Circuit Load Calculator

A circuit's safe continuous load equals the breaker rating times the voltage times 0.8. A 20-amp, 120-volt circuit can carry 20 × 120 × 0.8 = 1,920 watts continuously. It could technically pass 2,400 watts (the full 100%), but only for short, non-continuous bursts — sustaining that would trip the breaker and overheat the wiring. The 0.8 factor is the NEC 80% rule, and it is the single number that keeps circuits safe.

People also ask this as "how many amps on a 30 amp breaker?" The answer is 24 continuous amps (30 × 0.8), not 30. Every breaker follows the same logic. This calculator gives you the max continuous watts, the continuous amp limit, and the peak watts for any common circuit so you can plan what to plug in without nuisance trips or hazards.

Three related numbers describe a circuit's capacity:

The continuous figures are the ones that matter for planning, because most real usage — a space heater, a string of lights, a battery charger — runs for hours. The NEC caps continuous load at 80% of the breaker rating so the thermal element and conductor stay within their design margins. The peak figure is the theoretical ceiling for brief, intermittent loads; you should never design around it.

Voltage doubles the wattage capacity: the same 20A breaker carries 1,920W at 120V but 3,840W at 240V, because power equals volts times amps. That is why energy-hungry appliances move to 240V — they deliver more watts through the same current and wire. To turn a specific device's watts into the amps it will draw, use the watts to amps calculator; to size a breaker for a known load, use the breaker size calculator.

Example 1 — 20A/120V kitchen circuit: Max continuous = 20 × 120 × 0.8 = 1,920 W. A 1,500W toaster oven fits, but adding an 1,100W microwave (2,600W total) trips it. This is why the NEC requires at least two separate 20A small-appliance circuits in a kitchen.

Example 2 — 30A/240V circuit: Max continuous = 30 × 240 × 0.8 = 5,760 W. A 4,500W water heater fits comfortably (it draws about 3,600 continuous VA after the 80% rule is applied to sizing). "How many amps on a 30 amp breaker?" — 24 continuous amps.

Example 3 — 15A/120V bedroom circuit: Max continuous = 15 × 120 × 0.8 = 1,440 W. A 1,500W space heater alone exceeds this, which is the classic cause of winter breaker trips. See the dedicated watts on a 15 amp circuit guide.

Example 4 — 50A/240V range circuit: Max continuous = 50 × 240 × 0.8 = 9,600 W. This easily handles an electric range whose simultaneous demand, after the range demand factor, is well under the nameplate total.

This is one of the most-searched circuit questions, so it deserves a clear answer. A 30-amp breaker is rated for 30 amps of instantaneous current, but the continuous limit is 30 × 0.8 = 24 amps. In watts, that is 2,880W at 120V or 5,760W at 240V.

The distinction matters because a 30A breaker protecting a load that draws a steady 28–30 amps will run hot and eventually trip on thermal overload, even though 30A is "within rating." The 80% rule exists precisely to keep steady loads below the point where the breaker's thermal element accumulates heat. If your continuous load genuinely needs more than 24 amps, you should be on a 40A circuit (32A continuous) with the appropriate 8 AWG wire.

The same math answers the related questions: a 15A breaker allows 12 continuous amps, a 20A breaker allows 16, a 40A breaker allows 32, and a 50A breaker allows 40. Multiply any breaker rating by 0.8 to get its safe continuous current, then by the voltage to get watts.

1. Select the breaker size. Pick the rating of the breaker protecting the circuit — check the handle number in your panel (15, 20, 30, 40, or 50 A).
2. Select the voltage. Standard 120V branch circuits power lights and receptacles; 240V circuits serve dryers, ranges, water heaters, and EV chargers.
3. Read the max continuous watts. This is the largest steady load you should place on the circuit. Keep the sum of everything running for hours below this number.
4. Check the secondary values. The continuous amp limit tells you the safe steady current; peak watts is the short-burst ceiling you should not design around.

To add up the actual devices on a circuit, convert each appliance's watts to amps and total them, then compare against the continuous limit here. For sizing a new circuit from a known load instead, use the breaker size calculator, and verify the conductor with the wire size calculator.

