The fundamental unit of electric capacity for AC branch circuits is the Watt (or Volt-Ampere), constrained by the Ampere rating of the breaker and wire. For DC battery systems, the unit is the Amp-hour (Ah) or Watt-hour (Wh). On a standard 15A, 120V AC circuit, your maximum continuous capacity is exactly 1,440 Watts (12 Amps), dictated by the NEC 80% continuous load rule. On a 20A circuit, the continuous capacity is 1,920 Watts (16 Amps). If you exceed these thresholds for three hours or more, you risk thermal degradation of the wire insulation long before the breaker trips.
The True Unit of Electric Capacity: Amps, Watts, and Amp-Hours
When makers and electricians talk about "capacity," they are often conflating two different physics concepts depending on whether they are wiring a mains-powered workshop bench or building a 12V LiFePO4 solar bank.
For AC mains, capacity is a rate of flow. We measure it in Amperes (current) and Watts (power). A 20A breaker doesn't hold 20 amps like a bucket; it allows 20 amps to flow continuously. However, the National Electrical Code (NEC) Article 210.20 mandates that for continuous loads (anything expected to run for 3 hours or more), you must derate the breaker capacity to 80%. This means a 20A breaker is only rated for 16A of continuous draw. The NFPA NEC guidelines enforce this to prevent heat buildup in bundled wires inside conduit or NM-B sheathing.
For DC systems, the unit of electric capacity is the Amp-hour (Ah). A 100Ah battery can theoretically supply 10A for 10 hours. But because of Peukert's Law in lead-acid chemistry (and BMS limits in lithium), usable capacity is often 50% to 80% of the nameplate Ah. To bridge AC and DC planning, convert everything to Watt-hours (Wh). A 12V 100Ah LiFePO4 battery holds 1,280Wh of capacity. If your AC inverter is 90% efficient, you have roughly 1,152Wh of usable AC capacity to draw from.
Load Tally: Calculating Your Circuit and Battery Capacity
To plan a circuit or size a battery bank, you must tally the running watts, the continuous amps, and critically, the inrush current. Motors and compressors draw massive current for a fraction of a second when starting. This is measured in Locked Rotor Amps (LRA). While a thermal-magnetic breaker will tolerate a brief inrush spike, a heavily loaded circuit with multiple motors starting simultaneously will nuisance-trip.
| Device / Load | Running Watts | Continuous Amps (120V) | Inrush / LRA (Peak Amps) | Load Type |
|---|---|---|---|---|
| Desktop PC + 2 Monitors | 450W | 3.75A | ~6A (Capacitive) | Non-Continuous |
| 1/2 HP Sump Pump | 980W | 8.2A | ~35A (Inductive) | Non-Continuous |
| Space Heater (High) | 1500W | 12.5A | 12.5A (Resistive) | Continuous |
| LED Shop Lights (x6) | 240W | 2.0A | ~4A (Driver spike) | Continuous |
| 12V Fridge (via Inverter) | 60W (Avg) | 0.5A (Avg) | ~15A (Compressor) | Non-Continuous |
If you put the space heater and the sump pump on the same 15A circuit, the running load is 20.7A. The breaker will trip instantly. If you put the PC and the LED lights on a 15A circuit, the total is 5.75A—well under the 12A continuous limit, leaving ample headroom.
What Trips Before the Breaker? Heat, Voltage Drop, and Inrush
A common misconception is that the breaker is the weakest link in the circuit, protecting everything else. In reality, several components can fail or trip before a standard thermal-magnetic breaker does.
1. Voltage Drop and Motor Burnout: If you run a 1/2 HP motor 75 feet away on 14 AWG wire, the resistance of the wire causes voltage drop. According to Department of Energy electrical principles, if voltage drops by 5% (down to 114V), an induction motor must draw more current to produce the same mechanical work. This elevated current generates heat in the motor windings. The motor's internal thermal overload protector will trip, or the windings will melt, while the 15A branch breaker sits perfectly happy because the total current never exceeded 15A.
2. Receptacle Contact Degradation: Standard 15A residential duplex receptacles use friction-fit brass wipers. If you pull 14A continuously through a cheap $1.20 builder-grade receptacle, the contacts heat up, lose their spring tension, and arc. The plastic faceplate will melt and scorch long before the breaker's bimetallic strip bends enough to trip.
3. Inrush Fatigue on GFCI/AFCI: Electronic breakers (GFCI/AFCI) contain microprocessors. Repeated high-inrush loads (like a table saw starting up) can cause voltage sags that brownout the breaker's internal logic board, causing nuisance trips or degrading the sensing coil over time.
Headroom, Future Loads, and Dedicated Circuit Rules
Load planning isn't just about what you plug in today; it's about what you'll plug in next year. NEC 210.23 requires that any single cord-and-plug connected appliance that is fastened in place (like a window AC unit or a large freezer) and rated at 50% or more of the branch circuit rating must be on a dedicated circuit.
For a 20A circuit, 50% is 10A (1,200W). If your new chest freezer draws 11A on startup and 4A running, it legally and practically requires its own 20A dedicated line. Sharing it with a dehumidifier will result in spoiled food when the two compressors start simultaneously and trip the breaker.
The 20% Headroom Rule: When sizing a DC battery bank or an AC subpanel, always add a 20% buffer to your calculated maximum load. If your calculated peak workshop load is 3,500W, size your inverter and upstream wiring for 4,200W. This prevents operating transformers and inverters at their thermal limits, drastically extending their lifespan.
Decision Tree: Sizing Your Next Circuit or Battery Bank
Use this decision path to terminate your load planning with exact part numbers and wire gauges. Do not guess; follow the logic based on your load tally.
| Condition / Scenario | Decision Path | Concrete Pick / Specification |
|---|---|---|
| Continuous load is < 12A (1440W). No large motors. | Standard 15A branch circuit is sufficient. Use 14 AWG minimum, 12 AWG preferred for voltage drop. | Wire: 12/2 NM-B Breaker: Eaton BR115 (15A) |
| Continuous load is 12A - 16A, OR includes 1/2 HP+ motors. | Step to 20A circuit. Mandatory 12 AWG wire. Use commercial-grade receptacles. | Wire: 12 AWG THHN in 1/2" EMT Breaker: Eaton BR120 (20A) Receptacle: Leviton 5362 |
| Single fixed appliance draws > 10A (e.g., 1500W heater, freezer). | Do not share the circuit. Run a dedicated line directly from the panel. | Wire: 12/2 NM-B (Dedicated) Breaker: 20A Single-Pole |
| Off-grid / Backup DC bank needs to supply 1500W AC for 4 hours. | Total AC energy = 6000Wh. Inverter loss (10%) = 6600Wh DC. At 12V, this requires 550Ah. Step up to 24V to halve the Ah and reduce wire size. | Battery: 24V 200Ah LiFePO4 (e.g., Ampere Time 24V 200Ah) Inverter: 3000W 24V Pure Sine |
Final Recommendation for a Standard Workshop Bench: If you are wiring a new workbench for general electronics, soldering, and occasional power tool use, do not rely on a shared 15A bedroom circuit. Pull 12 AWG THHN wires through 1/2-inch EMT conduit, terminate them on a 20A Eaton BR120 breaker, and install two Leviton 5362 commercial receptacles. For your DC testing bus, pair this with a 12V 100Ah LiFePO4 battery (like the Ampere Time 12V 100Ah) and a 20A MPPT charge controller to safely handle 1200W of continuous DC bench loads without voltage sag.
