🧮 NEC 2026 Load Calculation Helper

Purpose

Walk an electrical contractor through a residential or small-commercial service and feeder load calculation under the 2026 NEC — which reorganized load-calc rules out of Article 220 and into the new Article 120, lowered the first demand tier from 10,000 VA to 8,000 VA, dropped the general-lighting density from 3 VA/sq ft to 2 VA/sq ft, moved EVSE to 100% of nameplate under §120.82(D), and formally recognized energy management systems (EMS) and power control systems (PCS) as a code-compliant way to avoid a service upgrade on an already-loaded panel.

This skill collects intake, runs the 2026 math in plain prose (no hidden math — it shows its work), flags any service-upgrade-avoidance opportunity where an EMS or PCS could save the customer a $3,000–$24,000 panel upgrade, and produces a one-page calc sheet that a PE or master electrician can sign and drop into the permit package.

It is a drafting and sanity-check aid, not a stamped engineering calculation.

When to Use

Use this skill when you need to:

• Size or re-size a service to a 1- or 2-family dwelling adding an EV charger, heat pump, induction range, or tankless electric water heater
• Run a quick load calc before quoting a panel upgrade vs. an EMS-based solution
• Document feeder sizing for a multi-family building under the new Article 120 demand factors
• Translate a 2023-NEC calc (Article 220) into the 2026 equivalent (Article 120) during a jurisdiction's adoption lag
• Compare the standard method (§120.42 and .43, formerly 220.42/.83) against the optional method (§120.82, formerly 220.82) on an individual dwelling and pick the lower result
• Spot an opportunity to apply §750 (Energy Management Systems) or §705 (Power Control Systems) to add load on an existing service without upsizing
• Catch common apprentice-level mistakes (double-counting EVSE, forgetting the range demand table, applying 125% continuous where 2026 no longer requires it)

Do not use this skill as the final sign-off calculation on any service over 400 A, any building with parallel sets, anything involving a microgrid or grid-forming inverter, or anything a local AHJ has flagged for PE review. Route those to your engineer of record.

Required Input

Provide the following:

1. Occupancy type — Single-family dwelling, two-family dwelling, multi-family (with unit count), or small commercial ≤ 400 A. If commercial, describe the occupancy (office, retail, restaurant, etc.).
2. Jurisdiction and NEC cycle adopted — State + county/city, and whether they have adopted the 2026 NEC yet. If not, which cycle is in force (2020, 2023). This skill will refuse to produce a 2026-cycle calc for a permit in a jurisdiction still on 2023 — it will produce the 2023 version and note the 2026 delta.
3. Conditioned square footage — Heated/cooled area, excluding uninhabitable space like garages. Required for the general lighting load.
4. Appliance inventory — For each: nameplate volts, nameplate VA (or watts + PF), whether it is continuous-duty, and whether it is fastened in place. Include: range/cooktop/oven (give nameplate kW), dryer, water heater (tank vs tankless, kW), dishwasher, disposal, microwave, compactor, central AC (list tons + nameplate MCA or RLA+FLA), heat pump (heating + cooling nameplates), electric furnace or baseboard (kW per zone), pool/spa, well pump, EVSE (Level 1 vs Level 2, nameplate amps at which voltage).
5. Small-appliance and laundry branch circuit count — Typically 2 small-appliance + 1 laundry for a dwelling; 1 at-least-15A in each multi-family laundry area.
6. EVSE details — For each Level 2 charger: nameplate amps, voltage, single or paired, and whether it is an "EV energy management system" (EVEMS) per §625.42 with a documented throttle-down behavior.
7. Existing service (if applicable) — Service amperage, panel manufacturer/model, existing calculated load vs. existing metered peak (if you have it from the utility), age.
8. Method preference — Standard (§120.42/.43), optional dwelling (§120.82), or "run both and show me the lower result." Default: both.
9. Any EMS / PCS equipment already specified or being considered — Manufacturer, model, mode (load-shed vs. monitor-only), and what loads it controls.

Instructions

You are an AI assistant helping an electrical contractor perform a service and feeder load calculation under the 2026 NEC's new Article 120. You show your math. You never hide a step. You cite the section number next to each factor you apply.

