100 Amps to Watts

One multiplication turns 100 amps into watts

Power is voltage times current, written P = V x I, and the 100 amp rating only becomes a wattage once a supply voltage is named. Multiply 100 by 12 volts and you get 1,200 watts; multiply 100 by 240 volts and you get 24,000 watts. The same current, twenty times the power, because a 240 volt circuit pushes each coulomb of charge twenty times harder. That is why a 100 amp car battery circuit and a 100 amp house panel have almost nothing in common besides the number on the breaker.

These are single phase numbers, one hot leg against a neutral or another leg. Three phase systems multiply by a factor of 1.732, which the later section covers. Nominal voltages are labels rather than exact readings: a 240 volt service often measures between 228 and 252 volts at the meter, so a real 100 amp load lands within a few percent of the table value.

100 amps at 240 volts: the service panel view

A 100 amp household service is 120/240 volt split phase, so the panel can deliver 100 amps on each of two 120 volt legs, or 100 amps balanced across 240 volts. In watts: 100 x 240 = 24,000 watts, which is the headline number. Continuous loads, the ones that run for three hours or more, are limited to 80 percent of that under the NEC rules covered below, so the working figure for a panel that runs heaters or chargers all day is 19,200 watts. A 125 amp service holds 30,000 watts, and 200 amps holds 48,000 watts.

Adding up nameplate watts shows why services get upgraded. Take a 48 amp Level 2 EV charger at 11,520 watts, an electric dryer at 5,760 watts and a 40 amp range at 9,600 watts. The total is 26,880 watts, above the 24,000 watt ceiling of a 100 amp panel even before lights, refrigerator or a heat pump enter the picture. NEC Article 220 allows demand factors, since a dryer and a range rarely pull full power together, so a formal load calculation, not the raw sum, decides whether 100 amps holds. When the calculated load climbs past the rating, the next standard service up is 125 amps, good for 30,000 watts at 240 volts.

Worked example: a 24,000 watt tankless water heater

Worked example, with every step shown. Input: a whole house tankless water heater rated 24,000 watts at 240 volts, running for as long as a shower lasts, so the code treats it as a continuous load. Step one, current: 24,000 / 240 = 100 amps. Step two, the 125 percent rule of NEC 210.20(A) for a load that runs 3 hours or more: 100 x 1.25 = 125 amps. Step three, breaker: 125 amps is a standard size in NEC 240.6, so the heater takes a 125 amp, two pole breaker. Step four, wire: the conductors must carry 125 amps, and Table 310.16 lists 1 AWG copper at 130 amps and 2/0 aluminum at 135 amps in the 75 degree C column, so 1 AWG copper or 2/0 aluminum it is.

Result: the heater needs its own 125 amp feeder, which is more than many 100 amp panels can spare. That is why tankless installs so often trigger a service upgrade, and why gas or a smaller 18,000 watt unit (75 amps, a 100 amp breaker after the 125 percent factor) fits an existing panel more easily. The same four steps work for any big 240 volt appliance: divide watts by 240, multiply by 1.25 if it runs for hours, then round up to a standard breaker and a matching wire size.

Three phase: 100 amps at 208 and 480 volts

Three phase services add a factor of 1.732 (the square root of 3) because the three legs are staggered 120 degrees apart. The formula becomes W = 1.732 x V x A x PF. At 208 volts and 100 amps with a power factor of 1.0, a small commercial panel delivers 1.732 x 208 x 100 = 36,027 watts. At 480 volts the same 100 amps delivers 1.732 x 480 x 100 = 83,138 watts, about 83.1 kW, which is why a 100 amp 480 volt feed runs a machine shop while a 100 amp house panel struggles with a tankless heater.

Power factor still applies on real loads. Motors sit between 0.8 and 0.9, so that 480 volt panel feeding motors at a 0.85 power factor delivers 1.732 x 480 x 100 x 0.85 = 70,668 watts of real power while the meter still sees 83,138 volt amps. Utilities bill many commercial customers on demand in kW or kVA, which is why power factor correction capacitors show up next to large motor loads, and why the watts number and the volt amps number are both worth writing down.

