How Many Watts Is 18 Amps at 277V?
A 18-amp circuit at 277V delivers 4,986 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 4,986W, this is equivalent to 4.99 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 3,988.8W.
For comparison at the same inputs: 4,986W 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 Uses 18A at 277V?
Monthly Running Cost
As a rough reference, running 4,986W for 8 hours daily at the US residential average of $0.17/kWh works out to about $203.43 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 18A
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 18A non-continuous load maps to the 20A standard size at or above the load, and a continuous 18A load maps to 25A 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, 18A at 277V delivers a full 4,986W. On AC single-phase with a power factor of 0.85, the same current only delivers 4,238.1W of real power because the remaining capacity goes to reactive current.
| Circuit Type | Formula | Result |
|---|---|---|
| DC | 18 × 277 | 4,986 W |
| AC Single Phase (PF 0.85) | 0.85 × 18 × 277 | 4,238.1 W |
Power Output by Load Type
The same 18A circuit at 277V 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 (18A at 277V, single-phase) |
|---|---|---|
| Resistive (heaters, incandescent) | 1 | 4,986 W |
| Fluorescent lamps | 0.95 | 4,736.7 W |
| LED lighting | 0.9 | 4,487.4 W |
| Synchronous motors | 0.9 | 4,487.4 W |
| Typical mixed loads | 0.85 | 4,238.1 W |
| Induction motors (full load) | 0.8 | 3,988.8 W |
| Computers (without PFC) | 0.65 | 3,240.9 W |
| Induction motors (no load) | 0.35 | 1,745.1 W |
Other Amperages at 277V
| Amps | DC Watts | AC Watts (PF 0.85) |
|---|---|---|
| 1A | 277 W | 235.45 W |
| 2A | 554 W | 470.9 W |
| 3A | 831 W | 706.35 W |
| 5A | 1,385 W | 1,177.25 W |
| 7.5A | 2,077.5 W | 1,765.88 W |
| 10A | 2,770 W | 2,354.5 W |
| 12A | 3,324 W | 2,825.4 W |
| 15A | 4,155 W | 3,531.75 W |
| 20A | 5,540 W | 4,709 W |
| 25A | 6,925 W | 5,886.25 W |
| 30A | 8,310 W | 7,063.5 W |
| 35A | 9,695 W | 8,240.75 W |
| 40A | 11,080 W | 9,418 W |
| 45A | 12,465 W | 10,595.25 W |
| 50A | 13,850 W | 11,772.5 W |