Electrical reference chart
Conductor Bundling Derating Chart
Use this worksheet after the bundling derating calculator result to record conductor count, count basis, adjustment factor, ambient factor, combined factor, and derated ampacity.
Quick reference table
Bundling derating starts with the number of current-carrying conductors that actually count for the installation condition. The calculator result should document count basis, adjustment factor, ambient correction, and derated ampacity before any decision to upsize conductors, split raceways, reroute circuits, or change the wiring method.
Bundling derating worksheet checkpoints
| Checkpoint | Record from calculator | Field verification |
|---|---|---|
| Conductor count | Current-carrying conductor quantity | Confirm neutrals, travelers, multiwire circuits, and parallel raceways |
| Base ampacity | Starting ampacity value | Confirm material, size, insulation, and selected ampacity basis |
| Count factor | Adjustment factor | Confirm raceway, cable, tray, nipple, or bundle condition |
| Ambient factor | Temperature correction factor | Confirm temperature basis and routing condition |
| Combined result | Derated ampacity | Compare with load, terminal cap, OCPD, and equipment markings |
Possible next decisions after derating
| Result condition | Likely next review | Do not skip |
|---|---|---|
| Derated ampacity still adequate | Carry lower value into wire-size record | Terminal rating and voltage drop check |
| Derated ampacity too low | Upsize conductor or split circuits | Conduit fill and pulling condition |
| High conductor count controls | Separate raceways or reduce grouping | Neutral-counting and circuit arrangement notes |
| Ambient factor controls | Reroute or change installation condition | Temperature basis and product data |
How to use this chart
Confirm conductor count
Use the calculator result only after documenting which conductors were counted and why they were treated as current-carrying.
Separate the factors
Keep conductor-count adjustment and ambient-temperature correction separate so the combined derating can be reviewed.
Compare routing choices
Use the worksheet to decide whether conductor upsizing, raceway splitting, rerouting, or equipment review should happen next.
Formula basis
Derated ampacity = base ampacity x conductor-count factor x ambient-temperature factor.
- Base ampacity is the starting conductor ampacity used by the calculator.
- Conductor-count factor reflects the counted current-carrying conductors in the raceway, cable, or bundle.
- Ambient-temperature factor reflects the installation temperature basis used in the calculator.
- Derated ampacity is the planning value to compare with load, terminals, equipment limits, and OCPD review.
Worked examples
Multiple branch circuits in one raceway
Record the counted current-carrying conductors, the adjustment factor, and the ambient factor as separate rows. The worksheet should show whether count or heat drove the derating result.
Derating result leads to a raceway split review
If derated ampacity falls below the load basis, the next action may be splitting conductors into another raceway rather than simply upsizing every conductor.
Related calculators
Cable Bundling Derating Calculator
Calculate ampacity derating for bundled conductors per NEC 310.15(C)
Temperature Correction Calculator
Screen conductor ampacity derating from ambient temperature, more than three current-carrying conductors, and the governing equipment termination limit.
Wire Ampacity Calculator
Calculate conductor ampacity with temperature correction, conductor-count adjustment, and 60°C or 75°C termination checks
Cable Ampacity Calculator
Calculate cable current carrying capacity with temperature and bundling corrections per NEC
Conduit Fill Calculator
Calculate conduit fill percentages, maximum wire counts, and conduit sizing per NEC requirements
Related charts
Temperature Correction Chart
Use a temperature correction chart to document starting ampacity, ambient condition, adjustment factors, terminal cap, and final usable ampacity after a calculator result.
Ampacity Chart
Review conductor ampacity as a heat problem: material, insulation, terminal rating, ambient correction, bundling adjustment, and equipment limits.
Conduit Fill Chart
Plan conduit fill from finished conductor area, raceway type, trade size, conductor count, bends, pulling conditions, and derating follow-up.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.