How Many Watts Is 37.9 Amps at 120V?
A 37.9-amp circuit at 120V delivers 4,548 watts to a resistive AC load at PF 1.0. Real-world AC loads with lower power factor deliver less real power per amp.
At 4,548W, this is equivalent to 4.55 kW. NEC 210.19(A) sizes the conductor and OCP at 125% of any continuous load (equivalently 80% of breaker rating), so the usable continuous capacity on this circuit is about 3,638.4W.
For comparison at the same inputs: 4,548W on DC. These are reference values for contrast; the canonical answer for this page is the one in the hero above.
Use this citation when referencing this page.
Assumes an AC single-phase resistive load at PF 1.0. Typing a commercial L-L voltage (208/400/480V) re-routes the result to three-phase; 277V stays on single-phase because it's the L-N lighting leg of a 480Y/277V wye; 12/24V re-routes to DC.
Formulas
DC: Amps to Watts
P(W) = I(A) × V(V)
AC Single Phase (PF = 0.85)
P(W) = PF × I(A) × V(V)
What Can You Run on 37.9A at 120V?
Appliances This Circuit Supports
A 37.9A circuit at 120V delivers 4,548W to a resistive AC load at PF 1.0. NEC 210.19(A) sizes the conductor and OCP at 125% of any continuous load (equivalently 80% of the breaker rating, about 3,638.4W here), so these appliances fit within the continuous-load allowance:
| Appliance | Watts | % of Circuit | Fits Continuous? |
|---|---|---|---|
| Air Conditioner (window) | 3,500W | 76.96% | Yes |
| Hair Dryer | 1,800W | 39.58% | Yes |
| Dishwasher | 1,800W | 39.58% | Yes |
| Space Heater | 1,500W | 32.98% | Yes |
| Toaster Oven | 1,500W | 32.98% | Yes |
| Electric Kettle | 1,500W | 32.98% | Yes |
Monthly Running Cost
As a rough reference, running 4,548W for 8 hours daily at the US residential average of $0.17/kWh works out to about $185.56 per month. Electricity rates change every tariff cycle and vary sharply by region, time of day, and utility; treat the number here as a ballpark and check your actual bill or the energy-cost calculator with your own rate for a real figure.
Standard Breaker Sizes Near 37.9A
This section is reference framing, not an install recommendation. NEC 240.6(A) lists the standard breaker amp ratings, and under the NEC 210.19(A) 125% continuous-load rule (equivalently 80% of breaker rating) a 37.9A non-continuous load maps to the 40A standard size at or above the load, and a continuous 37.9A load maps to 50A once the 125% factor is applied. Breaker ratings are expressed in amps, not watts: the real power associated with a given breaker size depends on the circuit type and the load's power factor, which is why the AC Conversion Detail section shows multiple wattage interpretations. None of these numbers is a breaker selection for a real install. Actual breaker and conductor selection depends on the equipment nameplate FLA, continuous-load treatment, conductor ampacity and termination temperature rating, bundling and ambient derates, any NEC 430/440 motor or HVAC provisions, and local code, and should be made by a licensed electrician against the specific install conditions.
AC Conversion Detail
On DC, 37.9A at 120V delivers a full 4,548W. On AC single-phase with a power factor of 0.85, the same current only delivers 3,865.8W of real power because the remaining capacity goes to reactive current.
| Circuit Type | Formula | Result |
|---|---|---|
| DC | 37.9 × 120 | 4,548 W |
| AC Single Phase (PF 0.85) | 0.85 × 37.9 × 120 | 3,865.8 W |
Power Output by Load Type
The same 37.9A circuit at 120V delivers different real power depending on the load, computed on the same single-phase basis the rest of the page uses:
| Load Type | PF | Real Power (37.9A at 120V, single-phase) |
|---|---|---|
| Resistive (heaters, incandescent) | 1 | 4,548 W |
| Fluorescent lamps | 0.95 | 4,320.6 W |
| LED lighting | 0.9 | 4,093.2 W |
| Synchronous motors | 0.9 | 4,093.2 W |
| Typical mixed loads | 0.85 | 3,865.8 W |
| Induction motors (full load) | 0.8 | 3,638.4 W |
| Computers (without PFC) | 0.65 | 2,956.2 W |
| Induction motors (no load) | 0.35 | 1,591.8 W |