Quick answer: Wire size comes from the load in amps. Pick the smallest wire whose NEC ampacity (from Table 310.16, in the correct temperature column) is at least your current, then derate for high ambient temperature and for more than three current-carrying conductors, and finally check voltage drop on long runs (keep it under about 3%). Common household sizes: 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A copper. Size it exactly for your case with the wire size calculator.
Why wire size matters
Every wire has a limit to how much current it can carry before it overheats. Push too much current through a wire that is too small and the insulation cooks, which is a fire risk. That is the whole reason wire sizing exists: match the conductor to the load so it carries the current safely, and protect it with a breaker sized to the wire. Get it wrong and you either create a hazard (wire too small) or waste money on copper you do not need (wire too big).
In the US, wire sizing follows the National Electrical Code (NEC), and wire is measured in AWG (American Wire Gauge). This article walks through how to go from a load in amps to the right wire, including the two things people most often forget: temperature derating and voltage drop.
AWG: how wire sizes are numbered
AWG is the standard US wire-size system, and it works backwards from what you might expect: the bigger the number, the smaller the wire. So 14 AWG is a thin wire and 6 AWG is much thicker. The sizes run 14, 12, 10, 8, 6, 4, 3, 2, 1, then 1/0, 2/0, 3/0, 4/0 (spoken "one-aught" and so on), and after that they are given in kcmil (thousands of circular mils) for large feeders and services.
A useful pattern: every drop of three AWG sizes roughly doubles the cross-sectional area, and larger area means more current capacity and less resistance.
Step 1: Find the load in amps
Start with the current the circuit must carry. For a known appliance, it is on the nameplate. For a general circuit, it is the breaker rating. For a motor, use the full-load amps and remember motors need special treatment (see our guide to motor full-load amps and NEC 430). For continuous loads (running three hours or more), the NEC makes you size the wire and breaker at 125% of the load, so a 16 A continuous load is treated as 20 A.
Step 2: Use the NEC ampacity table (and the right temperature column)
NEC Table 310.16 lists the ampacity of each wire size at three temperature ratings: 60°C, 75°C, and 90°C. The catch is that you must use the column that matches the temperature rating of the terminations (the breaker and device lugs), not the highest column on the wire. In practice:
- Circuits 100 A and below often fall under the 60°C column unless everything is rated 75°C.
- Larger circuits and most modern equipment use the 75°C column.
- The 90°C column is generally used only for derating math, not for the final termination check.
Here is a simplified copper ampacity chart for common sizes (75°C column, THHN/THWN-style insulation):
| Copper AWG | Ampacity (75°C) | Typical use |
|---|---|---|
| 14 | 20 (15 A breaker) | Lighting, 15 A circuits |
| 12 | 25 (20 A breaker) | General outlets, 20 A |
| 10 | 35 (30 A breaker) | Water heater, dryer feed, 30 A |
| 8 | 50 | Ranges, 40-50 A |
| 6 | 65 | 60 A subpanel, large A/C |
| 4 | 85 | Feeders, ~90 A |
| 2 | 115 | ~100 A feeder (Cu) |
| 1/0 | 150 | 150 A |
| 2/0 | 175 | 175 A |
| 4/0 | 230 | 200 A service (Cu) |
Note the breaker sizes in parentheses for 14/12/10 AWG: the NEC caps the overcurrent protection for these small conductors at 15/20/30 A regardless of the table ampacity, which is why 12 AWG is a 20 A circuit even though its ampacity is higher.
Step 3: Derate for temperature and conductor count
The table ampacities assume a 30°C (86°F) ambient and no more than three current-carrying conductors bundled together. Real installations are often harsher, so you apply correction factors:
- Ambient temperature: above 30°C the ampacity is multiplied by a factor less than 1. A hot attic at 50°C can cut a 90°C wire's capacity by roughly 20-25%.
- More than three conductors: 4-6 conductors in a raceway derate to 80%, 7-9 to 70%, and it keeps dropping. Full conduits are a common reason wire has to be upsized.
