Two circuits, one appliance. A 1,500 W heater plugged into an ordinary 120 V outlet pulls 12.5 A. Move the same heating element to a 240 V circuit and it pulls 6.25 A. Same watts, same heat in the room, half the current.
That halving is why dryers, ranges and water heaters in North American homes sit on 240 V while lamps and toasters stay on 120 V. It decides the breaker, the wire gauge, how warm the cable gets and how much copper an install swallows. It decides almost nothing about the bill.
The same 1,500 watts, side by side
Everything below is calculated, not quoted: the current and the wiring come from the amps calculator, the power and the running cost from the electrical power calculator, and the field in the insulation from the parallel plate capacitor calculator.
| 1,500 W heater | On 120 V | On 240 V |
|---|---|---|
| Current the load draws | 12.5 A | 6.25 A |
| Resistance of the element | 9.6 Ω | 38.4 Ω |
| Smallest breaker, short use | 15 A | 15 A |
| Smallest breaker, 3 hours or more | 20 A | 15 A |
| Copper wire, continuous use | 12 AWG | 14 AWG |
| Voltage lost in 0.1 Ω of wiring | 1.25 V | 625 mV |
| Heat wasted in that wiring | 15.625 W | 3.9063 W |
| Energy at 8 hours a day | 12 kWh | 12 kWh |
| Cost per year at $0.17/kWh | $744.60 | $744.60 |
| Field across 1 mil of insulation | 4,724.41 kV/m | 9,448.82 kV/m |
Two rows in that table are identical on purpose. Energy is power multiplied by time, and the power is the same 1.5 kW either way, so eight hours a day is 12 kWh daily, 365 kWh a month and 4,380 kWh a year on both circuits. At $0.17 per kWh that is $2.04 a day, $62.05 a month and $744.60 a year. Nobody saves money by rewiring a heater to 240 V. They save copper.
Where the halved current actually shows up
Heat in a conductor follows I² × R, so halving the current quarters the waste. Take a tenth of an ohm of wiring, which is what a long run of thin cable adds up to. At 12.5 A it drops 1.25 V and turns 15.625 W into heat inside the walls. At 6.25 A the same cable drops 625 mV and wastes 3.9063 W. The heater does not care. The cable does.
The breaker follows the same halving, with one twist. A load that runs three hours or more counts as continuous, and the code sizes the breaker at 125 % of it. Our heater at 120 V goes from 12.5 A to 15.625 A once it runs all evening, which pushes it past the 15 A breaker onto a 20 A one with 12 AWG wire. At 240 V the continuous figure is 7.8125 A, so a 15 A breaker and 14 AWG still cover it.
Push the wattage up and the gap gets expensive. A 4,500 W water heater takes 18.75 A at 240 V, which is a 20 A breaker on 12 AWG. The same appliance on 120 V would want 37.5 A, a 40 A breaker and 8 AWG copper, four gauge steps thicker. That is the real reason big appliances are not offered in a 120 V version.
Watts to amps, both ways
The pattern repeats for every load. On a 120 V circuit, 1,800 W is exactly 15 A, which is why so many space heaters stop at 1,500 W: it leaves headroom on a 15 A circuit.
| Load on 120 V | Current | Breaker, short use | Copper wire |
|---|---|---|---|
| 500 W | 4.1667 A | 15 A | 14 AWG |
| 800 W | 6.6667 A | 15 A | 14 AWG |
| 1,000 W | 8.3333 A | 15 A | 14 AWG |
| 1,200 W | 10 A | 15 A | 14 AWG |
| 1,500 W | 12.5 A | 15 A | 14 AWG |
| 1,800 W | 15 A | 15 A | 14 AWG |
| 2,000 W | 16.667 A | 20 A | 12 AWG |
| 2,400 W | 20 A | 20 A | 12 AWG |
| 3,000 W | 25 A | 25 A | 10 AWG |
On 240 V the same table starts where the first one ends, because the voltage does half the work.
| Load on 240 V | Current | Breaker, short use | Copper wire |
|---|---|---|---|
| 3,000 W | 12.5 A | 15 A | 14 AWG |
| 4,000 W | 16.667 A | 20 A | 12 AWG |
| 4,500 W | 18.75 A | 20 A | 12 AWG |
| 5,000 W | 20.833 A | 25 A | 10 AWG |
| 7,500 W | 31.25 A | 35 A | 8 AWG |
| 8,000 W | 33.333 A | 35 A | 8 AWG |
| 10,000 W | 41.667 A | 45 A | 6 AWG |
Read it backwards and you get the capacity question instead: what can a given circuit carry? At 120 V, 15 A is 1.8 kW and 20 A is 2.4 kW. At 240 V, 30 A is 7.2 kW and 50 A is 12 kW. Those are load figures, not permission to load a breaker to the limit: a continuous load should stay at or below 80 % of the rating.
When 120 volts wins
Anything portable. Every room has 120 V outlets and none of them needs a plan. A 240 V circuit exists only where somebody ran one.
Small loads. Below 1,800 W the current stays at or under 15 A and the wire stays 14 AWG, so the thicker-cable argument never gets started. A 800 W microwave draws 6.6667 A; nothing about that is improved by doubling the voltage.
