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2023 NEC • Article 220 Part III • Table 220.42 • 220.53 • 220.54 • 220.55 • 220.50 • 310.12 • 110.14(C)

Ultimate Dwelling Service & Feeder Calculation Tool

Full-screen, mobile-friendly, scoped HTML tool for one-family dwellings, individual dwelling-unit feeders, and building-service scenarios using NEC edition selection, corrected motor logic, conductor sizing review, terminal-limited ampacity review, and advisory voltage-drop review.

Step 1 — Company, Project, NEC, and Conductor Review Setup

Enter company details, project details, NEC edition, and conductor/termination review information. These details are included in the printed and exported summary.

2020 and 2023 use 3 VA/ft² for dwelling-unit general lighting/general-use receptacles. 2026 uses 2 VA/ft².
Use the actual conductor insulation rating. A 90°C conductor may be used for adjustment/correction review where permitted, but final conductor selection still has to satisfy terminal limits under 110.14(C).
Edition notice: This tool changes article references and dwelling-unit VA per square foot based on the NEC edition selected above.
Conductor ampacity notice: This version distinguishes between conductor insulation rating and terminal rating. A 90°C conductor may be used for adjustment/correction review where permitted, but the final selected conductor still has to satisfy the terminal-limited ampacity under 110.14(C).
Main motor logic notice: This version treats the motor rule as an extra 25% adder for the largest motor already included in the final selected load. It does not duplicate the entire motor load again.
Heat vs A/C notice: If A/C is larger than heat, the A/C compressor is eligible to be the motor that receives the extra 25% adder. If heat is larger and A/C is ignored, the A/C compressor is not used for the motor adder and the tool moves to the next largest included motor you enter for that unit.
Neutral motor notice: The neutral worksheet uses a separate field for the largest 120V motor that actually contributes to the neutral. The worksheet adds only the extra 25% there as well.

Step 2 — Dwelling Unit Entries

Complete one unit block for each dwelling unit included in the calculation. Each unit includes a dedicated cooking engine and corrected motor logic.

Step 3 — Common / Building Loads and Building-Level Motor Inputs

Use this for corridor lighting, house loads, site lighting, laundry room loads, mechanical room loads, clubhouse loads, or any other non-unit load. The building-level motor fields also use the extra 25% adder approach.

Step 4 — Conductor Sizing Engine

Select a conductor size and review its base ampacity, adjusted ampacity, terminal-limited ampacity, final allowable ampacity, and whether it passes the calculated load and entered OCPD. Ambient and adjustment factors are intentionally manual so the electrician can apply the correct NEC table values for the actual conditions.

Use 1 for a single conductor per phase. Use more only where parallel conductors are actually permitted and used.
Conductor sizing engine note: This engine is a field review aid. It does not replace actual NEC table selection, raceway fill review, equipment instructions, or installation-specific limitations. Final selection still requires code review.

Step 5 — Voltage Drop Review

This is an advisory design tool. It uses the selected conductor size, material, and parallel sets to estimate voltage drop. The electrician can compare against the calculated load current or a manually entered current.

Choose the actual voltage you want to evaluate the drop against.
Voltage drop note: This section is a design aid. It does not replace actual engineering review, conductor impedance analysis, or local practice. It is especially useful for feeder-length review and comparing conductor options.

Step 6 — Neutral Load Review

This is an organized neutral worksheet. It helps with review but does not replace final code analysis.

Neutral worksheet notice: This worksheet is intentionally user-controlled. It is designed to help the electrician apply judgment about what actually contributes to the maximum unbalanced neutral load.

Step 7 — Results

This area totals all units and added loads, converts VA to amperes, gives a rounded-up standard-size reference, and shows the edition in use.

Total Calculated Load 0 VA Units + common loads + other building load + all extra 25% motor adders
Calculated Current 0 A Calculated from the selected system
Rounded-Up Standard Size Reference — Helpful reference only
Units Included 0 Dwelling-unit blocks currently included

Step 8 — 310.12 Review

This section tries to avoid false positives. A building service is not the same thing as an individual dwelling-unit feeder.

Step 9 — Save, Share, and Print

The email button uses the user’s mail app. A true send-from-site button would need a backend mail endpoint.

Professional use note: Always verify final design decisions for ampacity, terminations, OCPD selection, grounded conductor sizing, local amendments, actual project conditions, and the specific NEC edition selected.
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