Panelboard Load Guide | Breaker Sizing Workflow

Quick Answer: Use this page as a panelboard load and breaker-sizing workflow: gather panel rating, phase, voltage, connected load, demand assumptions, continuous-duty status, conductor limits, spare-capacity target, and the candidate breaker context; then run the Electrical Panel Load Calculator and Breaker Sizing Calculator before treating any NEC table or standard breaker size as selected.

Panelboards sit between the service and the individual branch and feeder circuits. Good design answers three questions:

1. Is the panel rating adequate for the diversified load?
2. Are branch and feeder breakers sized correctly for their loads and conductors?
3. Is there enough spare capacity for future loads without overloading the panel?

This guide explains practical panelboard loading and breaker sizing concepts and shows how to use:

• Electrical Panel Load Calculator
• Circuit Breaker Sizing Calculator

Note: This guide is educational only. Always refer to the adopted NEC edition and local amendments for binding requirements.

Panel loading concepts

Connected, demand, and design load

For a given panel:

• Connected load – Sum of nameplate or calculated loads of all circuits served by the panel (often in kW or VA).
• Demand load – Connected load after applying applicable demand factors or diversity assumptions.
• Design load – Demand load further adjusted for diversity factor and future expansion.

The Electrical Panel Load Calculator models the progression:

1. Total connected load
2. Total demand load (after applying demand factors)
3. Diversified load
4. Final design load including future expansion allowance

Panel rating, design current, and load factor

Panel loading is often summarized with:

• Panel rating (A) – Main breaker or busbar rating (e.g., 100 A, 200 A, 400 A).
• Design current (A) – Design load (kW) converted to current using line-to-line voltage and number of phases.
• Load factor (%) – Design current ÷ panel rating × 100.

High-level interpretations (subject to AHJ and local practice):

• Load factor well below 80% – Typically comfortable margin.
• Load factor near or above 80% – Pay attention to future expansion and operation.
• Load factor above 100% – Indicates overload; panel rating or load assumptions must change.

The panel load calculator reports:

• designLoad (kW)
• designCurrent (A)
• loadFactor (%)
• availableCapacity (A and %)

Panel load calculation workflow

The Electrical Panel Load Calculator accepts:

• Panel type and voltage: residential main/sub, commercial main/sub, or industrial
• Panel rating (A)
• General lighting, small-appliance, and laundry loads
• Major appliance and HVAC kW values
• Motor, other, and future expansion figures

Internally, it:

1. Converts entered kW to watts/VA
2. Applies typical NEC-style demand logic:

• General lighting – first block at 100%, next block at reduced factors, remainder at lower factors
• Small-appliance and laundry circuits – modeled at 1,500 VA each in many residential contexts
• Range, dryer, water heater, HVAC, and motors – modeled with simplified demand multipliers consistent with NEC-style demand and diversity concepts (edition-dependent)

3. Sums demand loads to a total demand kW
4. Applies the project diversity assumption to account for non-simultaneity
5. Adds future expansion percentage to reach a design kW and design current

You can tune:

• Diversity factor – Lower project-specific factor → more conservative design
• Future expansion – Extra headroom for future loads (e.g., shops, EVs, heat pumps)

The goal is to ensure:

• Design current ≤ panel rating
• Enough spare capacity remains for credible expansions

Breaker sizing rules: continuous and non-continuous loads

Once panel loading is known, each branch and feeder breaker must be sized correctly for load and conductor.

The Circuit Breaker Sizing Calculator focuses on:

• Load current (A) – Expected operating current
• Load type – Continuous, non-continuous, motor, or mixed
• Wire gauge (AWG) – Selected conductor size
• Temperature and conduit derating factors – For ambient temperature and conductor bundling

Continuous vs non-continuous load sizing

Many NEC editions treat loads differently depending on duration:

• Continuous loads – Expected to operate at maximum current for 3 hours or more.
• Non-continuous loads – Operate intermittently or for shorter periods.

A typical rule for many branch and feeder circuits (for example, in NEC editions that include requirements similar to 210.20(A) and 215.3) is to size the overcurrent device at:

• 125% of continuous load current, plus
• 100% of non-continuous load current

Use the Breaker Size Finder Guide as the reference page after the calculator has fixed the load type, conductor, terminal rating, and standard OCPD review point. A table row alone does not prove that the breaker and conductor are coordinated for the installation.

The breaker-sizing calculator models this by applying a sizing factor, often 1.25, for continuous or mixed loads. For more detailed discussion of overcurrent protection rules (including Articles 210, 215, 240, and 430) and interrupting-capacity examples, see the Breaker Selection Guide.

