buncha/engineering/wiresize

Wire Size Calculator

Load amps

One-way distance (ft)

Source volts

Max drop %

Phase

Conductor material

Insulation temp rating

60°C = older TW · 75°C = THW/RHW (most common) · 90°C = THHN/XHHW. Lugs and breakers usually limit you to the 75°C column even with 90°C wire.

Use this wire

10 AWG

Driven by voltage drop over this distance.

Ampacity-required

14 AWG

20 A at 75°C

Voltage-drop-required (≤3%)

10 AWG

Stays under 3% over 100 ft

Voltage drop

4.97 V

Drop percent

2.07%

Wire ampacity

35 A

Run length

200 ft round-trip

Show full NEC ampacity table (Table 310.16)

Size	cmil	Cu 60°	Cu 75°	Cu 90°	Al 60°	Al 75°	Al 90°
14 AWG	4,110	15	20	25	—	—	—
12 AWG	6,530	20	25	30	15	20	25
10 AWG	10,380	30	35	40	25	30	35
8 AWG	16,510	40	50	55	35	40	45
6 AWG	26,240	55	65	75	40	50	55
4 AWG	41,740	70	85	95	55	65	75
3 AWG	52,620	85	100	115	65	75	85
2 AWG	66,360	95	115	130	75	90	100
1 AWG	83,690	110	130	145	85	100	115
1/0	105,600	125	150	170	100	120	135
2/0	133,100	145	175	195	115	135	150
3/0	167,800	165	200	225	130	155	175
4/0	211,600	195	230	260	150	180	205
250 kcmil	250,000	215	255	290	170	205	230
300 kcmil	300,000	240	285	320	195	230	260
350 kcmil	350,000	260	310	350	210	250	280
400 kcmil	400,000	280	335	380	225	270	305
500 kcmil	500,000	320	380	430	260	310	350
600 kcmil	600,000	350	420	475	285	340	385

NEC reference values. Table 310.16 ampacities for not more than three current-carrying conductors in raceway. Voltage-drop math uses K = 12.9 (Cu) or 21.2 (Al). Always confirm against your local code, conduit fill, bundling derate, and terminal temp limits — this tool is a starting point, not a permit.

Read the guide

What Wire Size Do I Need? NEC Ampacity + Voltage Drop, Demystified

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About Wire Size Calculator

Find the right AWG / kcmil for any branch circuit or feeder — NEC ampacity AND voltage drop in one shot.

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