Wire size & voltage drop calculator

Wire size calculator by amps, volts and distance: it checks ampacity, then voltage drop, then grows the ground with the wire. A 60 A subpanel 120 ft away takes 6 AWG copper (2.4 % drop) with a 10 AWG ground. A 30 A, 240 V dryer reaches 120 ft on 10 AWG at 3 %.

Drawing at true size…

Quick Project Finder · 60 A subpanel

Subpanel, 60 A · 120 ft one way, THHN copper in EMT

USE:

6 AWG COPPER WIRE · 60 A 2-POLE BREAKER · 3/4" EMT CONDUIT

Wire

6 AWG

copper THHN

Breaker

60 A

2-pole

Conduit

3/4" EMT

ground 10 AWG

Safe for up to 65 A (75 °C THHN terminals) on a 60 A breaker, and carries a 48 A load up to 152 feet one way with under 3% voltage drop.

Meets NEC 2023 Table 310.16 at 75°C — verify with your local AHJ/inspector110.14(C)
2.36 % drop at 120 ft210.19 IN

The answer

Wire: 3 × 6 AWG THHN copper + 10 AWG groundTHHN copper in EMT
Breaker: 60 A 2-pole2-pole, 240 V
Ground: 10 AWG copperTable 250.122 for 60 A
Conduit: 3/4" EMT32.5 % full of 40 %
Max distance at this size: 152 ft one wayunder 3 % drop at 48 A

How it was worked out

Allowable ampacity: 65 A6 AWG, 75 °C column
Load used for the drop: 48 AA subpanel feeder
Smallest size before distance: 6 AWG
With In-wall NM-B cable: 6 AWGNM-B is limited to the 60 °C column

6 AWG

Copper 60 / 75 / 90 °C: 55 / 65 / 75 ANM-B uses the 60 °C column (334.80)
Aluminum 60 / 75 / 90 °C: 40 / 50 / 55 A
Largest breaker (Cu, 75 °C): 60 Aampacity ≥ breaker
Solid diameter: 0.162"4.12 mm · ASTM B258 formula
Stranded (7 strands): 0.184"4.67 mm · Table 8
THHN outside: 0.254"6.45 mm · 0.0507 in² · Table 5
Area: 26,240 cmil13.30 mm² · nearest IEC 16 mm²

Wire size and distance tables 44 rows

Sort any column, print a shop sheet, download CSV. Click a row to load it in the calculator.

Wire size by breaker size

18 rows

Download CSV

Minimum wire size for each standard breaker from 15 to 200 A: copper NM-B at 60 °C, copper THHN at 75 °C, aluminum at 75 °C, and the Table 250.122 ground, with ampacity at least the breaker rating

14 AWG	14 AWG	12 AWG	14 AWG
12 AWG	12 AWG	10 AWG	12 AWG
10 AWG	10 AWG	10 AWG	10 AWG
10 AWG	10 AWG	8 AWG	10 AWG
8 AWG	8 AWG	8 AWG	10 AWG
8 AWG	8 AWG	8 AWG	10 AWG
6 AWG	8 AWG	6 AWG	10 AWG
6 AWG	8 AWG	6 AWG	10 AWG
4 AWG	6 AWG	4 AWG	10 AWG
4 AWG	4 AWG	3 AWG	8 AWG
3 AWG	4 AWG	2 AWG	8 AWG
2 AWG	3 AWG	2 AWG	8 AWG
—	3 AWG	1 AWG	8 AWG
—	2 AWG	1/0 AWG	6 AWG
—	1 AWG	2/0 AWG	6 AWG
—	1/0 AWG	3/0 AWG	6 AWG
—	2/0 AWG	4/0 AWG	6 AWG
—	3/0 AWG	250 kcmil	6 AWG

Ampacity at least the breaker rating (no 240.4(B) next size up). NM-B is shown up to 2 AWG, the largest made.

Longest run at 120 V for 3 % voltage drop

13 rows

Download CSV

Longest one-way distance in feet that keeps the voltage drop at or under 3 % at 120 V single-phase, copper, NEC Chapter 9 Table 8 resistance at 75 °C, for loads of 10, 15, 20, 30 A

57	38	28	19
90	60	45	30
145	96	72	48
231	154	115	77
366	244	183	122
584	389	292	194
734	489	367	244
927	618	463	309
1168	779	584	389
1475	983	737	491
1861	1240	930	620
2349	1566	1174	783
2960	1973	1480	986

Feet one way (the current goes out and back). Ampacity is a separate check: a long run may need a heavier wire for drop, never a lighter one for heat.

