What wire size do I need for 20 amps?

Use 12 AWG copper wire for 20-amp circuits per NEC Table 310.16. For runs over 50 feet, consider upsizing to 10 AWG to limit voltage drop below 3%. 20-amp circuits require 20-amp rated outlets (with T-slot) and a 20-amp breaker.

What wire size for 30 amps?

Use 10 AWG copper wire for 30-amp circuits (dryers, water heaters) per NEC Table 310.16. For longer runs or aluminum wire, upsize accordingly. 30-amp circuits use NEMA 10-30 or 14-30 outlets.

What is the difference between copper and aluminum wire?

Copper has better conductivity (requires smaller wire), is easier to work with, and is more durable. Aluminum is cheaper and lighter but requires 1–2 sizes larger wire for the same ampacity. Aluminum also requires anti-oxidant compound and compatible connectors per NEC 110.14.

What is voltage drop and why does it matter?

Voltage drop is the loss of voltage as electricity travels through wire. Excessive drop causes dim lights, slow motors, and equipment damage. NEC recommends max 3% drop for branch circuits. A 120V circuit with 3% drop delivers only 116.4V at the outlet.

What wire size do I need for a 20-amp circuit?

For a 20-amp circuit, NEC requires a minimum of 12 AWG copper wire. However, for runs longer than 50-75 feet, you may need to upsize to 10 AWG to keep voltage drop under 3%.

What is the difference between copper and aluminum wire?

Copper has better conductivity and requires smaller wire sizes, but costs more. Aluminum is cheaper and lighter but requires wire two sizes larger for the same ampacity. Aluminum also needs special connectors and anti-oxidant compound.

Why does distance matter for wire size?

Longer wire runs have more resistance, causing greater voltage drop. A wire size that works perfectly at 25 feet may cause unacceptable voltage drop at 100 feet, leading to dim lights, motor overheating, or equipment malfunction.

What voltage drop percentage is acceptable?

NEC recommends no more than 3% voltage drop for branch circuits and 5% for the combined feeder and branch circuit. While not a hard code requirement in most areas, exceeding these limits causes performance issues.

Can I use 14 AWG wire on a 20-amp circuit?

No. NEC requires 12 AWG minimum for 20-amp circuits. Using undersized wire creates a fire hazard because the wire can overheat before the breaker trips.

What wire size for a 100-amp sub-panel?

A 100-amp sub-panel typically requires 2 AWG copper or 1/0 AWG aluminum for short runs. For longer distances, use this calculator with your specific run length to check voltage drop.

Does conduit fill affect wire sizing?

Conduit fill does not change wire size directly, but when multiple current-carrying conductors share a conduit, NEC requires derating the ampacity. This may require upsizing wire to compensate.

Wire Size Calculator — AWG Gauge by Amps & Distance | DIYCalc

Wire Sizing Formula

The required circular mil area is calculated using:

CM = (2 × K × I × D) / V_d

Where:

K = Resistivity constant (10.4 for copper, 17.0 for aluminum)
I = Current in amperes
D = One-way distance in feet
V_d = Maximum allowable voltage drop in volts

The calculator selects the smallest AWG wire that meets both the voltage drop requirement and NEC ampacity rating at 75°C (Table 310.16).

Understanding Wire Gauge (AWG)

The American Wire Gauge (AWG) system is the standard for measuring electrical wire sizes in North America. Smaller AWG numbers indicate larger wire diameters with greater current-carrying capacity. For example, 4 AWG wire is significantly larger than 14 AWG wire.

Wire sizing involves two critical checks: the wire must handle the circuit amperage without overheating (ampacity), and it must keep voltage drop within acceptable limits over the run distance.

NEC Voltage Drop Recommendations

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for the combined feeder and branch circuit. While these are recommendations rather than requirements in most jurisdictions, exceeding them can cause equipment malfunction, motor overheating, and dimming lights.

Copper vs. Aluminum Conductors

Copper has lower resistivity (10.4 vs. 17.0 ohm-cmil/ft) making it more efficient, but aluminum is lighter and less expensive. For the same ampacity, aluminum wire must be approximately two sizes larger than copper. Aluminum requires special connectors and anti-oxidant compound at terminations.

Common Residential Circuits

15A general circuits: 14 AWG copper minimum
20A kitchen/bath: 12 AWG copper minimum
30A dryer: 10 AWG copper minimum
40A range: 8 AWG copper minimum
50A range/sub-panel: 6 AWG copper minimum

Long Run Considerations

Distance is the most commonly overlooked factor in wire sizing. A 20A circuit using 12 AWG copper is fine at 50 feet, but at 150 feet the voltage drop exceeds 5%, requiring an upgrade to 10 AWG or larger. Always measure the actual wire run length, including vertical runs through walls and along joists—it is typically 20-40% longer than the straight-line distance.

Key NEC Code References

NEC 310.16 — Ampacity tables for conductors rated 0–2000V (the primary reference for wire sizing)
NEC 210.19 — Branch circuit conductor sizing requirements
NEC 215.2 — Feeder conductor sizing requirements
NEC 310.15(B)(7) — Special ampacity allowance for dwelling unit service and feeder conductors

Temperature Ratings

This calculator uses 75°C ampacity ratings per NEC Table 310.16, which applies to THWN and THHN insulation types commonly used in residential and commercial wiring. Higher temperature ratings (90°C) allow slightly higher ampacity but are typically only used for derating calculations in bundled conduit runs.

