This article presents a precise NEC-based commercial feeder load calculator methodology for immediate, accurate sizing.
Engineered guidance, formulas, examples, and tables support NEC 220.12 and 220.42 compliance verification worldwide practices.
Commercial Feeder General Lighting Load by Area Calculator (NEC 220.12 & 220.42)
Scope and applicability under NEC 220.12 and 220.42
This technical article addresses methods to compute commercial feeder loads by area and to translate connected equipment into accurate feeder ampacity using the principles and referenced sections of NEC Article 220 (notably 220.12 and 220.42 where applicable). It is written for electrical engineers, designers, and plan reviewers who require a fast, defensible, code-compliant workflow to estimate feeder load from area-based loads, equipment loads, and demand factors.
Purpose and regulatory context
- Provide step-by-step arithmetic to convert area loads (VA/ft²) into feeder currents at common voltages (120/208V, 277/480V three-phase, 240/120V single-phase).
- Demonstrate application of continuous-load adjustments, power factor, and three-phase conversion factors so that conduit, conductor, and overcurrent protective device (OCPD) selections are defensible under NEC guidance.
- Show example calculations using conservative engineering assumptions while indicating where the NEC tables (220.x) and AHJ-adopted demand factors must be used.
Fundamental formulas and variables (HTML-only presentation)
Below are the core formulas used to convert loads to feeder currents. All formulas are presented using plain HTML/math characters.
Variable definitions and typical engineering values
- VA — Apparent power in volt-amperes (W when PF = 1).
- W — Real power in watts (actual measured/specified input).
- V — Line-to-line voltage for three-phase systems (e.g., 208, 480 V) or line-to-neutral for single phase (120, 277 V).
- I — Current in amperes.
- PF — Power factor (typical: 0.90–0.95 for HVAC/electronic loads; 0.70–0.85 for some motors during low loading). When unknown, use 0.9 for design conservative estimate for mixed loads.
- 1.732 — Square root of 3 for balanced three-phase systems conversion.
- Continuous load — Load expected to run for 3 hours or more; NEC requires conductor and OCPD sizing consideration (often 125% of continuous portion).
Stepwise method to compute commercial feeder load by area
- Inventory area-based loads (lighting VA/ft², receptacle/general-use VA/ft², specialized equipment VA/ft²) and fixed equipment (HVAC, compressors, elevators, kitchen equipment).
- Convert all real-power specifications (W or kW) into VA using PF where required: VA = W / PF.
- Identify continuous loads and mark them for the 125% sizing factor per NEC continuous-load rules.
- Sum VA for all loads to obtain total connected VA. Apply demand factors where permitted by the adopted NEC tables or local amendments; if none available, use 100% of connected for conservative calculations.
- Compute feeder current: I = Total VA / (V × 1.732) for three-phase; I = Total VA / V for single-phase.
- Apply continuous-load multiplier to relevant portion(s) to determine conductor/OCPD design current: Design_I = (Continuous_VA × 1.25 + Noncontinuous_VA) / (V × 1.732)
- Select conductor ampacity and OCPD rating equal to or greater than the calculated Design_I and consistent with NEC ampacity tables and terminal temperature ratings.
Common unit loads and typical engineering tables
Below are tables of common assumptions used for rapid commercial load estimation. These are engineering-typical values for initial design and must be replaced by actual equipment nameplate data for final calculations.
| Occupancy / Area Type | Typical Lighting Unit Load (VA/ft²) | Typical Receptacle/General Use (VA/ft²) | Notes |
|---|---|---|---|
| Office (open plan) | 1.5–2.5 | 1.0–2.0 | Lighting low to medium; computers and office equipment vary. |
| Retail (display-intensive) | 3.0–4.5 | 2.0–3.5 | Displays and accent lighting increase VA/ft². |
| Restaurant / Kitchen (dining) | 2.5–4.0 | 3.0–6.0 (kitchen much higher) | Kitchen equipment is typically large fixed loads; treat separately. |
| Warehouse | 0.5–1.5 | 0.5–1.0 | High bay lighting; large open areas. |
| Healthcare (clinic spaces) | 2.5–4.0 | 2.5–5.0 | Special medical equipment must be inventoried. |
| Typical Equipment PF / Efficiency | Typical PF | Design Notes |
|---|---|---|
| LED Lighting drivers | 0.9–0.99 | Often near unity; use manufacturer PF for accuracy. |
| VFD-driven motors (modern) | 0.85–0.95 | PF variable depending on load and drive settings. |
| HVAC packaged units | 0.85–0.95 | Use nameplate or manufacturer data for compressors. |
| Resistive heating | 1.0 | No PF correction required. |
| Quick VA-to-Amps mapping (Three-phase) | 208 V (A) | 480 V (A) | Remarks |
|---|---|---|---|
| 10 kVA | 10,000 / (208 × 1.732) ≈ 27.8 | 10,000 / (480 × 1.732) ≈ 12.0 | Useful quick conversion for field checks. |
| 25 kVA | ≈ 69.5 | ≈ 29.9 | Common small transformer sizes. |
| 50 kVA | ≈ 139 | ≈ 59.8 | Typical medium feeder loads. |
| 100 kVA | ≈ 278 | ≈ 119.6 | Large single feeder or small building service. |
Application of continuous-load rules and demand factors
NEC requires special treatment of continuous loads (operating 3 hours or more). For conductor sizing and OCPD selection, the continuous portion of the load must be multiplied by 125% per NEC requirements. Demand factors for lighting and other loads may be used where the code provides tables — always apply the adopted edition of the NEC and local amendments.
