Circuit Load Calculator
Enter total watts, voltage and breaker size to see amp draw, breaker capacity used and whether a circuit stays under the 80% continuous load guideline. A handy way for electricians to explain tripped breakers, and for homeowners to check before adding a space heater.
Free, no sign-up. Runs in your browser; nothing you type is stored unless you choose to email yourself the results.
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Email me these results. We'll send your numbers, plus how to turn this calculator into a lead magnet on your own website.
Three steps, about a minute
Step 1
Add up the wattage of everything running on the circuit and enter it as Total load on the circuit.
Step 2
Select the Voltage: 120 V for standard outlets, 240 V for dryers, ranges and similar loads.
Step 3
Choose the Breaker size, then check Current draw, Breaker capacity used and Status.
How the circuit load calculator works
Amps equal watts ÷ volts, so 1,800 watts on a 120 V circuit draws 15 A. The tool takes 80% of the breaker rating as the continuous limit, 16 A for a 20 A breaker, and shows what share of the full rating the load uses, here 75%. A draw at or under the 80% line reads as within the guideline. Anything above it gets a prompt to split the load or upgrade. That 80% figure is the common practice for continuous loads, ones running three hours or more. Motor startup surges and power factor are left out. Wire gauge must match the breaker, and a bigger breaker on the same wire is never the fix. This is for guidance only: follow local code and have a licensed electrician confirm any change.
| Total load on the circuit | 1,800 watts |
|---|---|
| Voltage | 120 V |
| Breaker size | 20 A |
| Current draw | 15 A |
|---|---|
| 80% continuous limit | 16 A |
| Breaker capacity used | 75% |
| Status | Within the 80% guideline |
Amps = watts ÷ volts. Continuous loads are commonly kept at or below 80% of the breaker rating. For guidance only; follow local code.
Using the numbers like a pro: tips for electricians
Ask which appliances were running when the breaker tripped. Space heaters, hair dryers and window units sharing a circuit are classic culprits, and the answer often points to a dedicated circuit job.
Turn a nuisance-trip call into a panel evaluation. Once you've shown the load math, offer a review of the whole panel and a written quote for new circuits or a service upgrade.
Explain why the wire, not the breaker, sets the limit. Customers who understand that stop asking for a bigger breaker and start trusting your recommendation for a proper fix.
Put a calculator like this on your own website
Tripping breakers send homeowners searching for answers before they call anyone. A load checker on your site gives them the math, then offers a callback or panel inspection when the result lands over the 80% line. Those visitors already know something is wrong, so the requests you get are real. We build these with your license number and service area on the page.
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Circuit Load Calculator questions
How many watts can a 20 amp breaker handle?
On a 120 volt circuit, a 20 amp breaker is rated for 2,400 watts. For continuous loads the common guideline is 80%, which is 16 amps or 1,920 watts. On a 240 volt circuit the same breaker handles 4,800 watts, or 3,840 watts for continuous loads.
What is the 80% rule for breakers?
It says a continuous load, one that runs three hours or more, should not exceed 80% of the breaker's rating. A 15 A breaker should carry no more than 12 A continuously, and a 20 A breaker no more than 16 A. Most standard breakers are not rated to carry their full nameplate current for hours at a time.
How do I convert watts to amps?
Divide watts by volts. A 1,500 watt space heater on 120 volts draws 12.5 amps, and a 4,800 watt water heater element on 240 volts draws 20 amps. That works well for resistive loads like heaters. Motors and some electronics draw extra current at startup, so leave headroom on those circuits.
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