Maximum Demand from Utility Data

Calculate electrical service sizing from 12-month utility interval meter data using the NEC 2026 §120.87 (formerly §220.87) 125% peak demand method.

NEC edition

Inputs

kW

The single highest kW demand recorded in the past 12 months

mo

Minimum 12 months required per §120.87

VA

Add EVSE, A/C, heat pump, etc. to be added — enter total VA

PF

Use 1.0 for resistive loads; 0.85–0.95 for mixed loads

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

Under NEC Section 120.87 (formerly Section 220.87), you are permitted to determine the existing load on an electrical service or feeder by utilizing actual recorded utility peak demand data (such as 15-minute smart meter interval data or electric bills) instead of performing a theoretical manual square-footage calculation.

NEC §120.87 Calculation Rules:

  • 12 Months Minimum Data: The maximum demand data must reflect the highest recorded peak load over a continuous 12-month period (or a continuous 30-day recorded recording if 1-year data is unavailable).
  • 125% Sizing Multiplier: The highest recorded maximum demand is multiplied by 125% (1.25) to provide a safe operating buffer.
  • Adding New Electrified Loads: The new added load (EV charger, heat pump, solar) is added to the 125% utility demand to determine total required service capacity.

Formulas & equations

Existing Demand (VA) = (Peak kW / Power Factor) × 1,000 Required Service Load = (Existing Demand VA × 1.25) + New Added Loads (VA) Calculated Service Amperes = Required Service Load / (Voltage × √3 if 3-Phase)

NEC reference table

Highest 12-Month Peak kW Demand Demand @ 125% (240V 1-Phase) Max Allowable Added Load (100A Panel) Max Allowable Added Load (200A Panel)
4.0 kW (e.g. Gas Home)5,000 VA (20.8 A)19,000 VA (79.2 A)43,000 VA (179.2 A)
6.0 kW7,500 VA (31.3 A)16,500 VA (68.7 A)40,500 VA (168.7 A)
8.0 kW (A/C Running)10,000 VA (41.7 A)14,000 VA (58.3 A)38,000 VA (158.3 A)
12.0 kW15,000 VA (62.5 A)9,000 VA (37.5 A)33,000 VA (137.5 A)
16.0 kW20,000 VA (83.3 A)4,000 VA (16.7 A)28,000 VA (116.7 A)

Worked example, step by step

Example Calculation: 12-Month Utility Data for Home EV & Heat Pump Upgrade

Scenario: A home with a 100A 240V panel pulled utility data showing a single peak hour of 6.4 kW during last summer. The homeowner wants to add a 32A EV charger (7,680 VA).

  • Step 1 (Convert kW to VA @ 125%): (6.4 kW × 1,000) × 1.25 = 8,000 VA (33.3 Amps).
  • Step 2 (Add EV Charger): 8,000 VA + 7,680 VA = 15,680 VA.
  • Step 3 (Calculate Amperage): 15,680 VA / 240V = 65.3 Amps.
  • Verdict: 65.3A is well below 100A! The inspector approves the permit without a panel upgrade.

Frequently asked questions

Most modern electric utility companies (e.g. PG&E, ConEd, FPL, ComEd) allow customers to download "Green Button" hourly or 15-minute interval energy data directly from their online customer portal. You can easily find the maximum kW value recorded over the previous 12 billing cycles.

Under NEC §120.87 Exception, if 12 months of utility meter data is unavailable, you are permitted to install an energy data logger on the service conductors for a continuous **30-day period**, provided the recording captures the seasonal maximum heating or cooling operational cycle.

Related NEC calculators

Other tools in the same install chain:

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Existing Dwelling Added Load Calculator

Calculate if new electrical loads (EV charger, heat pump, hot tub) can be added to an existing residential panel without a 200A service upgrade per NEC 2026 §120.83 (formerly §220.83).

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Service & Panel Upgrade Advisor

"Do I need a 200A panel?" — Evaluate if adding an EV charger, heat pump, or induction range requires upgrading your 100A or 150A panel under NEC Article 230 and §120.83.

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Residential Load Calculator

Calculate single-family and multifamily residential electrical service load using NEC 2026 Article 120 (formerly Article 220) standard & optional methods with 2026/2023 dual citations.

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