Calculation methodology
The load calculator implements the NEC 2023 (NFPA 70-2023) dwelling-unit calculations in Article 220, Part III (standard method) and 220.82 (optional method). This page lists the clauses it applies, what it deliberately leaves out, and how the engine was checked against the code's own worked examples.
Every NEC-derived number the engine uses lives in one fixtures module with its citation, and the reports cite NEC 2023 section numbers. Where NEC 2023 renumbered a table, the older number is shown in parentheses.
Edition
NEC 2023 (NFPA 70, 2023 edition). Jurisdictions adopt editions on their own schedules and add amendments; confirm which edition applies to your permit.
Informational estimate only — not a compliance document. Verify all calculations against the NEC edition adopted in your jurisdiction.
Clauses implemented
| Section | What it governs | Method |
|---|---|---|
| 220.41 | Dwelling unit general lighting and receptacle load, 3 VA/ft² | Both methods |
| Table 220.45 | Lighting load demand factors (Table 220.42 in NEC 2020) | Standard (Part III) |
| 220.52(A) | Small-appliance branch circuits, 1500 VA each | Both methods |
| 220.52(B) | Laundry branch circuit, 1500 VA each | Both methods |
| 220.53 | Appliances fastened in place, 75% when four or more | Standard (Part III) |
| 220.54 | Electric clothes dryers, 5000 VA or nameplate, Table 220.54 demand | Standard (Part III) |
| 220.55 | Household cooking appliances, Table 220.55 | Standard (Part III) |
| 220.60 | Noncoincident loads, larger of heating or cooling | Standard (Part III) |
| 220.50 / 430.24 | Motor loads, 125% of the largest motor | Standard (Part III) |
| 220.14 | Other loads at nameplate | Both methods |
| 230.42(A)(1) | Continuous loads at 125% for service conductors | Standard (Part III) |
| 220.82(B) | Optional method general loads, 100% of first 10 kVA + 40% of remainder | Optional (220.82) |
| 220.82(C) | Optional method heating and air-conditioning, largest of six selections | Optional (220.82) |
| 230.79(C) | One-family dwelling service disconnect not less than 100 A | Both methods |
| 240.6(A) | Standard ampere ratings | Both methods |
Scope limits
These are known gaps, stated so a reviewer can see them without reading the code. Items marked for a later phase are tracked in the fixtures module.
No feeder or service neutral calculation (220.61)
The calculator sizes the ungrounded conductors only. Neutral demand, including the 70% allowance for ranges and dryers, is not computed.
Single dwelling units only (no 220.84 multifamily, no 220.83 existing dwellings)
Multifamily demand factors and the existing-dwelling method for added loads are not implemented. Each run models one dwelling unit.
Single-phase services only
The 220.54 and 220.55 rule for single-phase ranges and dryers on a 3-phase, 4-wire feeder or service is not applied.
EVSE is entered as an other load at nameplate
Electric vehicle supply equipment is added under 220.14 at its nameplate rating. The 220.57 minimum of 7,200 VA is not applied automatically; enter it as the nameplate when it governs.
Table 220.55 Note 3 is applied automatically
When every cooking unit is 8¾ kW or less and the Column A or B result is lower than Column C, the lower value is used. It is not yet a selectable option.
Table 220.55 Note 2 averaging covers every unit
Ranges of unequal rating are averaged across all units. The NEC averages only units individually rated over 8¾ kW.
220.53 counts every fastened-in-place appliance
The four-or-more appliance test does not yet exclude units rated under ¼ hp or 500 W.
One floor-area input for both methods
Floor area follows 220.41 (attached garage included). The optional method excludes garages under 220.82(B)(1); enter the garage-free area for that method until a separate garage input ships.
The future-load allowance is a planning margin
The optional percentage added before service sizing is a user choice for spare capacity. It is not an NEC requirement and is labelled as such in the report.
Verification against Informative Annex D
The engine is tested against the worked examples in NEC 2023 Informative Annex D. Each fixture below records the printed result the engine must reproduce. VERIFIED means the fixture was checked against the NFPA 70-2023 text on the date in its source; PROVISIONAL means the values are still awaiting that check. 6 of 6 fixtures are verified.
| Fixture | Expected result | Status | Source |
|---|---|---|---|
Informative Annex D, Example D1(a) One-family dwelling, standard method | Net calculated load 18,600 VA; Calculated amps 77.5 A; Printed amps 78 A; Minimum service 100 A | VERIFIED | NFPA 70-2023 Informative Annex D, verified 2026-09-23 |
Informative Annex D, Example D2(a) Optional calculation, heating larger than air conditioning | Total load 21,480 VA; Calculated amps 89.5 A; Printed amps 90 A; Minimum service 100 A | VERIFIED | NFPA 70-2023 Informative Annex D, verified 2026-09-23 |
Informative Annex D, Example D2(b) Optional calculation, air conditioning larger than heating 2023 prints 122 A; 125 A service size derived from 240.6(A). | Total load 29,200 VA; Calculated amps 121.67 A; Printed amps 122 A; Minimum service 125 A | VERIFIED | NFPA 70-2023 Informative Annex D, verified 2026-09-23 |
Informative Annex D, Example D2(c) Optional calculation with heat pump, single-phase 240/120 V | Total load 34,790 VA; Calculated amps 144.96 A; Printed amps 145 A; Minimum service 150 A | VERIFIED | NFPA 70-2023 Informative Annex D, verified 2026-09-23 |
Informative Annex D, Example D6, ranges all 16 kW Maximum demand for 24 ranges at 16 kW (Note 1) | Maximum demand 46.8 kW | VERIFIED | NFPA 70-2023 Informative Annex D, verified 2026-09-23 |
Informative Annex D, Example D6, ranges of unequal rating Maximum demand for 30 mixed ranges (Note 2) | Maximum demand 49.5 kW | VERIFIED | NFPA 70-2023 Informative Annex D, verified 2026-09-23 |
Manifest generated from lib/nec/fixtures/nec2023.ts; the test suite fails if the two drift apart. Annex D item references inside the examples still carry pre-2023 220.82(C) numbering; this site cites the 2023 section numbers.
See it applied
The sample report runs Example D2(a) through both methods with every line item and its clause.