A standard 30-amp breaker can handle a maximum of 3,600 watts on a 120V circuit and 7,200 watts on a 240V circuit. However, under National Electrical Code (NEC) guidelines for continuous loads (equipment running for 3 hours or more), you must derate the breaker to 80% of its capacity. This makes the safe, legal continuous limit 2,880 watts at 120V and 5,760 watts at 240V. The exact wattage depends entirely on the circuit voltage, the number of phases, and the power factor of the connected load.
The Core Formula and the Assumptions That Fix It
To convert amps to watts, we use the fundamental DC and single-phase AC power formula:
Substituted for a 120V circuit: P = 120V × 30A = 3,600W
Substituted for a 240V circuit: P = 240V × 30A = 7,200W
This basic calculation relies on three critical assumptions that fix the final answer:
- Voltage: Are you measuring a standard US 120V branch circuit, a 240V split-phase appliance circuit, or a European 230V single-phase supply?
- Phase: Is this single-phase or three-phase power? Three-phase math introduces a square root multiplier.
- Power Factor (PF): Is the load purely resistive (like a space heater, PF = 1.0) or inductive (like an air compressor motor, PF = 0.8)?
When is this conversion meaningless? If you are sizing a breaker for an inductive load (motors, transformers, heavy ballasts) and the Power Factor is unknown, calculating watts is practically useless for breaker sizing. Inductive loads draw "apparent power" measured in Volt-Amps (VA), which is higher than the "real power" measured in Watts. A 30-amp breaker will trip on the total current (Amps), regardless of how many of those amps are doing real work (Watts) versus just sustaining a magnetic field (VARs). Always size breakers for inductive loads using the nameplate FLA (Full Load Amps) and VA ratings, not just the wattage.
Neighboring Amperage Values (±20% Range)
When designing a panel or evaluating load capacity, it helps to see how a 30-amp breaker compares to neighboring values within a ±20% range (24A to 36A). This illustrates why stepping up to the next standard breaker size (like 35A or 40A) dramatically changes your thermal limits.
| Current (Amps) | Max Watts @ 120V | 80% Continuous @ 120V | Max Watts @ 240V | 80% Continuous @ 240V |
|---|---|---|---|---|
| 24A (-20%) | 2,880W | 2,304W | 5,760W | 4,608W |
| 30A (Base) | 3,600W | 2,880W | 7,200W | 5,760W |
| 36A (+20%) | 4,320W | 3,456W | 8,640W | 6,912W |
Note: 24A and 36A are not standard residential breaker sizes in the US (standard sizes per NEC 240.6 are 25A, 30A, 35A, 40A), but they represent the exact mathematical ±20% boundaries of a 30A trip curve.
How the Answer Shifts: 120V vs 230V vs 3-Phase
The wattage a 30-amp breaker can handle shifts dramatically depending on the regional grid standard and the phase configuration of the panel.
120V Single-Phase (US Standard & RV)
In North America, a standard 120V single-pole 30-amp breaker is most commonly found in RV parks (the TT-30 receptacle) or for specific heavy-duty 120V shop equipment. Here, the absolute maximum is 3,600W, but you must cap continuous draws at 2,880W. If you are wiring this, NEC Table 310.16 dictates you must use a minimum of 10 AWG copper wire (THHN in conduit or NM-B in dry walls).
230V / 240V Single-Phase
For US residential split-phase (240V nominal), a 30-amp double-pole breaker handles 7,200W (5,760W continuous). This is the standard size for electric dryers, small welders, and RV 50-amp adapters split into two 30-amp legs. In Europe and the UK, the nominal single-phase voltage is 230V. Therefore, a 30A MCB (Miniature Circuit Breaker) in a UK consumer unit handles a maximum of 6,900 watts (230V × 30A), with a continuous safe limit of roughly 5,520W.
Three-Phase Power (Industrial & Commercial)
When dealing with 3-phase power, the formula changes to account for the 120-degree phase offset between the legs. The formula becomes:
Power = √3 × Voltage × Current × Power Factor
If you have a 30-amp breaker on a 208V 3-phase wye system (common in US commercial buildings) powering a resistive heater (PF = 1.0), the math is:
1.732 × 208V × 30A × 1.0 = 10,807 watts.
Apply the 80% continuous rule, and the safe limit is 8,645 watts. As you can see, three-phase power delivers significantly more wattage through the exact same 30-amp breaker footprint.
Frequently Asked Questions
Can I plug a 3,000-watt heater into a 30-amp 120V breaker?
No, not safely or legally. A space heater is considered a continuous load because it is expected to run for 3 hours or more. Under the NEC 80% rule, a 30-amp 120V breaker is limited to 2,880 continuous watts. A 3,000-watt heater will draw 25 amps continuously (3000W / 120V = 25A). Because 25A exceeds the 24A continuous limit (80% of 30A), the breaker will eventually overheat and trip. You must upgrade to a 35-amp or 40-amp breaker and correspondingly larger wire (8 AWG) to run a 3,000W continuous 120V load.
What size wire do I need for a 30-amp breaker?
For a standard 30-amp breaker, you must use a minimum of 10 AWG copper wire. According to NFPA 70 (NEC) Table 310.16, 10 AWG copper with 60°C insulation (like standard NM-B / Romex) is rated for exactly 30 amps. If you are using THHN wire in a conduit and terminating at 75°C rated lugs, 10 AWG is rated for 35 amps, but the breaker still caps the circuit protection at 30 amps. Never use 12 AWG or 14 AWG wire on a 30-amp breaker; the wire will melt before the breaker trips, creating a severe fire hazard.
How many watts can a 30-amp RV breaker handle?
A standard 30-amp RV service (the TT-30 plug) is a 120V single-phase circuit. Therefore, it can handle a maximum of 3,600 watts, and a continuous safe load of 2,880 watts. This is a common point of confusion because RVers often compare it to a 50-amp RV service. A 50-amp RV plug is actually 120/240V split-phase with two 50-amp legs, yielding 12,000 watts total. A 30-amp RV plug only has one 120V hot leg, strictly limiting you to 3,600 watts total across all appliances in the camper.
Does a 30-amp double-pole breaker give me 60 amps of power?
No. A 30-amp double-pole breaker provides 30 amps per leg, not 60 amps combined. It connects to two adjacent hot bus bars in your panel to deliver 240V to an appliance (like a dryer or water heater). The total wattage capacity is 7,200W (240V × 30A). If the appliance uses 120V internally for a control board or motor, it draws that 120V from just one of the 30-amp legs. The breaker will trip if either individual leg exceeds 30 amps. For a deeper breakdown of multi-wire branch circuits and double-pole sizing, Electrical Technology's breaker sizing guide offers excellent single vs. double-pole schematics.
