Five calculations electricians run every week, each with the rule, the steps, a worked example and the mistakes inspectors see. The numbers are computed by the same formulas and tables as the app's calculators.
Box fill
Add up the volume each item takes in cubic inches and compare it with the box's volume (NEC 314.16). Each conductor counts its size's allowance: 2 in³ for 14 AWG, 2.25 for 12 AWG, 2.5 for 10 AWG. A device yoke counts two of the largest conductor connected to it; all equipment grounding conductors together count one; internal clamps count one.
Voltage drop
The NEC recommends, in informational notes rather than requirements, that voltage drop stay at or below 3% on a branch circuit or a feeder and 5% for the two combined (NEC 210.19 and 215.2). Voltage drop is 2 × K × I × L ÷ circular mils on a single-phase circuit and 1.732 × K × I × L ÷ circular mils on a three-phase circuit, with K = 12.9 for copper and 21.2 for aluminum.
Conduit fill
Conductors may fill 53% of a conduit's internal area when there is one, 31% when there are two, and 40% when there are three or more (NEC Chapter 9, Table 1). Add the conductors' areas from Chapter 9, Table 5 and compare the total with that percentage of the conduit's area from Chapter 9, Table 4.
Wire ampacity
Start from the conductor's ampacity in NEC Table 310.16 for its size, material and insulation temperature rating, multiply by the adjustment factor for more than three current-carrying conductors and the correction factor for an ambient above 30 °C, then apply the small-conductor limits of 240.4(D): 15 A for 14 AWG copper, 20 A for 12 AWG and 30 A for 10 AWG.
Service load
The optional method for a dwelling (NEC 220.82; 120.82 in NEC 2026) adds the general lighting load per square foot, 1,500 VA for each small-appliance and laundry circuit and the nameplate of fixed appliances, takes the first part at 100% and the rest at 40%, then adds the larger of the air conditioning or heating load. NEC 2026 cut the lighting load from 3 to 2 VA per square foot and the first part from 10 to 8 kVA.