Amp to Wire Size Calculator

By Giorgi Gaprindashvili · Updated Jul 2026

Convert amps into a practical wire gauge (AWG) using common reference ampacity tables.

Amps

Material

Wiring Method

Your Wire Size Result

Recommended Wire Size

Ampacity Used

Input Amps

Breaker (next standard)

Table Choice

What This Result Means

How to Use

Enter amps - Type the current you want the wire to carry.
Pick copper/aluminum - Material affects ampacity and resistance.
Pick the wiring method - NM-B cable (Romex) uses the 60°C column; THHN in conduit uses 75°C (NEC 334.80).
Read the gauge - The calculator picks the smallest gauge meeting the input amps.
Use voltage drop separately - Long runs may need upsizing even if ampacity is sufficient.

Quick answer

Match the amps to the smallest gauge that carries them: 15 amps to 14 AWG, 20 amps to 12 AWG, 30 amps to 10 AWG, 40 to 50 amps to 8 AWG copper (6 AWG for NM-B cable). These are copper in conduit at the 75 degree column. Two rules keep the answer honest, and this tool applies both: NEC 240.4(D) never lets 14, 12, or 10 AWG copper exceed a 15, 20, or 30 amp breaker, and NEC 334.80 sizes NM-B cable from the 60 degree column. Enter your amps and wiring method above.

Amps to Gauge at a Glance

A quick map from circuit amps to copper wire gauge at the 75 degree column (THHN in conduit). NM-B cable sizes one column cooler, so a load near the top of a range often steps up one gauge on Romex.

Amps to Wire Size: What This Tool Does

This page is a quick ampacity-to-gauge lookup. Enter amps and select copper or aluminum, and the calculator returns a smallest wire size that meets the input using a simplified planning table.

It's designed for fast comparisons, not final design. Breaker sizes come in standard steps and some loads are treated as continuous, so the “right” breaker and wire can be one size higher depending on the scenario.

Example: a 30A circuit may map to a common copper gauge, but if the run is long the voltage drop may drive you to upsize. Use this with the Voltage Drop Calculator when distance is significant.

Amp‑to‑Wire Size vs Wire Size: What's the Difference?

Both tools point you to a practical AWG range, but they start from different inputs and are used at different stages.

Use Amp‑to‑Wire Size for quick planning: you have amps and want a starting gauge in seconds.

Use Wire Size when you want more context (material choice, wiring method, and related sizing checks like voltage drop).

If you are building a full circuit plan, a common flow is: Watts/Amps/Volts → Electrical Load → Breaker Size → Wire Size → Voltage Drop.

Typical Amps for Common Devices (Quick Planning)

Device	Watts	Voltage	Approx. amps
Space heater	1500 W	120 V	12.5 A
Microwave	1200 W	120 V	10.0 A
Electric kettle	1500 W	120 V	12.5 A
Small water heater	3500 W	240 V	14.6 A
EV charger (Level 2)	7200 W	240 V	30.0 A

These are rough conversions (PF assumed 1.0). Use the Electrical Load Calculator when summing many items.

Fast Lookups (Copper & Aluminum, Conduit 75°C)

Amps	Copper (THHN, conduit)	Aluminum (THHN, conduit)
15 A	14 AWG	12 AWG
20 A	12 AWG	10 AWG
30 A	10 AWG	8 AWG
40 A	8 AWG	8 AWG
50 A	8 AWG	6 AWG

Copper in conduit at the 75°C column, matching this calculator default. For NM-B cable (Romex) size one column cooler at 60°C: a 50 A circuit becomes 6 AWG copper. Remember NEC 240.4(D) caps 14/12/10 AWG copper at 15/20/30 A breakers.

Starter Gauge Logic (Amp → AWG)

This tool gives a fast starting gauge from your amps. It is intentionally conservative and meant for early planning.

Starter gauge = Map your input amps to the first gauge that meets/clears it in the selected column
Wiring method = NM-B cable uses the 60°C column, THHN in conduit the 75°C - the tool sets this from your selection
Sanity checks = Confirm breaker size and voltage drop before you buy materials

For a full circuit plan (multiple loads, phases, long distance), use Electrical Load and Voltage Drop calculators too.

Code Notes & Sources

Ampacities follow NEC Table 310.16 for copper and aluminum. The 240.4(D) small-conductor rule then caps overcurrent protection at 15, 20, and 30 amps for 14, 12, and 10 AWG copper regardless of the table value, and this tool enforces it.NFPA 70 (NEC) Table 310.16 and 240.4(D)
NM-B cable is sized from the 60 degree column even though its insulation is rated 90, so the wiring-method selector changes the recommended gauge.NFPA 70 (NEC) 334.80 - NM Cable Ampacity

Next Steps

A starting gauge is not the whole circuit. Confirm the rest:

Check long runsDistance can force a larger wire than ampacity alone - verify the drop.
Size the breakerConfirm the standard breaker for the load, with continuous-load margin.
Full wire sizingThe sister tool adds material and wiring-method context in one place.

Related Calculators

Conduit Fill CalculatorConduit for the conductors.Box Fill CalculatorBox cubic inches per NEC 314.16.Cable Tray Fill CalculatorTray fill percent check.Electrical Load CalculatorWatts to amps, kVA, and breaker.

Amps drive both wire gauge and breaker selection. Browse the electrical calculator collection.

FAQ

Do I always pick the smallest wire that matches amps?

Not always. Distance (voltage drop), ambient temperature, bundling, and terminations can require a larger conductor.

Why does the breaker result not equal my amps?

The tool rounds your load up to the next standard breaker size (15, 20, 25, 30, 40, 50, 60 A and up), then applies the NEC 240.4(D) cap so 14, 12, and 10 AWG copper never exceed 15, 20, and 30 A. It does not add the 125 percent continuous-load factor for you - if your load runs more than three hours continuously, size it at 125 percent yourself before entering it, or use the breaker-size calculator, which offers that option.

What wire size do I need for a 30 amp circuit?

A 30A circuit requires 10 AWG copper wire (or 8 AWG aluminum). This is common for dryers, EV chargers, and water heaters. Always verify with local code - some jurisdictions require larger wire for specific applications or longer runs where voltage drop becomes significant.

What wire size do I need for a 20 amp circuit?

A 20 amp circuit requires 12 AWG copper - 14 AWG is not allowed, because NEC 240.4(D) caps 14 AWG copper at a 15 amp breaker no matter what the ampacity table shows. 12 AWG is standard for kitchen, bathroom, and general 20 amp outlet circuits. Aluminum would be 10 AWG for the same 20 amp circuit.

Does insulation temperature rating change the wire size?

Yes, indirectly. NM-B cable (Romex) is sized from the 60 degree column and THHN in conduit from the 75 degree column, so the same AWG allows different amps depending on the wiring method you pick above, plus terminations and installation conditions.

Updated Jul 2026 · See our Methodology

This calculator uses standard electrical formulas and published reference tables. Local codes and amendments vary. Use for planning estimates only. See our Data Sources and Methodology.