Circuit Specifications
Set your breaker size, voltage, and load type before adding appliances.
Appliance Inventory
Add everything plugged into or wired to this circuit.
Appliance Preset
Value
Unit
Custom Appliance Name
No appliances added yet. Use the fields above to build your circuit inventory.
Total Amps Drawn
0.0 A
Circuit Overloaded: Risk of tripped breaker or fire hazard.
Total Watts Drawn
0
Watts
Max Safe Amps
16.0
Amps (NEC 80%)
Amps Remaining
16.0
Amps Available
0% 0% of safe capacity 100%
Key Terms Explained
Amperage (Amps)
The rate of electrical current flow through a circuit, measured in amperes (A). Think of it as the volume of electricity moving per second. Breakers are rated in amps.
Voltage (Volts)
The electrical pressure pushing current through the circuit. Standard US household outlets are 120V; large appliances like dryers and ranges use 240V circuits.
Wattage (Watts)
The rate of energy consumption. Watts = Amps x Volts. Most appliance labels list watts, which you can convert to amps by dividing by the circuit voltage.
Circuit Breaker
A safety switch that automatically cuts power when the circuit draws more current than it is rated for. It protects wiring from overheating and prevents fires.
Continuous Load
Any load that runs for 3 or more hours continuously. Space heaters, water heaters, and EV chargers are common examples. The NEC requires continuous loads to stay below 80% of breaker capacity.
NEC 80% Rule
National Electrical Code rule (210.19/210.20) limiting continuous loads to 80% of a circuit breaker's rated amperage. A 20A breaker may carry only 16A of continuous load safely.
Dedicated Circuit
A circuit that supplies power to only one appliance and no other outlets. Large appliances like refrigerators, microwaves, and dryers often require or benefit from a dedicated circuit.
Load Capacity
The maximum electrical load a circuit can safely handle. For continuous loads this is 80% of the breaker rating; for intermittent loads it equals the full breaker rating.

The Complete Guide to Household Circuit Loads

Overloaded circuits are one of the leading causes of house fires in the US. Understanding how much your outlets and appliances draw - and how that compares to your breaker's safe limit - is a practical skill every homeowner should have. This tool does the math instantly, but the guide below explains what the numbers mean and how to act on them.

How to Use This Estimator

Start by setting your breaker size (check the number printed on the breaker switch in your panel - common sizes are 15A and 20A for standard rooms). Choose the correct voltage (120V for standard outlets, 240V for heavy appliances like dryers and ranges). Then choose load type: select "Continuous" if any appliance runs 3 or more hours at a stretch, which is the more conservative and NEC-compliant setting.

Next, add every appliance or device that could be running on this circuit at the same time. Pick from the presets for common items or enter a custom watt or amp value for anything not listed. The results update in real time. If the load bar turns red and the overload banner appears, you need to either move appliances to a different circuit or talk to a licensed electrician about adding capacity.

How Breaker Ratings Work

A circuit breaker is a thermal and magnetic trip device. The thermal element responds to sustained overcurrent - heat builds up in the bimetal strip and eventually triggers the breaker. The magnetic element responds to short-circuit spikes almost instantly. This means a breaker at 110% of its rating will not trip immediately; it may take minutes or even hours. Running a breaker at or near its limit for extended periods degrades it and can cause nuisance tripping even at lower loads in the future.

Standard residential branch circuits use 14-gauge wire (rated for 15A) or 12-gauge wire (rated for 20A). The breaker size must match the wire gauge - a 20A breaker on 14-gauge wire is a code violation and a fire hazard, because the wire can overheat and melt before the breaker trips.

The NEC 80% Continuous Load Rule Explained

Article 210.19(A)(1) and 210.20(A) of the National Electrical Code require that branch circuits supplying continuous loads be sized at 125% of the load - which is mathematically equivalent to keeping the load at or below 80% of the circuit rating. The code defines "continuous load" as any load that is expected to continue for 3 hours or more. A space heater running all night, a grow light on a timer, a water pump on a well system - all of these qualify. When you select "Continuous Load" in this tool, the safe capacity shown reflects this 80% threshold.

