What Size Wire Do I Need?

Choosing the wrong wire size is one of the most common -- and most dangerous -- mistakes in electrical work. Too small and the conductor overheats; too large and you waste money on copper. This guide walks through every factor that governs the choice, from basic ampacity to voltage drop and installation method.

The Two Governing Rules

Every wire sizing decision is governed by two independent requirements, and you must satisfy both:

Ampacity -- the conductor must carry the design current continuously without exceeding the insulation's temperature rating.
Voltage drop -- the resistance of a long run causes a voltage loss at the load end; most codes and best practice limit this to 3 to 5 per cent of the supply voltage.

You calculate both requirements separately, then choose the wire size that satisfies whichever rule gives the larger conductor. Our wire size calculator handles both calculations simultaneously and flags which constraint is driving the result.

Already know the breaker rating?

If you are working back from a circuit rating rather than a load, these pages give the minimum conductor straight out, with the full 60/75/90°C table and the maximum run length before voltage drop forces a bigger size.

20 A 30 A 40 A 50 A 60 A 70 A 80 A 100 A 125 A 150 A 200 A 400 A

Understanding Ampacity

Ampacity is the maximum current a conductor can carry under defined conditions without damaging its insulation. The published tables in NEC Article 310 (US) and BS 7671 Appendix 4 (UK) assume specific conditions: a maximum ambient temperature, a specific number of current-carrying conductors bundled together, and a particular installation method (in free air, buried direct, in conduit, and so on).

Derating factors

The tabulated ampacity values apply only to the standard conditions. Real installations usually require derating -- multiplying the table value by a correction factor less than 1.0. The two most important derating factors are:

Ambient temperature. NEC tables assume 30 degC (86 degF) in air and 20 degC for underground. For every 10 degC above that, ampacity falls by roughly 10 to 15 per cent depending on insulation type. A cable routed through a roof space in summer can reach 50 degC ambient, which calls for a significant upsize.
Bundling / grouping. When multiple current-carrying conductors share a conduit or are tied in a bundle, each one's heat has nowhere to go. NEC Table 310.15(C)(1) applies a factor as low as 0.5 for nine or more conductors. UK Regulation 523.6 uses similar grouping factors from BS 7671 Table 4C1.

After applying all derating factors, the corrected ampacity of your chosen conductor must still exceed the circuit's design current -- and that design current is normally the breaker or fuse rating, not just the load, because the conductor must survive a breaker that has not yet tripped.

On a mechanical load you often have to establish the design current before you can size anything. A pump or fan is specified by duty, not by amps: start from flow and head on the pump power calculator to get the shaft power in kW, convert that to amps with the motor FLA calculator, and only then work through the ampacity tables below.

Understanding Voltage Drop

Resistance increases with conductor length and decreases with conductor cross-section. For a DC or single-phase AC circuit, the voltage drop formula is:

VD = 2 x L x R x I (where R is resistance per metre, L is one-way length, 2 accounts for live and return)

For three-phase circuits the factor changes to 1.732 (the square root of 3) instead of 2. Use our voltage drop calculator to model both single-phase and three-phase runs at any length without doing the arithmetic by hand.

When does voltage drop drive the wire size?

Voltage drop becomes the governing constraint on long runs at moderate current. Classic examples include:

A 30 m run of 2.5 mm² cable at 16 A will drop roughly 5 V on a 230 V single-phase circuit -- right at the 2 per cent limit before you account for the internal drop of the distribution board itself.
Garden outbuildings, EV charger sub-panels, and workshop feeds are the most frequent cases where a 6 mm² or 10 mm² cable is needed not because of load current, but because of run length.
LED lighting circuits are especially sensitive: a 3 per cent drop on a 230 V supply is 6.9 V, which at the end of a driver-fed strip can shift colour temperature noticeably.

AWG, mm2, and Circular Mils Explained

The US uses the American Wire Gauge (AWG) system, where a higher gauge number means a thinner wire -- 14 AWG is smaller than 10 AWG. Metric countries use cross-sectional area in square millimetres (mm²): 1.5, 2.5, 4, 6, 10, 16, and so on. Neither scale maps cleanly onto the other, but the rough equivalents most often needed are:

AWG	Approx. mm²	Typical use (residential)
14 AWG	2.1 mm²	15 A lighting / outlet circuit (US)
12 AWG	3.3 mm²	20 A kitchen / bathroom circuits (US)
10 AWG	5.3 mm²	30 A dryer / water heater (US)
8 AWG	8.4 mm²	40 A cooker circuit (US)
6 AWG	13.3 mm²	60 A sub-panel feed (US)

Note that the metric mm² values are closest-available standard sizes, not exact equivalents. Always confirm with your local code's ampacity table rather than converting and assuming the result is code-compliant.

