor and no 100-percent-rated assembly allowance. Under the ordinary continuous-load rule, the minimum load basis is 10,000 VA + (4,000 VA × 1.25) = 15,000 VA. Feeder current is I = 15,000 VA ÷ 240 V = 62.5 A.

That 62.5 A is not yet a conductor size. The next steps select conductor ampacity using the 2017 table and terminal-temperature rules, then select overcurrent protection where permitted. For a balanced three-phase load, current would instead be I = VA ÷ (√3 × V_L). State phase before choosing the formula.

Final audit

Check that every input appears once, each demand factor is attached to an eligible subtotal, and mutually exclusive loads are documented. Confirm units: watts and VA are not automatically equivalent for every AC load, and kVA must be converted consistently. Compare the final magnitude with the connected load. A demand calculation should not exceed the connected subtotal unless a required multiplier, such as a continuous or motor factor, explains the increase.