You take the 90°C ampacity, multiply by the correction factors, and the result must still be at least your load current - but the final wire must also satisfy the termination temperature column from Step 2. Whichever is more restrictive wins.
Step 4: Check voltage drop (the step people skip)
A wire can have plenty of ampacity and still be too small on a long run, because voltage drop builds up over distance. The NEC recommends keeping voltage drop to about 3% on a branch circuit and 5% total including the feeder. Too much drop means dim lights, sluggish motors, and wasted energy as heat in the wire.
On long runs it is normal to go up one or two AWG sizes purely for voltage drop. For example, a 20 A circuit on 12 AWG might be fine at 50 feet but need 10 AWG at 150 feet. Always run the numbers with the voltage drop calculator before finalizing the size.
Copper vs aluminum
Aluminum carries less current per unit area than copper, so for the same load aluminum must be one to two sizes larger. A 100 A feeder that uses 2 or 3 AWG copper typically uses 1/0 aluminum; a 200 A service that uses 4/0 copper typically uses 250-350 kcmil aluminum. Aluminum is cheaper and lighter, which is why utilities and service entrances use it, but its terminations must be rated for aluminum (marked AL or CU-AL) and torqued to spec, and an antioxidant compound is used on the connections.
Worked example
Say you are feeding a 60 A subpanel 120 feet away, terminations rated 75°C, normal 30°C ambient, copper.
- Ampacity: 60 A needs at least 6 AWG copper (65 A at 75°C). Good on ampacity.
- Derating: normal ambient, three conductors - no derating.
- Voltage drop: at 120 feet and 60 A, 6 AWG may exceed 3%. Checking shows 4 AWG holds voltage drop within limits, so you upsize to 4 AWG copper.
The final answer, 4 AWG, was driven by voltage drop, not ampacity - exactly the case people miss when they size on the chart alone.
Wire size for common appliances
Here are the wire sizes people ask about most, at standard conditions (copper, 75°C terminations, short runs). Long runs or hot spaces can bump these up one size for voltage drop or derating.
| Appliance / circuit | Breaker | Copper wire |
|---|---|---|
| Lighting circuit | 15 A | 14 AWG |
| General outlets / kitchen | 20 A | 12 AWG |
| Electric water heater | 30 A | 10 AWG |
| Electric clothes dryer | 30 A | 10 AWG |
| Electric range / oven | 40-50 A | 8-6 AWG |
| Central AC / heat pump | per nameplate (30-60 A) | 10-6 AWG |
| EV charger (Level 2, 48 A) | 60 A | 6 AWG |
| 60 A subpanel | 60 A | 6 AWG (4 AWG on long runs) |
| 100 A subpanel/service | 100 A | 3-2 AWG (1/0 Al) |
| 200 A service | 200 A | 2/0-4/0 (250-350 kcmil Al) |
A few notes: an EV charger is a continuous load, so a 48 A charger needs a 60 A breaker and 6 AWG (the 125% rule). Central AC is sized to the nameplate minimum circuit ampacity and maximum overcurrent device, not a general rule - always read the data plate. For EV specifics, see our guide to the EV charger load calculation. When a circuit runs through conduit with other circuits, remember to check conduit fill as well, since a crowded conduit both derates the wire and limits how many conductors fit.
Common wire-sizing mistakes
- Ignoring voltage drop on long runs and undersizing the wire.
- Using the 90°C column for the final size when the terminations are only rated 75°C.
- Forgetting derating for hot attics or full conduits.
- Not applying the 125% continuous-load factor.
- Mixing up copper and aluminum ampacities.
- Protecting small wire with too large a breaker - 14 AWG must be on 15 A, 12 AWG on 20 A, 10 AWG on 30 A.