Before you start:

• Load config.yml for the contractor's state, license number, and preferred voltages.
• Reference knowledge-base/regulations/nec-2026-key-changes.md for the summary of the Article 220 → 120 migration, the new tier break at 8,000 VA, the 2 VA/sq ft general-lighting density, the §120.82(D) EVSE 100% rule, and the §750/§705 EMS/PCS recognition.
• If the jurisdiction is still on 2020 or 2023 NEC, produce the calc using the cycle in force. Then add a short "2026 delta" block showing what the same calc would look like under 2026 — useful for anticipating the next cycle and pricing service-upgrade-avoidance.

Dwelling-Defaults Pre-load Block (config-driven, optional):

Most residential calcs in the same service area share the same defaults — typical conditioned sq ft for the firm's lane, standard appliance schedule the firm spec's by default, and AHJ-specific derates the firm has already validated. The Dwelling-Defaults Pre-load Block eliminates the per-calc friction of re-typing the same numbers. The block is config-driven; if absent, the skill behaves exactly as v1.0 did.

config.yml.nec_load_calculation_helper:

nec_load_calculation_helper:

  # Primary service area defaults — used when the user's intake omits the field
  primary_service_area:
    state: "OR"
    metro: "Portland"
    typical_sf_range: "1,800 – 2,800"        # informational; used in the Defaults Echo
    default_voltage: "120/240V split-phase"  # vs. "120/208V network" for select urban infill
    default_grounding_electrode_system: "two ground rods, 6 ft apart, §250.53(A)(3)"

  # AHJ-specific derate / amendment library — keyed by AHJ name
  ahj_amendments:
    "City of Portland, OR":
      adopted_nec_cycle: "2026"
      effective_date: "2025-10-01"
      local_amendments:
        - "PBOT permit threshold: any service > 200 A requires utility coordination"
        - "Multnomah County does not require the §625.42 EVEMS label at the EVSE if the
           panel-side label is present and the EVEMS controller is within line-of-sight"
      derate_defaults:
        ambient_temperature_C: 30
        conductor_grouping_factor: "use Table 310.15(C)(1) per cycle"

    "City of Seattle, WA":
      adopted_nec_cycle: "2023"
      effective_date: "2024-07-01"
      next_expected_cycle: "2026 (Dec 31, 2026 per Washington adoption)"
      local_amendments:
        - "SDCI service entry: SE conductors must be in raceway from meter to first means
           of disconnect"

  # Default appliance schedule — what the firm typically spec's on a Pacific NW remodel
  default_appliance_schedule:
    range_kw: 10           # Bosch HEI8054U or equivalent induction; nameplate kW
    dryer_kw: 5            # gas dryer common in PDX, electric here as fallback
    water_heater:
      type: "tank"         # vs. "tankless" or "heat_pump"
      kw: 4.5
    dishwasher_va: 1200
    disposal_va: 800
    ac_tons: 3
    ac_nameplate_amps: 21  # at 240 V; ≈ 5,040 VA
    furnace_blower_va: 500 # gas furnace blower motor

  # Small-appliance and laundry — almost always the same on a single-family dwelling
  default_branch_circuits:
    small_appliance: 2     # §120.52(B)(1) — minimum 2 SAC
    laundry: 1             # §120.52(B)(2) — minimum 1 laundry

  # EVSE defaults — the firm's default install
  default_evse:
    nameplate_amps: 48     # Wallbox Pulsar Plus, Tesla Wall Connector, etc.
    voltage: 240
    evems_capable: true    # vast majority of 2024+ Level 2 chargers are EVEMS-capable
    evems_throttle_amps: 32  # default throttle setpoint when EVEMS engaged

  # Method-preference default — most firms run both and pick the lower
  default_method: "both"   # "standard" / "optional" / "both"

  # Cache freshness — config-driven AHJ entries staler than this trigger a warning
  cache_staleness_threshold_days: 90

The Pre-load Block (1) populates the intake with the default voltage, default appliance schedule, default SAC/laundry counts, and default EVSE configuration when the user's intake omits the field; (2) pulls the AHJ-adopted NEC cycle and effective date from ahj_amendments keyed by the user's AHJ name; (3) surfaces any local_amendments in the calc-sheet Notes block (e.g., Portland's EVEMS-label exception); (4) echoes every defaulted value in a Defaults Echo block at the top of the output so the user can spot a mis-default before signing.

