Derate grouped conductors for trays, bundles, and conduits. Enter base ampacity and factors. Get adjusted ratings, plus downloadable CSV and PDF reports fast onsite.
This example reflects a commonly used current-carrying conductor adjustment approach for more than three conductors in a shared space. Always confirm with your governing standard and cable manufacturer.
| Current-carrying conductors (CCC) | Typical grouping factor | Meaning |
|---|---|---|
| 1–3 | 1.00 | No grouping adjustment applied |
| 4–6 | 0.80 | Moderate heating from shared space |
| 7–9 | 0.70 | Higher mutual heating between conductors |
| 10–20 | 0.50 | Significant grouping heat buildup |
| 21–30 | 0.45 | Severe grouping conditions |
| 31–40 | 0.40 | Very severe grouping conditions |
| 41+ | 0.35 | Extreme grouping; verify design assumptions |
Tip: If your project uses a different table, tick “Override with manual factor” and enter the correct multiplier.
Adjusted ampacity is calculated by multiplying the base ampacity by each applicable multiplier:
Adjusted Ampacity = Base Ampacity × Grouping Factor × Temperature Factor × Other Factor
Grouping factors account for heat buildup when multiple loaded conductors share the same raceway, cable, bundle, or tray. Temperature and other factors are optional multipliers you can supply.
When several loaded conductors share the same thermal space, heat dissipation drops. Temperature rises faster, insulation ages sooner, and allowable current must be reduced. Grouping derating keeps conductor temperature closer to its rating under realistic installation conditions.
The key input is the number of current-carrying conductors (CCC) in the raceway, cable, bundle, or tray. Equipment grounding conductors are not counted. Neutrals may count depending on loading and harmonics, so the calculator lets you include them intentionally.
A commonly applied set of multipliers is: 1–3 CCC = 1.00, 4–6 = 0.80, 7–9 = 0.70, 10–20 = 0.50, 21–30 = 0.45, 31–40 = 0.40, and 41+ = 0.35. Example: 8 CCC often uses 0.70.
The calculator multiplies your base ampacity by grouping, temperature, and any additional factor you provide. This mirrors common engineering workflow where independent corrections are applied together. It also keeps assumptions transparent for reviewers and inspectors. If your standard uses a different table, the manual override accepts the required multiplier.
Hotter ambient conditions, rooftop exposure, poor ventilation, or unfavorable soil thermal resistivity can further reduce capacity. Temperature multipliers are typically below 1.00 when the site is hotter than the reference condition. Use 1.00 when no correction applies.
With a design load entered, the tool back-calculates the minimum base ampacity needed after derating. This is useful when comparing conductor options from tables: you can confirm whether a candidate base value remains adequate once all factors are applied. This speeds sizing decisions in early design and budgeting stages.
Projects often include a margin (commonly 5–20%) to cover future circuit additions, uneven loading, or installation variability. The margin option reports a higher minimum base ampacity target. The CSV/PDF exports support design review notes, submittals, and commissioning records.
Suppose base ampacity is 100 A, CCC = 8, grouping factor 0.70, temperature factor 0.91, and other factor 1.00. Total factor = 0.70 × 0.91 × 1.00 = 0.637, so adjusted ampacity ≈ 63.7 A. A 55 A load passes.
Use correct factors from your governing standard for compliance.
1) What is “base ampacity” in this tool?
Base ampacity is the unadjusted conductor rating taken from your selected ampacity table before applying grouping, temperature, or other corrections.
2) Should I count equipment grounding conductors as CCC?
No. Grounding conductors are typically not current-carrying under normal operation, so they are not included in the CCC count for derating calculations.
3) When does the neutral count as current-carrying?
It depends on the circuit configuration, load balance, and harmonic content. Use your governing code and project criteria; the calculator lets you include or exclude it explicitly.
4) Can I use a different grouping table than the example?
Yes. Enable the manual override and enter the multiplier required by your standard or the cable manufacturer’s published guidance.
5) Why are multiple factors multiplied together?
Derating and correction multipliers typically represent independent reductions in allowable ampacity. Multiplying them provides a combined adjustment consistent with common engineering practice and code application.
6) What does the design check tell me?
It estimates the minimum base ampacity needed to carry your specified load after applying all adjustment factors, optionally increased by a safety margin.
7) Is the calculated result a final code approval value?
It is an engineering estimate that documents assumptions. Always confirm final ratings with local regulations, manufacturer data, and the specific installation method used on site.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.