Wire & Cable calculator
Wire Size Calculator
This wire size calculator owns the conductor gauge selection step after the load current, duty basis, ampacity basis, conductor material, and run length are known. Enter those inputs first so the result can balance load basis, standard breaker review, ambient correction, and the single-phase or three-phase voltage-drop check for the exact circuit being modeled.
Updated July 16, 2026
Enter amps, load type, phase, conductor material, ampacity basis, ambient temperature, run length, and voltage-drop target before selecting a wire size.
Method path: load basis -> standard breaker review -> ampacity basis -> ambient correction -> voltage-drop check.
Enter current, load type, phase, conductor material, and run length below to size wire by ampacity and voltage drop
Calculator Inputs
Calculation Results
Enter values above to see calculation results
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Calculation history
Example Calculations
How to Use
What this wire size calculator covers
This page is intended for practical conductor sizing questions such as wire size calculator, wire gauge calculator, what size wire do I need, and NEC small-conductor rule lookups. The calculator balances three decisions at the same time: conductor ampacity basis, the standard breaker size that the load calls for, and the voltage drop on the run.
Inputs used by the calculator
- Load current is the current the conductor must serve.
- Load type decides whether the conductor and standard breaker review use 100% or 125% of the entered current.
- Circuit phase selects the single-phase or three-phase voltage-drop formula.
- Conductor material selects copper or aluminum data.
- Ampacity basis lets you review the run on a 60°C or 75°C basis.
- Ambient temperature applies a simplified correction factor to the selected ampacity basis.
- Voltage-drop target limits the maximum voltage lost on the run.
How the calculation works
- For non-continuous loads, the load basis stays at 100% of current.
- For continuous loads, the calculator uses a 125% basis for both conductor ampacity and the standard breaker review.
- It selects the next standard breaker size at or above that load basis.
- It checks each candidate conductor against the selected ampacity basis and ambient correction factor.
- It also checks whether that conductor can support the standard breaker size, including small-conductor branch-circuit limits.
- Finally, it verifies voltage drop using the single-phase or three-phase formula and recommends the smallest wire that satisfies all modeled limits.
Small-conductor branch-circuit limits used in this calculator
Use the calculator above before treating this reference table as the answer for a specific circuit. The table explains the small-conductor breaker limits that the tool applies after it knows the load type, conductor material, ampacity basis, ambient condition, and run length.
| Conductor | General branch-circuit breaker limit | Common use |
|---|---|---|
| 14 AWG copper | 15A | 15A lighting and general circuits |
| 12 AWG copper | 20A | 20A kitchen, bathroom, garage, and appliance circuits |
| 10 AWG copper | 30A | 30A dryers, water heaters, and similar loads |
| 12 AWG aluminum | 15A | Small aluminum branch-circuit review only when equipment is rated for it |
| 10 AWG aluminum | 25A | Larger aluminum branch-circuit review where permitted |
60°C versus 75°C ampacity basis
| Basis | How the calculator uses it | Common reason to choose it |
|---|---|---|
| 60°C | Uses the lower ampacity column and the matching ambient correction factors | Typical branch-circuit review where terminals or cable rules point to a 60°C basis |
| 75°C | Uses the higher ampacity column and the matching ambient correction factors | Feeder or equipment review where the actual terminals are listed for a 75°C conductor basis |
What this page does not model automatically
- Current-carrying conductor adjustment for more than three current-carrying conductors; use the ampacity calculator for that review.
- Physical raceway fill; use the conduit fill calculator when the question is how many conductors fit in EMT, PVC, IMC, or RMC.
- Free-air ampacity tables.
- Motor branch-circuit conductor exceptions and detailed motor design rules.
Technical notes. Open for formula basis, assumptions, and validation notes.
Why wire size questions are never only about ampacity
Searchers often ask for a wire size chart or an ampacity table, but the actual field decision is not just a table lookup. A practical conductor choice also has to survive the standard breaker size you end up using and the voltage lost on the run. That is why this page checks all three at once.
Why this page keeps branch-circuit wire limits visible
A common source of confusion is seeing a table ampacity that looks higher than the breaker normally paired with a small branch-circuit conductor. That is why the calculator keeps the small-conductor breaker limit visible instead of pretending the raw table number answers every branch-circuit question by itself.
What to check after the wire size result
Use the wire size result as the conductor selection step, then close the surrounding workflow. Send the same load basis through the Ampacity Calculator if ambient temperature or current-carrying conductors matter, use the Voltage Drop Calculator for long runs, use the Conduit Fill Calculator before raceway layout, and finish the device review with the Breaker Sizing Calculator.
Common Applications
More applications. Open to review 1 additional use case.
Frequently Asked Questions
What size wire do I need for a 20A circuit?
Enter the circuit current, load type, conductor material, ampacity basis, distance, and voltage-drop target in the calculator. The result can change for continuous loads, long runs, aluminum conductors, ambient correction, and the standard breaker review.
Why can a continuous load force a larger wire even when the ampacity table looks close?
Because the calculator sizes continuous loads at 125% before it checks both conductor ampacity and the standard breaker size. A wire that looks close on a table can still be too small once the continuous-load basis and standard breaker review are applied.
Does 12 AWG always mean a 20A breaker?
No single conductor label should be used without the rest of the circuit data. The calculator checks the selected conductor against ampacity basis, ambient correction, small-conductor limits, and the standard overcurrent device instead of using raw table ampacity alone.
Does the calculator use a different formula for three-phase runs?
Yes. Single-phase runs use the 2 × I × R × L voltage-drop model, while three-phase runs use the √3 × I × R × L model. That usually reduces voltage drop for the same current, conductor, and one-way distance.
Should I use this page for motor branch circuits?
Use it for a general conductor screening only. Motor branch circuits follow their own rules for conductor sizing, short-circuit protection, and overload protection, so a motor-specific calculator or detailed design review is still required.
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