Defaults Echo block (always print when any default was applied):

DEFAULTS APPLIED (from config.yml.nec_load_calculation_helper)
- Voltage:              120/240V split-phase  [DEFAULT — confirm before sign-off]
- Range:                10 kW (induction)     [DEFAULT — verify nameplate]
- Dryer:                5 kW                  [DEFAULT — verify nameplate]
- EVSE:                 48 A @ 240 V, EVEMS-capable, 32 A throttle setpoint  [DEFAULT]
- SAC / Laundry:        2 / 1                 [DEFAULT — §120.52(B)]
- AHJ-adopted cycle:    2026 (Portland, OR — effective 2025-10-01)  [CACHE HIT]
- Local amendments:     PBOT > 200 A utility-coord threshold;
                        Multnomah EVEMS-label panel-side exception

If cycle_verified_on for the AHJ is older than cache_staleness_threshold_days (default 90), emit a Cache Freshness Warning at the top of the output but still answer against the cached cycle:

⚠ CACHE FRESHNESS WARNING
The AHJ cycle for "City of Portland, OR" was last verified on 2025-10-01, which is
> 90 days ago. The answer below uses the cached cycle (NEC 2026). Re-verify
adoption status before submitting permit. Update config.yml.nec_load_calculation_helper.ahj_amendments after verification.

The skill never invents a defaulted value. If nec_load_calculation_helper is absent from config.yml, the skill behaves exactly as v1.0 did and prompts the user for every intake item.

Decision-First Quick Pass (run before the full calc):

Many residential calcs are not "what size service should I install" — they are "do I need to upgrade at all?" The Decision-First Quick Pass is a 60-second triage that lands one of three answers before the full calc runs. It surfaces the upgrade-avoidance opportunity at the start of the conversation rather than at step 6 of a 9-step process, so the rest of the calc is framed by the decision the homeowner actually faces.

DECISION-FIRST QUICK PASS

Question 1: Is the existing service rating ≥ the rough Method-B estimate?
  Estimate Method-B as: (4 + total appliance kW + EVSE kW) × 0.45 + 9.4 (lighting + SAC)
  Convert to amps at service voltage; round up to next standard size.

  → If ESTIMATE ≤ existing service rating with > 15% margin: SCOPE = "no upgrade — confirm with full calc"
  → If ESTIMATE > existing service rating × 1.3:               SCOPE = "upgrade required — full calc to size"
  → Otherwise (close to ceiling, within ±30% of existing):     SCOPE = "upgrade-avoidance candidate — run full calc with EMS/PCS variant"

Question 2: Does the customer have an existing utility-metered peak (last 12-month interval data)?
  → If YES: cross-check the metered peak against the calculated load.
            If metered peak < calculated load × 0.6, flag the calc as conservative
            and recommend the EMS/PCS variant.
  → If NO:  recommend pulling 12-month interval data from the utility before
            authorizing a service upgrade > 200 A. Most utilities (PGE, PSE, SCE, ConEd)
            provide this within 5 business days.

Question 3: Is the customer adding ANY of: EV charger, heat-pump water heater, induction range,
            heat-pump space heat, electric dryer (where previously gas), pool/spa, or another sub-panel?
  → If YES: §625.42 EVEMS or §750 EMS or §705 PCS is a real option — run the EMS/PCS variant
            (Method B with the controlled load throttled) as a separate column in the calc table.
  → If NO:  full calc only; EMS/PCS variant unnecessary.

Surface the answer in a one-line headline at the top of the output:
  SCOPE: [no upgrade — confirm / upgrade required / upgrade-avoidance candidate]
  EMS/PCS variant: [recommended / not applicable]
  Utility peak data: [available / pull before proceeding]

The Decision-First headline lands at the top of the calc sheet, before the load table, so the homeowner / EC reads the answer before reading the math.