Longest run at 240 V for 3 % voltage drop

13 rows

Download CSV

Longest one-way distance in feet that keeps the voltage drop at or under 3 % at 240 V single-phase, copper, NEC Chapter 9 Table 8 resistance at 75 °C, for loads of 20, 30, 50, 60, 100 A

57	38	22	19	11
90	60	36	30	18
145	96	58	48	29
231	154	92	77	46
366	244	146	122	73
584	389	233	194	116
734	489	293	244	146
927	618	371	309	185
1168	779	467	389	233
1475	983	590	491	295
1861	1240	744	620	372
2349	1566	939	783	469
2960	1973	1184	986	592

Feet one way (the current goes out and back). Ampacity is a separate check: a long run may need a heavier wire for drop, never a lighter one for heat.

05 / Field guide

Heat sets the minimum,
distance sets the rest.

Sizing a wire is two checks in a row. First ampacity: the conductor must carry the breaker rating (or 125 % of a continuous load) at the right temperature column. Then voltage drop: on a long run the same current loses enough voltage that a heavier wire is worth it. If the drop forces a larger wire, 250.122(B) makes the ground grow in the same proportion.

Voltage drop

The drop is the current times the resistance of the whole loop, out and back for DC and single phase, √3 times one leg for three phase. The calculator uses NEC Chapter 9 Table 8 DC resistance at 75 °C, which lands on the same size as Cerrowire’s AC tables or one size heavier, never lighter.

A 30 A, 240 V dryer reaches 120 ft one way on 10 AWG at 3 % (24 A running); a 20 A, 120 V garage circuit at 16 A reaches 56 ft on 12 AWG and 90 ft on 10 AWG.

V drop = 2 × L (ft) × I × R (Ω/1000 ft) ÷ 1000 3-phase: √3 × L × I × R ÷ 1000 % = V drop ÷ V × 100

3 % and 5 % are recommendations

The NEC’s 3 % for a branch circuit and 5 % for feeder plus branch are informational notes (210.19(A) IN No. 4, 215.2(A) IN No. 2): good practice, not an enforceable rule. Sensitive loads, motors and long outbuilding runs are where they matter.

Grow the ground too

250.122(B): when you upsize the circuit conductors for distance, increase the wire-type equipment grounding conductor by the same circular-mil ratio. 6 → 4 AWG on a 60 A circuit multiplies the area by 1.59, so the 10 AWG ground becomes 8 AWG.

Worked example: a 100 A subpanel 150 ft away

At 80 A of load and 240 V, the 75 °C minimum for a 100 A feeder is 3 AWG copper or 1 AWG aluminum. Over 150 ft the drop pushes the copper to 3 AWG (2.5 %) and the aluminum to 1 AWG (2.5 %), and the ground grows with it to 6 AWG aluminum or 8 AWG copper. Aluminum feeders are usually the cheaper choice at this size, terminated on lugs rated for aluminum and torqued to the marked value.

Questions

Answered plainly.

What size wire for 100 feet at 30 amps?

At 240 V and 24 A of running load, 10 AWG copper reaches 120 ft at 3 %, so 100 ft is fine. At 120 V and 24 A, 10 AWG reaches only 60 ft: use 8 AWG.

What size wire for a 60 amp subpanel 150 feet away?

6 AWG copper or 3 AWG aluminum, four wires, figured at 80 % of the feeder rating with under 3 % drop; the ground grows with it.

Does voltage drop matter at 240 V?

Less than at 120 V: the same current loses the same volts, but they are half the percentage. That is why long runs to subpanels, wells and EV chargers are 240 V.

Should I use 3 % or 5 %?

3 % for a single branch circuit, 5 % for the feeder and the branch together, as the NEC recommends. Motors and electronics are happier at the low end.

Why is my answer bigger than the ampacity chart?

Because the run is long enough that voltage drop, not heat, sets the size. The card says when it upsized for distance and grows the ground to match.

Is it cheaper to use aluminum for a long feeder?

Usually, from about 60 A up: aluminum needs a size or two larger than copper, but it costs far less per foot. For a 100 A subpanel 150 ft away the answer is 1 AWG aluminum against 3 AWG copper.

Does voltage drop change the breaker size?

No. The breaker protects the conductor and is chosen for the load; a heavier wire for distance can stay on the same breaker. Only the ground grows with the wire (250.122(B)).

Wire size & voltage drop calculator

The answer

How it was worked out

6 AWG

Wire size and distance tables 44 rows

Wire size by breaker size

Longest run at 120 V for 3 % voltage drop

Longest run at 240 V for 3 % voltage drop

Heat sets the minimum,distance sets the rest.

Voltage drop

3 % and 5 % are recommendations

Grow the ground too

Worked example: a 100 A subpanel 150 ft away

Answered plainly.

What size wire for 100 feet at 30 amps?

What size wire for a 60 amp subpanel 150 feet away?

Does voltage drop matter at 240 V?

Should I use 3 % or 5 %?

Why is my answer bigger than the ampacity chart?

Is it cheaper to use aluminum for a long feeder?

Does voltage drop change the breaker size?

Browse all 20 gauges, 12 conduit sizes and 10 circuits

Heat sets the minimum,
distance sets the rest.