Commercial Generator Sizing Calculator (NEC / IEC)
Accurate generator sizing for commercial and industrial applications
Commercial Generator Sizing
Select NEC or IEC standard to calculate your load
IEC Load Calculation: Enter connected loads. Diversity factors (e.g., 70% for lighting) will be applied per IEC.
Sizing Recommendation
- Base Size Driver --
- Calculated Running kVA 0.00
- Est. Starting kVA 0.00
- Rec. Size (w/ 70% Headroom) 0.00 kVA
- Final Size (w/ 10% Safety) 0.00 kVA
Sizing Notes
Enter loads and click 'Calculate Size' to see the sizing driver.
The Base Size is the minimum kVA to run your loads. The Rec. Size (w/ 70% Headroom) then applies the 70% target loading factor (e.g., `Base Size / 0.70`) to find the optimal generator size. This ensures the generator operates efficiently (60-80% capacity) and is not overloaded.
The Estimated Starting kVA is a critical check. If this value is larger than the Base Size, it will become the driver for the recommendation, as the generator *must* be large enough to handle the motor start. Always verify this with the generator supplier's transient analysis software.
How to Use the Calculator
1Select Standard
- Use the NEC / IEC toggle to choose your calculation method.
- This determines which demand or diversity factors are applied.
2Add Your Loads
- Click Add Load for each item.
- Enter the load size in kW.
- Select the Load Type from the dropdown. This is critical for applying the correct calculation percentage.
- Be sure to mark your single largest motor as Largest Motor.
3Set Sizing Parameters
- Enter the Starting Multiplier (e.g., 6) and PF for your largest motor.
- Set the Generator Target PF (typically 0.8) and a final Safety/Growth Factor.
- Click Calculate Size to see the results.
1. Formulas & Input Data
Basic Formulas (3-Phase):
S = (√3 × V × I) ÷ 1000 (kVA)
P = S × PF (kW)
I = (1000 × P) ÷ (√3 × V × PF)
PF = P ÷ S
Required Input Data:
- Connected load in kW (or kVA and PF)
- Power factor (PF) of each load
- Continuous (3+ hrs) or intermittent
- Motor data: size, PF, starting type
- Demand or diversity factors (NEC/IEC)
- Target generator loading (normally 70%)
- Generator power factor (normally 0.8)
2. NEC Demand Calculation
Step 1. Receptacle (NEC 220.44):
S_R = P_R ÷ PF_R (e.g., 0.9)
S_Rcalc = min(S_R, 10) + 0.5 × max(S_R - 10, 0)
P_RNEC = S_Rcalc × PF_R
Step 2. Lighting:
P_LNEC = 1.25 × P_L (if continuous)
Step 3. Motors:
P_MNEC = (Σ motors kW) + (0.25 × largest motor kW)
Step 4. Other Continuous:
P_CNEC = 1.25 × P_C (e.g., Music, IT)
Step 5. Total NEC Demand:
P_NECcalc = P_LNEC + P_RNEC + P_MNEC + P_CNEC + P_NonCont.
3. IEC Method & Step-Load
IEC Max Demand (Diversity):
Apply diversity factors (D_x) to each load:
P_xIEC = D_x × P_x
- Lighting: 0.6 to 0.8
- Receptacles: 0.4 to 0.6
- Motors: 1.0 (if must run)
- HVAC: 1.0 (design condition)
P_IECMD = Σ(P_xIEC)
Step-Load Check (ISO 8528-5):
S_Mstart = k × (P_M ÷ PF_M)
(k = 5 to 8 for DOL start). Find largest S_stepmax.
4. A) NEC Example (Demand)
Assume:
- Lighting: 10 kW (cont)
- Receptacles: 80 kW (at PF 0.9)
- Motor: 5 kW (largest)
- Music: 5 kW (cont)
1. Receptacles:
S_R = 80 ÷ 0.9 = 88.9 kVA
S_Rcalc = 10 + 0.5 × (88.9 - 10) = 49.45 kVA
P_RNEC = 49.45 × 0.9 = 44.5 kW
2. Lighting: 1.25 × 10 = 12.5 kW
3. Motors: 5 + (0.25 × 5) = 6.25 kW
4. Music: 1.25 × 5 = 6.25 kW
Total NEC Load (P_NECcalc) = 12.5 + 44.5 + 6.25 + 6.25 = 69.5 kW
5. B) Example (Final Sizing)
1. Convert to Running kVA (@ 0.8 PF):
S_genrun = 69.5 kW ÷ 0.8 = 86.9 kVA
2. Check Motor Start (DOL, k=6, 0.85 PF):
S_M = 5 ÷ 0.85 = 5.9 kVA
S_start = 6 × 5.9 = 35.4 kVA
3. Find Base Size (Pre-Headroom):
Base kVA = MAX(S_genrun, S_start)
Base kVA = MAX(86.9, 35.4) = 86.9 kVA
4. Apply Headroom (70% loading):
S_headroom = (S_genrun ÷ 0.70)
S_headroom = 86.9 ÷ 0.70 = 124.1 kVA
Recommended Size:
Rec. Size = MAX(Base kVA, S_headroom)
Rec. Size = MAX(86.9, 124.1) = 124.1 kVA
Select: ~125 kVA (100 kW) Gen.
