How to use the wire size & voltage drop calculator
- Enter the load current in amps, the circuit voltage and the one-way length of the run.
- Choose DC, single-phase or three-phase, copper or aluminium, the terminal temperature rating and the voltage drop you will accept (3% is the usual limit).
- Read the minimum wire size: the larger of what ampacity and voltage drop each require, with the mm² equivalent.
Two checks, the larger wins
A conductor has to pass two tests. Ampacity is how much current it can carry without its insulation overheating; the values come from NEC Table 310.16 for up to three conductors in a raceway or cable at 30 °C ambient. Voltage drop is how much of the supply voltage is lost in the wire itself; it depends on current, length and the conductor's resistance, and it has nothing to do with heating. A 20 A circuit on 12 AWG copper is fine for ampacity at any length, but at 100 ft on 120 V it loses over 6% of its voltage, so the run needs 8 AWG to stay under 3%. The calculator finds the smallest size that passes both.
Single-phase and DC: Vdrop = 2 × I × R × L ÷ 1000 Three-phase: Vdrop = √3 × I × R × L ÷ 1000
I in amps, L the one-way length in feet, R in ohms per 1,000 ft from NEC Chapter 9 Table 8 (e.g. 12 AWG copper 1.93, 10 AWG 1.21, 6 AWG 0.491).
The factor 2 accounts for the current going out and coming back; in a balanced three-phase circuit the return currents cancel, which is why the factor is √3 instead. Metric input is converted to feet internally.
Which temperature column to use
The ampacity of a wire depends on what its insulation and its terminations can stand. In North American residential work: NM-B (Romex) cable must be sized from the 60 °C column; THWN/THHN in conduit on breakers and lugs rated 75 °C uses the 75 °C column; the 90 °C column is only for applying derating factors (more than three conductors in a conduit, high ambient) and the result must still not exceed the 75 °C terminal rating. Separately, NEC 240.4(D) caps the breaker on 14 AWG at 15 A, 12 AWG at 20 A and 10 AWG at 30 A no matter which column you use; the calculator notes this.
Continuous loads
Anything that runs for three hours or more at full current (EV chargers, water heaters, most lighting, heat pumps, pool pumps) must be served by conductors and breakers rated at 125% of the load (NEC 210.19 and 215.2). A 48 A Level 2 charger therefore needs 60 A wire and breaker: 6 AWG copper at 75 °C. Tick the box and the calculator applies the factor to the ampacity check (voltage drop still uses the real current).
Low-voltage DC: why 12 V needs thick cable
Voltage drop is a percentage of the supply, so at 12 V every tenth of a volt matters. A 30 A load 5 m from the battery on 10 AWG copper loses 1.2 V, which is 10% and enough to trip the low-voltage disconnect on an inverter; it takes 4 AWG (0.3 V, 2.5%) to stay under 3%. Campervan, boat and off-grid solar wiring is routinely two or three sizes larger than the current alone would need, and the fix is often to move to 24 or 48 V rather than buy heavier cable.
Metric sizes
The UK, Europe, Australia, New Zealand and Singapore sell cable by cross-section in mm²: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95 and so on. 12 AWG is 3.31 mm², so the nearest metric cable that is at least as large is 4 mm²; 6 AWG (13.3 mm²) maps to 16 mm². The tool shows the next metric size up, never down. Your national rating tables (BS 7671 Appendix 4, AS/NZS 3008) give the ampacity for a given installation method and may be lower or higher than the NEC column for the same copper.
Worked example: 100 A sub-panel in a detached workshop
Feeder at 240 V, 150 ft one-way, aluminium, 75 °C lugs. Ampacity: 1 AWG aluminium (100 A). Voltage drop on 1 AWG aluminium: 2 × 100 × 0.253 × 150 ÷ 1000 = 7.6 V, which is 3.2%, slightly over. 1/0 aluminium gives 6.0 V (2.5%) and passes, so the feeder is 1/0 AL, or 2 AWG copper if you prefer copper (5.8 V, 2.4%). With the 5% total feeder-plus-branch recommendation in mind, many electricians would go one size larger still so the branch circuits in the workshop have room.
This calculator is a sizing aid. Fixed wiring must comply with your local code and in most countries be installed or inspected by a licensed electrician. Conduit fill, derating for bundling or ambient temperature, and overcurrent protection are separate checks.
Questions and answers
What size wire for a 50 amp circuit?
6 AWG copper at 75 °C terminals (65 A) or 4 AWG aluminium (65 A) for ampacity. Over a long run voltage drop takes over: a 50 A, 240 V single-phase circuit stays under 3% drop up to about 70 ft (one-way) on 6 AWG copper; beyond that step up to 4 AWG.
What size wire for a 30 amp, 240 V circuit (dryer, RV, EV charger)?
10 AWG copper for runs up to about 100 ft one-way. A 30 A load on 10 AWG copper reaches 3% drop at roughly 100 ft at 240 V; at 150 ft use 8 AWG. If it is a continuous load such as an EV charger, the NEC requires the conductor to be rated for 125% of the load, so a 32 A charger needs a 40 A circuit and 8 AWG.
What size wire for a 100 amp sub-panel?
3 AWG copper or 1 AWG aluminium at 75 °C. For a detached garage 150 ft away, voltage drop at 100 A and 240 V pushes copper to 1 AWG or aluminium to 2/0, which is why aluminium feeders are so common for outbuildings.
What does 3% voltage drop mean and why does it matter?
Resistance in the wire turns some of the voltage into heat before it reaches the load. The NEC recommends (it does not require) keeping branch-circuit drop under 3% and total feeder-plus-branch under 5%. Motors and compressors draw more current as voltage drops, run hotter and fail earlier; LED drivers flicker; at 12 V a 3% drop is only 0.36 V, so low-voltage solar and marine runs need surprisingly thick cable.
Copper or aluminium?
Aluminium has about 61% of copper's conductivity, so it needs to be two sizes larger for the same job, but it costs a fraction as much and weighs half as much. It is standard for service entrances and feeders over 50 A; for branch circuits under 30 A most jurisdictions and most electricians use copper. Aluminium terminations must be rated AL/CU and torqued correctly.
Are these the right tables for the UK, Australia or Canada?
The ampacity and resistance data are from the US NEC (310.16 and Chapter 9 Table 8). The Canadian Electrical Code tables are nearly identical. The UK (BS 7671) and Australia/New Zealand (AS/NZS 3008) use metric cable sizes and different installation-method tables; the mm² equivalent and the voltage drop physics transfer directly, but use your national current-rating tables for the final selection, and remember that only a licensed electrician may do fixed wiring in the UK, Australia, New Zealand and Singapore.
Free to use, runs in your browser, nothing uploaded. Last updated 2026-10-07. Made by HandyToolkits; questions to [email protected].