Electrical Load & Breaker Size Calculator
List the loads of a building, apply demand factors and get the maximum demand, the main breaker, a breaker for each circuit and a transformer or generator size. Export the load schedule to PDF or Excel. Modelling the installation too? Browse our MEP Revit families.
1-phase and 3-phaseIEC or NEC breaker ratingsPhase balancingPDF and Excel exportFormula with your numbers
Supply
Line-to-line voltage for 3-phase.
Margin for future load on the main breaker and transformer.
Used as the title of the PDF and Excel files.
Loads
One row per kind of load. Picking a type fills a typical power factor and demand factor; edit them to your local code and the nameplate data.
Recommended main breaker
Enter your loads to see the result.
Preliminary sizing only. Demand factors and power factors are typical values, not code values for every country: check them against your local code (NEC Article 220, BS 7671, IEC 60364 or your utility rules). This calculator does not select cable sizes (use the Voltage Drop Calculator), and it does not check discrimination, short-circuit breaking capacity or motor starting.
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Request a projectA load schedule in three steps
Pick the supply
Choose a preset such as 230 V 1-phase or 400 V 3-phase, or type your own voltage. US presets switch to NEC breaker ratings.
Add your loads
Add a row per kind of load. The type fills a typical power factor and demand factor; change them to match your code and the nameplates.
Read and export
See the maximum demand, phase currents, breaker sizes and transformer size, check the formula with your numbers, then download PDF or Excel.
How the calculator works
Connected load = quantity × power per unit. For motors and lifts the input power is the rated output ÷ efficiency (1 HP = 745.7 W).
Maximum demand P = Σ connected load × demand factor. Reactive power Q = P × tan φ, and apparent power S = √(P² + Q²). The overall power factor is P ÷ S.
Current
3-phase: I = P ÷ (√3 × U × cos φ), U line-to-line
1-phase: I = P ÷ (U × cos φ), U line-to-neutral
Breakers: each circuit gets the smallest standard rating In ≥ Ib. Rows marked as continuous or motor loads use 1.25 × Ib. The main breaker uses the most loaded phase plus your spare capacity.
Example: a 7.5 kW motor, efficiency 90 %, cos φ 0.85 on 400 V 3-phase. Input = 7,500 ÷ 0.9 = 8,333 W. Ib = 8,333 ÷ (1.732 × 400 × 0.85) = 14.2 A. With 125 %: 17.7 A, so a 20 A breaker.
Typical defaults used by the calculator
| Load type | Power factor | Demand factor | Source of the demand factor |
|---|---|---|---|
| Lighting | 0.90 | 100 % | Schneider Electric Electrical Installation Guide, ks = 1 |
| Sockets / receptacles | 0.90 | 50 % | NEC 220.44: load above 10 kVA at 50 % |
| Air conditioning | 0.85 | 100 % | Schneider guide, heating and air conditioning ks = 1 |
| Motor (pump, fan) | 0.85 | 75 % | Schneider guide, utilization factor ku = 0.75 for motors |
| Water heater / heating | 1.00 | 100 % | Schneider guide ks = 1; NEC 220.51 |
| Kitchen / cooking | 0.95 | 100 % to 65 % | NEC 220.56 by number of units |
| Lift / elevator | 0.85 | 100 % to 72 % | NEC Table 620.14 by number of lifts |
These are typical values. Your local code or utility may require different ones, so every value is editable.
Standard breaker ratings, IEC: 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A (IEC 60898-1), then 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3200, 4000 A. NEC 240.6(A): 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 A.
Frequently asked questions
Add up the connected load of every item (quantity × power), multiply each group by its demand factor to get the maximum demand in kW, then convert to kVA with the power factor. The current is I = P ÷ (√3 × U × cos φ) for 3-phase or I = P ÷ (U × cos φ) for 1-phase.
Connected load is the total rating of everything installed. Maximum demand is the highest load that actually runs at the same time. The ratio between them is the demand factor, for example 50 % for general sockets.
Work out the design current Ib of the circuit and choose the next standard breaker rating at or above it. For continuous loads and motors many designers, and the NEC, use 125 % of Ib. The cable must then be rated at or above the breaker.
NEC 430.22 sizes motor circuit conductors at 125 % of the full-load current, and the breaker is commonly sized the same way. The breaker must also ride through the starting current, so use a curve D MCB, a motor-protection breaker or an MCCB with a suitable magnetic setting.
Take the maximum demand in kVA, add a spare margin (20 % is common) and choose the next standard size. For a generator, also check that it can start the largest motor.
No. It sizes breakers and the main supply. Use the Voltage Drop Calculator to choose a cable for each circuit.
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