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.

Open calculator

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

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

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

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 racewayRecord 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 reviewIf derated ampacity falls below the load basis, the next action may be splitting conductors into another raceway rather than simply upsizing every conductor.

Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.

The worksheet assumes the calculator count is limited to current-carrying conductors under the project interpretation.
The worksheet does not decide final raceway fill, neutral treatment, rooftop adders, cable tray conditions, or conductor sizing without separate review.
The adopted NEC edition, equipment markings, manufacturer data, utility requirements, and AHJ interpretation must be verified before installation.

Code and standard notes. Planning limits that should be checked before final equipment selection.

Use this worksheet as an educational planning record; verify the adopted NEC edition, manufacturer data, equipment markings, utility requirements, and AHJ interpretation before final installation.

How to use this chart

1Confirm conductor countUse the calculator result only after documenting which conductors were counted and why they were treated as current-carrying.

2Separate the factorsKeep conductor-count adjustment and ambient-temperature correction separate so the combined derating can be reviewed.

3Compare routing choicesUse the worksheet to decide whether conductor upsizing, raceway splitting, rerouting, or equipment review should happen next.

Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.

Record count basisWrite conductor count, raceway or cable grouping, neutral-counting assumption, and the condition used by the calculator.
Record factor stackDocument base ampacity, conductor-count factor, ambient factor, combined factor, and derated ampacity in order.
Record next decisionNote whether the result leads to conductor upsizing, raceway split, routing change, equipment review, or AHJ question.

Common mistakes to avoid. Review these before turning chart current into an equipment decision.

Counting all physical conductors without documenting which ones are current-carrying for the project condition.
Multiplying derating factors correctly but comparing the result with the wrong load, terminal limit, or OCPD basis.
Upsizing conductors after derating without checking conduit fill, pulling conditions, terminal size, and enclosure fit.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Should equipment grounding conductors be counted?

Do not assume every conductor counts. Use the project rule set, calculator notes, and AHJ interpretation to identify current-carrying conductors.

Can derating be solved by upsizing only?

Sometimes, but splitting circuits, changing routing, reducing conductor count, or addressing ambient conditions may be better project choices.

Why keep count factor and ambient factor separate?

Separate factors make the result reviewable and show whether conductor grouping, heat, or both conditions controlled the derated ampacity.