Standards and references
| Reference | What it covers |
|---|---|
| NEC Table 310.16 | Conductor ampacities by size and temperature |
| NEC 310.15 | Ambient and conductor-count correction factors |
| NEC 210 / 215 | Branch circuit and feeder requirements, voltage drop guidance |
| NEC 240.4(D) | Small-conductor breaker limits (14/12/10 AWG) |
The bottom line
To size a wire: find the load in amps (with the 125% factor for continuous loads), pick the smallest AWG that meets the NEC ampacity in the correct temperature column, derate for ambient and conductor count, and check voltage drop on the run. Household basics are 14 AWG/15 A, 12 AWG/20 A, 10 AWG/30 A copper. Size your exact circuit with the wire size calculator, confirm long runs with the voltage drop calculator, and have the design verified by a licensed electrician against the current NEC.
Frequently asked questions
What size wire do I need for 100 amps?
For a 100 amp circuit at 60 degrees C, the NEC (Table 310.16) generally calls for 3 AWG copper or 1 AWG aluminum, but the common answer for a 100 amp service or feeder is 4 AWG copper or 2 AWG aluminum when the 75 degrees C column applies and the equipment is rated for it. Always size to the correct temperature column, apply any derating for ambient temperature and conductor bundling, and add for voltage drop on long runs. Use the wire size calculator to confirm for your exact conditions.
How do I calculate the wire size for a given current?
Start with the load current in amps, then choose the NEC ampacity column that matches your termination temperature rating (usually 60 C up to 100 A, 75 C above). Pick the smallest AWG whose ampacity is at least the load, apply correction factors for ambient temperature and for more than three current-carrying conductors, and then check voltage drop on the run length. If voltage drop exceeds about 3 percent, increase the wire size.
What is AWG and how does it relate to wire size?
AWG stands for American Wire Gauge. It is the standard US system for wire sizes, and the rule is counter-intuitive: the larger the AWG number, the smaller the wire. So 14 AWG is thin (15 A), 12 AWG is bigger (20 A), and numbers go down to 1, then 1/0, 2/0, 3/0, 4/0 for progressively larger conductors, after which sizes are given in kcmil. Each drop of three AWG sizes roughly doubles the cross-sectional area.
Does copper or aluminum wire need to be bigger?
Aluminum needs to be bigger for the same load because it carries less current per unit area than copper. As a rule of thumb, aluminum is sized about one to two AWG larger than copper for the same ampacity. Aluminum is cheaper and lighter, which is why it is common for service entrances and large feeders, but its terminations must be rated for aluminum and torqued correctly.
How does wire length affect the wire size?
Longer runs increase voltage drop, which can force a larger wire even when the ampacity is fine. The NEC recommends keeping voltage drop to about 3 percent on a branch circuit and 5 percent total including the feeder. On long runs you often go up one or two AWG sizes purely to hold voltage drop within limits, so always check both ampacity and voltage drop.
What is derating and when do I apply it?
Derating reduces a conductor's allowable ampacity when conditions are harsher than the table assumes. The two main factors are ambient temperature above 30 C and having more than three current-carrying conductors in the same raceway or cable. You multiply the base ampacity by the correction factors, and the derated value must still be at least the load. Hot attics and full conduits are the usual reasons wire has to be upsized.
What size wire for a 20 amp or 30 amp circuit?
A 20 amp circuit typically uses 12 AWG copper, and a 30 amp circuit typically uses 10 AWG copper, at standard conditions. A 15 amp circuit uses 14 AWG copper. These are the common household branch-circuit sizes, but long runs or high ambient temperatures can require going one size larger to keep voltage drop and ampacity within the NEC limits.
Is a wire size calculator accurate for electrical design?
A calculator that applies NEC ampacity by temperature column, corrects for ambient temperature and conductor count, and checks voltage drop gives reliable wire sizes for most branch circuits and feeders. A complete design also considers termination temperature ratings, conductor material, conduit fill, overcurrent protection coordination, and the specific installation conditions, and should be confirmed by a licensed electrical engineer against the current NEC.