Authoritative Data Source and Methodology
Primary Code Reference: NFPA 70 — National Electrical Code (NEC), Article 220 (Branch-Circuit, Feeder, and Service Calculations), Articles 430 (Motors) and 600 (Electric Signs). Latest adopted edition in your jurisdiction governs. Official access via NFPA: NFPA 70.
- NEC 220.12 — Lighting Loads for Specified Occupancies (unit VA/ft² by occupancy)
- NEC 220.14 — Other Loads — Including Receptacles (180 VA per outlet minimum in many cases)
- NEC 220.44 — Receptacle Load Demand Factors (non-dwelling)
- NEC 220.50 — Noncoincident Loads (take the larger of heating vs cooling)
- NEC 430.24 — Feeders/Services Supplying Multiple Motors (add 25% of largest motor)
- NEC 600.5(A) — Sign branch-circuit requirements
Secondary reading (industry guidance): Commercial Electrical Load Calculations (EC&M).
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formulas Explained
Lighting VA (raw): L_raw = A × U
Lighting VA (demanded): L = L_raw × (D_L / 100) × C_L
Receptacles VA (count mode): R_raw = N × 180
Receptacles VA (known): R_raw = R_known
Receptacles VA (demand per Table 220.44): R = 10,000 + 0.5 × max(0, R_raw − 10,000)
Receptacles VA (custom %): R = R_raw × (D_R / 100)
HVAC VA (noncoincident): H = max(H_cool, H_heat)
Motors VA: M = (Σ m_i) + 0.25 × max(m_i)
Total kVA: kVA_total = (L + R + S + W + H + M) / 1000
Current (single-phase): I = VA_total / V_L-L
Current (3-phase): I = VA_total / (√3 × V_L-L)
Where: A = area (ft²), U = unit lighting load (VA/ft²), D_L = lighting demand (%), C_L = 1.25 if continuous else 1.00, N = receptacle outlets, S = signage VA, W = show window VA.
Glossary of Variables
- Area (A)
- Gross floor area served by the service (ft² or m² converted to ft²).
- Unit lighting load (U)
- VA/ft² from NEC 220.12 for the occupancy.
- Lighting demand (D_L)
- Demand factor (%) applicable to the general lighting portion per Article 220 (where permitted).
- Continuous lighting factor (C_L)
- 1.25 if lighting is treated as continuous; otherwise 1.00.
- Receptacle raw load (R_raw)
- Either count × 180 VA (NEC 220.14(I)) or a known total VA.
- Table 220.44 demand
- First 10 kVA at 100%, balance at 50% for non-dwelling receptacles (where applicable).
- HVAC noncoincident (H)
- Larger of cooling vs heating per NEC 220.50.
- Motors (M)
- Sum of all motors plus 25% of the largest motor per NEC 430.24.
- Total kVA
- Sum of all demanded VA divided by 1000.
- Service current
- Calculated line current based on selected service voltage system.
How It Works: A Step-by-Step Example
Assume an office with:
- Area A = 5,000 ft²
- Unit lighting U = 1.3 VA/ft² (example only; verify code)
- Lighting demand D_L = 100%; continuous C_L = 1.25
- Receptacles: 60 outlets → R_raw = 60 × 180 = 10,800 VA; apply Table 220.44
- Signage S = 1200 VA; Show window W = 0 VA
- HVAC: Cooling 18,000 VA; Heating 25,000 VA → H = 25,000 VA
- Motors: 5 kVA, 3 kVA, 2 kVA → Σ = 10 kVA; largest = 5 kVA → M = 10 + 0.25×5 = 11.25 kVA → 11,250 VA
Lighting: L_raw = 5,000 × 1.3 = 6,500 VA → L = 6,500 × 1.00 × 1.25 = 8,125 VA.
Receptacles: R = 10,000 + 0.5 × (10,800 − 10,000) = 10,400 VA.
Total VA = L + R + S + W + H + M = 8,125 + 10,400 + 1,200 + 0 + 25,000 + 11,250 = 55,975 VA.
Total kVA = 55.975 kVA. On 3-phase 120/208 V: I = 55,975 / (√3 × 208) ≈ 155.5 A.
Frequently Asked Questions (FAQ)
Do I need to enter unit lighting loads per occupancy?
Yes. NEC 220.12 provides unit loads by occupancy; enter the appropriate VA/ft². Local amendments may adjust values.
When should lighting be treated as continuous at 125%?
Where the lighting meets the NEC definition of continuous load and applicable sections require 125% for feeders/services, enable the option. Apply engineering judgment.
Can I use Table 220.44 for all receptacles?
Only where permitted by NEC 220.44. The tool lets you toggle this and otherwise use a custom demand percentage.
How are motors handled if one motor is part of the HVAC I already entered?
Avoid double counting. Either include HVAC as HVAC only, or include that motor in the motor list and remove it from HVAC.
Does this tool account for kitchen equipment demand factors (commercial kitchens)?
This version focuses on general non-dwelling loads. For commercial kitchens and specialized equipment, apply the specific NEC sections/tables and add the resulting VA to motors or “other” categories as appropriate.
Is the computed current equal to the required main breaker rating?
No. The calculator reports calculated load current. OCPD sizing must follow additional NEC articles and may require oversizing, temperature corrections, and conductor ampacity checks.