Wire Size Calculator
Enter the load in amps, the voltage, the one-way run, copper or aluminum, and whether the load is continuous. You get the wire gauge from the NEC ampacity table, the breaker that protects it, and the voltage drop over your run, with the neighbouring sizes shown so you can see why.
Nameplate amps, or watts ÷ volts.
Continuous = runs 3+ hours (EV charger, water heater, HVAC). NEC sizes these at 125%.
NEC recommends 3% on a branch circuit, 5% total.
Wire size
8 AWG
copper, 50 A at 75°C
Breaker
40 A
design load 37.5 A (125%)
Voltage drop
1.43%
3.44 V over 75 ft
Sized by
Ampacity
run is short enough
Nearby sizes for this load
| AWG | Ampacity 75°C | Drop at 75 ft | Verdict |
|---|---|---|---|
| 12 | 25 A | 3.62% | too small for the load |
| 10 | 35 A | 2.27% | too small for the load |
| 8 | 50 A | 1.43% | OK |
| 6 | 65 A | 0.92% | OK |
| 4 | 85 A | 0.58% | OK |
Common circuits, copper, by run length
Wire size and breaker for typical household loads at 50 and 150 feet one way, holding voltage drop to 3%.
| Load | Amps | Breaker | Wire at 50 ft | Wire at 150 ft |
|---|---|---|---|---|
| 15 A lighting circuit | 15 | 15 A | 12 AWG | 8 AWG |
| 20 A receptacle circuit | 20 | 20 A | 10 AWG | 6 AWG |
| Water heater 4,500 W | 18.75 ×1.25 | 25 A | 10 AWG | 10 AWG |
| Dryer 30 A | 24 | 25 A | 10 AWG | 8 AWG |
| EV charger 32 A | 32 ×1.25 | 40 A | 8 AWG | 6 AWG |
| EV charger 48 A | 48 ×1.25 | 60 A | 6 AWG | 6 AWG |
| Range 50 A | 40 | 40 A | 8 AWG | 6 AWG |
| Hot tub 50 A GFCI | 40 ×1.25 | 50 A | 8 AWG | 6 AWG |
| Subpanel 60 A | 48 | 50 A | 8 AWG | 6 AWG |
| Subpanel 100 A | 80 | 80 A | 4 AWG | 3 AWG |
Loads marked ×1.25 are continuous. 60°C terminals or conduit derating can push any row up a size; a 50 A hot tub or range is commonly wired in 6 AWG for that reason.
Wire size chart: ampacity, resistance and breaker limits (NEC 310.16 at 75°C, 240.4(D))
Every AWG and kcmil size this site's calculators use, in one table -- 75°C ampacity for copper and aluminum, resistance per 1,000 ft (NEC Chapter 9, Table 8), and the largest standard breaker each size can carry.
| AWG | Copper ampacity | Aluminum ampacity | Cu Ω/1,000 ft | Al Ω/1,000 ft | Max breaker (Cu) |
|---|---|---|---|---|---|
| 14 | 20 A | — | 3.07 | 5.06 | 15 A |
| 12 | 25 A | 20 A | 1.93 | 3.18 | 20 A |
| 10 | 35 A | 30 A | 1.21 | 2 | 30 A |
| 8 | 50 A | 40 A | 0.764 | 1.26 | 50 A |
| 6 | 65 A | 50 A | 0.491 | 0.808 | 60 A |
| 4 | 85 A | 65 A | 0.308 | 0.508 | 80 A |
| 3 | 100 A | 75 A | 0.245 | 0.403 | 100 A |
| 2 | 115 A | 90 A | 0.194 | 0.319 | 110 A |
| 1 | 130 A | 100 A | 0.154 | 0.253 | 125 A |
| 1/0 | 150 A | 120 A | 0.122 | 0.201 | 150 A |
| 2/0 | 175 A | 135 A | 0.0967 | 0.159 | 175 A |
| 3/0 | 200 A | 155 A | 0.0766 | 0.126 | 200 A |
| 4/0 | 230 A | 180 A | 0.0608 | 0.1 | 225 A |
| 250 kcmil | 255 A | 205 A | 0.0515 | 0.0847 | 250 A |
| 300 kcmil | 285 A | 230 A | 0.0429 | 0.0707 | 250 A |
| 350 kcmil | 310 A | 250 A | 0.0367 | 0.0605 | 300 A |
| 400 kcmil | 335 A | 270 A | 0.0321 | 0.0529 | 300 A |
| 500 kcmil | 380 A | 310 A | 0.0258 | 0.0424 | 350 A |
How the calculator decides
- Design current: load amps, times 1.25 if continuous.
- Breaker: the next standard size (NEC 240.6(A)) at or above the design current.
- Ampacity: the smallest conductor whose 75°C rating is at least the design current and can be protected by that breaker, respecting the 240.4(D) caps.
- Voltage drop: if that conductor drops more than your limit over the run, step up until it does not. The result says which of the two rules set the size.
Not applied: terminal temperature ratings, conduit fill and ambient derating, motor and HVAC nameplate rules (which allow different breaker ratios), and local amendments. Those are exactly what the electrician and the inspector are for.
Frequently Asked Questions
What size wire for a 20 amp circuit? 30 amp? 50 amp?
Copper at 75°C: 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 50 A, 6 AWG for 65 A, 4 AWG for 85 A, 3 AWG for 100 A. Those are the NEC 310.16 values with the 240.4(D) small-conductor limits; a 50 A circuit is often run in 6 AWG anyway because many breakers and receptacles have 60°C terminals, which drops 8 AWG to 40 A. Long runs push each of these up a size for voltage drop.
What is the 125% rule for continuous loads?
NEC 210.20(A) and 215.3: a load that runs for three hours or more -- an EV charger, a water heater, most HVAC, commercial lighting -- must have its conductor and breaker sized at 125% of the load current. A 32 A EV charger is therefore a 40 A circuit on 8 AWG copper, not a 32 A load on 10 AWG.
Why does the calculator limit 14, 12 and 10 gauge to 15, 20 and 30 amp breakers?
NEC 240.4(D). Even though 12 AWG copper is rated 25 A at 75°C, the code caps its overcurrent protection at 20 A (14 AWG at 15 A, 10 AWG at 30 A) for general branch circuits. The rule exists because the small-wire ratings assume ideal conditions that residential wiring rarely meets.
Which temperature column should I use?
The 75°C column, which this calculator uses, is correct when the wire is THHN/THWN and the breaker and device terminals are rated 75°C -- standard for anything over 100 A and most modern equipment. Terminals rated 60°C, common on 15-30 A receptacles and older breakers, require the 60°C column, which is one size more conservative. The 90°C column is only for derating calculations, never for the final ampacity.
What about conduit fill, ambient temperature and bundling?
Ambient temperature (310.15(B)) and bundling both derate ampacity and this calculator does not apply them, sometimes reducing the allowed current 20-50%. Conduit fill is a separate NEC limit on how many conductors physically fit in a conduit, not on ampacity -- see the conduit fill calculator for that check on the same run.
Is there a full wire size chart, not just my specific answer?
Yes -- the table below lists every AWG and kcmil size this site uses, with copper and aluminum ampacity, resistance per 1,000 ft, and the largest standard breaker each size can carry, all in one reference.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.