Wire Size Calculator

Find the wire gauge (AWG) for a circuit from the load in amps, run length, voltage and allowed voltage drop, using NEC ampacity tables for copper and aluminum.

By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a wire, calculate the circular mils needed to hold voltage drop to 3%, then pick the smallest gauge that meets both that figure and the NEC ampacity for the load. A 20 A load 50 ft from a 120 V panel needs 7,167 circular mils, so 10 AWG copper, which drops 2.07% (2.49 V). This is a planning estimate; a licensed electrician and local code govern.

Your measurements

A length built from stock pieces: total length divided by piece length, rounded up, with the last piece cut.

Load current (amps)Use the breaker size for a general circuit or the nameplate MCA for an appliance or air conditioner. Do not multiply by 1.25 here; the continuous load row does that.One-way distanceftFrom the panel to the load along the cable path through the attic or crawlspace, one way, not the straight line.System voltage120 V for lights and receptacles; 240 V for ranges, dryers, water heaters, EV chargers and subpanels; 208 and 480 are commercial.Maximum voltage drop%NEC recommends 3% for a branch circuit and 5% for feeder plus branch combined.ConductorNM-B (Romex) and THHN are copper. Aluminum is for 6 AWG and larger feeders such as SER and URD; 12 and 10 AWG aluminum branch circuits are special order and need CO/ALR devices.Phase

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Results

Recommended wire size: 10 AWG copper; Needs 7,167 circular mils; 10 AWG has 10,380
Voltage drop at that size: 2.07%; 2.49 V drop, 117.5 V at the load
Minimum size for ampacity alone: 12 AWG copper; NEC 75°C column with small-conductor limits, ignoring voltage drop
Continuous load (80% rule): 25 A; If the load runs 3 hours or more, size the breaker and wire for 125% of the load
Planning estimate only: 50 ft run; Verify with a licensed electrician; local code, conduit fill, temperature and termination ratings govern

Wire Size formula

Circular mils = 2 x K x Amps x Distance (ft) / (Volts x Drop %)   (K = 12.9 copper, 21.2 aluminum; use 1.732 instead of 2 for three-phase)
Wire size = smallest AWG with circular mils >= required and ampacity >= amps
Voltage drop (V) = 2 x K x Amps x Distance / circular mils of the chosen wire
Continuous load: Amps x 1.25

Worked example

A 30 A, 240 V circuit to a workshop 150 ft away in copper with a 3% limit: 2 x 12.9 x 30 x 150 = 116,100. Divide by 240 x 0.03 = 7.2 to get 16,125 circular mils. 10 AWG (10,380 CM) is too small even though it carries 30 A; 8 AWG at 16,510 CM is the answer. Actual drop: 116,100 / 16,510 = 7.03 V, or 2.9%.

A 50 A, 240 V feeder 100 ft long in aluminum: 2 x 21.2 x 50 x 100 = 212,000, divided by 7.2 = 29,444 circular mils. 4 AWG aluminum (41,740 CM, 65 A) covers both requirements, with a drop of 212,000 / 41,740 = 5.08 V, or 2.1%. 6 AWG aluminum would carry 50 A but drops 8.1 V, or 3.4%.

Quick reference

Maximum one-way run for 3% voltage drop, copper, single-phase
Wire size	Load	120 V	240 V
14 AWG	15 A	38 ft	76 ft
12 AWG	20 A	46 ft	91 ft
10 AWG	30 A	48 ft	97 ft
8 AWG	40 A	58 ft	115 ft
6 AWG	50 A	73 ft	146 ft

How wire size is chosen

Two rules decide the gauge. The first is ampacity: the wire must carry the load without overheating, and the NEC tables give the allowed current for each size. In a house the practical limits are 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 40 A and 6 AWG for 50 to 55 A in copper. NM-B cable is held to the 60°C column (334.80): 8 AWG Romex is 40 A and 6 AWG is 55 A, versus 50 and 65 A as THHN in conduit.

The second is voltage drop. Long runs lose voltage in the wire itself. The NEC recommends no more than 3 percent on a branch circuit. The calculator finds the circular mils that hold the drop to your limit and then picks the smallest gauge that satisfies both rules. On short runs ampacity wins; past about 50 ft at 120 V, voltage drop usually forces a larger wire.

