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Circuit load calculator

Enter the breaker and what runs on it. You get the amps, with long-running loads counted the way the code counts them.

Checked Nov 20261 source citedSee the working

This circuit load figure is for planning, using the 2021 IRC's parallels to the National Electrical Code. A breaker's rating says nothing about the wire behind it, and a bigger breaker on small wire can start a fire in your walls. Motors have their own rules. List everything on that breaker, and have a licensed electrician check before you add any load or change a breaker.

Breaker

It is printed on the breaker's handle.

Voltage

A space heater, an EV charger or lights left on all evening.

Everything is worked out in this page. Nothing you type is sent.

Drawn to scale
0510152014 amps, with continuous load at 125%The breaker's rating
Load, the way the code counts it
14.00
amps
Of the breaker
93%
Fits the breaker
Yes
Plain total
12.00 amps
If it was all continuous, the limit is
12.0 amps
Or in watts
1440 W
Largest plug-in fits the 80 percent limit
Yes

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Add this, then whatever else the job needs. Everything you need to buy ends up here.

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Electricity costVoltage drop

How we got 14.00 amps

  1. Continuous amps8.00
  2. Noncontinuous amps4.00
  3. Load, with continuous at 125%14.00 amps
  4. Limit if it was all continuous12.0 amps

Figures used

Factor on a continuous load when sizing a circuit
1.25
Largest share of a circuit one plug-in device can use
0.8

1 source cited further down. Checked Nov 2026 by Anil, developer of this site.

Working

How to add up a circuit

Start at the panel. Turn off the breaker and walk through the house to see what stops working, as most circuits serve more rooms than you would guess. Write down every light and plug that goes dead. Then read the watts from each label on what you run there. Multiply a label in amps by the volts if that is how it is shown.

Next, sort the list by how long things run. The code treats a load running for 3 hours or more as continuous, and it does not matter what the device is called. A space heater on a cold evening is continuous. So are lights left on all evening. A vacuum cleaner is not, however much it draws.

0510152014 amps, with continuous load at 125%The breaker's rating
8 amps continuous and 4 amps noncontinuous on a 15 amp breaker count as 14 amps, which fits.

Then look at the largest plug-in device. Under E3702.3 of the 2021 IRC, one cord-and-plug device can use at most 80 percent of the circuit. Fixed equipment has its own limit of 50 percent where lighting or plug-in equipment shares the circuit, so a built-in heater on a general circuit is a different story.

A worked example

Here is a 15 amp, 120 volt circuit with a 960 watt space heater running all evening, and a 480 watt load running part of the time. These lines come from the same formula the calculator above runs.

15 A circuit: 960 W continuous, 480 W not

  1. Continuous: 960 ÷ 120 = 8 amps
  2. Noncontinuous: 480 ÷ 120 = 4 amps
  3. Load, with 8 amps counted at 125%: 4 + 10 = 14 amps
  4. Of the breaker: 93%, which fits

A flat 80 percent rule would put a 15 amp circuit at 12 amps and stop there. That total of 12 amps looks like it is at the limit, and the code's sum shows it has 1 amp to spare. If everything were continuous, the limit would be 12 amps. A long run may also lose voltage, which the voltage drop calculator checks, and to see what a load costs to run, use the electricity cost calculator.

What fits on a common breaker

These are the limits if a whole circuit is continuous, and for one plug-in device. A shorter load can use more of the breaker. These numbers tell you nothing about the wire, which has to match the breaker, so never use them to choose a larger one.

Worked out by this calculator.
BreakerVoltsAll continuous, ampsAll continuous, wattsOne plug-in device, amps
15 A120 V12144012
20 A120 V16192016
30 A120 V24288024
15 A240 V12288012
20 A240 V16384016
30 A240 V24576024

Mistakes that overload a circuit

  • A bigger breaker. It feels like a fix. The breaker protects the wire, so going up on small wire removes the protection and can start a fire inside the wall.
  • A flat 80 percent. It is correct only for a load that is all continuous. For a mix, use the sum above.
  • Forgetting what else is on the breaker. Many circuits run through several rooms, and something plugged in elsewhere counts too.
  • Going by the name. The 3 hour test decides. A hobby machine or a charger left on overnight is a continuous load.
  • Label watts for a motor. Motors draw more to start, and they have their own rules in the code. Get advice before adding one.

