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About this tool
Check whether the appliances on one extension board exceed its amp rating, the cord ampacity, the 80% continuous rule or safe voltage drop.
The Extension Cord Load Checker adds up everything plugged into one extension board and converts it to amps with I = watts / (volts x power factor), then compares that current against the lower of two limits: the amp rating printed on the board and the ampacity of its flexible cord. It also applies the 80% continuous-load ceiling from NEC 210.19(A)(1) for anything running three hours or more, and estimates voltage drop from the two-way copper run so a long thin cord does not quietly cook itself. Use it before plugging a heater, kettle or air conditioner into a strip that was only ever meant for chargers and a lamp.
Open Extension Cord Load Checker on AltFTool — it loads instantly in your browser.
Set "Mains supply" (India / EU / UK — 230 V, North America — 120 V or Japan — 100 V), the "Board rating printed on the strip (A)", the "Cord conductor size" such as "1.5 mm² flexible — 16 A", and "Cord length (m)".
Add appliances from the chip row under "What is plugged in" — Room heater 2000 W, Electric kettle 1500 W, Fridge 250 L — adjust "Watts each" and Quantity, and tick "Runs 3 hours or more (continuous)" to apply the 80% ceiling.
The bar reports the percentage of the rating in use, and the rows give "Safe limit" with whichever limit binds, "Continuous ceiling (80% rule)", "Headroom left", "Maximum watts you may plug in", "Peak start-up current" and "Voltage drop along the cord".
Compares the load against the board rating AND the cord ampacity, whichever is lower.
Applies the 80% de-rating that governs anything running three hours or more.
Warns when a fridge or AC compressor start-up spike would exceed the board's rating.
About 1380 W at 230 V (6 A x 230 V), and only 1104 W if the load runs three hours or more, because the 80% continuous-load ceiling applies. That rules out a 2000 W heater or geyser on a 6 A strip entirely.
Only into a 16 A board on 1.5 mm² or thicker cord at 230 V, and never into a light-duty strip. A 2000 W heater draws 8.7 A at 230 V continuously, which is above the 4.8 A continuous ceiling of a 6 A board. At 120 V the same heater draws 16.7 A, which overloads anything short of a 20 A board on 12 AWG cord — and even that combination sits above the 16 A continuous-load ceiling, so it only avoids a caution if the heater will not run for hours at a stretch. For 120 V circuits, a dedicated wall socket is the safer choice for a 2000 W heater.
Thicker than you would use short, because voltage drop rises with length: keep the two-way drop under 3% of supply voltage. A 1000 W load on 120 V through 15 m of 16 AWG already drops about 2.7%, so beyond that go to 14 AWG or 12 AWG.
Heat means the current is at or above what the contacts and cord can carry — usually a high-wattage appliance, a coiled cord that cannot shed heat, or a daisy-chained second board. Unplug the largest appliance immediately and have an electrician inspect the strip and the wall socket before using it again.
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