Wire & Cable Size Calculator: Size Conductors for Ampacity and Voltage Drop

Every conductor in a wiring system must satisfy two independent requirements: it must carry the design current without overheating (ampacity), and it often must do so with an acceptable voltage drop so that equipment at the far end sees a stable voltage.

The calculator finds the smallest AWG (or kcmil) size from a simplified 75°C table that passes both tests for the inputs you provide.

Planning tool only. Final conductor selection requires compliance with your local electrical code (NEC, BS 7671, AS/NZS 3000, or equivalent), manufacturer listing data, and review by a qualified electrician. Ambient temperature, conduit fill, parallel runs, terminal ratings, and many other factors are not modelled here.

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The two sizing criteria

1. Ampacity

Ampacity is the maximum continuous current a conductor can carry without exceeding its insulation temperature rating. It depends on:

• Conductor material — copper has higher ampacity than aluminum of the same AWG
• Insulation type — 60°C, 75°C, or 90°C rated
• Installation method — conduit fill, direct burial, free air, cable tray
• Ambient temperature — higher ambient means less headroom before the conductor overheats
• Number of current-carrying conductors — more bundled conductors = more derating

This tool uses a single simplified column for 75°C, not more than three current-carrying conductors — a common planning assumption.

2. Voltage drop

Voltage drop across the conductors means the load sees less voltage than the source supplies. Excessive drop causes:

• Motors running slower or overheating
• LED drivers, VFDs, and sensitive electronics misbehaving
• Voltage out of equipment specification

The calculation used here is the simplified single-phase two-wire model:

Vdrop ≈ 2 × I × (L_ft ÷ 1,000) × RΩ/kft

Where the factor of 2 accounts for both the outgoing and return conductors, I is amperes, L is one-way length in feet, and R is the conductor’s resistance in ohms per 1,000 ft at 75°C.

Percent drop = 100 × V_drop ÷ V_circuit

Common planning limits:

• Branch circuits: 3% max (NEC-style guidance; some codes permit 5%)
• Feeders: 3–5% (check your standard)
• Combined feeder + branch: 5% total (NEC FPN guidance)

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What governs: ampacity or voltage drop?

On short runs, ampacity almost always governs. A 20 A circuit in a panel room needs a conductor that can safely carry 20 A — voltage drop over 10 feet is negligible.

On long runs, voltage drop governs. A 20 A circuit running 200 feet at 120 V may need 8 AWG or larger even though 12 AWG is fine for 20 A ampacity — the drop on 12 AWG would exceed 3%.

The calculator tells you which constraint drove the final size selection.

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Copper vs aluminum

Aluminum conductors have:

• Lower ampacity per AWG — 6 AWG aluminum (40 A) is roughly equivalent to 8 AWG copper (40 A)
• Higher resistance — leads to more voltage drop for the same gauge and run length
• Oxidation concerns — aluminum forms an insulating oxide layer; anti-oxidant compound and proper terminations are required

Aluminum is common for service entrances and large feeders where the weight and cost savings are significant. For branch circuits and small feeders, copper remains standard in most regions.

Note: the tool shows — for aluminum at 14 AWG because aluminum is not typically used at that small a size in most codes.

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Ampacity quick reference (75°C simplified)

• 14 AWG — Copper 15 A, Aluminum —, ~2.1 mm²
• 12 AWG — Copper 20 A, Aluminum 15 A, ~3.3 mm²
• 10 AWG — Copper 30 A, Aluminum 25 A, ~5.3 mm²
• 8 AWG — Copper 40 A, Aluminum 35 A, ~8.4 mm²
• 6 AWG — Copper 55 A, Aluminum 40 A, ~13.3 mm²
• 4 AWG — Copper 70 A, Aluminum 55 A, ~21.2 mm²
• 2 AWG — Copper 95 A, Aluminum 75 A, ~33.6 mm²
• 1/0 AWG — Copper 125 A, Aluminum 100 A, ~53.5 mm²
• 2/0 AWG — Copper 145 A, Aluminum 115 A, ~67.4 mm²
• 4/0 AWG — Copper 195 A, Aluminum 150 A, ~107.2 mm²

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Real-world examples

Short 20 A branch circuit

20 A, 120 V, 30 ft copper, 3% drop limit:

Ampacity: 12 AWG handles 20 A. Drop on 12 AWG: 2 × 20 × (30/1000) × 1.93 = 2.32 V → 1.9% ✓

Result: 12 AWG — ampacity governs, drop is well within limit.

Long lighting run

20 A, 120 V, 150 ft copper, 3% drop limit:

Drop on 12 AWG: 2 × 20 × (150/1000) × 1.93 = 11.6 V → 9.6% ✗

Drop on 8 AWG: 2 × 20 × (150/1000) × 0.764 = 4.6 V → 3.8% ✗

Drop on 6 AWG: 2 × 20 × (150/1000) × 0.491 = 2.9 V → 2.5% ✓

Result: 6 AWG — voltage drop governs, even though 12 AWG covers ampacity.

Aluminum feeder, 100 A, 200 ft, 240 V, 3%

Ampacity: 2/0 AWG aluminum handles 115 A ✓ Drop on 2/0 AWG Al: 2 × 100 × (200/1000) × 0.159 = 6.36 V → 2.65% ✓

Result: 2/0 AWG aluminum — both criteria met.

Three-phase loads

The current-from-power feature supports three-phase, but the voltage-drop model uses single-phase two-wire math. For long three-phase runs, use engineering software with proper three-phase formulas.

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What this tool does not cover

• Ambient temperature derating — at > 30 °C, ampacity must be reduced by a correction factor
• Conduit fill derating — more than 3 current-carrying conductors require derating (NEC Table 310.15(C)(1))
• Parallel conductors — splitting large loads across multiple smaller conductors in parallel
• Terminal temperature limits — equipment rated at 60°C may not allow 75°C ampacity of the conductor
• Motor conductors — motors use FLA-based sizing with separate code tables
• Grounding conductors — sized separately per NEC Table 250.122 or equivalent
• Metric-native designs — the tool shows approximate mm² but is built around AWG/kcmil tables

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FAQ

Why doesn’t this match my jurisdiction’s table?

Ampacity tables vary by code edition, conductor type (THHN, XHHW, USE), installation method, and special conditions. This tool uses one simplified 75°C planning column.

I entered amps and watts — which takes priority?

If the load current field has a valid positive value, it is used. Power fields are ignored until you clear amps.

Does this size grounding conductors?

No — equipment grounding conductors are sized separately by code (e.g. NEC Table 250.122), not by ampacity.

Can I use this for DC circuits?

The ampacity values apply to DC as well, but the voltage-drop formula uses two-conductor (phase + return) math which also works for DC. Just enter the DC voltage and one-way length.

How does this relate to the Circuit Breaker Size Calculator?

Size the wire first (ampacity + drop), then confirm the breaker rating does not exceed the conductor’s ampacity. For most branch circuits the code maximum OCPD is tied directly to the conductor size (e.g. 20 A breaker with 12 AWG).

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Try the Tooladex Wire & Cable Size Calculator — enter current (or watts and voltage), circuit voltage, one-way run length, conductor material, and your voltage-drop limit to instantly find the minimum conductor size.