What size wire do I need for a 50 amp breaker?

A 50 amp breaker needs 6 AWG copper if you are running NM-B (Romex) cable, or 8 AWG copper if you are using THHN/THWN conductors in conduit with 75 °C-rated terminations. The difference comes from NEC 334.80, which caps NM cable at the lower 60 °C ampacity column, where 8 AWG is only good for 40 A. When you are not sure which applies, run the heavier 6 AWG, which is safe for either method. Aluminum conductors are larger again and fall outside this calculator’s copper-only scope.

Why does a continuous load need a bigger breaker?

NEC 210.20(A) requires the breaker to be rated for at least 125% of any load that runs three hours or more, which is the code’s definition of a continuous load. Put the other way around, a standard breaker can only carry 80% of its rating continuously, so a 20 A breaker tops out at 16 A for a load that stays on all day. The extra margin keeps the breaker and conductors from overheating during long, steady operation. Heaters, EV chargers, and store lighting are the loads that usually trigger this rule.

Can I use this calculator to size a breaker for a motor?

No, because motors follow a different NEC rule and this tool would undersize them. Motor branch circuits size from the motor’s full-load amps using the percentages in NEC 430.52, which commonly allow a breaker rated 175% to 250% of the running current so it can ride out the starting inrush. Air conditioners and heat pumps use the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) on the nameplate instead. For a motor, start with our motor FLA calculator and have an electrician apply the 430.52 factors.

Is it safe to put a bigger breaker on a circuit that keeps tripping?

No, and it is one of the most dangerous shortcuts in home wiring. The breaker protects the wire, not the appliance, so a breaker rated larger than its conductor lets the wire overheat before the breaker ever trips, which is a real fire risk. A breaker that trips repeatedly is usually telling you the circuit is overloaded or faulted, not that the breaker is too small. The right fix is to move some load to another circuit, add a dedicated circuit, or have an electrician find the fault, never to swap in a higher-rated breaker on the same wire.

Should I use the 60°C or 75°C wire column?

Use the 60 °C column whenever you are running NM-B (Romex) cable, because NEC 334.80 limits it to that lower ampacity regardless of the wire’s printed insulation rating. The 75 °C column applies to THHN/THWN conductors in conduit when the breaker and lugs are listed for 75 °C, which most modern equipment is. For circuits of 100 A or less, NEC 110.14(C) defaults you to the 60 °C column unless the terminations are specifically rated higher. When you cannot confirm the rating, size to the 60 °C column to stay on the safe side.

Breaker Size Calculator — Wire & Amp Sizing

7 min read

Get the right breaker before you buy it. The breaker size calculator turns a load in amps into the correct circuit breaker and the minimum copper wire to go with it, using the rules electricians follow: the 125% factor for continuous loads, the standard breaker sizes in NEC 240.6(A), and the small-conductor limits in 240.4(D). Enter the load current and whether it runs three hours or more, and you get the breaker plus the wire gauge for both NM cable and conduit. For long runs where voltage drop matters, pair it with our wire distance calculator.

Circuit breaker sizing overview showing load amps, the 125 percent continuous rule, and matching copper wire gauge.

How to Size a Breaker in Three Steps

Start with the load, not the breaker. The calculator follows the same three steps an electrician uses, all from the National Electrical Code:

Find the load current in amps. Read it off the nameplate, or convert watts to amps (watts ÷ volts). A 1,500 W heater on 120 V draws 12.5 A.
Apply the 125% rule if the load is continuous. A load that runs three hours or more is sized at 125% per NEC 210.20(A), so 12.5 A becomes 15.6 A.
Round up to the next standard breaker, then match the wire. NEC 240.6(A) lists the standard sizes (15, 20, 30, 40, 50 A and up), so 15.6 A rounds up to a 20 A breaker on 12 AWG copper.

The breaker protects the wire, so the conductor always comes from the breaker rating, never from the load directly. The next two sections show the exact wire pairings and why continuous loads change the answer.

