Bus Bar Size Calculator (IEC & NEC Compliant)
Size busbars with a safety factor or perform a full IEC/NEC verification
Bus Bar Sizing Tool
Standard Sizing, IEC 61439 & NEC (NFPA 70) Modes
Results:
Total Design Current: 0.00 A
Required Area: 0.00 mm²
Recommended Busbar: N/A
How to Use the Calculator
1. Standard Sizing
- Choose to calculate by Current (Amps) or Power (kW).
- Enter your system's parameters (e.g., 669 A or 500 kW @ 480V).
- Select the busbar Material (Copper or Aluminum).
- Adjust the Safety Factor if needed (default is 25%).
- Click Calculate to see the required area and recommended size.
2. Full IEC Verification
- Enter your base parameters as in the standard method.
- Check the Perform Full IEC Verification box.
- This disables the safety factor and reveals IEC-specific inputs.
- Enter derating factors, short-circuit current, and fault time.
- Click Calculate to get a full compliance report.
3. Full NEC Verification
- Enter your base parameters.
- Check the Perform Full NEC Verification box.
- This reveals NEC-specific inputs like equipment temperature rating.
- Enter the short-circuit and fault parameters.
- Click Calculate for a size recommendation and NEC compliance check.
Busbar Selection & Sizing (IEC Explained)
Busbars carry massive current safely through switchboards. Their design must satisfy thermal, mechanical, and fault requirements according to IEC standards to ensure they won’t overheat, deform, or fail during faults.
⚡ Step 1: Define Applicable Standards
First, know which IEC standards guide your design:
- IEC 61439-1/-2: Main LV switchboard standard.
- IEC 60364-5-52: Defines current capacity and derating.
- IEC 60949: Adiabatic formula for thermal short-circuit withstand.
- IEC 60890: Calculation methods for temperature rise.
🔢 Step 2: Calculate Design Current (Ib)
Use the 3-phase power formula, rearranged for current:
I = P / (√3 × V × cosφ)
Example: For a 500 kW load at 400V with 0.9 PF, the design current (Ib) is 801 A.
🌡️ Step 3: Apply Derating Factors
Busbars in hot or enclosed environments can't carry as much current. Divide the design current by the relevant derating factors.
Example: If the ambient temperature is 50°C (derating factor of 0.9):I_required = 801 A / 0.9 = 890 A
📏 Step 4: Estimate Required Cross-Section
Pick a preliminary size based on a conservative current density (J), typically 1.6 A/mm² for copper.
S_pre = I_required / J
Example: For 890 A:S_pre = 890 / 1.6 = 556 mm²
Let's choose a standard size of 2 x (40x8 mm) bars = 640 mm².
♨️ Step 5: Verify Temperature Rise
IEC 61439 limits temperature rise (typically 70°C). We can check our design by calculating the actual current density.
Example:J_actual = 890 A / 640 mm² = 1.39 A/mm²
Since 1.39 A/mm² is safely below the typical 1.6-2.5 A/mm² limit, this busbar is thermally acceptable.
🔥 Step 6: Check Short-Circuit Thermal Withstand
The busbar must survive the heat from a short-circuit fault. Use the IEC 60949 adiabatic formula: $S \ge \frac{I_k \times \sqrt{t}}{k}$
- S: Required cross-section (mm²)
- I_k: RMS short-circuit current (A)
- t: Fault clearing time (s)
- k: Material constant (~143 for copper)
Example: For a 50 kA fault for 1s, required area is 350 mm². Our 640 mm² bar passes easily.
🧲 Step 7: Check Mechanical (Dynamic) Withstand
Faults create huge magnetic forces. The peak current ($I_{pk} = \kappa \times I_k$) determines this force. For a 50 kA fault, $I_{pk}$ could be 100 kA.
Your design must use busbar supports and spacing rated for this force, as specified by IEC 61439 or manufacturer data.
✅ Step 8: Final Verification Summary
A compliant busbar design must pass all checks:
- Design Current: Calculated correctly (801 A).
- Derated Current: Accounted for conditions (890 A).
- Continuous Temp Rise: OK (1.39 A/mm² is safe).
- Thermal Short-Circuit: OK (640 mm² > 350 mm²).
- Mechanical Withstand: OK (with proper supports).
Understanding Busbar Derating Factors
A busbar's capacity is rated for ideal lab conditions. Real-world factors like heat, altitude, and bundling reduce its effective rating. You must apply derating factors to ensure safety by calculating the current the busbar must be rated for under these specific conditions.
🌡 Ambient Temperature Derating
When surrounding air is hotter, the busbar cannot cool efficiently. To prevent overheating, you must reduce its allowable current.
Formula: I_required = Ib / Ca
Example: For a design current (Ib) of 801 A where the ambient temperature factor (Ca) is 0.9, the required rating is 890 A.
👥 Grouping Derating (Proximity Effect)
When several busbars are installed close together, their combined heat makes cooling harder, requiring a reduction in each busbar's current rating.
