Panelboard Sizing Calculator – NEC

Accurate panelboard sizing is critical for electrical safety and compliance with NEC standards. Proper calculations prevent overloads and ensure system reliability.

This article explores the NEC-based panelboard sizing calculator, providing formulas, tables, and real-world examples. Learn to size panelboards confidently and precisely.

Artificial Intelligence (AI) Calculator for “Panelboard Sizing Calculator – NEC”

• Calculate panelboard size for a 120/240V residential service with 100A load.
• Determine panelboard rating for a commercial building with 200A main breaker and 50kVA transformer.
• Find panelboard size for a 3-phase, 480V industrial load of 150A.
• Estimate panelboard capacity for a mixed-use facility with 75kW total connected load.

Comprehensive Tables for Panelboard Sizing According to NEC

Table 1: Common Panelboard Ratings and Corresponding Maximum Continuous Load (NEC 408.36)

Table 2: NEC 2023 Panelboard Sizing Based on Load Type and Demand Factors (NEC Article 220)

Table 3: Typical Panelboard Busbar Ratings and Corresponding Breaker Sizes

Essential Formulas for Panelboard Sizing According to NEC

Panelboard sizing requires understanding load calculations, demand factors, and NEC requirements. Below are the key formulas with detailed explanations.

1. Total Load Calculation

The total load (I_total) is the sum of all individual loads connected to the panelboard, adjusted by demand factors.

• I_total: Total panelboard load current (Amps)
• I_load: Individual load current (Amps)
• Demand Factor: Percentage (decimal) applied to account for non-simultaneous use

2. Continuous Load Adjustment

NEC 210.20(A) requires continuous loads to be multiplied by 125% to ensure safety margins.

• I_adj: Adjusted continuous load current (Amps)
• I_continuous: Continuous load current (Amps)

3. Motor Load Sizing

Per NEC 430.22, motor branch-circuit conductors and overcurrent protection must be sized at 125% of the motor full-load current.

• I_motor: Motor load current for sizing (Amps)
• FLC: Motor full-load current from manufacturer or NEC tables (Amps)

4. Panelboard Rating Selection

The panelboard rating must be equal to or greater than the total adjusted load current.

5. Transformer Secondary Load Calculation

When sizing panelboards fed from transformers, calculate the secondary current based on transformer kVA and voltage.

• I_secondary: Transformer secondary current (Amps)
• Transformer kVA: Transformer rating in kilovolt-amperes
• V_line-line: Line-to-line voltage (Volts)
• V_line-neutral: Line-to-neutral voltage (Volts)

Detailed Real-World Examples of Panelboard Sizing Using NEC Guidelines

Example 1: Residential Panelboard Sizing for a 120/240V Service

A single-family home has the following loads:

• General lighting and receptacles: 8,000 VA
• Small appliance circuits: 1,500 VA × 2 circuits
• Electric water heater (continuous load): 4,500 VA
• Air conditioning unit (motor load): 3 HP, 240V single-phase

Calculate the minimum panelboard rating required.

Step 1: Calculate general lighting load current

Assuming 120/240V single-phase service, current is calculated as:

Step 2: Calculate small appliance circuits load

Total VA = 1,500 VA × 2 = 3,000 VA

Step 3: Calculate continuous load (water heater)

Continuous load current:

Apply 125% factor for continuous load:

Step 4: Calculate motor load (air conditioning unit)

From NEC Table 430.248, a 3 HP, 240V single-phase motor has a full-load current (FLC) of approximately 14 A.

Apply 125% factor:

Step 5: Sum all loads

Step 6: Select panelboard rating

Choose the next standard panelboard rating above 86.77 A, which is 100 A.

Result: A 100A panelboard is sufficient for this residential service.

Example 2: Commercial Panelboard Sizing for a 3-Phase 480V System

A commercial facility has the following loads:

• Lighting load: 30 kVA
• Receptacle load: 20 kVA
• HVAC motors: 3 motors, each 10 HP, 480V 3-phase
• Continuous process load: 15 kW

Calculate the minimum panelboard rating required.

Step 1: Calculate lighting and receptacle load current

Total VA = 30 kVA + 20 kVA = 50,000 VA

Step 2: Calculate motor load current

From NEC Table 430.250, a 10 HP, 480V 3-phase motor has an FLC of approximately 14 A.

Apply 125% factor for each motor:

Total motor load current:

Step 3: Calculate continuous process load current

Continuous load current:

Apply 125% factor for continuous load:

Step 4: Sum all loads

Step 5: Select panelboard rating

Choose the next standard panelboard rating above 135.1 A, which is 150 A.

Result: A 150A panelboard is required for this commercial application.

Additional Technical Considerations for NEC Panelboard Sizing

• Voltage Rating: Ensure the panelboard voltage rating matches the system voltage (e.g., 120/240V, 208Y/120V, 480V).
• Short-Circuit Current Rating (SCCR): Panelboards must have an SCCR equal to or greater than the available fault current at the installation point (NEC 110.10).
• Space for Future Expansion: Consider additional capacity for future loads or circuits to avoid premature panelboard replacement.
• NEC Article 408: Governs panelboard construction, installation, and sizing requirements.
• Load Diversity: Apply demand factors per NEC Article 220 to avoid oversizing.
• Continuous vs Non-Continuous Loads: Continuous loads require 125% sizing; non-continuous loads do not.
• Motor Loads: Must be sized per NEC Article 430, including conductor and breaker sizing.

Authoritative Resources and References

• National Fire Protection Association (NFPA) – NEC Official Site
• EC&M Magazine – Electrical Codes and Standards
• Eaton – Panelboard Technical Specifications
• Schneider Electric – Electrical Design Resources

By following NEC guidelines and using precise calculations, engineers and electricians can ensure safe, compliant, and efficient panelboard installations. This comprehensive approach minimizes risks and optimizes electrical system performance.