The direct answer to the most common electrical question: a standard US residential 120V 15 amp circuit wattage capacity is 1800 watts peak, and 1440 watts continuous. The National Electrical Code (NEC) Article 210.20 mandates that continuous loads (those running for 3 hours or more) cannot exceed 80% of the branch circuit rating. Therefore, 15A × 120V = 1800W maximum, but 12A × 120V = 1440W for continuous duty.

While you cannot safely breadboard a 120V AC mains circuit, the exact same mathematical principles and protection topologies apply to 12V or 24V DC off-grid and solar branch circuits. A 12V 15-amp DC circuit caps at 180W peak and 144W continuous. In this guide, we will design a solid-state 15A DC wattage limiter. This active topology monitors current, calculates the equivalent wattage threshold, and cuts power before a thermal breaker ever needs to trip, giving you a safe, low-voltage testbed to master branch circuit protection.

Topology & Node Labels: The 15A Solid-State Limiter

Why use an active solid-state limiter topology instead of a simple 15A automotive blade fuse? A fuse is a passive thermal device. It does not calculate wattage, it ignores the 80% continuous NEC derating rule, and it suffers from fatigue over time. An active topology uses a shunt resistor to measure voltage drop, a comparator to evaluate that drop against a reference threshold, and a MOSFET to sever the load path instantly when the 15A (or 12A continuous) limit is breached.

Pro-Tip: When designing for continuous loads, set your reference voltage to trip at 12A (80% of 15A) rather than the hard 15A peak. This mimics the NEC continuous load rule perfectly.

Here is the topology mapped to specific node labels for your schematic:

  • V_SOURCE: The positive terminal from your 12V DC supply.
  • SHUNT_P: The high-side node of the current-sense shunt resistor.
  • SHUNT_N: The low-side node of the shunt, connecting to the load.
  • V_LOAD: The positive terminal feeding the actual appliance or dummy load.
  • V_REF: The precise reference voltage set by a resistor divider, representing the 12A or 15A trip point.
  • COMP_OUT: The output pin of the LM393 comparator, driving the gate logic.
  • GATE_DRV: The gate node of the N-channel MOSFET, pulled low to sever the circuit.
  • GND: The common ground for the supply, load, and control logic.

Behavior Matrix: Element Changes & Failure Extremes

Understanding how a circuit degrades is just as important as knowing how it operates. Below is the behavior matrix detailing what happens when specific elements drift, followed by the hard failure modes at the extremes.

Element Parameter Change Circuit Behavior Result
Shunt Resistor Resistance increases (thermal drift) Voltage drop increases for the same current; circuit trips prematurely below 15A.
V_REF Divider Reference voltage drops The comparator threshold lowers; the 15 amp circuit wattage limit is artificially reduced.
MOSFET (IRF3205) Rds(on) increases (heating) Higher voltage drop across the switch; reduces voltage available to the load, lowering total wattage delivered.
Comparator (LM393) Input offset voltage drifts Minor shift in the exact amperage trip point (usually negligible at the 75mV shunt scale).

Failure Extremes: Open and Short Scenarios

Shorting the Shunt (SHUNT_P to SHUNT_N): The voltage drop across the shunt becomes zero. The comparator sees 0V on the sense pin, which is below V_REF. The MOSFET stays permanently ON. The circuit loses all overcurrent protection, and the load will draw until the upstream power supply folds back or wires melt. Never bypass the shunt in a hardwired installation.

Opening the Gate Drive (COMP_OUT to GATE_DRV trace breaks): The MOSFET gate floats or is pulled low by a 10kΩ pulldown resistor (which you must include). The MOSFET turns OFF. The circuit fails safe, cutting power to the load entirely. This is the preferred failure mode for branch circuit protection.

Design Walkthrough: Picking Real Component Values

Let's pick exact components to enforce a hard 15A peak limit on a 12V DC branch (180W). If you want the 80% continuous limit (144W / 12A), adjust the V_REF divider accordingly.

  1. The Shunt Resistor: We need a low resistance to minimize power loss. Let's use a Vishay WSL3637 5mΩ (0.005Ω) surface mount shunt, or a through-hole equivalent. At 15A, the voltage drop is V = I × R = 15 × 0.005 = 0.075V (75mV). Power dissipation is I²R = 225 × 0.005 = 1.125W. A 2W or 3W rated shunt is mandatory here to prevent thermal drift.
  2. The Comparator: The TI LM393 dual differential comparator is ideal. It operates on a single supply and has an open-collector output, making it easy to interface with pull-up resistors and MOSFET gate drivers.
  3. Setting V_REF (75mV): The LM393 cannot easily compare signals near the ground rail without a negative supply. To fix this, we elevate both the shunt signal and the reference using a simple op-amp differential stage, OR we use a high-side current sense IC like the INA180. For a pure discrete breadboard build, we will use an INA180A1 (gain of 20V/V). 75mV × 20 = 1.5V. We then set V_REF to exactly 1.5V using a 10kΩ and 1.2kΩ resistor divider from a clean 3.3V or 5V logic rail.
  4. The Switch: The IRF3205 N-channel MOSFET. It has an Rds(on) of roughly 8mΩ and handles up to 110A. At 15A, it will dissipate about 1.8W. While it can survive briefly without a heatsink, a small TO-220 clip-on heatsink is required for continuous 15 amp circuit wattage testing to keep the silicon below 100°C.

