# Space heater tripping the breaker: the watts-to-amps arithmetic, what else is on the circuit, and the signs that mean stop using it > A 1,500 W space heater draws 12.5 A at 120 V, above the 12 A (80 percent) figure ESFI gives for a 15 A circuit. Give it its own circuit or run it at 900 W. | | | |---|---| | Source | https://ownerspec.com/air/guides/space-heater-tripping-breaker-watts-amps/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-10-03 | | Updated | 2026-10-03 | | Facts verified | 2026-10-03 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer A space heater tripping a breaker is usually arithmetic, not a defect. Amps are watts divided by volts (ESFI and Virginia Tech Extension print the formula), so a 1,500 W space heater on a 120 V circuit draws 12.5 A. Lasko's support page states the same figure on high, and about 7.5 A on the 900 W low setting. ESFI's booklet says a 15 A circuit at 120 V carries 1,800 W and recommends not exceeding 80 percent of that, 1,440 W or 12 A. The heater alone is over that figure, so a 150 W television or a 1,600 W hair dryer on the same circuit tips the breaker. The fix is a different circuit or the low setting, never a bigger breaker: CPSC says to plug heaters directly into a wall outlet, never into an extension cord or power strip. A plug or wall plate that is hot, not just warm, is a stop sign: ESFI says unplug it and call a licensed electrician. ## Fact sheet - **Answer:** Amps equal watts divided by volts, so a 1,500 W space heater on a 120 V circuit draws 12.5 A (Lasko: 12.5 A on high, 7.5 A on 900 W low). ESFI says a 15 A circuit carries 1,800 W and recommends staying under 80 percent, 1,440 W or 12 A, so the heater alone exceeds that figure and any other load trips the breaker. Fix: another circuit or low, never a bigger breaker. - **Basis:** The Volts x Amps = Watts formula and the 15 A / 1,800 W / 80 percent (1,440 W) circuit figures as printed in ESFI's Indoor Electrical Safety Check booklet; the 12.5 A and 7.5 A heater figures as printed on Lasko's support page; CPSC's and ESFI's rule that a heater plugs directly into a wall outlet, never an extension cord or power strip. - **Assumptions:** Household circuits are 120 V and the heater's label says 1,500 W on high; a heater with a different nameplate draws its own watts divided by 120; The 80 percent figure is ESFI's recommendation as the booklet prints it; the National Electrical Code's continuous-load rule was not opened for this page, so no NEC section number is cited; Appliance wattages in the table are the typical figures and ranges printed by ESFI and Virginia Tech Extension, not measurements; the amps column is those watts divided by 120 - **Method:** Spec and source comparison of the circuit arithmetic and safety wording published by ESFI, CPSC, NFPA, Virginia Tech Extension, Lasko, Leviton and Schneider Electric, read on 2026-10-03. No heater, breaker or outlet was tested. Prices on the product card are Amazon's live figures and are never written into this page. - **Primary source:** [ESFI, Indoor Electrical Safety Check booklet (PDF)](https://www.esfi.org/wp-content/uploads/2021/08/Indoor-Electrical-Safety-Check-Booklet-3FB3.pdf) (manufacturer or vendor document) - **Facts verified:** 2026-10-03 - **Last updated:** 2026-10-03 - **Canonical URL (cite this):** https://ownerspec.com/air/guides/space-heater-tripping-breaker-watts-amps/ If the plug or wall plate is hot to the touch, close this page, unplug the heater and call an electrician: that is a wiring problem, and no table below fixes it. ## Why does a 1,500 W heater trip a 15 A breaker? Because 12.5 A is more than the 12 A ESFI suggests as the ceiling. ESFI's Indoor Electrical Safety Check booklet prints Volts x Amps = Watts, says a 15 A circuit at 120 V carries 1,800 W, and adds that "it is not recommended that you exceed 80% of the total circuit capacity at any given time, in this case 1,440 watts." Divided by 120 V, that is 