My circuit keeps tripping. Can I just fit a bigger breaker?

No — that is the one change that turns an inconvenience into a fire risk. The breaker is sized to the conductor, and a bigger breaker lets the same wire carry more current than it is rated for before anything intervenes. A circuit that trips is either overloaded, which means moving some of the load, or faulty, which means finding the fault. If the load genuinely belongs there, the wire has to grow before the breaker can.

Why 125% for a continuous load?

Because breakers and conductors are rated on the assumption that they get to cool down. A load that runs for three hours or more at full current never gives them that chance, so the code requires the device and the conductor to be sized at 125% of it — equivalently, a standard breaker is loaded to no more than 80% continuously. Three hours is the NEC's own dividing line, not a rule of thumb.

What is the next-standard-size-up rule?

NEC 240.4(B) lets you protect a conductor with the next larger standard device where the conductor's ampacity falls between two standard ratings, for circuits up to 800 A and where the circuit does not supply receptacles for cord-and-plug-connected portable loads. It is a rule about protecting a conductor, not about serving a load — and it is worth keeping the two apart, because the first says what a given wire may be protected at while the second says what a given load demands.

Does this work for a motor?

No, and putting a motor's full-load current in will give you a breaker that trips every time it starts. Article 430 sizes motor protection from Table 430.52, and an inverse-time breaker is commonly permitted at 250% of full-load current so that inrush passes without tripping. The motor's overload protection is a separate device sized close to the running current — the two jobs are split, which is exactly why the general rule does not apply.

Can I use this outside North America?

For the load arithmetic, yes — watts, volts and the square root of three do not change at a border. For the device, no. BS 7671 and the IEC standards use different standard ratings, different breaker curves (B, C and D types chosen by inrush rather than by a flat percentage) and a different treatment of continuous duty. Take the current from here and size the protective device to the local standard.

Why is the breakdown showing the load as a percentage of the breaker?

Because it is the number an inspector thinks in, and it makes the continuous rule visible. A continuous load correctly sized lands at or below 80% of the breaker; anything above that on a continuous circuit means the 125% has not been applied. On an intermittent circuit a higher figure is acceptable, which is exactly the distinction the percentage makes easy to see.

Electrical

Breaker Size Calculator

Find the smallest standard breaker rating for a load in watts or amps, with the continuous-load rule applied and the conductor ampacity it then demands.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

I know the load as

Whether the appliance's rating plate gives you power or current.

If the plate gives amps, use amps — it is the figure the manufacturer measured and it already accounts for power factor and efficiency. Converting from watts assumes a power factor of one, which is right for a resistive load such as a heater or a hob and optimistic for anything with a motor or a switching supply in it.

Load (W or VA)

The nameplate power rating of the equipment on this circuit.

Where several pieces of equipment share the circuit, add their ratings. Use the nameplate figure rather than a measured one — a breaker is sized for what the equipment may draw, not for what it happened to draw when you looked.

Circuit voltage

The nominal voltage of the circuit the equipment is connected to.

For a three-phase circuit this is the line-to-line voltage — 208 or 480 in North America, 400 in Europe. Using a line-to-neutral figure with the three-phase setting is the commonest error here and it overstates the current by about three quarters.

Phase

Whether the circuit is single or three phase.

A three-phase load of the same power draws about 58% of the single-phase current, because the power is shared across three conductors and the line-to-line voltage does the work of the square root of three. This assumes the load is balanced across the phases.

Continuous load

Whether the load runs for three hours or more at maximum current.

Three hours is the NEC's own threshold. Electric heating, EV charging, commercial lighting and most fixed process equipment are continuous; a kettle, a hob and a power tool are not. Where you are unsure, treating a load as continuous is the conservative answer — it raises the calculated minimum by a quarter.

Minimum breaker size

25 A

Medium confidence

The arithmetic is the code's and it is exact. What makes this a screening figure rather than a design is everything it cannot see: the conductor's insulation rating, the ambient temperature, how many conductors share the raceway, the length of the run, and what the terminals at each end are listed for. Any of those can move the required conductor — and therefore the acceptable device — by a size or more.

