Inputs
Enter project values to estimate switchgear ratings and fault capacity selection.
Example Data Table
| System | Voltage (V) | Load (kW) | PF | Demand | Diversity | Margin | Fault (kA) | Recommended Breaker | Bus |
|---|---|---|---|---|---|---|---|---|---|
| Three\u2011phase | 415 | 250 | 0.90 | 0.85 | 1.10 | 20% | 35 | 630 A | 630 A |
| Three\u2011phase | 400 | 120 | 0.92 | 0.80 | 1.20 | 15% | 25 | 315 A | 400 A |
| Single\u2011phase | 230 | 18 | 0.95 | 1.00 | 1.00 | 10% | 10 | 100 A | 125 A |
Formula Used
Design kW = Coincident kW \u00d7 (1 + Margin%)
Single\u2011phase: I = P(W) \u00f7 (V \u00d7 PF \u00d7 \u03b7)
Design I = I \u00d7 Continuous Factor
I\u00b2t (kA\u00b2\u00b7s) = (Isc_kA)\u00b2 \u00d7 t
How to Use This Calculator
- Enter system type, voltage, connected load, power factor, and efficiency.
- Set demand and diversity factors to reflect realistic coincidence.
- Add future margin and continuous factor for design conservatism.
- Provide the site short\u2011circuit level from a study or utility data.
- Click Calculate Switchgear to view recommended ratings and download reports.
Define coincident demand from connected load
Switchgear sizing should start with coincident demand, not the sum of nameplates. Apply a demand factor to reflect likely maximum usage, then apply diversity to account for non-simultaneous operation. This converts connected kW into a realistic design kW suitable for construction sequencing and temporary services. For mixed-use boards, separate essential and non-essential loads to avoid oversizing and improve selectivity later. Record assumptions early to speed approvals and procurement.
Convert design power into design current
The calculator converts design kW to current using voltage, power factor, and efficiency. For three-phase systems, current depends on √3 × V × PF × efficiency, while single-phase uses V × PF × efficiency. Converting kW to kVA clarifies transformer loading and feeder utilization during reviews.
Apply margins for continuous duty and growth
Continuous loads can run for long periods and should be sized conservatively. A continuous factor (often 1.25) increases the calculated current to reduce overheating risk. A future margin adds capacity for equipment additions, tenant changes, and commissioning modifications. Higher ambient temperatures may also require additional capacity; treat the ambient adjustment as an advisory check.
Select standard breaker and bus ratings
After the design current is established, choose the next standard breaker rating above the requirement. The bus rating should meet or exceed the breaker rating so the assembly is not internally limited. Installation location and IP rating support practical decisions for dust, moisture, and maintenance access on site.
Check fault level and document assumptions
Interrupting capacity must be at least the available short-circuit current at the board. Example data: three-phase 415 V, 250 kW, PF 0.90, efficiency 0.95, demand 0.85, diversity 1.10, margin 20%, continuous 1.25, fault 35 kA for 1.0 s. Use the CSV/PDF exports to capture inputs and results for reviews, procurement, and coordination studies.
FAQs
1) What is the main output of this calculator?
It estimates design current from load factors and suggests standard main breaker, bus rating, and minimum interrupting capacity based on the entered fault level.
2) How do I pick demand and diversity factors?
Use historical operation, scheduling, and load studies where available. Demand reflects maximum usage, while diversity accounts for non-simultaneous operation across multiple circuits.
3) Why does the result change when I edit power factor?
Lower power factor increases current for the same kW. That raises the selected breaker and bus ratings, and can affect upstream transformer utilization.
4) Do I need the continuous factor for all projects?
Use it when loads can operate continuously or near-continuously. If most loads are intermittent, a lower factor may be justified by project criteria and standards.
5) Is the ambient adjustment mandatory?
No. It is a quick advisory for high temperatures. Final derating should follow manufacturer temperature-rise data, enclosure ventilation, and installation conditions.
6) Where does the short-circuit value come from?
From a short-circuit study or utility data combined with transformer impedance and cable details. Fault current typically decreases with distance from the source.
7) Can I use the CSV/PDF exports in submittals?
Yes, as supporting documentation of assumptions and preliminary sizing. For final submittals, attach coordination studies and manufacturer ratings for the selected equipment.