Formula: I_required = Ib / Cg
Example: If three circuits are grouped (Cg = 0.8), the required rating for a design current of 801 A becomes 1001 A.
🏔 Altitude Derating
At high altitudes (above 2000m), thinner air cannot cool conductors effectively, so the current rating must be reduced.
Formula: I_required = Ib / Calt
Example: At 4000m (Calt = 0.91), the required rating for a design current of 801 A becomes 880 A.
Combined Derating (All Factors Applied)
In most cases, multiple derating conditions apply. The correction factors are multiplied together to find the total derating effect.
I_required = Ib / (Ca × Cg × Calt)
Example: For a design current of 801 A with Ca=0.9, Cg=0.8, and Calt=0.96, the final required rating becomes 1159 A. You must select a busbar rated for at least this value.
✅ Derating Factors Summary Table
| Factor | Symbol | Typical Range | Main Cause |
|---|---|---|---|
| Ambient temperature | Ca | 0.7 – 1.0 | High surrounding air temperature |
| Grouping | Cg | 0.6 – 1.0 | Mutual heating between busbars |
| Altitude | Calt | 0.86 – 1.0 | Thinner air at high altitude |
Quick IEC Compliance Checklist
Calculate Design Current (Ib)
Start with the actual current your load will draw.
Apply Derating Factors
Correct for temperature, grouping, and altitude to find the required rating.
Verify Temperature Rise
Ensure the chosen busbar cross-section stays within thermal limits.
Check Thermal Withstand
Verify the busbar can survive the heat from a short-circuit fault.
Check Mechanical Withstand
Ensure supports can handle magnetic forces during the fault's peak current.
✅ NEC Compliance Quick Checklist
Calculate Load Current
Determine the full-load current (FLA) your system will draw.
Apply 125% Rule
Size busbar ampacity to at least 125% of the continuous load.
Check Temperature Limits
Ensure the busbar doesn't exceed equipment termination ratings (e.g., 75°C).
Verify Thermal Withstand
Confirm the busbar can survive the heat from a short-circuit fault.
Verify Mechanical Withstand
Ensure supports can handle magnetic forces from the fault current.
Check Voltage Drop
Keep voltage drop below the NEC's recommended 3% limit for feeders.
Busbar Size Chart (Quick Reference)
This chart provides recommended busbar sizes for common continuous current ratings. The configurations shown are verified to pass typical IEC and NEC checks for thermal and short-circuit withstand, assuming a 50 kA fault and standard derating conditions. Use the calculator above for precise sizing based on your specific parameters.
| Current (A) | Busbar Size (mm) | No. of Bars / Phase | IEC Check | NEC Check |
|---|---|---|---|---|
| 400 A | 50 x 10 | 1 | PASS | PASS |
| 800 A | 80 x 10 | 1 | PASS | PASS |
| 1200 A | 100 x 10 | 1 | PASS | PASS |
| 1600 A | 80 x 10 | 2 | PASS | PASS |
| 2000 A | 100 x 10 | 2 | PASS | PASS |
| 2500 A | 120 x 10 | 2 | PASS | PASS |
Frequently Asked Questions (FAQs)
What size is a 400 amp busbar?
There is no single size, as it depends on material, ambient temperature, and installation conditions. However, a common rule of thumb for a copper busbar is approximately 1.2 A/mm². This suggests a cross-sectional area of about 333 mm². A standard size like 50 mm x 10 mm (500 mm²) is often a safe choice for a 400A copper busbar in moderate conditions. For aluminum, the required area would be larger. Always verify with manufacturer data and apply necessary derating factors.
What is the formula for earthing busbar size?
The size of an earthing (grounding) busbar is determined by its ability to handle a fault current for a specific duration without overheating. The commonly used adiabatic formula is: $$ S = \frac{\sqrt{I_{fault}^2 \times t}}{k} $$ Where:
- S = Minimum cross-sectional area in mm².
- $I_{fault}$ = RMS prospective fault current in Amperes (A).
- t = Fault clearing time in seconds (s).
- k = A material-dependent constant (e.g., ~143 for copper).
What size is an 800 amp busbar?
Using the rule of thumb of 1.2 A/mm² for copper, an 800A busbar would require a cross-sectional area of at least 667 mm². A standard copper busbar size of 80 mm x 10 mm (800 mm²) would typically suffice for this rating under favorable conditions. As with any busbar, this is an estimate; the final size must account for derating factors for temperature, grouping, and altitude. For aluminum, a larger cross-section would be necessary.
What size is a 600 amp busbar?
Based on a typical current density of 1.2 A/mm² for copper, a 600A load requires a busbar with a cross-sectional area of approximately 500 mm². A standard copper busbar of 60 mm x 10 mm (600 mm²) is a common and safe selection for a 600A rating in good installation conditions. Always consult manufacturer specifications and apply derating for real-world conditions like high ambient temperatures.