Installation Method Matters

The same conductor carries very different currents depending on how it is installed. A 2.5 mm² twin-and-earth cable clipped directly to a wall surface can carry 27 A; the same cable buried in insulation drops to around 16 A because it cannot shed heat. Common installation methods and their approximate derating impact (BS 7671 / NEC equivalent):

Clipped direct to surface -- baseline, no derating. Good heat dissipation.
In conduit on a wall -- moderate derating, typically 0.77 to 0.85 of clipped-direct value for standard 3-conductor runs.
Enclosed in thermal insulation -- severe derating, down to 0.50 of the open-air value or lower. This catches many DIY jobs out: a cable run along a joist and then buried under loft insulation must be treated as fully enclosed.
Direct buried -- soil type and depth affect ratings; standard tables assume a soil thermal resistivity of 2.5 K.m/W.

When wiring runs pass through conduit, you also need to check that the conduit is not overfilled. Too many cables reduce airflow and raise temperature. Use our conduit fill calculator to check that your conduit selection meets NEC Chapter 9 Table 1 limits.

Worked Example: Sizing a 32 A EV Charger Feed

A homeowner in the UK wants to install a 7.4 kW (32 A single-phase) EV charger. The run from the consumer unit to the garage is 18 m. The cable will be run in a plastic surface-mounted trunking in an unheated garage.

Design current: 7400 W / 230 V = 32.2 A, so a 32 A circuit breaker is correct.
Ampacity check: 6 mm² twin-and-earth in trunking (Method C, BS 7671) has a rated current of 47 A. After a grouping factor of 0.8 (two circuits in the same trunking) the derated value is 37.6 A. That comfortably exceeds 32 A.
Voltage drop check: 6 mm² copper has a resistance of approximately 3.08 mohm/m per conductor. VD = 2 x 18 x 0.00308 x 32 = 3.55 V, which is 1.54 per cent of 230 V -- well within the 3 per cent BS 7671 guidance for final circuits.
Result: 6 mm² two-core plus earth satisfies both constraints.

If the run had been 40 m, the voltage drop would have risen to 7.9 V (3.4 per cent), which would require stepping up to 10 mm². That is exactly the kind of long-run trade-off the wire size calculator evaluates automatically when you enter your run length and load.

Frequently Asked Questions

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

In the US, NEC requires 12 AWG copper (or 10 AWG aluminium) for a 20 A circuit protected by a 20 A breaker. In the UK on a 230 V system, a 20 A circuit in normal domestic conditions typically uses 2.5 mm² twin-and-earth, though installation method and run length can push this to 4 mm². Always confirm with your local wiring regulations and measure the run length to check voltage drop.

Does wire size affect voltage drop?

Yes, directly. A larger cross-section conductor has lower resistance per metre, so the voltage drop over a given length and current is proportionally smaller. Doubling the cross-section roughly halves the voltage drop. On runs longer than about 15 to 20 metres at typical domestic currents, voltage drop often requires a larger conductor than ampacity alone would demand.

Can I use a smaller wire if I install a smaller breaker?

The breaker size and the wire size must be coordinated: the breaker must trip before the wire overheats. You can protect a larger conductor with a smaller breaker -- that is simply conservative and safe. But you cannot protect a smaller conductor with a larger breaker. The conductor's ampacity, after all derating, must meet or exceed the breaker rating. Fitting a 16 A breaker on 1.5 mm² cable that has a derated ampacity of 13 A is a code violation and a fire risk.

Is aluminium wire allowed for residential wiring?

Aluminium is permitted in many jurisdictions for service entrance conductors and feeder runs of large cross-section (typically 35 mm² / 2 AWG and above), but it is generally not used for branch circuits in new residential construction in the US or UK due to its lower conductivity (roughly 61 per cent of copper), higher coefficient of thermal expansion, and the additional termination care required. Where aluminium feeders are used, only aluminium-rated lugs and terminals may be used, and anti-oxidant compound is required at all connections.

What happens if I use wire that is too small?

Undersized wire resists current flow, converting electrical energy to heat. At moderate overload the insulation softens and becomes brittle over time, eventually failing and creating a short circuit or ignition source. At severe overload the insulation can melt within minutes. The overcurrent protection device (breaker or fuse) is designed to trip before this happens, but only if the conductor and the protective device are correctly coordinated. Running a 30 A load on 14 AWG wire protected by a 30 A breaker means the breaker will not trip until the wire is already hot enough to start a fire inside a wall cavity.