Wire size calculator

How to size a wire

A wire has to pass two tests. It must carry the current without overheating, which is what the NEC ampacity tables cover. And it must not lose too much voltage on the way, which depends on the distance. The calculator runs both and gives you the larger size. Here it is with the values it opens with: a 20 amp breaker, 120 volts, 40 feet one way, copper NM-B cable and a 3% limit.

1. 1

   Pick the breaker

   If you know the load instead, the breaker is the next standard size at or above it, or above 125% of it for a continuous load. Here it is 20 A.

2. 2

   Find the smallest wire rated for it

   In the 60 °C column of NEC Table 310.16, 14 AWG copper carries 15 A and 12 AWG carries 20 A. So 12 AWG passes. Rule 240.4(D) also caps 14, 12 and 10 AWG copper at 15, 20 and 30 A breakers.

3. 3

   Work out the voltage drop

   Current flows out and back, so the wire is twice the distance: 2 x 40 ft = 80 ft. 12 AWG copper has 1.98 ohms per 1,000 ft, so 80 ft is 0.158 ohms. 20 A x 0.158 = 3.17 volts.

4. 4

   Compare with the limit

   3.17 ÷ 120 = 2.64%, under the 3% target. 12 AWG passes this test too.

5. 5

   Take the larger of the two

   Both say 12 AWG, so that is the answer: 12/2 NM-B with ground. With 10% slack plus 6 ft for the panel and box, buy a 50 ft roll.

Now stretch the same circuit to 100 feet. Ampacity still says 12 AWG, but the drop at 12 AWG would be 6.6%, and 10 AWG still loses 4.1%. It takes 8 AWG to get under 3% (2.6%). That is why long runs to a shed or a detached garage often use much bigger wire than the breaker alone suggests. The voltage drop is figured at the full breaker rating, which is the worst case; if you know the real load, choose "Load current" and enter it.

Wire size for common circuits

These are typical minimums for copper on short runs. Always go by the appliance's nameplate and installation manual, which may ask for more.

Air conditioners and heat pumps are the exception to "breaker sets the wire". Their nameplate lists a minimum circuit ampacity (MCA) for the wire and a maximum breaker (MOCP), and the breaker can be larger than the wire's rating because the unit has its own overload protection. Size those from the nameplate, not from this calculator.

60 °C or 75 °C: which column applies

NEC Table 310.16 lists three ampacities for every size, for insulation rated 60, 75 and 90 °C. You can only use as much as the weakest part of the circuit allows. NM-B cable (Romex) is held to the 60 °C column by NEC 334.80, even though the wires inside are rated 90 °C. Rule 110.14(C) adds that circuits of 100 amps or less use the 60 °C column unless every terminal is marked for 75 °C; above 100 amps, 75 °C is the norm. Most modern breakers and panels are marked 60/75 °C, which is why individual THHN wires in conduit can run one size smaller than cable for the same breaker.

* Capped by NEC 240.4(D): 14 AWG copper at 15 A, 12 AWG at 20 A and 10 AWG at 30 A; 12 AWG aluminum at 15 A and 10 AWG at 25 A. The table assumes no more than three current-carrying wires together and a room-temperature space. Hot attics, rooftops and bundles of cables lower these numbers, and an electrician applies those corrections.

Voltage drop: why distance matters

Every foot of wire has a little resistance, and current through it turns some voltage into heat. On a long run, a motor or a compressor gets less voltage than it was built for, runs hotter and starts harder. The formula for a single-phase circuit is:

Voltage drop = 2 x one-way length (ft) x amps x ohms per 1,000 ft ÷ 1,000

The NEC's informational notes recommend no more than 3% on a branch circuit and 5% for the feeder and branch together. That is 3.6 volts on 120 V and 7.2 volts on 240 V. Doubling the voltage halves the current for the same power, which is why long runs to a pump or a shop are cheaper at 240 V. The calculator uses resistance values for stranded wire at 75 °C from NEC Chapter 9, Table 8; at room temperature real drop is a little lower, so the result has a small margin built in.

Copper or aluminum?

