Electrical

How to Calculate Wire Size

Wire size is not chosen from current alone. A safe conductor has to satisfy ampacity first, then it may need to be larger if the run is long enough for voltage drop to become excessive.

Final answer from the example

Required ampacity

20 A

Non-continuous load, k = 1

Ampacity minimum

14 AWG

20 A copper table value

Final size

8 AWG

First size under 3% voltage drop

Key formulas

Required ampacity

Required ampacity = Load current × continuous-load factor

Use 1.25 for loads expected to run continuously for 3 hours or more; otherwise use 1 for this simplified workflow.

Ampacity selection

Choose the smallest gauge with Ampacity ≥ Required ampacity

Ampacity comes from code tables by material, size, temperature column, installation condition, and derating factors.

Single-phase voltage-drop check

Vdrop% = (2 × I × ρ × L / A) ÷ V × 100

This checks whether the selected conductor keeps voltage drop under the chosen limit for a one-way run length.

Upsizing rule

If Vdrop% > limit, try the next larger conductor

Voltage drop can force a larger conductor than ampacity alone requires, especially on low-voltage long runs.

Variables and units

Symbol	Meaning	Typical units
I	Load current	A
k	Continuous-load factor	1 or 1.25
L	One-way conductor run length	m or ft
V	Nominal supply voltage	V
ρ	Conductor resistivity	Ω·m
A	Conductor cross-sectional area	m², mm², kcmil

Quick reference conversions

Continuous load factor

1.25

For loads expected to run 3 hours or more.

Voltage-drop target

Common branch or feeder design target, not a standalone ampacity rule.

Single-phase multiplier

The formula uses one-way length and accounts for the return path.

Copper 14 AWG

20 A

75°C baseline table value; final use may be lower under code rules.

Copper 8 AWG

50 A

The selected size in the worked long-run example.

Aluminum note

larger size

Aluminum has lower ampacity and higher resistance than copper at the same size.

Step-by-step solved example

Example problem

Choose a copper wire size for a 20 A non-continuous load on a 120 V single-phase circuit with a 30 m one-way run. Use a 3% voltage-drop target.

1. Calculate required ampacity

The load is non-continuous in this example, so k = 1. Required ampacity = 20 A × 1 = 20 A.

2. Pick the ampacity-minimum conductor

Using the 75°C copper ampacity table baseline used by this calculator, 14 AWG copper is rated 20 A. That makes 14 AWG the ampacity-minimum size in this simplified table check, before any small-conductor, terminal-temperature, ambient, bundling, or local-code limits are applied.

3. Check voltage drop at the ampacity-minimum size

For 14 AWG copper, area A = 2.081×10⁻⁶ m². Vdrop% = (2 × 20 × 1.7241×10⁻⁸ × 30 / 2.081×10⁻⁶) ÷ 120 × 100 = 8.28%.

4. Upsize until voltage drop passes

12 AWG gives about 5.21%, 10 AWG gives about 3.28%, and 8 AWG gives about 2.06%. With a 3% target, 8 AWG is the first size in this table that passes.

5. Interpret the recommendation

The final recommendation is 8 AWG copper. It is larger than the ampacity-minimum 14 AWG because the 30 m run on a 120 V circuit makes voltage drop the governing check.

Practical field notes

Ampacity is the safety gate

Do not use voltage drop to justify a wire smaller than the required ampacity. Ampacity protects the conductor insulation from overheating.

Voltage drop is a performance gate

Voltage drop affects equipment voltage, efficiency, and performance. It often governs long low-voltage runs even when the ampacity-minimum wire is thermally acceptable.

Small-conductor and termination rules matter

Real code sizing can be limited by terminal temperature ratings, small-conductor overcurrent rules, conductor insulation, ambient temperature, raceway fill, bundling, and local amendments. Treat the table result as a calculation example, not permission to install that conductor on a breaker.

Use one-way run length

The single-phase voltage-drop formula already includes the out-and-back path with the factor 2. Entering round-trip length would double-count the run.

Common mistakes to avoid

Choosing wire size only from breaker size without checking the actual load and code conditions.
Forgetting the 1.25 factor for continuous loads.
Ignoring voltage drop on long 120 V runs.
Entering round-trip length into a formula that already includes the return path.
Using a 75°C table value without considering small-conductor rules, terminal rating, ambient temperature, bundling, or local code requirements.

When to use the calculator instead

Use the calculator when you want to compare copper vs aluminum, continuous vs non-continuous loads, optional run length, supply voltage, and voltage-drop limits. It automates the gauge-by-gauge upsize check.

Calculation FAQs

How do I calculate wire size from amps?

Start by calculating required ampacity from load current and continuous-load factor. Then choose the smallest conductor whose allowed ampacity meets or exceeds that requirement.

Why did the example choose 8 AWG instead of 14 AWG?

14 AWG meets the simplified 20 A ampacity check, but it drops about 8.28% over the 30 m run. 8 AWG is the first listed copper size that brings the drop below 3%.

Is wire gauge the same as wire size?

In AWG-based systems, people often use wire gauge, wire size, and AWG size to mean the same conductor-size selection.

Can I use this as final electrical design?

No. This is a calculation walkthrough. Final conductor sizing must follow the applicable electrical code and should be checked by a licensed electrician or electrical engineer.

What is the difference between ampacity and voltage drop?

Ampacity is about safe current-carrying capacity. Voltage drop is about how much voltage is lost before power reaches the load. A conductor must pass both checks when voltage drop is relevant.

References

Calculators
Southwire
Southwire describes conductor sizing tools that account for current carrying capacity and voltage drop.
Overcurrent Protection and the NEC
IAEI Magazine
Explains the NEC branch/feeder concept of noncontinuous load plus 125 percent of continuous load.
NEC 310.16 conductor ampacity table
Solar Scripto
Reference-style ampacity table for copper and aluminum conductors at 60°C, 75°C, and 90°C columns.

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