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Wire Size Calculator

Conductor size from the larger of the voltage-drop limit and the NEC 310.16 ampacity limit, for copper or aluminum.

Inputs

The load

The current the circuit carries, from the nameplate or the load calculation.

Continuous loads are sized at 125 percent of the load under NEC 210.19(A)(1) and 210.20(A). A storage water heater up to 120 gallons is treated as continuous by 422.13.

Used for the load in volt-amperes and for the voltage-drop limit.

Single phase uses the factor 2 in the drop formula; three phase uses 1.732.

The run

Distance from the source to the load along the conductor, not the round-trip. The formula applies the 2 or 1.732 itself.

3 percent is the usual branch-circuit target and 5 percent the total from the service, per the Informational Note to NEC 210.19(A)(1).

The conductor

Copper or aluminum. Aluminum needs roughly two sizes larger for the same ampacity and is not made at 14 AWG.

The usable column is set by the weakest termination, not by the insulation (NEC 110.14(C)). 60 C for older equipment, 75 C for most modern equipment rated 100 A and below, 90 C only when every termination is 90 C rated.

Percent of the Table 310.16 ampacity after the ambient and conductor-count corrections. Leave 100 if none apply.

Result

Recommended conductor size
—
Load after the continuous factor
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Voltage drop at that size
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Voltage drop in volts
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Circular mils provided
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Circular mils required
—
—
Where this stops applying
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Smaller size the ampacity alone would allow
—
Table range exceeded
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Calculation details
Largest ampacity in the selected column
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Table ampacity needed
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Circular mils the load demands
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Largest conductor area in the table
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Circular mils the drop limit demands
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Drop demand minus load demand
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Drop budget in volts
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Voltage-drop constant K
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Phase multiplier
—

What to check

  • Load amperes and whether the load is continuous
  • One-way length of the run from source to load
  • The voltage-drop budget you will accept

Standards referenced

  • NFPA 70 National Electrical Code (NEC) — 2023 edition · Branch circuits, conductors, protection, raceways, and services. Local AHJ adoptions vary; the enforcing edition is set by the jurisdiction.

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