When to Upsize Wire

Consider upsizing beyond the minimum in these situations:

Future load growth anticipated on the circuit
Multiple conductors in conduit (derating factors apply)
High ambient temperatures reduce ampacity
Motor loads with high starting current
Critical equipment sensitive to voltage fluctuations

How to Determine Wire Size

Choosing the correct wire size is critical for electrical safety. Undersized wire overheats, creating fire hazards and tripping breakers. The National Electrical Code (NEC) specifies minimum wire sizes based on amperage, but distance and voltage drop also affect the optimal choice.

Wire Size Formula

Minimum wire size = Per NEC Table 310.16 based on ampacity
Voltage drop = (2 × Length × Current × Resistance per ft) ÷ 1000
Max voltage drop: 3% for branch circuits, 5% total per NEC 210.19(A) Informational Note

Common Wire Sizes (Copper, NEC Table 310.16)

14 AWG: 15 amps. Lighting circuits, general outlets.
12 AWG: 20 amps. Kitchen, bathroom, outdoor, and garage outlets.
10 AWG: 30 amps. Dryers, water heaters, window AC units.
8 AWG: 40 amps. Ranges, cooktops, large AC units.
6 AWG: 55 amps. Sub-panels, large equipment, EV chargers.
4 AWG: 70 amps. Sub-panels, service entrance.
2 AWG: 95 amps. Large sub-panels.
1/0 AWG: 150 amps. Main service entrance.

NEC Code Requirements

All conductors must be rated for the circuit ampacity per NEC 310.16
All branch circuits must be protected by appropriately sized breakers
AFCI protection required for bedroom, living room, and other habitable room circuits per NEC 210.12
GFCI protection required for kitchen, bathroom, outdoor, garage, and basement circuits per NEC 210.8
Aluminum wire connections require anti-oxidant compound and AL/CU rated devices per NEC 110.14

Frequently Asked Questions

What wire for 20 amps?

12 AWG copper per NEC. Upsize to 10 AWG for runs over 50 feet to control voltage drop.

What wire for 30 amps?

10 AWG copper per NEC. Common for dryers and water heaters.

How does distance affect wire size?

Longer runs increase voltage drop. Keep voltage drop under 3% by upsizing wire for long runs.

Copper vs aluminum?

Copper is better conductivity and easier to work with. Aluminum is cheaper but requires 1–2 sizes larger and special connectors.

What is voltage drop?

Loss of voltage through wire resistance. Excessive drop (over 3%) causes equipment problems. Use our voltage drop calculator for specific calculations.

Related Calculators

Voltage Drop Calculator — calculate exact voltage drop
BTU Calculator — HVAC load for equipment sizing
Square Footage Calculator — room area for lighting circuits

Electrical work must comply with NEC and local codes. All electrical work should be performed by or inspected by a licensed electrician. Wire sizes shown are for standard conditions per NEC Table 310.16 at 75°C. Derating may be required for high-temperature environments or conduit fill.

Wire Sizing Formula

The required circular mil area is calculated using:

CM = (2 × K × I × D) / V_d

Where:

K = Resistivity constant (10.4 for copper, 17.0 for aluminum)
I = Current in amperes
D = One-way distance in feet
V_d = Maximum allowable voltage drop in volts

The calculator selects the smallest AWG wire that meets both the voltage drop requirement and NEC ampacity rating at 75°C (Table 310.16).

Understanding Wire Gauge (AWG)

Wire sizing involves two critical checks: the wire must handle the circuit amperage without overheating (ampacity), and it must keep voltage drop within acceptable limits over the run distance.

NEC Voltage Drop Recommendations

Copper vs. Aluminum Conductors

Common Residential Circuits

15A general circuits: 14 AWG copper minimum
20A kitchen/bath: 12 AWG copper minimum
30A dryer: 10 AWG copper minimum
40A range: 8 AWG copper minimum
50A range/sub-panel: 6 AWG copper minimum

Long Run Considerations

Key NEC Code References

NEC 310.16 — Ampacity tables for conductors rated 0–2000V (the primary reference for wire sizing)
NEC 210.19 — Branch circuit conductor sizing requirements
NEC 215.2 — Feeder conductor sizing requirements
NEC 310.15(B)(7) — Special ampacity allowance for dwelling unit service and feeder conductors

Temperature Ratings

When to Upsize Wire

Consider upsizing beyond the minimum in these situations:

Future load growth anticipated on the circuit
Multiple conductors in conduit (derating factors apply)
High ambient temperatures reduce ampacity
Motor loads with high starting current
Critical equipment sensitive to voltage fluctuations

Wire Size Calculator

Wire Size Calculator

How This Calculator Works

Wire Sizing Formula

Understanding Wire Gauge (AWG)

NEC Voltage Drop Recommendations

Copper vs. Aluminum Conductors

Common Residential Circuits

Long Run Considerations

Key NEC Code References

Temperature Ratings

When to Upsize Wire

Frequently Asked Questions

How to Determine Wire Size

Wire Size Formula

Common Wire Sizes (Copper, NEC Table 310.16)

NEC Code Requirements

Frequently Asked Questions

What wire for 20 amps?

What wire for 30 amps?

How does distance affect wire size?

Copper vs aluminum?

What is voltage drop?

Related Calculators

Wire Size Calculator

How This Calculator Works

Wire Sizing Formula

Understanding Wire Gauge (AWG)

NEC Voltage Drop Recommendations

Copper vs. Aluminum Conductors

Common Residential Circuits

Long Run Considerations

Key NEC Code References

Temperature Ratings

When to Upsize Wire

Frequently Asked Questions