How do I know what size wire I need?

Wire size selection requires checking two separate things: the wire must be rated to carry the load current safely without overheating (ampacity), and it must keep voltage drop over the run within an acceptable range for long distances. This calculator checks both and recommends whichever wire size satisfies the stricter of the two requirements.

What size wire do I need for a given number of amps?

The minimum size for ampacity alone depends on the conductor material (copper or aluminum), the insulation temperature rating (60°C, 75°C, or 90°C), and any derating for ambient temperature or bundling with other conductors. As a common rough reference at standard conditions, 15A commonly uses 14 AWG, 20A commonly uses 12 AWG, and 30A commonly uses 10 AWG copper — but always verify against the applicable ampacity table and any derating factors for the actual installation.

Why does wire distance matter for sizing, not just amperage?

A wire can be rated to carry a given current safely (ampacity) while still producing too much voltage drop if the run is long. Voltage drop increases with both current and distance, so long runs — such as to a detached garage, well pump, or outbuilding — often need a larger wire than ampacity alone would require, purely to keep the voltage at the load within an acceptable range.

What is the difference between ampacity-limited and voltage-drop-limited sizing?

Ampacity-limited means the smallest wire that safely carries the current without overheating is also small enough to keep voltage drop acceptable — the ampacity requirement determines the final size. Voltage-drop-limited means that same small wire would overheat-safe but produce too much voltage drop over the distance, so a larger wire is needed to control voltage drop even though it exceeds the ampacity requirement.

How does continuous load affect wire sizing?

For loads expected to operate continuously (typically 3 hours or more), conductors and overcurrent protection are commonly sized for 125% of the continuous load current — equivalently, the continuous load should not exceed 80% of the conductor and breaker rating. This calculator applies that 125% factor when the continuous load option is selected.

Does ambient temperature affect wire size?

Yes. Ampacity tables are based on a reference ambient temperature (commonly 30°C / 86°F). Higher ambient temperatures — such as an attic, rooftop conduit, or hot outdoor location — reduce a conductor's safe current-carrying capacity, so a correction factor below 1.0 is applied, which can push the calculation to a larger wire size.

Why do more conductors in a raceway require a bigger wire?

When multiple current-carrying conductors share a conduit or cable, they generate heat that affects each other, reducing the safe current-carrying capacity of each conductor. Adjustment factors — for example, roughly 0.8 for 4–6 conductors, and lower for more — are applied to the wire's base ampacity to account for this bundling effect.

Should I always size up from what a calculator recommends?

Rounding up to the next standard wire size (rather than the exact calculated minimum) is common practice, since it adds thermal and voltage-drop margin, tends to reduce future upgrade needs, and is inexpensive relative to labor cost on new installations. This calculator shows the minimum size that satisfies both checks; choosing one size larger is a reasonable and common safety margin.

Engineering

Wire Size Calculator

Find the minimum wire gauge for a circuit by checking both ampacity (with temperature and bundling derating) and voltage drop over distance.

This calculator uses simplified, representative ampacity and derating values for common planning scenarios. It is not a substitute for the applicable electrical code's full ampacity tables and derating rules — see code cautions below.

Load

Load current (A)

Load type

Conductor & installation

Material

Insulation temp. rating

Ambient temperature

Current-carrying conductors in raceway

Circuit & distance

Circuit type

System voltage (V)

One-way circuit length

Length unit

Max acceptable voltage drop

Recommended minimum wire size

—

Sizing driven by—

Required ampacity (with 125% & derating)—

Adjusted ampacity of chosen size—

Voltage drop at chosen size—

NoteRounding up one size adds margin

How the recommended wire size is determined

This calculator runs two independent checks and recommends whichever wire size satisfies both:

Ampacity check: the load current is scaled up by 125% if continuous, then the required table ampacity is found by dividing by the ambient temperature correction factor and the conductor bundling adjustment factor. The smallest standard wire size (for the selected material and insulation rating) whose base ampacity meets or exceeds this adjusted requirement is the ampacity-limited size.

Voltage drop check: using the same approach as a standard voltage drop calculation, each candidate wire size is checked to see whether it keeps percentage voltage drop at or below your selected maximum over the specified one-way distance, current, and circuit type. The smallest size that meets this is the voltage-drop-limited size.

The calculator starts from the smallest wire size and increases the gauge until both conditions are satisfied, then reports which requirement ultimately determined the final size.