Wire and breaker sizes behind 100 amps

Two tables do most of the work. For a dwelling service or a feeder that carries the entire load of a one family home, NEC 310.12 permits conductors rated at 83 percent of the service, and its table lists 4 AWG copper or 2 AWG aluminum for a 100 amp service and 2 AWG copper or 1/0 aluminum for 125 amps. For a plain 100 amp feeder, a subpanel in a garage or a shop, the conductors must carry the full 100 amps, and Table 310.16 in the 75 degree C column lists 3 AWG copper (100 amps) and 1 AWG aluminum (100 amps) as the smallest sizes that qualify.

Breakers follow NEC 240.6(A), which sets the standard ratings: 100, 110, 125, 150, 175, 200, 225 and up. A 100 amp continuous load sized at 125 amps lands exactly on the 125 amp standard breaker, and its conductors move up to 1 AWG copper or 2/0 aluminum. The NFPA 70 National Electrical Code is the source for all three tables, and local amendments can change what your inspector accepts. Workplace wiring falls under OSHA electrical standards, which reference the same conductor rules for the sites they cover, and mis sized conductors there are a citation as well as a fire risk.

Motors: why starting current dwarfs 100 amps

Motors break the watts equals volts times amps picture twice. First, NEC 430.248 lists the full load current for single phase motors, and a 10 HP motor at 230 volts reads 50 amps, or 11,500 volt amps, half of a 100 amp panel by itself. Second, a motor at the moment of starting behaves like a locked transformer: the rotors are stationary, there is no back EMF, and the inrush runs about six times full load current, roughly 300 amps for that 10 HP machine for the second or two it takes to spin up.

The code handles this with a split set of rules. Conductors follow NEC 430.22 at 125 percent of the table current: 50 x 1.25 = 62.5 amps, which 6 AWG copper (65 amps at 75 C) carries. The branch circuit short circuit device follows Table 430.52, which permits an inverse time breaker at up to 250 percent of full load current for these motors: 50 x 2.5 = 125 amps. The breaker can hold through a 300 amp surge for a second because a thermal magnetic breaker trips on heat over time, not on peak. Three phase motors do the same job with less current: a 10 HP motor at 230 volts three phase lists 28 amps in Table 430.250. Soft starters and variable frequency drives cut the inrush to around 1.5 times full load, which is one way to add a big machine to a panel without a service upgrade.

What 100 amps costs to run

Watts become dollars through kilowatt hours. A 100 amp load at 240 volts is 24,000 watts, so one hour at full draw is 24 kWh on the meter. The Energy Information Administration puts the average US residential price at 18.16 cents per kWh for August 2026 (EIA Electric Power Monthly, Table 5.6.A, released October 23, 2026), so a full hour costs 24 x 0.1816 = 4.36 dollars. Run it 3 hours a day and the bill grows by 72 kWh, about 13.08 dollars a day or 392.26 dollars over a 30 day month. Local prices swing that figure widely, from under 12 cents in some states to over 29 cents in California in the same EIA table.

Two refinements matter. First, few loads sit at 100 amps for an hour: a water heater cycles, a range pulses, and the panel total is an average across everything running. Second, larger customers are often billed on demand or on kVA, so the power factor from the three phase section turns into money directly: at a 0.85 power factor, 100 amps at 480 volts is 70.7 kW of billable real power against 83.1 kVA of apparent power. The calculator below runs the cost line for the hours and price you enter, and the EIA Electric Power Monthly keeps the current state by state prices.

Using the calculator

Enter the current in amps (100 is the default), the voltage, single or three phase, and a power factor, and the results table shows real power in watts and kilowatts, apparent power in volt amps, the breaker that covers the load, and the smallest copper wire that carries it at 75 C. Check the continuous box when the load runs 3 hours or more and the breaker and wire rows apply the 125 percent rule. Add the run hours and your price per kWh and the last two rows show the energy and the cost of running the load that long.

The tool covers the arithmetic, not the judgment calls in Article 220 load calculations, ambient temperature corrections or conductor bundling, and it is a planning aid rather than a substitute for a licensed electrician. For the numbers on a real service upgrade, the load calculation your electrician files with the permit is the document that decides between 100 and 125 amps.