Aluminum carries about 60% of the current of copper the same size, so it needs to be about two sizes larger. In exchange it costs much less, which adds up on a 100-foot feeder. Today's aluminum building wire is a special alloy (AA-8000 series), and it is widely used for service entrances, subpanels and large appliances in sizes from 8 AWG up. The calculator only offers aluminum from 8 AWG. The trouble with old aluminum came from small branch-circuit wiring from the 1960s and 1970s, not from modern feeders. Lugs must be rated for aluminum (marked AL or CU-AL), connections get antioxidant paste where the maker calls for it, and terminals must be tightened to the torque on the label.

What it costs

Wire prices follow copper and moved a lot in 2025 and 2026. In October 2026, 12/2 NM-B ran about $0.56 to $0.90 per foot on a 250 ft roll, 8/3 about $3 and 6/3 about $3.80 to $4 per foot on larger reels, and short cut lengths cost up to twice as much per foot. The calculator's default price for each size is an estimate from those figures; type your store's price for a real number. A standard two-pole breaker is about $18, while AFCI and GFCI breakers cost several times more. Hiring an electrician to add a dedicated circuit usually runs about $570 to $1,000, mostly labor, and more for long runs, finished walls or a panel that is full.

Safety and the code

• Never put a bigger breaker on existing wire. If a 15 A circuit trips, the fix is a new circuit, not a 20 A breaker on 14 AWG wire. The breaker protects the wire.
• Bigger wire is always allowed. Upsizing for voltage drop is fine, but the ground wire must be upsized in proportion (NEC 250.122(B)), and large wire may not fit the device terminals.
• Kitchens, baths, garages and outdoors need GFCI protection, and most living areas need AFCI breakers under current codes.
• Turn off and test. Shut off the breaker, then confirm the circuit is dead with a tester before touching anything. Panels stay live on the line side even with the main off.
• Get the permit. Most towns require a permit and an inspection for new circuits, and an unpermitted circuit can complicate an insurance claim or a sale. Some places only allow licensed electricians to do the work.

Planning an EV charger or backup power? See the EV charging cost calculator and the generator size calculator.

Common questions

Sources and assumptions

The sizing follows the 2017 to 2023 editions of the National Electrical Code (NFPA 70), whose tables did not change for these sizes. Check which edition your state or town has adopted, and any local amendments.

• Ampacity: NEC Table 310.16, 60 and 75 °C columns, 30 °C ambient, up to three current-carrying conductors. Values cross-checked with Cerrowire's ampacity chart. Small-conductor limits from NEC 240.4(D); terminal temperature rule from 110.14(C); NM cable at 60 °C from 334.80; standard breaker sizes from 240.6(A); continuous loads at 125% from 210.19(A) and 210.20(A); ground wire size from Table 250.122.
• Resistance: NEC Chapter 9, Table 8, DC resistance at 75 °C for stranded uncoated copper and aluminum, as reproduced in this voltage drop study guide. Voltage drop uses the single-phase formula with resistance only, which is accurate for these sizes; above about 250 kcmil, AC effects add a little.
• 3% and 5%: informational notes to NEC 210.19 and 215.2. They are advisory unless your jurisdiction adopts them.
• Prices: 12/2 NM-B at $0.56–$0.62 per foot on a 250 ft roll from Elliott Electric; 12/2 about $0.90, 10/3 $3.12, 8/3 $3.00, 6/3 $3.80–$3.97 and 4/3 $10.81 per foot, and a 50 A two-pole breaker at $18.24, from Voltflow's 240 V line cost guide (October 2026); dedicated circuit cost of $570 to $1,000 from HomeAdvisor.
• Our estimates: default cable prices per foot of $0.60 (14), $0.90 (12), $1.80 (10), $3.00 (8), $3.80 (6) and $8.50 (4 AWG) for copper, larger sizes scaled up by conductor area, and aluminum at 35% of copper. Breakers at $8 (single-pole up to 20 A), $10 (single-pole above 20 A), $18 (two-pole up to 60 A), $45 (up to 100 A) and $90 above. $15 for staples, connectors and a box on cable runs. Cable length is the one-way distance plus 10% and 6 ft, rounded up to a 25, 50, 100 or 250 ft roll for 10 AWG and smaller, and to the next 5 ft (cut to length) for larger sizes.