Questions

How many amps can be on a 15 amp circuit?

12 amps, or 1,440 watts at 120 volts, if the load runs for 3 hours or more. A shorter load can go higher, up to the breaker's 15 amps. One plug-in device should use no more than 80 percent, so 12 amps.

What is the 80 percent rule?

It is a shorthand for a rule that counts continuous loads at 125 percent, because 1 divided by 1.25 is 0.8. It only works when the whole load is continuous. A mix is the noncontinuous amps plus 1.25 times the continuous amps, under 2021 IRC section E3701.2.1.

How much can I plug into a 20 amp circuit?

16 amps or 1,920 watts at 120 volts if it runs for 3 hours or more. One plug-in device should be no more than 80 percent of the circuit, which is also 16 amps. Add everything else on that breaker, since most circuits serve more than one thing.

Can I replace a breaker with a bigger breaker?

No, not without checking the wire, and usually not at all. The breaker protects the wire, and 14 AWG on a 20 amp breaker is the classic error that overheats wiring. Have an electrician look at any breaker that trips often.

How many outlets on a 20 amp circuit?

We found no primary statement of a limit for a home, so we will not give one. Count what you plug in, not the outlets. Your local code or an electrician can tell you if there is a rule where you live.

When do I need a professional?

You need a licensed electrician if a breaker keeps tripping, if you want a new circuit for a heater or a charger, or if you see heat, buzzing or scorch marks. Motors have their own rules, under E3702.6 and E3702.7 of the 2021 IRC.

This circuit load figure is for planning, using the 2021 IRC's parallels to the National Electrical Code. A breaker's rating says nothing about the wire behind it, and a bigger breaker on small wire can start a fire in your walls. Motors have their own rules. List everything on that breaker, and have a licensed electrician check before you add any load or change a breaker.

Sources and review

Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.

How this is calculated

Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.

The arithmetic

Load

Load = Noncontinuous amps + 1.25 × Continuous amps

Continuous
A load that runs for 3 hours or more

Worked through: 4 + 10 = 14 amps

Amps

Amps = Watts ÷ Volts

Watts
From the appliance's label

Worked through: 1440 ÷ 120 = 12 amps

The constants, and where they come from

CONTINUOUS_FACTOR

Published figure

Factor on a continuous load when sizing a circuit

1.25

Applies to: Branch circuits under the 2021 IRC, which parallels NEC 210.19 and 215.2

Source: 2021 residential code, chapter 37, as adopted in Virginia, UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.

Also from: 2021 residential code, chapter 35, as adopted in Virginia, UpCodes, published 2021, read 2026-11.

E3701.2.1, read through UpCodes' copy of Virginia's adoption, because we could not read NFPA 70. A continuous load runs for 3 hours or more. The 80 percent shorthand is only 1 ÷ 1.25, and it only works if the whole load is continuous.

PLUG_IN_SHARE

Published figure

Largest share of a circuit one plug-in device can use

0.8

Applies to: Cord-and-plug equipment not fastened in place

Source: 2021 residential code, chapter 37, as adopted in Virginia, UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.

E3702.3, which parallels NEC 210.23. It also says fastened-in-place equipment is at most 50 percent where lighting or plug-in equipment share the circuit.

What the labels mean

Published figure:
taken directly from the source named beside it.

If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.

How to use the circuit load calculator

  1. 1

    Find the breaker, and turn it off to see what else goes dead

  2. 2

    List what runs on that circuit, with the watts from each label

  3. 3

    Split the list into what runs 3 hours or more, and the rest

  4. 4

    Read the load, and whether it fits

  5. 5

    Ask an electrician before you change anything

What this calculator assumes

Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.

  • A continuous load is one that runs for 3 hours or more, whatever the device
  • Amps are watts divided by volts. Motors and other loads with a low power factor draw more than this says
  • It covers one breaker. It does not check the wire, which has to match the breaker
  • It does not count outlets, as we found no primary statement of a limit for a home

Pro tips

  • Switch the breaker off and walk the house, to learn what is on it
  • Space heaters and EV chargers are usual continuous loads, so give them their own circuit
  • A breaker that trips is doing its job, so find out why before anything else