Breadboard Testing Step-by-Step (Low-Voltage Safe)

We are testing the control logic and trip threshold using a 12V DC bench supply. Do not attempt to wire this directly into a 120V AC mains panel. This procedure validates the 15A trip logic safely.

Safety Callout: Ensure your DC bench supply is current-limited to 20A maximum before beginning. Never test branch circuit protection topologies on live mains voltage without a licensed electrician and proper arc-flash PPE.
  1. Prep the Power Rail: Connect your 12V DC bench supply positive to the breadboard's top red rail (V_SOURCE) and negative to the bottom blue rail (GND). Set the supply's current limit knob to 20A.
  2. Seat the Shunt & Amplifier: Place your 5mΩ shunt in series with the positive rail. Wire the SHUNT_P and SHUNT_N nodes to the input pins of your INA180 current sense amplifier. Power the INA180 with 5V from a secondary logic supply or a 7805 regulator.
  3. Establish the Reference: Build your voltage divider on the breadboard to output exactly 1.5V. Connect this to the V_REF node (the inverting input of the LM393). Connect the INA180 output to the non-inverting input.
  4. Wire the MOSFET Gate: Connect the LM393 open-collector output to the IRF3205 gate via a 100Ω gate resistor. Add a 10kΩ pulldown resistor from the gate to GND to ensure the MOSFET fails safe (OFF) if the comparator loses power.
  5. Connect the Dummy Load: Connect a 12V DC load (like a bank of power resistors or a 12V 50W halogen bulb array) to the drain of the MOSFET. The source of the MOSFET connects to GND.
  6. Execute the Ramp Test: Turn on the 12V supply. The load should power on. Slowly decrease the resistance of your dummy load (or increase the bench supply voltage if using a fixed resistor) to draw more current. Monitor the shunt voltage with a multimeter. At exactly 15A (75mV across the shunt, 1.5V out of the amp), the LM393 should trip high, pulling the gate low and instantly killing power to the load.

Frequently Asked Questions

How many watts can a standard 15 amp 120V circuit handle continuously?

A standard 15 amp 120V circuit can handle 1440 watts continuously. The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. For these loads, the branch circuit must be derated to 80% of its capacity. 15 amps multiplied by 0.80 equals 12 amps. 12 amps multiplied by 120 volts equals 1440 watts. If your load runs for less than 3 hours (like a vacuum cleaner or microwave), you can pull the full 1800 watts.

Why does my 15 amp breaker trip when I run a 1500 watt heater?

A 1500 watt space heater draws exactly 12.5 amps on a 120V circuit (1500W / 120V = 12.5A). While this is under the 15A peak limit, space heaters are often run for more than 3 hours, classifying them as continuous loads. The continuous limit for a 15 amp circuit is 12A (1440W). Furthermore, if the heater is sharing the circuit with any other loads (lights, a TV, or a fan), the combined wattage easily exceeds the 1800W hard peak or the 1440W continuous threshold, causing the bimetallic strip inside the breaker to heat up and trip.

What size wire is required for maximum 15 amp circuit wattage?

You must use a minimum of 14 AWG copper wire for a 15 amp circuit, as specified in NEC Table 310.16 (60°C column for standard NM-B Romex). 14 AWG copper is rated for 15 amps. If you are running a long distance (over 50 feet) to a shed or garage, you must calculate voltage drop. To keep voltage drop under 3% at full 15A wattage, you will need to upsize to 12 AWG or even 10 AWG copper wire, though the breaker must remain sized to protect the smallest wire in the circuit (or you must upsize the breaker to 20A if the entire run is 12 AWG).

Can I upgrade a 15 amp circuit to 20 amp to increase wattage?

You cannot simply swap a 15A breaker for a 20A breaker to get more wattage. A 20 amp breaker allows up to 2400W peak and 1920W continuous, but it requires the entire branch circuit wiring to be 12 AWG copper. If your walls are wired with 14 AWG (the standard for 15A circuits), installing a 20A breaker is a severe fire hazard. The 14 AWG wire will overheat and melt its insulation long before the 20A breaker trips. To legally and safely increase your 15 amp circuit wattage capacity to 20A, you must pull new 12 AWG wire from the panel to every outlet on that branch.