Load current: 20 A
Required minimum at 125% (continuous): 25 A
Minimum conductor ampacity after corrections: 25 A
Headroom to the next trip point: 0 %
Load as a share of the breaker: 80 %

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

NEC 240.6(A): standard ampere ratings for inverse-time circuit breakers and fuses — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A
NEC 210.20(A) and 215.3: the overcurrent device rating shall be not less than 100% of the noncontinuous load plus 125% of the continuous load
NEC 210.19(A)(1): branch-circuit conductors shall have an ampacity not less than the same figure, before any temperature or conduit-fill correction
NEC 240.4: conductors shall be protected at their ampacity — the device rating follows the conductor, not the load it happens to serve
NEC Article 430 governs motor branch circuits: short-circuit and ground-fault protection is sized from Table 430.52 and may substantially exceed the motor's full-load current, so this page does not apply to motors

Inputs used

I know the load as: Watts (or VA) from the nameplate
Load (W or VA): 4800
Load current (A): 20
Circuit voltage: 240 V (North American large appliance)
Phase: Single phase
Continuous load: Yes — runs three hours or more at a time

Intermediate steps

Load current: 20 A
Required minimum at 125% (continuous): 25 A
Minimum conductor ampacity after corrections: 25 A
Headroom to the next trip point: 0 %
Load as a share of the breaker: 80 %

Final result25 A

Confidence note: The arithmetic is the code's and it is exact. What makes this a screening figure rather than a design is everything it cannot see: the conductor's insulation rating, the ambient temperature, how many conductors share the raceway, the length of the run, and what the terminals at each end are listed for. Any of those can move the required conductor — and therefore the acceptable device — by a size or more.

What this calculation does not cover

A breaker protects the conductor, not the appliance. The rating above is the minimum the LOAD demands; it is only the right breaker if the conductor behind it is rated to carry it after every applicable correction. Fitting a larger breaker to stop a circuit tripping, without changing the wire, removes the protection the wire depends on and is the most common serious mistake made in a domestic panel.
Motors are not sized this way. NEC Article 430 sizes motor branch-circuit short-circuit and ground-fault protection from Table 430.52, and an inverse-time breaker on a motor is routinely rated well above the motor's full-load current so that starting inrush does not trip it — while the overload protection is a separate device sized much closer to the running current. Putting a motor's nameplate current into this page gives a breaker that will trip on starting.
Converting watts to amps assumes a power factor of one. That is correct for resistive loads — heaters, hobs, water heaters, incandescent lighting — and optimistic for motors, welders, electronic supplies and anything with a substantial inductive component, where the real current is higher than the power rating alone suggests. Where the plate gives amps, use amps.
Conductor ampacity has not been calculated here, only stated as a minimum. The actual conductor is chosen from the ampacity tables at the insulation's temperature rating, then corrected for ambient temperature and for the number of current-carrying conductors bundled together, and finally limited by the temperature rating of the terminations at both ends — which on most equipment is 75 °C regardless of what the cable itself can take.
Voltage drop is a separate constraint and often the binding one. On a long run the conductor that satisfies ampacity may still deliver too little voltage at the load, and the fix is a larger conductor — which does not change the breaker, because the breaker still has to match the load it protects.
This is NEC arithmetic. Circuit protection outside North America follows BS 7671, the relevant IEC standards or the local equivalent, which use different device characteristics, different standard ratings and a different treatment of continuous duty. The voltages offered here include European and Australian values because the load arithmetic is universal; the 125% rule and the standard rating list are not.

Add the equipment this sizes

This result is a specification — 25 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

QtyItemPrice each ($)

Part of a bigger job

This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.

Service panel upgrade Check a quote
also measuresElectrical Panel Load (NEC Service Sizing)Home Electrical Service Panel Amperage Sizing

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-11 · v1.0.0

Regulatory standards & verification citations5

NEC 240.6(A): standard ampere ratings for inverse-time circuit breakers and fuses — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A
NEC 210.20(A) and 215.3: the overcurrent device rating shall be not less than 100% of the noncontinuous load plus 125% of the continuous load
NEC 210.19(A)(1): branch-circuit conductors shall have an ampacity not less than the same figure, before any temperature or conduit-fill correction
NEC 240.4: conductors shall be protected at their ampacity — the device rating follows the conductor, not the load it happens to serve
NEC Article 430 governs motor branch circuits: short-circuit and ground-fault protection is sized from Table 430.52 and may substantially exceed the motor's full-load current, so this page does not apply to motors

Which documents these citations point at

National Electrical Code (NFPA 70) — 240.6(A), 210.20(A), 210.19(A)(1), 240.4, Article 430 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Protection this work requires

Stop and check first. Textured ceiling coatings, board behind panels, floor tiles and bitumen adhesive in older buildings — built before asbestos was banned or phased out where you are — can contain asbestos, and drilling or sanding them releases fibres. Do not disturb it — have it sampled first. This is not a job for better PPE.