What this calculator does not cover

Ampacities here are the 75°C column with no derating. More than three current-carrying conductors in one conduit or bundle derates to 80 percent (Table 310.15(C)(1)), and an attic over 86°F derates by Table 310.15(B)(1). Conduit fill is capped at 40 percent (Chapter 9, Table 1): three 6 AWG THHN plus a ground fit 3/4 inch EMT, four 4 AWG need 1 inch. Aluminum is normal for 6 AWG and larger feeders (SER, URD); it needs AL-rated lugs and anti-oxidant paste.

Continuous loads are sized at 125 percent (210.19(A)): a 4,500 W water heater at 18.75 A becomes 23.4 A, so 10 AWG on a 30 A breaker; a 48 A EV charger becomes 60 A, so 6 AWG copper in conduit or 4 AWG aluminum. Motors and air conditioners go by nameplate MCA and MOCP (Articles 430 and 440). Adding a circuit, a subpanel or an EV charger needs an electrical permit and a rough-in inspection before drywall.

What this calculator leaves out

The result is the conductor size. Not included: AFCI for bedrooms, living rooms, kitchens and hallways (NEC 210.12) and GFCI for kitchens, baths, garages, outdoors and laundry (210.8). Also box fill (314.16), staples within 12 inches of a box and every 4-1/2 feet (334.30), nail plates (300.4), the breaker, and the permit, typically $50 to $250.

Code limits that apply

NEC 334.80checked
NM-B cable held to the 60°C column: 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A, 6 AWG 55 A
If you miss it: 8 AWG Romex on a 50 A range breaker is flagged at rough-in; the cable is repulled as 6-3 NM-B or 8 AWG THHN in conduit
NEC 210.19(A)
Conductors sized for 125% of any continuous load that runs 3 hours or more
If you miss it: A 48 A EV charger is a 60 A circuit; 8 AWG copper at 50 A is refused, so it is 6 AWG copper on a 60 A breaker
NEC Table 310.16
75°C column: 8 AWG copper 50 A, 6 AWG 65 A, 4 AWG 85 A; aluminum 6 AWG 50 A, 4 AWG 65 A
If you miss it: 6 AWG aluminum SER picked from the copper column feeds a 60 A subpanel at 50 A ampacity; the feeder is repulled in 4 AWG
NEC 210.19(A) IN 4
Branch circuit voltage drop held to 3%, and 5% for feeder plus branch circuit combined
If you miss it: A 12 AWG run past about 46 ft at 120 V dims lights and stalls motors at full load; the circuit is repulled in 10 AWG
NEC 240.4(D)
14 AWG copper capped at 15 A, 12 AWG at 20 A, 10 AWG at 30 A whatever the table ampacity
If you miss it: 12 AWG on a 25 A breaker is refused at the panel inspection; the breaker is swapped to 20 A or the circuit repulled in 10 AWG
NEC 110.14(C)
Breaker and device terminations rated 75°C, so 90°C THHN is sized from the 75°C column
If you miss it: 8 AWG THHN sized at 55 A from the 90°C column overheats the breaker lug; the circuit is held to a 50 A breaker at inspection
NEC Table 310.15(C)(1)
Four to six current-carrying conductors in one raceway or bundle derate to 80% of ampacity
If you miss it: Four 20 A circuits bundled through one 3/4 inch EMT drop 12 AWG below the breaker rating; the runs are split or repulled in 10 AWG