12 A. The watts are on the heater's label. Lasko's support page says most of its heaters pull 1,500 W on high and 900 W on low, "about 12.5 amps on high and about 7.5 amps on low", and that one sharing a 15 A line with a TV, DVD player and anything else "will likely exceed the threshold for that line and will trip the breaker." The delay before the trip is the breaker working as designed. CPSC's AFCI technology paper describes a conventional breaker's thermal trip as a bimetal strip that heats when current exceeds the rating; a small overload takes minutes to heat it, while a short circuit fires the magnetic trip at once. The National Electrical Code is the usual basis quoted for the 80 percent figure; its text was not opened for this page, so the figure is ESFI's and carries no section number. ## What else is on the circuit? A bedroom circuit usually feeds several outlets, so the heater shares its 15 A with everything on the run. Typical wattages as two public tables print them, with amps worked out at 120 V: | Appliance | Typical watts | Amps at 120 V (watts / 120) | Printed by | | --- | --- | --- | --- | | Portable heater, high | 1,500 (range 750–1,500) | 12.5 | ESFI booklet; Virginia Tech Extension | | Portable heater, low | 900 | 7.5 | Lasko support page | | Hair dryer | 1,600 (range 1,200–1,875) | 13.3 (10–15.6) | ESFI booklet; Virginia Tech Extension | | Clothes iron | 1,000 (range 1,000–1,800) | 8.3 (8.3–15) | ESFI booklet; Virginia Tech Extension | | Microwave oven | 750–1,100 | 6.3–9.2 | Virginia Tech Extension | | Toaster | 800–1,400 | 6.7–11.7 | Virginia Tech Extension | | Coffee maker | 800 (range 900–1,200) | 6.7 (7.5–10) | ESFI booklet; Virginia Tech Extension | | Vacuum cleaner | 600 | 5 | ESFI booklet | | Television | 150 | 1.25 | ESFI booklet | | Computer | 150 | 1.25 | ESFI booklet | Against the 1,800 W circuit total, a 1,500 W heater leaves 300 W, or 2.5 A: a television fits, a vacuum cleaner or hair dryer does not. Lasko's diagnostic is to try the heater in a different room and note what else switches off with it: those devices share the breaker. ## Which symptom means which cause? | Symptom | Likely cause | Fix | Source | | --- | --- | --- | --- | | Trips a few minutes after switch-on, other outlets go dead too | Overload: heater plus other loads above the breaker rating | Move the heater to a circuit with nothing else running, or use the 900 W setting | ESFI booklet; Lasko support; CPSC AFCI paper | | Trips the instant it is switched on, on any circuit | Short circuit or fault in the heater or its cord | Stop using the heater; inspect for cracked plugs or loose connections; contact the maker | ESFI space heater page; Schneider FAQ | | Trips only on one outlet, runs fine elsewhere | That circuit already carries other loads, or its wiring needs checking | Note what else goes off with it; frequent trips are a reason to call a licensed electrician | Lasko support; ESFI booklet | | Breaker is a GFCI, trips with the heater plugged in | Ground fault: current leaking along an unintended path, such as a damaged cord | Unplug, inspect the cord and plug for fraying, replace a warm or discoloured outlet | Leviton GFCI note | | Breaker is an AFCI, trips about 2 seconds after the load starts | Arc fault detected (Schneider QO, QOB and HOM diagnostics) | Remove the other loads from the circuit before troubleshooting; a misread signature is possible | Schneider FAQ; Leviton AFCI blog | | Plug or wall plate hot, buzzing or discoloured | Heat build-up at a worn or loose connection | Unplug immediately, do not use the outlet, call a qualified, licensed electrician | ESFI booklet; Leviton GFCI note | | Breaker will not reset, or sits between ON and OFF | Still overloaded, or not fully cycled | Unplug the loads first, move the handle fully OFF then ON | ESFI booklet | ## Is a warm plug or outlet normal? ESFI's booklet puts the line at hot: "warm to the touch may be okay, but hot is not." For