Common Causes of Tripped Breakers

The most common cause is a circuit shared between a space heater and other devices. A 1500W space heater draws 12.5A alone on a 15A circuit - leaving only 2.5A for everything else. Add a 150W TV (1.25A) and a phone charger (0.2A) and you are at 14A on a 15A breaker before accounting for the continuous load rule.

Other common culprits include hair dryers (often 1875W, drawing 15.6A alone on 120V - enough to trip a 15A breaker by itself), vacuum cleaners, and motor-start surges from refrigerators and air conditioners. Motor loads have a high inrush current on startup that can be 3-6 times the running current, which can trip a breaker even when running load is within limits.

When to Add a Dedicated Circuit

Any appliance that frequently trips a breaker, or any device the NEC specifically lists as requiring a dedicated circuit, should be on its own circuit. The NEC requires dedicated 20A circuits for kitchen small appliance circuits, refrigerators, dishwashers, garbage disposals, and bathroom outlets. It requires dedicated circuits for ranges, dryers, water heaters, and EV chargers as well, but at higher amperages (30A to 60A). If this estimator shows your circuit is consistently over the safe limit, hiring an electrician to add a new circuit from the panel is the proper fix - not using extension cords or power strips to redistribute the load.

Frequently Asked Questions

What is the 80% rule for circuit breakers? +
The NEC 80% rule (National Electrical Code section 210.19 and 210.20) states that a circuit breaker must not carry more than 80% of its rated amperage for loads that run continuously - meaning 3 hours or longer. A 20-amp breaker on a continuous circuit should carry no more than 16 amps. This rule exists because breakers are thermal devices: running at 100% capacity for extended periods generates enough heat to degrade the breaker, cause nuisance trips, and in worst cases create a fire hazard. For short, intermittent loads, the full breaker rating is usable.
Why does my space heater keep tripping the breaker? +
A typical 1500-watt space heater draws 12.5 amps on a 120V circuit and runs continuously, putting it alone at 78% of a 15-amp breaker's safe continuous capacity. If anything else is on that circuit - a lamp, a phone charger, a TV - you push past the 80% threshold and the breaker trips. The fix is to plug the heater into a dedicated 20-amp circuit (it draws only 62.5% of that breaker's capacity), or to run it on a circuit with nothing else plugged in.
How do I calculate amps if my appliance only lists watts? +
Divide the appliance's wattage by the circuit voltage. For a standard 120V outlet: Amps = Watts divided by 120. For a 240V heavy appliance outlet: Amps = Watts divided by 240. Example: a 1200-watt microwave on a 120V circuit draws 10 amps. A 4800-watt electric water heater on a 240V circuit draws 20 amps. This calculator does the conversion automatically - just enter watts for any appliance and it will show the amp draw.
What requires a dedicated 240V circuit? +
Large appliances that require their own dedicated 240V circuit include: electric ranges and ovens (40-50 amp), clothes dryers (30 amp), central air conditioners (varies, typically 30-60 amp), electric water heaters (30 amp), EV chargers (Level 2, 30-50 amp), hot tubs and spas (50-60 amp), and electric furnaces. The NEC requires dedicated circuits for these because they draw far more power than a standard 120V branch circuit can safely supply. Sharing a circuit with other loads is a fire and equipment hazard.
Can I add more outlets to an existing circuit? +
You can physically add more outlets to a circuit, but whether you should depends entirely on the total load. Use this calculator to map out everything that will ever run simultaneously on that circuit and compare to the breaker's safe capacity (80% for continuous loads). If the total is under the safe threshold, adding outlets is fine. If you are already near the limit, adding outlets means you need to be careful about what you plug in - or better yet, add a new dedicated circuit from the panel.
This tool provides estimates for informational and planning purposes only. It is not a substitute for professional electrical inspection or licensed electrician advice. Always consult a licensed electrician before modifying circuits, adding loads, or making electrical changes to your home. Electrical code requirements vary by jurisdiction.