Provision	Limit	If you miss it
NEC 334.80checked	NM-B cable held to the 60°C column: 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A, 6 AWG 55 A	8 AWG Romex on a 50 A range breaker is flagged at rough-in; the cable is repulled as 6-3 NM-B or 8 AWG THHN in conduit
NEC 210.19(A)	Conductors sized for 125% of any continuous load that runs 3 hours or more	A 48 A EV charger is a 60 A circuit; 8 AWG copper at 50 A is refused, so it is 6 AWG copper on a 60 A breaker
NEC Table 310.16	75°C column: 8 AWG copper 50 A, 6 AWG 65 A, 4 AWG 85 A; aluminum 6 AWG 50 A, 4 AWG 65 A	6 AWG aluminum SER picked from the copper column feeds a 60 A subpanel at 50 A ampacity; the feeder is repulled in 4 AWG
NEC 210.19(A) IN 4	Branch circuit voltage drop held to 3%, and 5% for feeder plus branch circuit combined	A 12 AWG run past about 46 ft at 120 V dims lights and stalls motors at full load; the circuit is repulled in 10 AWG
NEC 240.4(D)	14 AWG copper capped at 15 A, 12 AWG at 20 A, 10 AWG at 30 A whatever the table ampacity	12 AWG on a 25 A breaker is refused at the panel inspection; the breaker is swapped to 20 A or the circuit repulled in 10 AWG
NEC 110.14(C)	Breaker and device terminations rated 75°C, so 90°C THHN is sized from the 75°C column	8 AWG THHN sized at 55 A from the 90°C column overheats the breaker lug; the circuit is held to a 50 A breaker at inspection
NEC Table 310.15(C)(1)	Four to six current-carrying conductors in one raceway or bundle derate to 80% of ampacity	Four 20 A circuits bundled through one 3/4 inch EMT drop 12 AWG below the breaker rating; the runs are split or repulled in 10 AWG

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

NM-B is a 60°C cable (NEC 334.80): 8 AWG Romex is 40 A, 6 AWG 55 A, so a 50 A range needs 6-3.
THHN is rated 90°C but breaker lugs are 75°C (NEC 110.14(C)); size to the termination column, not the insulation.
Box fill (NEC 314.16): 12 AWG counts 2.25 cu in, an 18 cu in box holds two 12-2 cables and a device; three need 22.5.
Every 15 or 20 A, 120 V circuit in living areas needs AFCI (NEC 210.12); kitchens, baths, garages, outdoors and laundry need GFCI (210.8).
Aluminum below 8 AWG is a special order and needs CO/ALR devices; use it for 6 AWG and up feeders with Noalox and AL-rated lugs.
NM-B comes in 25, 50, 100, 250 and 1,000 ft coils; per-foot cuts cost 30 to 50 percent more, so buy the 250 ft roll.

Key facts

The NEC recommends limiting voltage drop to 3% on a branch circuit and 5% on the feeder and branch circuit combined (informational notes to 210.19 and 215.2).
K is the resistance of a 1 ft conductor with a cross section of one circular mil: about 12.9 ohms for copper and 21.2 for aluminum at 75°C.
NEC 240.4(D) limits 14 AWG copper to a 15 A breaker, 12 AWG to 20 A and 10 AWG to 30 A regardless of table ampacity.
Every three AWG steps roughly doubles the conductor area, so 7 AWG has about twice the circular mils of 10 AWG.
A load that runs for 3 hours or more is continuous, and the NEC requires conductors and breakers rated for 125% of it.

Frequently asked questions

What size wire do I need for 50 amps?

8 AWG THHN copper in conduit (50 A at 75°C), 6-3 NM-B if you run Romex (8 AWG NM-B is only 40 A under 334.80), or 4 AWG aluminum. For runs beyond about 145 ft at 240 V with 6 AWG, step up to 4 AWG copper to keep voltage drop under 3%.

What size wire for a 20 amp circuit?

12 AWG copper is the minimum. If the one-way run is longer than about 45 ft at 120 V, use 10 AWG to limit voltage drop.

How far can I run 12 gauge wire on a 20 amp circuit?

About 46 ft one way at 120 V or 91 ft at 240 V before voltage drop exceeds 3% at full load. Lightly loaded circuits can go farther.

Can I use 8 gauge Romex for a 50 amp range or dryer?

No. NM-B is limited to the 60°C column (NEC 334.80), where 8 AWG copper is 40 A. A 50 A range needs 6-3 NM-B (55 A) or 8 AWG THHN in conduit, which is 50 A at 75°C. A 30 A dryer takes 10-3 NM-B.

Sources and references

NFPA 70, National Electrical Code (Table 310.16 ampacities, 240.4(D), 210.19 and 215.2 informational notes) (NFPA)
Voltage Drop Formulas (International Association of Electrical Inspectors)
Re3 Voltage Drop Calculator (Southwire)

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