unusually hot outlet or switch plates it says to unplug immediately and have a qualified, licensed electrician check the wiring. Its other reasons to call one: breakers that trip often, discoloured face plates, crackling or buzzing outlets, and cords or power strips in permanent use. A bigger breaker is not a fix: ESFI calls replacing a correct-size fuse with a larger one a serious fire hazard, because it lets excessive current flow and can overload the house wiring. ## Can you use an extension cord or power strip? No. CPSC's wording is to "always plug electric space heaters directly into a wall outlet and never into an extension cord or power strip", with overloading and fire as the reason. ESFI's space heater page adds: "Do not plug any other electrical devices into the same outlet as the heater." ESFI's booklet says older No. 18 gauge cords overheat at 15 or 20 A, so a 1,500 W heater on a light-duty cord is a heater on wire never rated for it. NFPA's heating page puts space heaters and heating stoves behind 30 percent of home heating-equipment fires and 73 percent of the deaths. ## What if the breaker is a GFCI or an AFCI? A GFCI (ground-fault circuit interrupter) trips on current leaking along an unintended path, such as water or a person, not on overload; Leviton's application note says to inspect the plugged-in cords for frayed wires. An AFCI (arc-fault circuit interrupter) senses the signature of an arc. Schneider Electric's FAQ for its QO, QOB and HOM breakers reads the timing: instant is a fault to ground, about 2 seconds an arc fault, about 5 seconds thermal overload or a short circuit. Leviton says nuisance trips come from a load's signature misread as an arc, with a new vacuum as its example; no source opened here names space heaters, so a 5-second trip reads as overload. **Before you buy anything** Do not buy a bigger breaker or a heavier extension cord. Measure first: a plug-in meter such as the P3 Kill A Watt P4400 sits between heater and wall and reads volts, amps and watts. Its manual caps it at 15 A and 1,875 VA: enough for one 1,500 W heater, so it shows what the heater draws, not what else shares the breaker. If the heater is the problem, the [best space heater](/air/reviews/best-space-heater/) comparison lists units with a 900 W setting, and the [space heater running cost calculator](/air/calculators/space-heater-running-cost-calculator/) turns the watts into a monthly bill. ## What should you do first? 1. Unplug everything on the circuit, then move the breaker fully OFF and back ON (ESFI's booklet). 2. Divide the label watts by 120: 1,500 W is 12.5 A, above ESFI's 12 A (80 percent) figure for a 15 A circuit. 3. Try the heater alone in another room, Lasko's test, and note which devices switch off with it. 4. Run it at 900 W (7.5 A) or on a circuit with nothing else drawing, plugged straight into the wall. 5. Hot plug, buzzing outlet or frequent trips: stop using the outlet and call a licensed electrician. ## The picks - **P3 P4400 Kill A Watt electricity usage monitor**: Wants the heater's real watts and amps before changing anything (P3 manual specifications: 115 VAC operating, 125 VAC maximum, 15 A maximum current, 1,875 VA maximum power; displays volts, amps, watts, Hz, power factor and VA) Amazon: https://www.amazon.com/dp/B00009MDBU/?tag=ownerspec-20 ## Frequently asked questions ### How many amps does a 1,500 watt space heater draw? 12.5 A on a 120 V circuit, because amps are watts divided by volts: 1,500 divided by 120. Lasko's support page states that most of its heaters pull 1,500 W on high and 900 W on low, which it puts at about 12.5 A and about 7.5 A. A heater with a different nameplate wattage draws that number divided by 120. ### Can a space heater run on a 15 amp circuit? ESFI's booklet says a 15 A circuit at 120 V carries 1,800 W and recommends not exceeding 80 percent of that, 1,440 W or 12 A, at any one time. A 1,500 W heater at 12.5 A is already above that figure