800 Watts at 120 Volts: Amps, Breaker and Wire Size

Amps, breakers and wire gauge on one page

How many amps does a 1,500 W heater draw on a 120 V outlet? 12.5 A. The calculator gets current from watts and volts, from volts and ohms, or from watts and ohms, and on AC it takes the power factor and the three-phase √3 into account. Every answer comes with the smallest standard breaker and the smallest copper wire that cover it, once for an ordinary load and once for a load that runs three hours or more.

12.5 A

1,500 W at 120 V

14.151 A

10 kW, 480 V three-phase, power factor 0.85

16 A

the most a 20 A breaker should carry for three hours or more

Filling in the calculator

What you know - watts and volts (DC or a resistive load), watts, volts and power factor on single-phase AC, the same on three-phase AC, volts and ohms, or watts and ohms.
Power and its unit - the nameplate rating in W or kW. A 4.8 kW water heater can go in as 4.8 with kW selected.
Voltage - 120 V or 240 V in a US home, 12 V in a car. On three-phase use the line-to-line voltage, such as 208 V or 480 V.
Resistance - ohms, for the two modes that use it.
Power factor - AC modes only, between 0 and 1; 1 for heaters, the nameplate figure for motors.
Read the result - the current and its formula, the other quantities, and the breaker and wire table for both load types.

What sets the current in a circuit

An ampere is a flow of one coulomb of charge per second. The supply sets the voltage, the load decides how much current it takes at that voltage, and the power is the product of the two. Turn P = V × I around and you get the formula most people want: I = P ÷ V. The same 1,500 W takes 12.5 A at 120 V and only 6.25 A at 240 V, which is why large appliances in North American homes sit on 240 V circuits.

Ohm's law gives the other two routes. With voltage and resistance known, I = V ÷ R. With power and resistance known, I = √(P ÷ R), because P = I² × R. A 100 W load of 4 Ω therefore carries 5 A and sees 20 V.

Alternating current adds two corrections. The power factor is the ratio of real power to apparent power, so a motor with a power factor of 0.8 draws 1 ÷ 0.8 = 1.25 times the current its wattage alone suggests. On a balanced three-phase supply the power is split across three lines and the voltage between two lines is √3 times the voltage of one phase, which gives I = P ÷ (√3 × V × PF).

The breaker side follows the National Electrical Code. Standard breaker ratings are listed in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125 A and upward. A load that runs 3 hours or more counts as continuous, and NEC 210.20(A) requires the breaker to be rated for at least 125% of it. The wire then has to carry the breaker rating, with the small-conductor limits of NEC 240.4(D) capping 14, 12 and 10 AWG copper at 15, 20 and 30 A.

Watts to amps at five common supplies

Current for purely resistive loads (power factor 1). The 12 V column is a car or a battery bank, the next two are ordinary US circuits, and the last two are three-phase supplies with line-to-line voltages of 208 V and 480 V. Values are rounded to two decimals.

Load	12 V DC	120 V	240 V	208 V three-phase	480 V three-phase
100 W	8.33 A	0.83 A	0.42 A	0.28 A	0.12 A
500 W	41.67 A	4.17 A	2.08 A	1.39 A	0.6 A
1,000 W	83.33 A	8.33 A	4.17 A	2.78 A	1.2 A
1,500 W	125 A	12.5 A	6.25 A	4.16 A	1.8 A
3,000 W	250 A	25 A	12.5 A	8.33 A	3.61 A
5,000 W	416.67 A	41.67 A	20.83 A	13.88 A	6.01 A
10,000 W	833.33 A	83.33 A	41.67 A	27.76 A	12.03 A

Read across a row and the pattern is plain: doubling the voltage halves the current, and three-phase at 480 V carries 10,000 W on 12.03 A per line where a 240 V single-phase circuit needs 41.67 A.

Copper wire gauges and what they may carry

Ampacity is the current a conductor can carry continuously without exceeding its insulation temperature. The figures below are for copper from NEC Table 310.16: no more than three current-carrying conductors in a raceway or cable, 30 °C (86 °F) ambient. Which column applies depends on the insulation and on the temperature rating of the terminals at both ends, and the calculator uses the 60 °C column because it is the conservative one. Diameters and areas are from the American wire gauge definition.

Gauge	Diameter	Area	60 °C	75 °C	90 °C	Breaker cap, 240.4(D)
14 AWG	0.0641 in	2.08 mm²	15 A	20 A	25 A	15 A
12 AWG	0.0808 in	3.31 mm²	20 A	25 A	30 A	20 A
10 AWG	0.1019 in	5.26 mm²	30 A	35 A	40 A	30 A
8 AWG	0.1285 in	8.37 mm²	40 A	50 A	55 A	-
6 AWG	0.1620 in	13.3 mm²	55 A	65 A	75 A	-
4 AWG	0.2043 in	21.2 mm²	70 A	85 A	95 A	-
3 AWG	0.2294 in	26.7 mm²	85 A	100 A	115 A	-
2 AWG	0.2576 in	33.6 mm²	95 A	115 A	130 A	-
1 AWG	0.2893 in	42.4 mm²	110 A	130 A	145 A	-
1/0 AWG	0.3249 in	53.5 mm²	125 A	150 A	170 A	-

The last column is why a 14 AWG circuit stays on a 15 A breaker even though its 90 °C ampacity reads 25 A. Small conductors have little thermal mass, so the code limits their overcurrent protection regardless of the ampacity table.