Conductor protection and compatibility

Breaker selection must also respect conductor ampacity (per Article 310 tables and applicable adjustment/correction factors). The calculator checks:

1. Base wire ampacity for the selected AWG
2. Adjusted ampacity using temperature and conduit-fill factors
3. Whether the breaker review point exceeds adjusted ampacity

Outputs include:

• recommendedBreaker (shown as the next standard breaker review point, A)
• minimumBreaker (A)
• wireAmpacity (A, adjusted)
• wireCompatibility and necCompliance flags

If breaker size exceeds adjusted ampacity, the installation would typically be considered noncompliant with conductor-protection rules and must be corrected by:

• Increasing conductor size (for example, confirmed with the Wire Size Calculator and, where voltage drop is critical, the Voltage Drop Calculator), or
• Reducing breaker size and/or load.

Calculator workflow examples

Residential main panel diversified-load check

Use this as an input checklist rather than a fixed answer:

• Panel rating, system voltage, and phase
• General lighting, receptacle, small-appliance, laundry, cooking, dryer, water-heating, HVAC, motor, and other fixed-load inputs
• Project diversity assumption
• Future expansion allowance

Using the Electrical Panel Load Calculator:

1. Select the panel type, voltage, phase, and rating.
2. Enter the actual connected-load and demand assumptions for the project.
3. Review key outputs:

• Total connected load (kW)
• Total demand load after NEC-style factors
• Diversified load and final design load
• Design current (A) and load factor (%)

After the calculator result, decide whether the panel rating, spare capacity, and future-load assumptions are still reasonable. If the result is close to the planning limit for the project, review the service size, feeder, panel schedule, and AHJ expectations before treating the panel as adequate.

Branch-circuit breaker sizing for a continuous load

Use this as the input sequence for a branch or feeder load that may be continuous:

Using the Circuit Breaker Sizing Calculator:

1. Enter:

• Load current
• Load type
• Selected conductor
• Voltage
• Temperature and conduit factors as appropriate

2. The calculator:

• Applies the appropriate load-type sizing factor
• Selects the next standard breaker review point
• Checks adjusted conductor ampacity against the breaker review point

3. Interpretation:

• If adjusted conductor ampacity supports the breaker review point, continue to equipment, terminal, and panel-schedule checks.
• If not, revise conductor size, derating assumptions, routing, or load assumptions before treating the breaker as selected.

A similar process applies for motor and mixed loads, using load types and recommendations consistent with motor-protection rules and continuous-load considerations. For detailed motor operating-point and starting-duty calculations, see tools such as the Three-Phase Motor Calculator and Motor Starting Current Calculator.

Coordination between panel load and breaker sizing

Panel loading and breaker sizing interact in several ways:

• Panel rating vs main breaker – Main breaker rating typically matches panel bus rating; its selection follows from service or feeder calculations.
• Feeder breakers – Need to be large enough for downstream panel demand load while coordinating with upstream protection.
• Branch breakers – Must protect conductors and equipment, and collectively their loads should not push the panel beyond its design load.

A practical coordination workflow is:

1. Use Residential Load and Electrical Service Size calculators to determine service and main panel requirements.
2. Use the Electrical Panel Load Calculator to check panel load factor and available capacity.
3. Size individual branch and feeder breakers with the Circuit Breaker Sizing Calculator, verifying conductor compatibility with the Ampacity Calculator for each.
4. Iterate if:

• Design current approaches panel rating too closely
• Future expansion requirements increase
• Motor starting or large non-linear loads require additional headroom.

When the next question is available fault current, interrupting rating, or SCCR, move from breaker sizing to the Short Circuit Calculator before updating the panel schedule.

Practical caveats and AHJ considerations

While the calculators encode common NEC-style logic, real projects must account for:

• Exact code edition and local amendments – Some jurisdictions adopt specific service-size minimums or vary demand-factor interpretations.
• Equipment nameplate data – Always reconcile calculator assumptions with manufacturer ratings.
• Special occupancies – Health care, hazardous locations, and similar occupancies have additional requirements.
• Protection coordination studies – Large or complex systems may require time–current coordination studies beyond simple breaker sizing.

The authority having jurisdiction (AHJ) ultimately interprets how code language applies in your project. Document:

• Calculation method and key assumptions
• Selected panel and breaker ratings
• Any deviations or engineering judgements used

Summary and next steps

Key points:

1. Panel loading is about converting connected loads to a diversified design load and checking panel rating and spare capacity.
2. Breaker sizing must respect both load type (continuous/non-continuous/motor) and conductor ampacity after derating.
3. Calculators like Electrical Panel Load and Circuit Breaker Sizing tools help automate arithmetic, but code and AHJ interpretations remain authoritative.
4. Good documentation of assumptions and selected ratings supports smoother design review and inspection.

To deepen your design workflow:

• Use Residential Load Calculations and Service Sizing per NEC for upstream context.
• Apply Wire Sizing: Ampacity, Voltage Drop, and NEC Examples to translate panel and breaker decisions into conductor sizes.
• Refer back to NEC Overview to locate the specific articles that back each calculation step.