Breaker size to copper wire chart pairing 15 to 200 amp breakers with NM cable and conduit wire gauges. — Minimum copper wire by breaker size — NM cable (60 °C) needs a thicker gauge than THHN in conduit (75 °C) at 50 A and up. Motors and A/C size separately under NEC 430/440.

Breaker Size to Copper Wire Size

Once you know the breaker, the minimum copper wire follows from NEC Table 310.16 — but with a catch. NM-B cable (the Romex in most homes) is limited to the lower 60 °C ampacity column by NEC 334.80, while THHN/THWN conductors in conduit can use the 75 °C column when the terminations are rated for it. The two diverge at 50 A and above, so the table gives both:

Breaker	NM-B cable (60 °C)	THHN in conduit (75 °C)
15 A	14 AWG	14 AWG
20 A	12 AWG	12 AWG
30 A	10 AWG	10 AWG
40 A	8 AWG	8 AWG
50 A	6 AWG	8 AWG
60 A	4 AWG	6 AWG
70 A	4 AWG	4 AWG
80 A	3 AWG	4 AWG
90 A	2 AWG	3 AWG
100 A	1 AWG	3 AWG
110 A	1 AWG	2 AWG
125 A	1/0 AWG	1 AWG
150 A	3/0 AWG	1/0 AWG
175 A	4/0 AWG	2/0 AWG
200 A	250 kcmil*	3/0 AWG

*Past 4/0 AWG, NM cable is no longer practical: large feeders and services use THHN or SER and should be sized by an electrician. A long run can also force a thicker wire to hold voltage drop in check, which our guide to voltage drop explains. These figures assume copper, no more than three current-carrying conductors, and a 30 °C ambient.

Why Continuous Loads Need a Bigger Breaker

A breaker cannot carry its full rating around the clock. NEC 210.20(A) requires it to be rated for 125% of a continuous load, which is the same as saying a standard breaker only carries 80% of its rating for anything that runs three hours or more. That is why a 1,500 W heater at 12.5 A needs a 20 A breaker, not a 15 A one: 12 A is all a 15 A breaker will hold continuously.

Breaker	Max standard load	Max continuous load (3 hrs+)
15 A	15 A	12 A
20 A	20 A	16 A
30 A	30 A	24 A
40 A	40 A	32 A
50 A	50 A	40 A
60 A	60 A	48 A
100 A	100 A	80 A

Set the load type to continuous above and the calculator applies the 125% factor for you. Tallying every circuit for a whole panel or service is a separate job: our electrical load calculator handles dwelling demand by the NEC method.

What This Calculator Does Not Cover

This tool sizes general branch circuits and feeders — receptacles, lighting, heaters, ranges, and similar resistive or mixed loads. It does not size circuits for motors, air conditioners, heat pumps, or welders. Those loads draw a heavy starting surge and follow separate NEC rules: motors size from full-load amps by the percentages in NEC 430.52, and HVAC equipment uses the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) printed on the nameplate. Run a motor load through the general rule and you will undersize its breaker.

Treat every result as a starting estimate, not a wiring permit. The figures assume copper conductors, no more than three current-carrying conductors in a run, and a 30 °C ambient; heat, conduit fill, and bundling all cut ampacity and can push you to a larger wire. Size and install real circuits with a licensed electrician, and confirm everything against the current NEC and your local Authority Having Jurisdiction (AHJ) before energizing.

Worked Examples

Sizing the Circuit for a 1,500 W Space Heater

Context

You are adding a 1,500 W plug-in space heater on a 120 V circuit, and it will run all evening — well over three hours, so it counts as a continuous load. What breaker and wire does it need?