Six loads and the rows they produce

Each line below is a calculator run. The first figure is the current, the rest is what the breaker and wire table prints for it.

A 1,500 W heater on 120 V
12.5 A - 15 A breaker and 14 AWG if it cycles; running all evening it counts as continuous, 15.625 A, so 20 A and 12 AWG

A 4.8 kW water heater on 240 V
20 A - 20 A and 12 AWG for a short draw; as a continuous load 25 A, which calls for a 25 A breaker and 10 AWG

A 9.6 kW charger on 240 V
40 A - 40 A and 8 AWG on paper; charging for hours is continuous, 50 A, so a 50 A breaker and 6 AWG

A 60 W cooler on a 12 V socket
5 A - far below the smallest standard breaker of 15 A; on low-voltage DC the cable length matters more than the breaker

A 1,200 W motor at power factor 0.8 on 120 V
12.5 A - the same current as the 1,500 W heater, because 1,200 ÷ 0.8 = 1,500 VA

An 11 kW three-phase load on 208 V
30.533 A - per line at power factor 1; a 35 A breaker and 8 AWG, or 40 A and 8 AWG if continuous (38.166 A)

Relationships behind the numbers

Voltage and current trade places. For a fixed wattage, current falls in step with voltage. A 1,500 W load needs 125 A at 12 V, 12.5 A at 120 V and 6.25 A at 240 V, which is why a car inverter running a kitchen appliance needs cables far thicker than the appliance cord.

Heat in the wire grows with the square of the current. The loss in a conductor is I² × R, so doubling the current through the same wire gives four times the heat. That is the physical reason behind thicker gauges for higher currents, and it is also why a slightly undersized wire runs disproportionately warm.

A low power factor costs current, not watts. A 1,200 W motor at a power factor of 0.8 takes 12.5 A at 120 V; the same 1,200 W as a heater takes 10 A. The wiring and the breaker see the larger figure.

The 125% rule is the 80% rule seen from the other side. Sizing the breaker at 1.25 times a continuous load is the same as loading it to at most 1 ÷ 1.25 = 80% of its rating: 16 A on a 20 A breaker, 24 A on a 30 A breaker, 40 A on a 50 A breaker.

How to read the breaker and wire rows

The result prints two rows because the same appliance can land on either side of the continuous-load line. Use the first row when the load switches on and off, like a hair dryer or a microwave. Use the second when it runs for three hours or more at a stretch, like a space heater on a cold night, a water heater recovering, or an EV charger.

Situation	Interpretation	Example
Current at or below 80% of the breaker	Fits either load type	12.5 A on a 20 A breaker
Between 80% and 100% of the breaker	Only for loads under 3 hours	16.5 A on a 20 A breaker
Above the breaker rating	Needs a larger breaker and wire	21 A on a 20 A breaker
Above 125 A	Outside this table, service or feeder sizing	130 A continuous

These rows are a planning aid, not a design. Motor circuits follow NEC Article 430, long runs need a voltage-drop check, more than three current-carrying conductors or a hot attic reduce ampacity, and local codes amend the NEC. A licensed electrician should confirm any circuit before it is wired.

Amps questions, with the numbers worked

How many amps is 1,500 watts?

At 120 V, 1,500 ÷ 120 = 12.5 A. At 240 V it is 6.25 A, and from a 12 V battery through an ideal inverter it would be 125 A.

How do I convert watts to amps on three-phase?

Divide the watts by √3, the line-to-line voltage and the power factor. 10 kW at 480 V and a power factor of 0.85: 10,000 ÷ (1.7321 × 480 × 0.85) = 14.151 A per line.

What size breaker does a 12.5 A load need?

If it runs under three hours at a time, the smallest standard rating that covers it is 15 A on 14 AWG copper. If it runs three hours or more, 12.5 × 1.25 = 15.625 A, which moves it to a 20 A breaker and 12 AWG.

Why does exactly 16 A land on a 20 A breaker?

Because the next standard rating at or above 16 A is 20 A; there is no 16 A rating in NEC 240.6(A). Treated as continuous, 16 × 1.25 = 20 A still fits the 20 A breaker exactly.

Can I put 14 AWG wire on a 20 A breaker?

Not under NEC 240.4(D), which caps 14 AWG copper at 15 A overcurrent protection. The 90 °C ampacity of 25 A does not change that limit.

How many amps is a 12 V, 100 W device?

100 ÷ 12 = 8.3333 A. Low-voltage DC needs a lot of current for modest power, so cable length and voltage drop matter more than they do at 120 V.

Does the power factor matter for a heater?

No. A resistive heater has a power factor of 1, so the single-phase AC mode and the plain watts and volts mode give the same current. It matters for motors, compressors and many power supplies.

Related tools

Electrical Power Calculator

Watts, VA and kWh from volts and amps, with single- and three-phase AC - Open the calculator

Parallel Plate Capacitor Calculator

Capacitance of two or more plates from their area, gap and dielectric, in pF, nF or µF - Open the calculator

Ohm's Law Calculator

Volts, amps or ohms from the other two, and the series resistor for an LED - Open the calculator

Electrical Cable Calculator

Cable length and cross-section for an installation run - Open the calculator

Electricity Usage Calculator

Monthly kWh and cost for household appliances - Open the calculator

Thermal Conductivity Calculator

Heat flow through a material from its conductivity, area and thickness - Open the calculator

Mechanical Power Calculator

Watts from work and time, or from force and speed - Open the calculator

How many amps is 800 watts at 120 volts?

Parameters