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Breaker Size Calculator

Find the smallest breaker rating a long-running or short-running load needs under the NEC 125% rule, and the next standard breaker size.

Level: Grades 6–7

  • Common Core standard 7.RP.A.3Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.
Printable worksheet

Breaker Size Calculator

Inputs
Load given in

For example a space heater, a water heater or an EV charger, which run for long stretches.

Rare in homes. Only tick this if the equipment is specifically listed for it; otherwise the 125% rule applies.

Result

Standard breaker size

20 A

A load of 16 A (16 A continuous) needs at least 20 A, so the next standard size is a 20 A breaker. The wire must be rated for it; have a licensed electrician confirm.

Minimum rating required
20 A
Total load current
16 A
Most continuous load allowed on it
16 A
Load as a share of the rating
80%

See the step-by-step solution ↓

Step-by-step solution

  1. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed

    0+1.25×16=20 A

  2. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))

    smallest standard size ≥ 20 A is 20 A

  3. See how much long-running load this breaker may carry: its rating ÷ 1.25

    201.25=16 A

  4. Check how much of the breaker's rating your total load uses

    1620×100=80%

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find the smallest circuit breaker your load needs, and the standard breaker size to look for. A circuit breaker protects a circuit's wiring from overheating. It trips (switches off) when too much current flows. In the United States, the National Electrical Code (NEC, NFPA 70) sets how large the breaker must be for a given load. Loads that run for three hours or more are called continuous loads. They count at 125% of their current. So in normal use, a breaker carries no more than 80% of its rating for long stretches.

This breaker size calculator applies that rule. Enter the continuous and noncontinuous (shorter-running) parts of a load. You can use amps, or watts plus the circuit voltage. The calculator finds the minimum rating: the noncontinuous load plus 125% of the continuous load. Then it picks the next standard breaker size from the NEC list. It's a learning and planning aid only. Local codes vary and change the NEC, and the wire size must match the breaker. Electrical work should be done or checked by a licensed electrician.

Where it's used

Why this math matters outside the classroom:

  • Space heaters. A 1,500 W heater on 120 V draws 12.5 A, and 125% of that is 15.625 A, more than a 15 A breaker allows.
  • EV charging. Homeowners planning a 40 A charger find 1.25 × 40 = 50 A, so the circuit needs a 50 A breaker.
  • The 80% rule. Electricians know a 30 A breaker can carry 30 × 0.8 = 24 A continuously.
  • Shop circuits. A workshop with 10 A of intermittent tools and 24 A of continuous dust collection needs 10 + 30 = 40 A.
  • Percent practice. Students compute 125% of a value and its inverse, 80%, and see that multiplying by 1.25 then 0.8 returns the start.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.RP.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose whether you know the load in amps or in watts.
  2. Enter the continuous part of the load (equipment that runs three hours or more at a time) and any noncontinuous part.
  3. For watts, enter the circuit voltage. In a US home it's usually 120 V or 240 V.
  4. Leave the 100% box unticked unless the breaker and its box are specifically listed for 100% operation. Then read the breaker size.

What the inputs mean

Continuous load (runs 3 hours or more)
The NEC calls a load continuous when its highest current is expected to last 3 hours or more. The code also names some equipment that must always count as continuous. Examples are fixed space heating, water heaters and EV chargers.
Noncontinuous load
Loads that run for shorter times, such as most kitchen appliances and power tools, count at 100% of their current.
Circuit voltage
Amps are found as watts ÷ volts. That's exact for heating loads like heaters, which turn power straight into heat. Motors and electronics draw a little more current than this, because of their power factor (how efficiently they use current). Use the amps on their nameplate where you can.
Breaker and enclosure are listed for 100% continuous operation
An exception lets equipment rated for 100% carry its full rating continuously. The listing covers the breaker and its enclosure (the box it sits in) together. Such equipment is uncommon in homes.

Formula and method

amps = watts ÷ volts
minimum rating = noncontinuous load + 1.25 × continuous load
breaker = smallest standard size ≥ minimum rating
most continuous load on that breaker = rating ÷ 1.25 = 80% of the rating

Amps measure current, watts measure power and volts measure the circuit's voltage. The rating is the current a breaker is built to carry. The minimum rating is the smallest rating the code allows for your load.

NEC 210.20(A) sets the rule for the breaker on a branch circuit. Its rating must be at least the noncontinuous load plus 125% of the continuous load. Multiplying by 1.25 is the same as keeping the continuous load at or below 80% of the breaker rating. That's where the 80% rule comes from.

Breakers come in standard ratings listed in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60 A and up. The calculator picks the first size that isn't below the minimum.

Worked example

A 1,500 W space heater on a 120 V circuit, treated as a continuous load.

  1. Current: 1,500 ÷ 120 = 12.5 A.
  2. Minimum rating: 0 + 1.25 × 12.5 = 15.625 A.
  3. The smallest standard size at or above 15.625 A is 20 A.
  4. A 20 A breaker may carry 20 ÷ 1.25 = 16 A continuously. This load uses 62.5% of its rating.

Result: The heater needs a 20 A breaker rather than a 15 A one, because 125% of its continuous current is more than 15 A.

Understanding the result

The breaker size is the smallest standard rating that meets the 125% rule for the loads you entered. A larger breaker isn't safer. The breaker also must be no larger than the wire and the equipment allow.

If the load is close to the limit for a size, think about whether more equipment will be added to the circuit later.

Assumptions and limitations

  • Loads are single-phase (the normal household supply). Their current is watts ÷ volts (power factor 1), unless you enter nameplate amps.
  • The breaker protects a branch circuit under the NEC. The continuous-load rule and standard sizes are as described above.
  • It doesn't choose a wire size, and the wire must be rated for the breaker. Small wires have their own maximum breaker sizes under the NEC.
  • Motors, air conditioners, welders and other special equipment have their own sizing rules and nameplate maximums. Those come first.
  • Local codes vary, and they adopt different NEC editions with their own changes. This is general information, not electrical advice. Have a licensed electrician design and install circuits, and get permits where required.

Frequently asked questions

What is the 80% rule for breakers?

It's the 125% rule seen from the other side. A continuous load must be no more than the rating divided by 1.25. So a standard breaker carries at most 80% of its rating continuously: 16 A on a 20 A breaker, or 24 A on a 30 A breaker.

What counts as a continuous load?

Any load whose highest current is expected to last 3 hours or more. Examples are lighting in a store, a space heater or a car charging overnight. The code treats some equipment, like EV chargers and water heaters, as continuous no matter how long it actually runs.

Can I just put in a bigger breaker if mine keeps tripping?

No. The breaker protects the wire. A breaker larger than the wire is rated for can let the wire overheat. Frequent tripping means the circuit is overloaded or has a fault. Reduce the load or have an electrician look into it.

Does this work for 240 V appliances?

Yes, for the math: enter the watts and 240 V and it finds the current. A 9,600 W charger at 240 V draws 40 A continuously and needs a 50 A breaker. Appliance instructions often state the breaker they need, and you must follow those instructions.

Sources

How this calculator is checked

Method: NEC (NFPA 70) 210.20(A): overcurrent device rating ≥ noncontinuous load + 125% of continuous load, with the 100%-rated assembly exception; standard ratings from NEC 240.6(A); amps = watts ÷ volts at unity power factor. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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