Calculation methodology

Editions

Informational estimate only — not a compliance document. Verify all calculations against the NEC edition adopted in your jurisdiction. Jurisdictions adopt editions on their own schedules and add amendments; confirm which edition applies to your permit.

Clauses implemented

Scope limits and readings

Known gaps and the readings taken where the change summaries left room, stated so a reviewer can see them without reading the code.

• Power control systems (NEC 2026, 120.7): implemented, 2026 only

  120.7 permits a listed power control system (Article 130 Part II) in branch-circuit, feeder and service calculations with its control setting, established by qualified persons, at no more than 80% of the monitored circuit’s overcurrent device. The calculated load is the noncontrolled loads per Parts II to VII plus the controlled loads: at the control setting when the PCS monitors only controlled loads (C)(1)(1), or at their minimum operating current, zero permitted, when it monitors controlled and noncontrolled loads (C)(1)(2). The rule and Informative Annex D Example D15(a) to (d) were verified against the NFPA 70-2026 text on 2026-09-24 and are reproduced exactly by the engine. Printed cross-reference discrepancy: the informational note to 120.7(C) cites Annex D "D14(a) to (d)" while the Annex numbers the examples D15; this site cites D15. NEC 2023 has no equivalent, so on a 2023 calculation the load-management panel routes to the 220.83 existing-dwelling calculation.

• Not implemented (Phase 6b backlog)

  120.84 multifamily (Annex D4 and D5), 430.24 Exception No. 4 (Design B/E and C/E motors) and 120.61(C) are out of scope for single-phase dwellings. Annex D1(b) prints per-leg (Line A, neutral, Line B) currents; the engine has no per-leg model, so that example is recorded but not asserted.

• Attached garage: editions differ in the optional method

  Under NEC 2023 the standard method counts an attached garage (220.41) and the optional method excludes it (220.82(B)(1)). Under NEC 2026, 120.82(B)(1) takes floor area per 120.5(C), which excludes only detached garages, so an attached garage is counted in both methods. Detached garage area is never counted.

• Single dwelling units only (no 220.84 / 120.84 multifamily)

  Multifamily demand factors are not implemented. Each run models one dwelling unit; the existing-dwelling method models one dwelling being upgraded.

• 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.

• Neutral load (220.61 / 120.61) depends on the 120 V flag

  The feeder/service neutral counts the general lighting demand, loads flagged 120 V after their demand factors, and 70% of the range and dryer demand, with the 70% reduction above 200 A. Loads without the flag are treated as 240 V line-to-line. Heating, cooling and the motor and continuous adders are not placed on the neutral.

• EVSE rules by edition

  NEC 2023: 220.57 (Part III) takes the larger of 7,200 VA or nameplate for the standard method and the neutral; the Part IV methods take EVSE at nameplate, the optional method inside the 220.82(B) general load (220.82(B)(3)a) and the existing-dwelling method inside its tiers (220.83(A)(3)a, (B)(3)a). NEC 2026: 120.57 takes the nameplate if available, otherwise 7,200 VA, and 120.82(D) adds 100% of that value outside the optional-method tiers; added EVSE under 120.83 takes the same value.

• Existing-dwelling method readings

  NEC 2023 220.83: in (B), cooling and heating take the larger per 220.60, and four or more separately controlled heating units are not listed in Table 220.83(B) and fall under all other loads (confirmed). 220.83(A)(1)/(B)(1) cite "220.42" for 3 VA/ft², a stale NFPA cross-reference; this site cites 220.41. NEC 2026 120.83 / Table 120.83: the first 8 kVA of existing and new load combined at 100%, filled with existing load first and then new load lowest percentage first (other 50%, then EVSE and central resistance heating 80%; the conservative reading); remainder of existing at 40%; larger of A/C or heating, not both; existing EVSE is existing load at nameplate. No NFPA worked example exists, so the checked examples are derived and were confirmed against the text.

• Table 120.55 Note 4 editorial leftover

  The NFPA 70-2026 text of Table 120.55 Note 4 still cites "Table 220.55". This site cites 120.55 throughout.

• Table 220.55 Note 3 defaults to the lower path

  When every cooking unit is 8¾ kW or less, the default takes the lower of Column C and Columns A/B; the choice can be forced either way and is printed on the line item and in the PDF.

• Fixed-appliance count threshold uses VA

  Appliances are entered by nameplate VA, so the 220.53 threshold of ¼ hp or 500 W is applied as 500 VA; units under it stay at 100% and do not count toward the four.

• 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 status

VERIFIED means the entry was checked against the NFPA 70 text on the date in its source; PROVISIONAL means it is awaiting that check. Worked examples list the printed result the engine must reproduce; the test suite fails if any drifts.

Manifest generated from lib/nec/fixtures (nec2023.ts, nec2026.ts, existingDwelling.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 and 2026 section numbers.