Wire Size Calculator
Updated Formulas shown
Enter the load in amps, the voltage and the one-way distance. The calculator picks the smallest copper or aluminum conductor that both carries the current (NEC Table 310.16) and keeps voltage drop under your limit. A 20 A, 120 V circuit 50 ft long is fine on 12 AWG for ampacity, but needs 10 AWG copper to stay under 3% drop.
| Size | Ampacity | Drop | OK? |
|---|---|---|---|
| 14 AWG | 15 A | 5.23% | Amps |
| 12 AWG | 20 A | 3.3% | Drop |
| 10 AWG | 30 A | 2.07% | Yes |
| 8 AWG | 40 A | 1.3% | Yes |
| 6 AWG | 55 A | 0.82% | Yes |
Ampacity from NEC Table 310.16 (30 °C ambient, ≤ 3 current-carrying conductors) with 240.4(D) limits for 14–10 AWG. Hot attics, bundled cables and conduit fill need derating. Always confirm with your local code and a licensed electrician.
How to choose a wire size
- Enter the load current in amps (from the appliance nameplate or the breaker size), the circuit voltage and the system type — 120/240 V homes are single-phase.
- Enter the one-way length of the cable run from the panel to the load, following the actual route through walls and ceilings.
- Choose the temperature column: 60 °C for NM-B (Romex) and UF cable, 75 °C for THWN in conduit, SER cable and most breaker terminals over 100 A.
- Tick “Continuous load” for loads that run 3 hours or more, such as EV chargers and water heaters — the wire must carry 125% of the load.
- Use the recommended size, and make sure the breaker doesn't exceed the wire's ampacity. Have a licensed electrician verify the design and pull the permit.
Wire ampacity chart (NEC Table 310.16)
| Size | Cu 60 °C | Cu 75 °C | Cu 90 °C | Al 60 °C | Al 75 °C | Al 90 °C |
|---|---|---|---|---|---|---|
| 14 AWG | 15 | 15* | 15* | — | — | — |
| 12 AWG | 20 | 20* | 20* | 15 | 15* | 15* |
| 10 AWG | 30 | 30* | 30* | 25 | 25* | 25* |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 AWG | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 300 kcmil | 240 | 285 | 320 | 195 | 230 | 260 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 400 kcmil | 280 | 335 | 380 | 225 | 270 | 305 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
Source: NFPA 70, National Electrical Code (2023), Table 310.16 and section 240.4(D). The 90 °C column is only for derating calculations unless every termination is rated 90 °C — in practice size to the 60 °C or 75 °C column. Adjust for more than three conductors in a raceway (310.15(C)(1)) and for high ambient temperatures such as attics and rooftops (310.15(B)).
Common household circuits
| Circuit | Breaker | Copper | Aluminum |
|---|---|---|---|
| Lighting / general outlets | 15 A | 14 AWG | 12 AWG |
| Kitchen, bath, laundry outlets | 20 A | 12 AWG | 10 AWG |
| Water heater, small AC | 30 A | 10 AWG | 8 AWG |
| Range, dryer (some) | 40 A | 8 AWG | 6 AWG |
| Range, EV charger (48 A) | 50 A | 6 AWG | 4 AWG |
| Subpanel, large EV/heat | 60 A | 6 AWG | 4 AWG |
| Subpanel | 100 A | 3 AWG | 1 AWG |
Copper vs. aluminum
Aluminum conductors have about 61% of copper's conductivity, so they need to be roughly two AWG sizes larger for the same current. Modern AA-8000 aluminum is common and code-compliant for feeders, service entrance cable and subpanels, where it saves a lot of money, but it requires terminals rated AL/CU and proper torque. Branch circuits under 30 A are almost always copper.
Safety note: this calculator is an estimating aid. Electrical work is regulated — consult a licensed electrician and your local inspector, who apply the full NEC, local amendments and the equipment's listing instructions.
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Frequently asked questions
What size wire for a 20 amp circuit?
12 AWG copper for a 20 A circuit (NEC 240.4(D)). For runs longer than about 50 ft at full load on 120 V, step up to 10 AWG to keep voltage drop under 3%.
What size wire for a 50 amp circuit?
6 AWG copper (NM-B, 60 °C column = 55 A) or 4 AWG aluminum. For a 50 A EV charger circuit (40 A continuous), 6 AWG copper is the usual choice; long runs may need 4 AWG.
What size wire for a 100 amp subpanel?
Typically 3 AWG copper or 1 AWG aluminum at 75 °C (e.g. SER or THWN in conduit). Many electricians use 2 AWG copper or 1/0 aluminum for margin and voltage drop on long runs.
What size wire for a 30 amp circuit?
10 AWG copper, or 8 AWG aluminum. A 30 A dryer or water heater circuit is usually 10/3 or 10/2 NM-B copper.
Does distance affect wire size?
Yes. Ampacity doesn't change with length, but voltage drop does. On long runs you often have to go one or two sizes up to keep drop under 3% for branch circuits.
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