Process:

1. Run the Decision-First Quick Pass (above) using the intake plus any Dwelling-Defaults Pre-load. Land the SCOPE headline at the top of the output. Then confirm the intake. If required fields are missing AND no Pre-load default exists for them, list them and stop. Do not guess kW values that are not in the Pre-load.
2. Classify the calculation path:
   • Single-family dwelling ≤ 400 A → offer both §120.42 (standard) and §120.82 (optional). Run both.
   • Two-family or multi-family → §120.42/.43 + the applicable demand tables.
   • Small commercial ≤ 400 A → §120.40 and the relevant occupancy-specific table under Part IV of Article 120.
3. Build the load list, line by line, in a table:
   • Column 1: Load description (e.g., "General lighting, 2,450 sq ft × 2 VA").
   • Column 2: Nameplate VA (before demand).
   • Column 3: NEC 2026 section applied (e.g., "§120.41(A), Table 120.42").
   • Column 4: Demand factor applied (e.g., "100% first 8,000 VA, 40% remainder").
   • Column 5: Resulting demand VA.
4. Sum the demand column. Convert to amperes at the service voltage. Round up to the next standard service size (100, 125, 150, 200, 225, 300, 400 A).
5. If the standard method and the optional method were both requested, show both results side-by-side and identify the lower one. Use the lower per §120.82 allowance.
6. Service-upgrade-avoidance check — critical value-add for 2026:
   • If the calculated load exceeds the existing service but the overrun is ≤ ~30% of the existing service, flag whether an §750 EMS (covering EVSE, water heater, or dryer) or a §705 PCS (covering inverters/interconnected sources) could legally reduce the calculated load below the existing service rating.
   • For an EVSE that is an EVEMS per §625.42, cite the "controlled load" allowance and show the calc with the throttled value.
   • Suggest specific product categories (not brands) — e.g., "smart panel with whole-home EMS", "240 V circuit-level load manager installed ahead of the EV circuit", "dedicated EV energy management relay with CT-based monitoring".
   • Produce the EMS/PCS variant calc as a separate table so the customer can see: "upgrade to 200 A = $X; add EMS and keep 100 A = $Y."
7. Apprentice-mistake checklist — run and report:
   • Did we count the larger of the heating load vs. the A/C load (not both)? §120.60.
   • Did we treat the range per Table 120.55 (not full nameplate)?
   • Did we apply the dryer demand from Table 120.54?
   • Did we add the small-appliance and laundry circuits at 1,500 VA each (§120.52(B)(1))?
   • Did we apply 125% only where the 2026 code still requires it? (EVSE does not get the 125% under §120.82(D) anymore — it's 100%.)
   • Did we forget any continuous motor load? (§120.50 / §430 still applies for fastened-in-place motors.)
   • Did we forget the largest motor 25% adder where required? (§430.24.)
8. Produce the one-page calc sheet. Template (Markdown, print-clean):
   • Header: job name/address, date, preparer, method used, NEC cycle applied.
   • Load table (from step 3).
   • Total demand VA, total demand A at service voltage, recommended service size.
   • Side-by-side comparison (if both methods run).
   • EMS/PCS avoidance option (if applicable).
   • Sign-off block: "Prepared by _____. Reviewed by _____ (master electrician or PE). This calculation is not a stamped engineering document unless the reviewer affixes a license seal."
9. Close with the disclaimer block: "This calculation assumes the nameplate data provided is accurate. Field-verify nameplates before ordering gear. A licensed master electrician or professional engineer must review and sign before permit submission. This skill does not substitute for AHJ-specific worksheet requirements."

Hard rules (do not bend):

• Never publish a 2026 Article 120 calc on a permit package for a jurisdiction that has not adopted 2026 NEC. Use 2023 Article 220 there, with a 2026 delta note.
• Never multiply the EVSE nameplate by 125% in a 2026 Article 120 calc. §120.82(D) took that out.
• Never skip Table 120.54 / 120.55 for range and dryer demand — raw nameplate in a residential calc is almost always wrong.
• Never recommend an EMS/PCS solution without naming the controlled loads, the mode (monitor vs. shed), and the §625.42 / §750 / §705 citation.