on its own, so ESFI's advice to make certain the circuit can handle the added demand means nothing else shares it, or the heater runs at 900 W. ### Why does the breaker trip after the heater has run for a few minutes rather than instantly? A conventional breaker has two trip mechanisms, as CPSC's AFCI technology paper describes them: a magnetic trip for short circuits, and a thermal trip, a bimetal strip that heats and bends when current exceeds the breaker's rating. An overload slightly above the rating takes minutes to heat the strip, so the heater runs and then the circuit opens. An instant trip points to a short or a fault. ### Is it safe to plug a space heater into a power strip or extension cord? No. CPSC's wording is to always plug electric space heaters directly into a wall outlet and never into an extension cord or power strip, and the reason it gives is overloading and fire. ESFI's space heater page adds that a cord or strip could overheat, and that no other electrical device should be plugged into the same outlet as the heater. ### The heater's plug is warm. Is that a problem? ESFI's booklet draws the line at hot: warm to the touch may be acceptable, hot is not. If an outlet or switch plate is unusually hot, ESFI says to unplug cords from it immediately, stop using it and have a qualified, licensed electrician check the wiring. Leviton's GFCI note says a discoloured or warm outlet with a loose plug connection should be replaced immediately. ## Sources - [ESFI, Indoor Electrical Safety Check booklet (PDF)](https://www.esfi.org/wp-content/uploads/2021/08/Indoor-Electrical-Safety-Check-Booklet-3FB3.pdf) (manufacturer or vendor document) - [ESFI, Space Heater Safety Tips](https://www.esfi.org/space-heater-safety-tips/) (manufacturer or vendor document) - [CPSC, There's a Chill in the Air: Stay Warm Safely (2025 news release)](https://www.cpsc.gov/Newsroom/News-Releases/2025/Theres-a-Chill-in-the-Air-Stay-Warm-Safely-Be-Cautious-When-Using-Generators-Furnaces-and-Space-Heaters) (government or regulatory source) - [CPSC, New Technology for Preventing Residential Electrical Fires (AFCI paper, PDF)](https://www.cpsc.gov/s3fs-public/AFCIFireTechnology.pdf) (government or regulatory source) - [NFPA, Safety with heating equipment](https://www.nfpa.org/education-and-research/home-fire-safety/heating) (manufacturer or vendor document) - [Lasko support, Heater is tripping the breaker. Why?](https://laskoproducts.zendesk.com/hc/en-us/articles/360059286671-Heater-is-tripping-the-breaker-Why) (manufacturer or vendor document) - [Virginia Tech Cooperative Extension, Estimating Appliance and Home Electronic Energy Use (2901-9014)](https://www.pubs.ext.vt.edu/2901/2901-9014/2901-9014.html) (university or extension service) - [Leviton, What to do when your GFCI trips (application note, PDF)](https://leviton.com/content/dam/leviton/residential/product_documents/application_note/Leviton%20GFCI%20Trip%20Application%20Note.pdf) (manufacturer or vendor document) - [Leviton, Understanding AFCI challenges and addressing nuisance tripping](https://leviton.com/support/literature/blogs/understanding-afci-arc-fault-circuit-interrupter-challenges-and-addressing-nuisance-tripping) (manufacturer or vendor document) - [Schneider Electric FAQ FA125415, QO, QOB and HOM arc fault and dual function breaker troubleshooting](https://www.se.com/us/en/faqs/FA125415/) (manufacturer or vendor document) - [P3 International, Kill A Watt P4400 operation manual (PDF)](https://images10.newegg.com/UploadFilesForNewegg/itemintelligence/P3%20International/Kill_20A_20Watt1453363762061.pdf) (manufacturer or vendor document) --- Citation: OwnerSpec, "Space heater tripping the breaker: the watts-to-amps arithmetic, what else is on the circuit, and the signs that mean stop using it", https://ownerspec.com/air/guides/space-heater-tripping-breaker-watts-amps/, facts verified 3 October 2026. 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