- When portions of the load are continuous, calculate the continuous VA separately and multiply by 1.25 before summing with noncontinuous VA for sizing purposes.
- Apply any permitted demand factors from the NEC tables for specific load types (e.g., some HVAC, electric heating, or cooking equipment demand allowances) only as allowed by your code edition.
- If in doubt, use 100% of connected load (conservative) and apply the 125% rule to continuous items.
Example 1 — Small retail store (practical worked example)
Project: Single-floor retail unit, 2,000 ft². Supply: 208Y/120 V three-phase system. Assumptions and nameplate data furnished by client.
Given data and engineering assumptions
- Area = 2,000 ft²
- Lighting unit load = 3.0 VA/ft² (assumed typical for retail) → Lighting VA = 2,000 × 3.0 = 6,000 VA
- Receptacles/general loads = 2.0 VA/ft² → Receptacle VA = 2,000 × 2.0 = 4,000 VA
- HVAC packaged unit (nameplate) = 9.0 kW electrical input (9,000 W); manufacturer PF = 0.95 → HVAC VA = 9,000 / 0.95 ≈ 9,474 VA
- One small display motor-driven conveyor: nameplate = 0.5 HP, 208 V three-phase; assume motor PF = 0.85, efficiency = 0.85. We compute motor current below.
- Lighting is expected to operate continuously (open more than 3 hours), so treat lighting as continuous load for sizing.
Step A — Motor current calculation
Motor electrical input (approximate) calculated using HP conversion:
Convert to VA using assumed PF and efficiency (approximate design method): Motor_VA = Motor_W / (PF × Efficiency) = 373 / (0.85 × 0.85) ≈ 517 VA
Step B — Total connected VA
Step C — Apply continuous load adjustment for conductor sizing
Step D — Compute feeder current on 208 V three-phase
Step E — Select conductor and OCPD
- Round-up per NEC practice: choose next standard conductor and breaker rating consistent with ampacity tables and temperature ratings. For example, a conductor ampacity rating of 75 A (copper, 75°C terminals) provides conservative margin.
- OCPD must comply with conductor sizing: if conductor ampacity is 75 A, an OCPD may be sized at 80 A or 70 A depending on conductor and OCPD coordination; ensure 125% continuous rule is not violated for breaker selection (NEC 240.x guidance).
Result: For the assumed loads and continuous lighting, a feeder rated for approximately 60 A design current is required; choose conductors and overcurrent protection with ampacity ≥ 60 A, commonly selecting a 75 A conductor ampacity and compatible 70–80 A OCPD per coordination and ampacity tables.
Example 2 — Office floor with mixed equipment (comprehensive calculation)
Project: Single open office floor, 10,000 ft². Supply: 480Y/277 V three-phase distribution. Nameplate data provided for HVAC and server room. Designer must ensure NEC compliance using the adopted edition.
Given data and assumptions
- Area = 10,000 ft²
- Lighting unit load = 1.8 VA/ft² → Lighting VA = 10,000 × 1.8 = 18,000 VA (lighting continuous)
- Receptacle/general load = 1.5 VA/ft² → Receptacle VA = 15,000 VA (noncontinuous)
- Packaged rooftop HVAC units combined electrical input = 25 kW → HVAC_W = 25,000 W; assume PF = 0.92 → HVAC_VA = 25,000 / 0.92 ≈ 27,174 VA
- Server rack bank: nameplate 7.5 kW total (7,500 W); assume PF = 0.98 → Server_VA = 7,500 / 0.98 ≈ 7,653 VA (often continuous)
- Two small three-phase motors: each 3 HP at 480 V, assume PF = 0.85 and efficiency = 0.90; compute below.
Motor calculations (two x 3 HP at 480 V)
Step A — Connected VA summary
Step B — Continuous load adjustments
Lighting and server are continuous. Apply 125% to those portions for sizing:
Step C — Compute three-phase feeder current at 480 V
Step D — Conductor and OCPD selection
- Design current ≈ 96.34 A. Choose conductor with ampacity ≥ 96.34 A; using ampacity tables select conductor rated at 115 A or higher depending on insulation and temperature rating (e.g., copper 3 AWG or 2 AWG depending on table and temperature correction).
- For OCPD sizing, ensure the breaker rating coordinate with conductor ampacity and continuous loads per NEC 240 requirements. Consider 125% continuous loads when selecting breaker and conductor combination.
- Consider transformer sizing, neutral sizing for 277/480 V if unbalanced loads, and inrush or motor starting currents for HVAC and motor loads that may require diversity or coordination devices (soft-starters, VFDs).
