Wire Size Calculator

What gauge wire you need for the amps and distance: NEC ampacity and voltage drop for copper or aluminium, or mm² cable sized by voltage drop.

The right wire size has to do two things: carry the current safely, and keep the voltage drop small over the length of the run. A 20 A circuit at 120 V can use 12 AWG copper for a short run, but over 100 ft 12 AWG loses 6.6% of the voltage, and it takes 8 AWG to stay under 3%. This calculator checks both. It finds the smallest copper or aluminium wire allowed the current in NEC Table 310.16, in the 60 °C or 75 °C column, with the 240.4(D) limits on 14, 12 and 10 AWG and 125% for continuous loads. Then it finds the smallest wire that keeps the drop within your limit, and recommends the larger of the two. Every size from 14 AWG to 4/0 is listed with its allowed current and drop. For metric cable it finds the smallest size from 1 to 120 mm² that keeps the drop within your limit. The current a UK or Australian cable may carry depends on how it's installed, so that isn't covered.

Units

Country

Supply

Voltage

Current

Cable length (one way)

Maximum voltage drop

Conductor

Ampacity column

Continuous load (3 hours or more): size for 125%

Wire size: 10 AWG
Sized for current: 12 AWG
Sized for voltage drop: 10 AWG
Drop with that size: 2.07%

Size	Allowed	Carries the load	Drop
14 AWG	15 A	No	5.23% (over 5%)
12 AWG	20 A	Yes	3.29% (3–5%)
10 AWG	30 A	Yes	2.07% (under 3%)
8 AWG	40 A	Yes	1.3% (under 3%)
6 AWG	55 A	Yes	0.82% (under 3%)
4 AWG	70 A	Yes	0.52% (under 3%)
3 AWG	85 A	Yes	0.41% (under 3%)
2 AWG	95 A	Yes	0.32% (under 3%)
1 AWG	110 A	Yes	0.26% (under 3%)
1/0 AWG	125 A	Yes	0.2% (under 3%)
2/0 AWG	145 A	Yes	0.16% (under 3%)
3/0 AWG	165 A	Yes	0.13% (under 3%)
4/0 AWG	195 A	Yes	0.1% (under 3%)

How it was worked out

Smallest copper wire allowed 20 A in the 60 °C column: 12 AWG (20 A, the most 240.4(D) allows)
Smallest that keeps the drop within 3% over 50 ft: 10 AWG (2.07%)
The larger of the two: 10 AWG

Allowed currents are from NEC 2023 Table 310.16 (30 °C ambient, no more than three current-carrying conductors together), capped for 14, 12 and 10 AWG by 240.4(D). Hot spaces such as attics and rooftops, or more conductors bundled together, lower them (310.15). NM cable is limited to the 60 °C column, and terminals on most circuits of 100 A or less are rated 60 °C unless marked otherwise. This doesn't cover services, feeders sized under 310.12, or motor circuits. Resistance is taken at the conductor's working temperature (75 °C for the US K-factor, 70 °C for metric cables, as BS 7671 does). Reactance is left out, which matters little below about 1/0 AWG or 35 mm²; above that, AC drops read a little low.

Common use cases

Choose the wire gauge for a new circuit to a garage, shed, workshop or outbuilding
Check whether a long run needs a bigger wire than the breaker size suggests
Size wire for a continuous load such as an EV charger or water heater
Compare copper and aluminium for a feeder before pricing cable

How to use

Choose AWG and feet or mm² and metres.
Pick the supply and check the voltage, then enter the current and the one-way length of the run.
Set the most voltage drop you'll accept: 3% is the usual target for a branch circuit.
For AWG, choose copper or aluminium and the ampacity column, and tick continuous load if the current runs for 3 hours or more.
Read the recommended size, why it was chosen, and the table of every size.

FAQ

What size wire do I need for 15, 20 and 30 amps?

For copper building wire, the NEC allows 14 AWG on circuits up to 15 A, 12 AWG up to 20 A and 10 AWG up to 30 A (section 240.4(D)). Those are the smallest sizes; a long run may need a bigger wire to keep the voltage drop down.

What size wire do I need for 40 and 50 amps?

In the 60 °C column, which applies to NM cable, 8 AWG copper is allowed 40 A and 6 AWG copper 55 A, so a 50 A circuit takes 6 AWG. Where the wire and the terminals at both ends are rated 75 °C, 8 AWG copper is allowed 50 A.

What size wire do I need for a 100 ft run?

It depends on the current and voltage. At 20 A and 120 V over 100 ft, 12 AWG drops 6.6%, 10 AWG 4.1% and 8 AWG 2.6%, so 8 AWG keeps the drop under 3%. At 240 V the percentages halve, so 10 AWG (2.1%) would do.

What is a continuous load?

The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. The wire and breaker are sized for 125% of it, so a 24 A EV charger needs a 30 A circuit and at least 10 AWG copper. Storage water heaters of 120 gallons or less are treated as continuous loads too.

How does aluminium compare with copper?

Aluminium has more resistance, so it needs a bigger size for the same current. In the 75 °C column, 1/0 aluminium is allowed 120 A, close to 2 AWG copper at 115 A. Aluminium also drops more voltage: the calculator uses K = 21.2 for aluminium against 12.9 for copper.

Can I use this for UK or Australian cable?

Only for voltage drop. The current a cable such as twin and earth may carry depends on how it's installed: clipped to a wall, run through insulation or in conduit. Those ratings come from BS 7671 Appendix 4 or AS/NZS 3008.1.1. In England and Wales much electrical work must be notified under Part P of the Building Regulations, and in Australia it must be done by a licensed electrician.

Results are estimates worked out from the figures you enter and published tables, for planning only. They are not electrical, engineering or professional advice. Mains electricity can kill and start fires. Check cable and circuit sizes against the wiring rules where you live (the NEC and local amendments in the US, BS 7671 in the UK, AS/NZS 3000 and 3008 in Australia and New Zealand), and use a licensed electrician where the law requires one.