Calculator Inputs
Example Data Table
| Case | Load (A) | Base (A) | Method | Temp (°C) | Group | Overall | Derated (A) | Status |
|---|---|---|---|---|---|---|---|---|
| Feeder A | 95 | 125 | In conduit | 40 | 3 | 0.546 | 68.25 | REVIEW |
| Motor B | 42 | 63 | Cable tray | 35 | 2 | 0.730 | 45.99 | PASS |
| Buried C | 110 | 160 | Buried direct | 30 | 1 | 0.893 | 142.88 | PASS |
Formula Used
The calculator applies a combined derating factor based on selected conditions:
Factors are examples meant for quick checks. For final sizing, confirm with applicable codes, cable datasheets, and site-specific installation conditions.
How to Use This Calculator
- Enter the load current and your cable’s base ampacity.
- Select insulation and installation method that match your layout.
- Set ambient temperature, grouping count, altitude, and harmonics.
- For buried routes, enter soil resistivity and burial depth.
- Choose built-in factors or enter custom factors for your standard.
- Click Calculate to see results above the form instantly.
- Download CSV or PDF to attach in your submittals.
Practical Guide to Cable Derating on Site
Cable current ratings published in tables or datasheets assume reference conditions: a specified ambient temperature, limited grouping, and a defined installation arrangement. Real projects rarely match those assumptions. When cables are placed in conduits, bundled on trays, routed through warm rooms, or buried in soil with poor thermal properties, heat cannot escape as efficiently. Derating converts a “base ampacity” into a realistic allowable current so the insulation temperature limit is not exceeded during operation.
This calculator applies a combined adjustment factor using temperature, grouping, installation method, altitude, harmonics, and—when buried—soil resistivity and burial depth. The overall factor is the product of each individual factor. A factor below 1.00 reduces capacity; a factor above 1.00 is occasionally seen at cooler temperatures. The output “Derated ampacity” is the base rating multiplied by the overall factor. For design checks, the tool also applies an optional safety margin to the load current and compares it with the derated capacity to indicate whether the selection needs review.
Example using the included table: assume a feeder with a load of 95 A, a base ampacity of 125 A, ambient temperature of 40 °C, and three grouped circuits in conduit with continuous duty and 20% harmonic THD. The factors combine to an overall value of approximately 0.546, producing a derated ampacity near 68.25 A. With a 10% margin, the required current is about 104.50 A, so the case is flagged for review. This does not automatically mean the design is unsafe; it indicates you should select a higher-rated cable, improve installation conditions, reduce grouping, or validate factors using your project standard.
Use custom factors when your project follows a specific code table or manufacturer correction chart. Enter the factors directly, keeping a clear record of the source and assumptions in your calculations package. After verifying thermal capacity, remember that cable selection also depends on voltage drop, short-circuit withstand, protective device coordination, and termination limits. Export the results to CSV or PDF to support drawings, submittals, inspection notes, and handover documentation.
FAQs
1) What does “base ampacity” mean?
Base ampacity is the cable’s current rating under reference conditions from a datasheet or a code table. Derating adjusts this value to reflect your actual installation environment.
2) Why does grouping reduce ampacity?
Grouped cables heat each other and reduce effective cooling. More circuits in the same space raise conductor temperature, so the allowable current must be reduced to stay within insulation limits.
3) When should I use soil resistivity and burial depth?
Use them only for buried installations. Soil resistivity represents how well heat moves through the ground, and deeper burial generally reduces heat transfer, requiring additional derating.
4) How do harmonics affect cable heating?
Harmonics increase losses in conductors and can add neutral heating in certain systems. If THD is significant, a conservative factor helps account for extra heat and reduces the allowable current.
5) What does the “Minimum base ampacity needed” output tell me?
It estimates the minimum table/datasheet rating required to carry your load plus margin after derating. Use it to shortlist cable sizes before confirming with full design checks.
6) Can a factor be greater than 1.00?
Yes. Cooler ambient conditions may allow a slight increase in capacity compared with the reference temperature. Always confirm that your standard permits factors above 1.00 for the scenario.
7) Is a “REVIEW” status a failure?
Not necessarily. It flags that derated capacity is below the required current including margin. Re-check inputs, confirm factors from your standard, and consider design changes or larger cable selection.