Result: Design feeder current is approximately 96–100 A at 480 V three-phase for the assumed loads and continuous portions; select conductors and protective devices accordingly while confirming with adopted NEC ampacity tables and local AHJ requirements.
Practical notes on motor loads, starting currents, and protection
- Motor locked-rotor and starting currents are significantly higher than running FLA; size branch-circuit OCPD and motor protective devices according to NEC motor rules and manufacturer starting data.
- For feeders supplying large motor loads, consider voltage drop and coordination of protective devices. Use motor starting methods (soft starters, VFDs) where necessary to reduce feeder size or upstream voltage impact.
- Include diversity for multiple motors only where NEC permits; otherwise assume full nameplate FLA for conductor sizing and motor branch circuits unless using code-permitted demand factors.
Voltage drop, transformer, and neutral considerations
Designers must check voltage drop for long feeders. The general engineering recommendation is to keep voltage drop at 3% or less for feeders plus 3% for branch circuit for a total of 5%–6% depending on authority having jurisdiction.
Three-phase approximation: Vdrop ≈ I × R × 1.732 × L
Where R is conductor resistance per unit length and L is one-way length; compute with material resistivity and conductor size. Always use manufacturer or NEC tables for conductor resistance values.
Checklist for code-compliant feeder load calculation
- Confirm applicable NEC edition and local amendments.
- Inventory all loads by nameplate or calculated area loads and classify continuous vs noncontinuous.
- Apply correct power factor conversions for W→VA conversions when necessary.
- Apply NEC required continuous load multipliers (125%), and apply demand factors only where code tables permit.
- Compute three-phase or single-phase currents using the formulas given.
- Select conductors and OCPD based on ampacity tables, terminal temperature ratings, and continuous load rules.
- Verify voltage drop and provide mitigation where required (increase conductor size or reduce length through design changes).
- Document assumptions (VA/ft², PF, efficiency, continuous load designations) in the calculation package for AHJ review.
Common mistakes and how to avoid them
- Neglecting continuous loads or failing to apply the 125% multiplier — always check operating hours for lighting, server rooms, and other continuous equipment.
- Using inconsistent power factor assumptions — use measured or manufacturer PF where available; otherwise apply conservative PF assumptions.
- Applying demand factors incorrectly — verify the specific NEC table or local rule that allows demand reductions for the load type.
- Failing to document assumptions — record all unit loads, PFs, and demand factors in the calculation report for transparency and AHJ acceptance.
Normative references and authoritative resources
- NFPA 70, National Electrical Code (NEC) — consult the adopted edition in your jurisdiction: https://www.nfpa.org/NEC
- NEC Article 220, Branch-Circuit, Feeder, and Service Calculations — see NFPA resources and the NEC Handbook for explanatory guidance: https://www.nfpa.org
- Institute of Electrical and Electronics Engineers (IEEE) — for system modeling and coordination: https://www.ieee.org
- National Electrical Manufacturers Association (NEMA) — for motor and transformer standards and tables: https://www.nema.org
- International Association of Electrical Inspectors (IAEI) — technical articles and plan review resources: https://www.iaei.org
- Manufacturer datasheets — always use equipment nameplate and manufacturer technical data for final design computations.
Appendix — Quick reference formulas and worked conversion table
| Formula | Use case | Example with values |
|---|---|---|
| I = VA / (V × 1.732) | Three-phase current | For 50,000 VA at 480 V: I = 50,000 / (480 × 1.732) ≈ 60.1 A |
| I = VA / V | Single-phase current | For 10,000 VA at 240 V: I = 10,000 / 240 ≈ 41.67 A |
| VA = W / PF | Convert real power to apparent power | For 25,000 W and PF = 0.92: VA = 25,000 / 0.92 ≈ 27,174 VA |
| Design_I = (Continuous_VA × 1.25 + Noncontinuous_VA) / (V × Factor) | Design current for conductor sizing (Factor = 1.732 for 3Φ) | Example See Example 2 calculation |
Final compliance and documentation recommendations
- Include a load summary table in the project electrical submission that lists area, loads (VA/ft²), nameplate data, continuous designations, PF, and calculated VA and currents.
- Reference the NEC sections used to apply demand factors or continuous load rules. For example: cite NEC Article 220 and the specific section numbers from the adopted edition.
- Provide transformer schedules, conductor ampacity selections, OCPD selections, and voltage-drop checks as separate supporting calculations.
- Coordinate with mechanical engineers early to obtain accurate HVAC electrical input data and with equipment vendors for precise PF and inrush/starting characteristics.
Accurate, defensible feeder load calculations for commercial projects require disciplined inventory of loads, application of continuous-load multipliers, conservative power-factor assumptions where manufacturer data are lacking, and strict adherence to the NEC and local amendments. Use the formulas and worked examples above as a reproducible template for rapid estimation, then replace assumptions with nameplate data for final design and AHJ submittals.
Selected authoritative links for further technical reading
- NFPA (NEC) — https://www.nfpa.org/NEC
- NEMA motor and transformer guides — https://www.nema.org/
- IEEE standards and white papers on power distribution — https://www.ieee.org/
- IAEI technical resource articles — https://www.iaei.org/