Where the 80 percent figure comes from
A breaker is a thermal device. It is calibrated to carry its rated current indefinitely under standard test conditions and to open at some margin above that, with a time-current curve that lets short surges pass and opens progressively faster as the overload grows. Sitting at exactly its rating for hours in a warm enclosure alongside other warm breakers is the condition it is least comfortable in.
The widely applied convention handles this from the other direction. Rather than derating the breaker, a load that runs for three hours or more at a stretch is counted at 125 percent of its actual current when the circuit is sized. Multiply a load by 1.25 and compare it against 100 percent of the breaker, or leave the load alone and compare it against 80 percent of the breaker — the two are the same arithmetic and produce the same answer. This calculator does the first, because it keeps mixed circuits honest when only some of the loads are continuous.
What counts as continuous is a duration question, not a device question. Electric heat, a car charger, shop lighting, a server, a well pump on a long cycle: three hours or more without stopping. A kettle, a microwave, a vacuum, a circular saw: not continuous, however hard they pull while running.
What actually fits on a circuit
| Circuit | Full rating | Continuous working figure |
|---|---|---|
| 15 A at 120 V | 1,800 W | 1,440 W |
| 20 A at 120 V | 2,400 W | 1,920 W |
| 30 A at 240 V | 7,200 W | 5,760 W |
| 40 A at 240 V | 9,600 W | 7,680 W |
| 50 A at 240 V | 12,000 W | 9,600 W |
Those numbers explain most domestic trip complaints in one line. A 1,500 watt space heater is 12.5 amps and eats 83 percent of a 15 amp circuit on its own. Two of them on the same circuit cannot work, and no amount of moving them between receptacles in the same room will change that if those receptacles are on one breaker. A kettle is another 12.5 amps. The arithmetic is brutal and it is why kitchen and bathroom circuits are treated specially in most wiring practice.
Reading a nameplate honestly
Enter the load in whatever unit the equipment gives you. Resistive things — heaters, kettles, toasters, incandescent lamps, elements — state watts and the conversion to amps is watts divided by volts with nothing else in it. Motor-driven equipment usually states amps directly, and that figure is the one to use, because a motor nameplate's watt rating is often the mechanical output rather than the electrical input and the two differ by the efficiency.
Power factor is where watts-to-amps quietly goes wrong on inductive loads. A motor drawing 1,000 watts at a power factor of 0.8 pulls 10.4 amps at 120 volts, not 8.3. The breaker responds to amps. If the equipment gives you amps, use amps and leave the power factor alone. If a piece of equipment states a VA rating rather than watts, treat the VA figure as if it were watts at a power factor of 1, because VA is already the product of volts and amps.
The trips that are not overloads
If a breaker is opening and this calculator says the circuit is nowhere near full, the load total is not the problem and adding it up again will not help. A few of the other causes, so you know what you are looking at: a ground-fault or arc-fault device tripping on leakage or on a signature it reads as arcing, which usually feels different from an overload because it happens at switch-on or seemingly at random rather than after a period of heavy use; a motor starting surge that the breaker's curve does not tolerate, common with compressors on marginal circuits; a loose or corroded connection producing heat at a termination; a genuinely faulty breaker, which does happen with age and thermal cycling. All of those are diagnosis on a live system, and all of them belong with an electrician rather than with a replacement breaker from a hardware store. Fitting a larger breaker to stop the tripping removes the protection from the wire in the wall, which is the one part of the system nobody can inspect after it fails.
Questions people ask
How many outlets can I put on a 20 amp circuit?
The count of receptacles and the load on the circuit are separate questions, and different rules cover them in different occupancy types, so there is no single number that is true everywhere. In dwellings, general-purpose receptacle outlets are commonly not counted individually at all, and the circuit layout is driven by the spacing rules for the rooms instead. In commercial work a fixed volt-ampere allowance per outlet is commonly applied, which does produce a maximum count. What matters practically is what gets plugged in: a 20 amp circuit carries 2,400 watts at full rating, and it makes no difference whether that arrives through two receptacles or twelve. Your Authority Having Jurisdiction decides what applies where you are.
What is the difference between this and a panel load calculation?
Scope and method. This adds up what is connected to a single branch circuit and compares it against that breaker, which is what you want when you are deciding whether one more thing can go on a circuit. A service or panel load calculation totals the whole building and then applies demand factors, because the assumption that every appliance in a house runs simultaneously is false and sizing a service that way would produce an absurd result. Those demand factors are specified in the adopted code and vary by occupancy type and by the calculation method chosen. If you need a service size, that is a different calculation and it belongs with someone qualified to sign it.
Can I just fit a bigger breaker?
No, and this is the single most consequential misunderstanding in domestic electrical work. The breaker is not protecting the appliance and it is not protecting you from the appliance. It is protecting the conductor in the wall from carrying more current than its insulation can tolerate. The breaker rating and the conductor size are chosen together, and replacing a 15 amp breaker with a 20 amp one on 14 AWG wire means the wire can now run hot enough to degrade its insulation while the breaker sits there content. The failure is inside a wall cavity where nobody sees it. If a circuit is genuinely undersized for what you need, the fix is a new circuit with conductors sized for it.
Do I count the continuous 125 percent on everything?
Only on the loads that actually run three hours or more without stopping, which is why this page marks them individually rather than applying a blanket factor. On a mixed circuit the non-continuous loads are counted at their actual current and only the continuous ones get the multiplier. Where it is close, err toward marking a load continuous: the cost of being wrong in that direction is a slightly conservative circuit, and the cost of being wrong in the other direction is a breaker that trips on the afternoon of the hottest day. Where a circuit serves a single continuous load, the arithmetic collapses to the familiar version, which is that the load should not exceed 80 percent of the breaker rating.