• Loading to the full breaker rating. A 15A circuit is not a 1,800W circuit for continuous use — it is a 1,440W circuit. Planning to the 100% number causes trips and overheating.
• Adding up peak watts instead of running watts for motors. Motors surge at startup but run lower; size the circuit for running load and the breaker for surge.
• Chaining power strips. Multiple strips on one receptacle do not increase circuit capacity — the breaker still limits the total.
• Forgetting shared circuits. Bedroom outlets and lights often share one 15A breaker; a heater in one room plus a vacuum in another can trip it.
• Ignoring voltage. A 20A/240V circuit carries twice the watts of a 20A/120V circuit; do not assume all 20A circuits are equal.

A circuit breaker is not a simple on/off switch — it is a calibrated protective device that trips in two distinct ways, and understanding both explains why the 80% rule matters so much. The thermal trip uses a bimetallic strip that bends as it heats. A modest overload — say, 22 amps on a 20A breaker — does not trip instantly; instead the strip heats gradually and opens after seconds to minutes, matching the way a conductor heats under sustained overcurrent. The magnetic trip uses an electromagnet that snaps open almost instantly on a large fault current, such as a short circuit drawing hundreds of amps. This inverse-time behavior is why a breaker tolerates a brief motor-startup surge but will not tolerate a steady overload.

The continuous-load margin exists because that thermal element is affected by ambient heat and by its own long-term operating temperature. A breaker run near 100% of its rating for hours sits at an elevated baseline temperature, so it both ages faster and trips more easily on small fluctuations — the nuisance trips people blame on a "bad breaker" are frequently just an overloaded one. Keeping continuous load at 80% leaves the thermal element cool enough to ride through normal variation while still opening promptly on a real overload.

Headroom also protects the conductor, which is the breaker's real job. Wire insulation is rated for a maximum temperature (60°C, 75°C, or 90°C depending on type), and current heats the wire continuously. A conductor loaded to its full ampacity for hours runs hot; the 80% practice, combined with correct wire sizing, keeps the insulation well within its rating and dramatically extends its life. This is why you can never fix repeated trips simply by installing a bigger breaker — doing so removes the protection the wire depends on and invites an insulation failure inside the wall.

When you plan a circuit with genuine headroom, you get three benefits at once: fewer nuisance trips, longer breaker and wire life, and a safety margin for the inevitable day someone plugs in one more device. The calculator's continuous figure is the number to design around; the peak figure is only for understanding the short-term ceiling, never for planning steady loads.

Knowing each circuit's continuous capacity lets you plan appliances without guesswork.

In the kitchen, two 20A circuits give 3,840W of continuous capacity — enough to run a microwave on one and a toaster or kettle on the other, but not two high-wattage heating appliances on the same circuit. In the bathroom, a 20A circuit (1,920W) comfortably runs a 1,500W hair dryer with margin. In a home office, a 15A circuit (1,440W) handles computers, monitors, and a laser printer, but a space heater will push it over.

Garages and workshops often need dedicated 20A or higher circuits because power tools and air compressors have high surge and running loads. For EV charging, size the circuit to the charger's continuous draw: a 32A charger needs a 40A circuit (7,680W at 240V). When several new loads are involved, confirm the whole panel with the electrical load calculator for house so the service is not overloaded even when individual circuits are fine.

A useful habit is to label each breaker in your panel not just by location but by its continuous capacity in watts. Writing "Kitchen counter — 1,920W" or "Garage — 1,920W" beside each 20A circuit turns an abstract amp rating into a concrete budget you can plan against at a glance. When you later buy an appliance, you simply check its wattage against the remaining budget on the intended circuit. This small discipline prevents the most common household electrical frustration — the mystery trip that happens every time two particular appliances run together — because you will have already reserved enough capacity for both, or knowingly split them across separate circuits from the start. A few minutes spent mapping your panel and noting each circuit's watt budget pays for itself the first time you avoid a trip during holiday cooking or a winter cold snap when every heater is running at once.