Formula steps

Ampacity path: Required current = Load current × (1.25 if continuous, else 1.0) Adjustment factor = Ambient correction × Bundling factor Required table ampacity = Required current ÷ Adjustment factor → Pick smallest wire size where base table ampacity (for material + insulation rating) ≥ Required table ampacity Voltage drop path: Multiplier = 2 (DC / single-phase) or √3 (three-phase) VD (V) = Multiplier × R(Ω/1000ft) × Length(ft) ÷ 1000 × I(A) %VD = (VD ÷ System voltage) × 100 → Pick smallest wire size where %VD ≤ selected max % Final recommended size = larger of the two candidate sizes

Safety derating

Two derating steps are built into the ampacity check, and both push toward a larger wire size when conditions are less favorable than the reference test conditions used in standard ampacity tables:

Continuous load (125% rule): for loads running 3 hours or more, sizing at 125% of the load current (equivalent to keeping the load at or below 80% of the conductor/breaker rating) leaves thermal margin for sustained operation.
Ambient temperature and bundling: conductors in hotter environments or bundled with several other current-carrying conductors dissipate heat less effectively, so their safe current-carrying capacity is reduced by the correction and adjustment factors applied here.

Beyond these two factors, many designers round up to the next standard wire size above the calculated minimum as an additional margin — inexpensive on new installations and helpful if the load grows later.

Electrical code cautions

The ampacity values, temperature correction factors, and bundling adjustment factors used here are simplified, representative figures modeled on common tables such as NEC Table 310.16 and NEC Table 310.15(B)(1)/(C)(1) in the United States — they are not a substitute for the full, current tables in the code that applies to your installation. Actual required conductor and overcurrent device sizing can also be affected by terminal temperature ratings, equipment listing requirements, local amendments, and other code sections not modeled here. For any fixed wiring, branch circuit, feeder, or service conductor sizing, verify against the applicable code and consult a licensed electrician or engineer.

Design limitations of this calculator

Simplified ampacity table: uses representative values for common AWG/kcmil sizes rather than the complete set of conditions in a full code ampacity table.
Generic temperature correction: a single correction factor set is used across insulation ratings rather than the distinct 60°C/75°C/90°C correction columns some codes specify.
No terminal temperature limitation: many breakers and devices are only rated for use with 60°C or 75°C terminations regardless of the conductor's insulation rating; this is not checked here.
Resistance-only voltage drop: as with the standalone voltage drop calculator, reactance, skin effect, and connector resistance are not modeled.
Balanced three-phase loads assumed: unbalanced loading is not modeled.
Not a permit or inspection tool: this calculator supports planning and comparison; it does not confirm code compliance for a specific jurisdiction or installation.

Frequently asked questions

Wire size selection requires checking two separate things: the wire must be rated to carry the load current safely without overheating (ampacity), and it must keep voltage drop over the run within an acceptable range for long distances. This calculator checks both and recommends whichever wire size satisfies the stricter of the two requirements.
The minimum size for ampacity alone depends on the conductor material (copper or aluminum), the insulation temperature rating (60°C, 75°C, or 90°C), and any derating for ambient temperature or bundling with other conductors. As a common rough reference at standard conditions, 15A commonly uses 14 AWG, 20A commonly uses 12 AWG, and 30A commonly uses 10 AWG copper — but always verify against the applicable ampacity table and any derating factors for the actual installation.
A wire can be rated to carry a given current safely (ampacity) while still producing too much voltage drop if the run is long. Voltage drop increases with both current and distance, so long runs — such as to a detached garage, well pump, or outbuilding — often need a larger wire than ampacity alone would require, purely to keep the voltage at the load within an acceptable range.
Ampacity-limited means the smallest wire that safely carries the current without overheating is also small enough to keep voltage drop acceptable — the ampacity requirement determines the final size. Voltage-drop-limited means that same small wire would overheat-safe but produce too much voltage drop over the distance, so a larger wire is needed to control voltage drop even though it exceeds the ampacity requirement.
For loads expected to operate continuously (typically 3 hours or more), conductors and overcurrent protection are commonly sized for 125% of the continuous load current — equivalently, the continuous load should not exceed 80% of the conductor and breaker rating. This calculator applies that 125% factor when the continuous load option is selected.
Yes. Ampacity tables are based on a reference ambient temperature (commonly 30°C / 86°F). Higher ambient temperatures — such as an attic, rooftop conduit, or hot outdoor location — reduce a conductor's safe current-carrying capacity, so a correction factor below 1.0 is applied, which can push the calculation to a larger wire size.
When multiple current-carrying conductors share a conduit or cable, they generate heat that affects each other, reducing the safe current-carrying capacity of each conductor. Adjustment factors — for example, roughly 0.8 for 4–6 conductors, and lower for more — are applied to the wire's base ampacity to account for this bundling effect.
Rounding up to the next standard wire size (rather than the exact calculated minimum) is common practice, since it adds thermal and voltage-drop margin, tends to reduce future upgrade needs, and is inexpensive relative to labor cost on new installations. This calculator shows the minimum size that satisfies both checks; choosing one size larger is a reasonable and common safety margin.