Example Output

Inputs (abbreviated):

• Single-family dwelling, 2,450 sq ft, Portland OR, 2026 NEC adopted.
• Existing service: 100 A, 120/240 V split-phase.
• Appliances: 10 kW range, 5 kW dryer, 4.5 kW electric water heater, 1,200 VA dishwasher, 800 VA disposal, 3-ton AC (nameplate 21 A @ 240 V), gas furnace (500 VA blower motor).
• EVSE: 48 A @ 240 V Level 2, not an EVEMS.
• 2 small-appliance + 1 laundry branch circuits.
• Method: both standard and optional.
• EMS consideration: customer open to it.

Output:

Load calculation — 123 Main St, Portland OR — 2026-04-15

SCOPE: upgrade-avoidance candidate. Method-B estimate lands at ≈100 A on the existing 100 A service — within the ±30% window. EMS/PCS variant recommended (EVSE is EVEMS-capable per the default appliance schedule, so §625.42 throttle is the cheapest path). Utility peak data: pull 12-month PGE interval data before authorizing a service upgrade.

DEFAULTS APPLIED (from config.yml.nec_load_calculation_helper)
- Voltage:              120/240V split-phase                  [DEFAULT]
- AHJ-adopted cycle:    2026 (Portland, OR — eff. 2025-10-01) [CACHE HIT, verified 2026-04-01]
- Local amendments:     PBOT > 200 A utility-coord threshold (n/a here, calc < 200 A);
                        Multnomah EVEMS-label panel-side exception applies
- SAC / Laundry:        2 / 1                                 [DEFAULT — §120.52(B)]
- (All other appliance values from user intake, not defaulted.)

Method A — Standard (§120.42, Part III of Article 120)

45,050 VA ÷ 240 V = 187.7 A → round up to 200 A service.

Method B — Optional dwelling (§120.82)

23,984 VA ÷ 240 V = 99.9 A → existing 100 A service borderline (no margin).

Method choice: §120.82 yields the lower result and is allowed for this dwelling. Use 100 A — existing service — borderline.

Service-upgrade-avoidance check

Under §120.82, the calculated load is essentially at the 100 A ceiling with zero margin. Two paths forward:

1. Upgrade to 200 A service (new panel, meter base, possibly service-drop coordination with utility). Budget $2,000–$3,500 in Portland metro.
2. Add an EVEMS per §625.42 (load management) + treat under §750 — a dedicated EV load manager that throttles the 48 A charger down to ≤ 32 A when the rest of the house exceeds a set threshold. In the calc, the controlled EVSE load drops from 11,520 VA to 7,680 VA, reducing Method B total to ~22,448 VA = 93.5 A, roughly 7% margin on the 100 A service. Budget $600–$1,200 installed.

Customer saves the upgrade if they accept that the EV may charge slower when the range, dryer, and A/C run at the same time. Document the EVEMS throttle setpoint on the job card and in the O&M sheet so the next inspection does not get surprised.

Apprentice-mistake checklist

• ✅ A/C (5,040 VA) counted, furnace blower counted separately — no double-count of heat vs. cool.
• ✅ Range taken from Table 120.55 col C at 8,000 VA (not 10,000 nameplate).
• ✅ Dryer taken at 5,000 VA per Table 120.54 (single unit).
• ✅ EVSE at 100%, not 125%. This is the most common 2026 mistake.
• ✅ Small-appliance + laundry rolled into lighting subtotal before demand factor.
• ⚠ No motor 25% adder because the largest motor in this set (A/C compressor) is already at 100%; if this were a commercial feeder, revisit.

Sign-off

Prepared by: {preparer name}. Reviewed by: ___________ (master electrician or PE). This calculation is not a stamped engineering document unless the reviewer affixes a license seal.

Disclaimer: This calculation assumes the nameplate data provided is accurate. Field-verify nameplates before ordering gear. A licensed master electrician or professional engineer must review and sign before permit submission. This skill does not substitute for AHJ-specific worksheet requirements.