Tools and safety for this job

To first-fix electrical. Generic types, no brands, no prices.

Assume every cable is live until proved dead at the point you will touch, with a two-pole tester you have just proved on a known source.

First-fix electrical: This site does not publish a tool list for electrical installation work. In every market it serves, fixed wiring is either reserved to a registered electrician or notifiable to a building authority, and the failure mode is a fire or an electrocution months later rather than a visibly bad job on the day. The calculator gives you the quantities to discuss and to buy against. The installation is a job for a qualified electrician, and the certificate they issue is the point of them.

Now that you have the number

These guides cover the work this quantity is for.

Setting Up a Site Compound

Called something else where you work? Consumer unit / breaker panel · Residual current and ground-fault protection · Fused connection unit — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Breaker Trip Rating vs Interrupting Rating — the factors that actually differ, with no invented prices.

Already gone wrong? A breaker keeps tripping

How to calculate breaker size in 7 steps

I know the load asWhether the appliance's rating plate gives you power or current.
Load (W or VA)The nameplate power rating of the equipment on this circuit.
Load current (A)The nameplate current draw of the equipment on this circuit.
Circuit voltageThe nominal voltage of the circuit the equipment is connected to.
PhaseWhether the circuit is single or three phase.
Continuous loadWhether the load runs for three hours or more at maximum current.
Minimum breaker sizeThe tool computes the minimum breaker size from those figures and shows the formula, its sources, and a confidence rating alongside it.

Minimum breaker size by load (W or VA)

Page defaults, not your figures above.

Load (W or VA)	Minimum breaker size (A)
500	15
1,000	15
2,000	15
5,000	30
10,000	60
20,000	110

Frequently asked questions

My circuit keeps tripping. Can I just fit a bigger breaker?: No — that is the one change that turns an inconvenience into a fire risk. The breaker is sized to the conductor, and a bigger breaker lets the same wire carry more current than it is rated for before anything intervenes. A circuit that trips is either overloaded, which means moving some of the load, or faulty, which means finding the fault. If the load genuinely belongs there, the wire has to grow before the breaker can.
Why 125% for a continuous load?: Because breakers and conductors are rated on the assumption that they get to cool down. A load that runs for three hours or more at full current never gives them that chance, so the code requires the device and the conductor to be sized at 125% of it — equivalently, a standard breaker is loaded to no more than 80% continuously. Three hours is the NEC's own dividing line, not a rule of thumb.
What is the next-standard-size-up rule?: NEC 240.4(B) lets you protect a conductor with the next larger standard device where the conductor's ampacity falls between two standard ratings, for circuits up to 800 A and where the circuit does not supply receptacles for cord-and-plug-connected portable loads. It is a rule about protecting a conductor, not about serving a load — and it is worth keeping the two apart, because the first says what a given wire may be protected at while the second says what a given load demands.
Does this work for a motor?: No, and putting a motor's full-load current in will give you a breaker that trips every time it starts. Article 430 sizes motor protection from Table 430.52, and an inverse-time breaker is commonly permitted at 250% of full-load current so that inrush passes without tripping. The motor's overload protection is a separate device sized close to the running current — the two jobs are split, which is exactly why the general rule does not apply.
Can I use this outside North America?: For the load arithmetic, yes — watts, volts and the square root of three do not change at a border. For the device, no. BS 7671 and the IEC standards use different standard ratings, different breaker curves (B, C and D types chosen by inrush rather than by a flat percentage) and a different treatment of continuous duty. Take the current from here and size the protective device to the local standard.
Why is the breakdown showing the load as a percentage of the breaker?: Because it is the number an inspector thinks in, and it makes the continuous rule visible. A continuous load correctly sized lands at or below 80% of the breaker; anything above that on a continuous circuit means the 125% has not been applied. On an intermittent circuit a higher figure is acceptable, which is exactly the distinction the percentage makes easy to see.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.