Wire Size Chart

Use this wire size chart after the calculator result to document why a conductor size was selected, which inputs controlled the result, and what must be checked before ordering wire or pulling a circuit.

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Quick reference table

A wire size chart is useful only when it keeps the field sequence visible: calculated load first, usable ampacity second, terminal temperature and derating third, then voltage drop and equipment instructions. Use the calculator result as the starting point and verify the adopted NEC edition, manufacturer data, and AHJ requirements before treating the conductor as ready for the job.

Wire size field decision order

Wire size field decision order
Step	Record from the job	Why it can control size
Load basis	Calculated amps, voltage, phase, and continuous duty	Sets the minimum current the conductor must carry
Conductor family	Copper or aluminum, insulation type, raceway or cable method	Changes resistance, ampacity basis, and termination expectations
Termination limit	Equipment lug temperature rating and conductor material allowed	Can cap the usable ampacity below the insulation rating
Adjustment conditions	Ambient temperature, rooftop exposure, grouping, and current-carrying count	Can reduce the conductor ampacity used for planning
Performance check	One-way distance and project voltage-drop target	May make a larger conductor practical for long runs

Common result-area follow-up checks

Common result-area follow-up checks
Calculator result shows	Field question to answer	Next action
Ampacity controls	Are terminals, ambient, and bundling already reflected?	Review ampacity chart and document the limiting factor
Voltage drop controls	Is the run length or voltage-drop target driving a larger size?	Keep the larger size and note the performance basis
Aluminum selected	Are lugs listed and torque instructions available?	Verify equipment instructions before procurement
Borderline result	Would a future load or field routing change affect the selection?	Review spare capacity before the pull is scheduled

Formula basis

Conductor planning screen = calculated load plus duty factors, then compare usable ampacity, terminal limit, voltage-drop target, and equipment instructions.

Calculated load is the amperes from the load schedule, nameplate, or calculator result.
Usable ampacity is the conductor value after material, insulation, terminal temperature, ambient correction, and conductor-count adjustment are considered.
Voltage drop is a performance screen that can require a larger conductor even when ampacity is acceptable.

Worked examples

Long 240 V feeder resultThe calculator may show that the minimum ampacity size passes, but the voltage-drop result pushes the feeder one size larger. Keep the performance reason in the worksheet so the field crew does not downsize it later.

Aluminum branch circuit reviewA planning result using aluminum conductors still needs terminal material, lug listing, oxide inhibitor requirements where specified, and torque instructions checked before the conductor choice is released.

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

This reference table does not reproduce NEC ampacity or conductor-size tables.
The chart assumes the actual calculator result, job drawings, equipment labels, and conductor product data remain the project record.
Voltage drop targets are project-performance decisions unless a contract, equipment instruction, or adopted rule makes a specific limit mandatory.

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

Verify conductor selection with the adopted NEC edition, equipment listings, manufacturer terminal data, local amendments, and the Authority Having Jurisdiction (AHJ).
Use manufacturer data and site conditions when insulation type, cable assembly, conduit grouping, rooftop exposure, or ambient temperature differs from the planning worksheet.

How to use this chart

1Start from the result, not memoryUse the calculator output as the job record, then confirm which input controlled the conductor size: load current, terminal rating, derating, or voltage drop.

2Separate heat from performanceAmpacity protects the conductor from overheating, while voltage drop protects equipment performance. Keep both screens visible before calling the size acceptable.

3Close with equipment notesBefore ordering conductor, document lug material, temperature rating, torque instructions, raceway plan, and any manufacturer requirements that affect the installation.

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

Record load and dutyWrite calculated amperes, voltage, phase, continuous-load status, and the source of the load value before comparing conductor options.
Document conductor limitsRecord copper or aluminum, insulation family, terminal temperature, ambient condition, number of current-carrying conductors, and raceway or cable method.
Mark the controlling reasonAfter the calculator result, identify whether ampacity, terminal rating, derating, voltage drop, or equipment instructions controlled the selected conductor.

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

Choosing wire size only from breaker amperes without checking calculated load, continuous duty, conductor material, terminal rating, derating, and voltage drop.
Using the insulation temperature rating as the final ampacity when the connected equipment terminals are rated for a lower temperature basis.
Reducing a long-run conductor after installation planning because the larger size was treated as optional instead of documenting that voltage drop controlled the result.

Frequently asked questions

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

Can I pick the conductor directly from this chart?

No. Use the chart to keep the work sequence organized, then use the wire size calculator and verify the result against the adopted NEC edition, equipment instructions, and AHJ requirements.

Why can the calculator show a larger conductor than the ampacity minimum?

Long distance, aluminum material, voltage-drop target, terminal limits, or adjustment conditions can all move the practical conductor size above a simple ampacity screen.

Should breaker size decide wire size by itself?

No. Breaker size is part of the overcurrent protection review, but conductor selection starts with calculated load and must also consider ampacity, terminals, derating, voltage drop, and equipment instructions.