Calculation

Current: 1,500 W ÷ 120 V = 12.5 A

Continuous, so size at 125%: 12.5 A × 1.25 = 15.6 A required

Next standard breaker ≥ 15.6 A: 20 A (a 15 A breaker only allows 12 A continuously)

Wire for a 20 A breaker: 12 AWG copper (both the 60 °C and 75 °C columns)

Interpretation

The heater needs a 20 A circuit with 12 AWG copper, not the 15 A / 14 AWG circuit its 12.5 A draw might suggest — the 125% continuous rule is what bumps it up a size. If you only know an appliance in watts, our amps draw calculator converts watts to amps first.

Takeaway

Any load that runs three hours or more — heaters, EV chargers, pool pumps — gets the 125% treatment, so set the load type before you pick a breaker. Distance is a separate check: a long run can force a thicker wire than the breaker alone calls for.

A 50 A Welder Circuit: NM Cable vs Conduit

Context

A stick welder pulls 50 A and runs in bursts (non-continuous). You will run the circuit from the panel out to the garage. What breaker does it need, and does the wire change if you use Romex versus THHN in conduit?

Calculation

Non-continuous, so no 125% factor: required current = 50 A

Next standard breaker ≥ 50 A: 50 A

NM (Romex) cable, 60 °C column: 6 AWG copper

THHN in conduit, 75 °C column: 8 AWG copper

Interpretation

Same 50 A breaker either way, but the wire depends on the method: 6 AWG for Romex, 8 AWG for THHN in conduit. NM cable is held to the lower 60 °C ampacity by NEC 334.80, so it needs the thicker conductor. When in doubt, run the heavier 6 AWG.

Takeaway

The breaker protects the wire, so the wire must match the breaker and the wiring method, never the other way around. Motor-driven tools are the exception: a welder is a steady load, but an actual motor circuit sizes by NEC 430.52, not this rule.

Frequently Asked Questions

Glossary

Overcurrent Protective Device (OCPD)

The breaker or fuse that opens a circuit when current exceeds its rating. NEC 240.6(A) lists the standard breaker sizes — 15, 20, 30, 40, 50, 60 A and up — and you always round up to the next standard size, never down. The OCPD exists to protect the wire, so its rating is tied to the conductor ampacity, not to the appliance plugged in.

Continuous Load

A load where the maximum current is expected to continue for three hours or more, per NEC Article 100. Continuous loads must be sized at 125% on both the breaker (210.20(A)) and the conductor (210.19(A)(1)). Electric heat, EV charging, and store lighting are continuous; a microwave or a hair dryer is not.

Ampacity

The maximum current a conductor can carry continuously without exceeding its temperature rating, from NEC Table 310.16. The same copper wire has different ampacities in the 60 °C, 75 °C, and 90 °C columns; the column you use depends on the cable type and the equipment terminations. For breaker sizing, the conductor ampacity must be at least the breaker rating.

Small-Conductor Rule (NEC 240.4(D))

A code limit that caps the breaker on the thinnest conductors regardless of their table ampacity: 14 AWG copper at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A. It exists because thin wires heat up fast under a fault. This is why 12 AWG is never placed on a 25 A breaker even though its 75 °C ampacity is 25 A.

Wiring an off-grid system instead of a grid circuit? The inverter and battery side has its own overcurrent protection, and our battery size for inverter calculator helps you spec the bank that feeds it.

Two numbers decide a safe circuit: the breaker that protects the wire, and the wire that can carry the breaker. Get them in that order — size the breaker to the load, then size the wire to the breaker, never the reverse. The most common and most dangerous shortcut is swapping a tripping breaker for a bigger one without upsizing the wire, which strips out the very protection the breaker exists to provide. When the load is a motor, an air conditioner, or anything with a large startup surge, stop here and size it from the equipment nameplate or with a licensed electrician instead.

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Browse all electrical calculators — Motor FLA, wire sizing, voltage drop, load calculations, and transformer fusing based on NEC standards.

Last updated: June 29, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Editorial review by Doc. dr. sc. Danijel Jerković-Štil, Assistant Professor, FERIT Osijek.