6. Practical Notes & Summary
Practical Notes:
- Normal generator loading range is 60% to 80% of its rated kW.
- Standby generators are usually rated at 0.8 PF lagging.
- Always check transient performance per ISO 8528-5 to confirm voltage/frequency dip.
- Ensure cables/breakers are rated for at least 115% of generator current (NEC 445).
- Add 10–20% capacity margin (the Safety/Growth Factor) for future expansion.
Formula Summary:
S_run = P_calc ÷ 0.8
S_start = k × (P_M ÷ PF_M)
Base kVA = MAX(S_run, S_start)
Rec. kVA = MAX(Base kVA, (S_run ÷ 0.70))
Example Sizing Chart (NEC vs. IEC)
Shows Recommended Size kVA (w/ 70% Headroom) based on load type and standard (at 0.8 PF).
| Load (kW) | Load Type | NEC Generator Size (kVA) | IEC Generator Size (kVA) |
|---|---|---|---|
| 10 kW | Lighting (Continuous) | 21.9 kVA (Headroom) | 12.3 kVA (Headroom) |
| 80 kW | Receptacles | 78.0 kVA (Headroom) | 70.0 kVA (Headroom) |
| 5 kW | Largest Motor (6x start, 0.85PF) | 35.4 kVA (Starting) | 35.4 kVA (Starting) |
| 50 kW | Largest Motor (6x start, 0.85PF) | 353 kVA (Starting) | 353 kVA (Starting) |
| 200 kW | HVAC (Cooling) | 357 kVA (Headroom) | 357 kVA (Headroom) |
| 200 kW | Largest Motor (6x start, 0.85PF) | 1412 kVA (Starting) | 1412 kVA (Starting) |
| 500 kW | Receptacles | 454 kVA (Headroom) | 446 kVA (Headroom) |
| 500 kW | IT / UPS (Continuous) | 1116 kVA (Headroom) | 893 kVA (Headroom) |
| 100 kW (Mixed) | From Example Above | 124 kVA (Headroom) | 100 kVA (Headroom) |
| 100 kW (Mixed) | ...with 50kW Largest Motor | 353 kVA (Starting) | 353 kVA (Starting) |
Frequently Asked Questions
How do I calculate what size generator I need?
List all items you need to power. Add up their Running kW. Find the single Largest Motor and calculate its Starting kVA (kW × multiplier / PF). Your generator's kVA must be higher than *both* the total Running kVA (Total kW / 0.8) and the motor's Starting kVA.
How to size a generator based on kVA?
Sizing by kVA (kilo-volt-amperes) is more accurate as it accounts for power factor. The formula is: kVA = kW / Power Factor (PF). A typical PF for mixed commercial loads is 0.8. So, if your running load is 400 kW, you would need: 400 kW / 0.8 PF = 500 kVA. Always size for the larger of the running kVA or the largest motor's starting kVA.
How many kW generator for 2000 sq ft home?
A 2,000 sq ft home typically needs a 20 kW to 25 kW generator to run essentials like an air conditioner, refrigerator, lights, and small appliances. If you have larger loads (e.g., electric heat, multiple AC units), you may need 30 kW or more. A detailed load calculation is the only way to be certain.
What is the 20/20/20 rule for generators?
This is a rule of thumb for home standby generators, suggesting a 20 kW generator can power a 200-amp service panel and can be installed 20 inches from the home (check local codes, as 18" is also common). It's a marketing rule, not an engineering one. Always perform a proper load calculation.
What will a 12000 watt generator run in a house?
A 12,000-watt (12 kW) generator can run most essentials simultaneously, including a central AC unit (up to 3-4 tons), refrigerator, freezer, sump pump, lights, and outlets for TVs/computers. It may struggle if you add a second large load like an electric range or water heater at the same time.
How many AC can run on a 10 kVA generator?
A 10 kVA generator (about 8 kW at 0.8 PF) can typically run one central AC unit (up to 2.5-3 tons) *or* two to three large window/portable AC units, along with other small loads like lights and a refrigerator. The high starting current of AC compressors is the main limiting factor.
What's the difference between NEC and IEC sizing?
NEC (National Electrical Code - USA): Very prescriptive. It uses fixed demand factors, such as 125% for all continuous loads and tiered rules for receptacles. It also requires adding 25% of the largest motor's running load to the total.
IEC (International Electrotechnical Commission - Global): More flexible. It relies on the engineer's judgment to apply diversity factors (how many loads run at the same time) to calculate the maximum demand. This can result in a smaller calculated running load, but the final generator size is still often determined by the motor starting (transient) load, just like with NEC.