Calculator
Formula Used
Single phase current: I = VA ÷ V
Three phase current: I = VA ÷ (√3 × V)
Continuous load adjustment: Continuous VA × 1.25 for standard 80 percent rated breakers.
Motor adjustment: Motor VA + 25 percent of largest motor VA.
Demand adjustment: Adjusted VA × Demand Factor.
Reserve adjustment: Demand Adjusted VA × (1 + Reserve Percent ÷ 100).
Derated breaker need: Calculated Current ÷ Derating Factor.
Selected breaker: Next available standard breaker size above the required rating.
How to Use This Calculator
Choose the system phase first. Enter the service voltage. Select whether your load values are entered as apparent load or watt load.
Enter non-continuous and continuous loads separately. Add motor full load amps when motors are part of the service load.
Use the demand factor when allowed by your design method. Add derating for temperature, enclosure limits, or equipment conditions.
Add future reserve if the panel may serve more load later. Press the calculate button. Review the selected standard breaker size.
Example Data Table
| Example | Phase | Voltage | Continuous Load | Non-Continuous Load | Motor FLA | Reserve | Typical Result |
|---|---|---|---|---|---|---|---|
| Small workshop | Single | 240 V | 4,000 VA | 8,000 VA | 15 A | 15% | 100 A range |
| Commercial panel | Three | 208 V | 18,000 VA | 35,000 VA | 30 A | 20% | 225 A range |
| Light industrial | Three | 480 V | 60,000 VA | 110,000 VA | 60 A | 25% | 300 A range |
Main Circuit Breaker Calculation Guide
Why Breaker Sizing Matters
A main circuit breaker protects the main feeder and panel. It also limits current before branch breakers divide the load. A poor estimate can cause nuisance trips. A larger unsafe choice can expose conductors to damage. This calculator helps you compare connected load, continuous load, motor load, reserve, demand, and derating.
Understanding Load Types
Non-continuous loads are usually counted at full value. Continuous loads may run for long periods. Many designs treat continuous load at one hundred twenty five percent when using common rated breakers. Motors also need special attention. The largest motor often adds an extra twenty five percent because starting and running conditions are harder.
Phase and Voltage
Single phase systems use volts directly in the current formula. Three phase systems use the square root of three multiplied by line voltage. That is why the same load can need less current on a higher voltage three phase system. The calculator handles both methods automatically.
Demand, Derating, and Reserve
Demand factor reduces the calculated load when diversity is allowed. It should only be used when supported by a real design method. Derating increases the needed breaker rating when heat, equipment limits, or installation rules reduce capacity. Future reserve adds planning room for later equipment. Reserve is useful for workshops, commercial suites, garages, and growing facilities.
Reading the Result
The calculated current shows the working load after adjustments. The minimum breaker rating includes derating. The selected standard breaker is the next common size above that rating. Spare capacity shows estimated room after derating. Always compare the result with conductor ampacity, equipment labels, local rules, and professional judgment. This tool supports planning. It does not replace a licensed electrical design review.
FAQs
What is a main circuit breaker?
It is the main protective switch for a panel or service. It limits current before power reaches branch circuits. It also provides a main disconnect point.
Can I choose the next larger breaker?
Only when conductors, panel ratings, equipment labels, and local rules allow it. A breaker must protect the wiring and connected equipment, not just match the load.
Why is continuous load multiplied by 1.25?
Many standard breaker applications require extra capacity for loads that run for long periods. The calculator applies this when 80 percent rated breaker mode is selected.
What does derating mean?
Derating reduces usable capacity because of heat, enclosure limits, grouping, or equipment conditions. The calculator raises the required breaker rating when derating is entered.
Should I enter watts or VA?
Use VA when available. If you enter watts, add the power factor. The calculator converts watts to apparent load before calculating current.
Does this calculator size wires?
No. It estimates breaker size only. Wire size must be checked separately using ampacity, insulation rating, installation method, temperature, and local code requirements.
Why add future reserve?
Reserve gives space for possible later loads. It can reduce panel replacement risk. Use a practical reserve based on expected expansion and available service capacity.
Can this result be used for permits?
Use it as a planning estimate. Permit work may require formal load calculations, code references, equipment data, and approval from a qualified electrical professional.