Existing motors and electronics. Voltage is built into a motor winding and into a power supply. Feeding a 120 V motor from 240 V does not make it efficient, it makes it scrap.
When 240 volts wins
High power in one place. A 10,000 W load is 41.667 A at 240 V, already a 45 A breaker on 6 AWG. At 120 V the same load would double that and leave the range of ordinary house wiring.
Long runs. Voltage lost in the cable scales with the current, so at half the amps the same run loses half the volts and a quarter of the power. A detached garage is the classic case.
Hours-long loads. The 125 % rule bites hardest where the current is already high. Our heater is the example: one wire gauge and one breaker size apart, purely because of how long it runs.
What the voltage does not change
The energy, as the table showed. The heat delivered into the room, which is the point of a heater. And the arithmetic for anything that is not a plain resistor: a 1,200 W motor with a power factor of 0.8 draws 12.5 A on 120 V, the same current as our 1,500 W heater, because the wiring sees 1.5 kVA of apparent power either way.
Three-phase takes the idea one step further. The same 10 kW is 27.757 A per line at 208 V and 12.028 A at 480 V. Higher voltage, thinner conductors, same watts. It is the reason large buildings distribute at 480 V and step down near the load.
The bill higher voltage does send
Doubling the voltage doubles the electric field the insulation has to hold back. A capacitor makes that visible, because its field is simply volts divided by the gap. One square inch of plate with 1 mil of polyimide between the plates sits at 4,724.41 kV/m on 120 V and 9,448.82 kV/m on 240 V, the same capacitance of 764.65 pF in both cases.
Restoring the old field means doubling the thickness, and thickness is the enemy of capacitance.
| Polyimide gap at 240 V | Capacitance | Field in the gap | Charge stored |
|---|---|---|---|
| 0.5 mil (12.7 µm) | 1.5293 nF | 18,897.64 kV/m | 367.03 nC |
| 1 mil (25.4 µm) | 764.65 pF | 9,448.82 kV/m | 183.52 nC |
| 2 mil (50.8 µm) | 382.32 pF | 4,724.41 kV/m | 91.758 nC |
Halve the gap and the capacitance doubles to 1.5293 nF, but the field doubles too, to 18,897.64 kV/m. Double the gap and the field returns to the 120 V level, at 382.32 pF. Capacitor datasheets sell that compromise as a voltage rating, and it is the same compromise the wiring in a wall makes: higher voltage, thinner conductors, thicker insulation.
The verdict, which is not universal
For a load that is small, portable or short-lived, 120 V is the practical answer and the arithmetic barely matters. For anything above roughly 1,800 W that runs for hours in a fixed spot, 240 V halves the current and with it the breaker, the wire and the waste in the cable. What it never does is lower the bill.
| Your situation | Better circuit | Why |
|---|---|---|
| A portable appliance you move between rooms | 120 V | Every outlet in the house fits it, and under 1,800 W the wire stays 14 AWG anyway |
| A 1,500 W heater running all evening | 240 V | The continuous load drops from 15.625 A to 7.8125 A, so 14 AWG and a 15 A breaker are enough |
| A 4,500 W water heater | 240 V | 18.75 A instead of 37.5 A, 12 AWG instead of 8 AWG |
| Anything above 1,800 W you plan to run for hours | 240 V | At 120 V it needs a dedicated circuit with thicker wire |
| An old appliance with a 120 V motor | 120 V | The voltage is built into the motor; a converter costs more than the savings |
One caveat that applies to every row: breaker and wire figures here are a starting point from the 60 °C copper column, with no allowance for long runs, hot spaces, bundled conductors or local amendments. A licensed electrician signs off on the circuit, not a calculator.
Tools discussed in this article
- Amps Calculator: watts to amps on DC, single-phase or three-phase AC, or amps from volts and ohms, with the smallest standard breaker and copper wire gauge for short and continuous use
- Electrical Power Calculator: watts from volts and amps, amps and ohms or volts and ohms, with power factor, apparent power in VA and optional kWh and cost per day, month and year
- Parallel Plate Capacitor Calculator: capacitance from plate area, gap and dielectric, stacked plates, charge, stored energy and the field in the gap, with eleven dielectrics compared
Ready-made calculations
The same questions with the numbers already filled in. Watts to amps on a 120 V circuit: 500 W, 800 W, 1,000 W, 1,200 W, 1,500 W, 1,800 W, 2,000 W, 2,400 W, 3,000 W.
Watts to amps on a 240 V circuit: 3,000 W, 4,000 W, 4,500 W, 5,000 W, 7,500 W, 8,000 W, 10,000 W.
Amps to watts on 120 V: 2 A is 240 W, 5 A is 600 W, 10 A is 1.2 kW, 15 A is 1.8 kW, 20 A is 2.4 kW, 25 A is 3 kW, 30 A is 3.6 kW.
Amps to watts on 240 V: 15 A is 3.6 kW, 20 A is 4.8 kW, 25 A is 6 kW, 30 A is 7.2 kW, 40 A is 9.6 kW, 50 A is 12 kW, 60 A is 14.4 kW.
More physics tools
The rest of the physics shelf, in the order it was built:
- Momentum Calculator, Force Calculator and Kinetic Energy Calculator
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