Why is the amperage lower for higher wattage devices?

Can anyone explain how electrical amperage associates to device wattage? From what I understand amperage is the amount of electrical current running through the circuit (if I imaging the AC current as a sine wave, then the amplitude is the distance from the base line to the peak of the sine wave. Though this gets a bit more complex with RMS amplitude).

As far as I understand wattage, that's the power consumption of a device in a given time unit. So for example my oven is a 12 kiloWatt oven. Which means it'll consume 12 kiloWatts every hour? Is that right?

So my question is, why is my oven on a 30 amp circuit when it's 12 kW, and my 8kW shower is on a 45 amp circuit?

Oh yes, I'm in the UK by the way so 240 Volts into the house.

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"Amp" is short for Ampere ( André-Marie Ampère ) rather than "amplitude". I think of amplitude being related to voltage (but I could be wrong). – Dennis Williamson Sep 14 '10 at 22:33
Amplitude is measured strength of a particular electrical property (voltage or current). There are current and voltage amplifiers. Power amplifiers multiply both voltage and current. Which probably doesn't have anything to do with Amp which is what should be used in the question. – Fiasco Labs Dec 15 '12 at 22:30

4 Answers

(Both of those circuits are 240V- 2 hot wires on a double-pole breaker. U.S. power used to be 110V per leg, then 115; now it's 120.)

The oven wattage is inflated. 30A circuit * 2 * 120V = 7,200W. Any more than that for an extended period of time, and the breaker will trip. You aren't getting 12,000W to it unless your breaker has failed, and then you'd be at risk of an electrical fire.

Since the oven is heating air, it needs a lot less average power than something that's heating water- water requires a ton of energy to raise its temperature. Hence the smaller wire to the oven.

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Thanks for your answer. Could you expand on what 2 hot wires are? Does that mean 2 live wires? – Ceilingfish Aug 22 '10 at 17:34
Not really. 3 wires come into the building from the transformer in your standard residential installation. Two of them are "hot", connected to the wires coming from the power plant. The third is "neutral", connected to ground potential. The hot wires alternate pushing and pulling current from the utility service. When current is pushed through one wire, it is also being pulled back through the other wire to complete the circuit. Both are "live" and able to kill you quite easily. This may help explain it: hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html – nstenz Aug 24 '10 at 20:17

You likely answere your own question -- wattage is Voltage times Amperage. What you might not know is that in higher wattage situations, they'll typically run a higher voltage (eg. 220V) circuit, so it's possible that the 30Amp circuit is actually getting more total power than the 45Amp circuit.

But those aren't your only considerations when sizing the breakers -- motors tend to draw about 50% over whatever their rating is when they first kick on. You can either place them on a high-rated circuit, or you can put them on a 'slow trip' fuse (or circuit breaker) that won't trip the instant it sees a surge; naturally, this is not something you want to install in a wet area where you'll need a GFI breaker. Because of this, particularly if they're going to be installed in wet area (where you don't want to use a slow-trip), you'll see higher than expected breakers for anything with a compressor, pump, or other motor.

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Short answer:

The 12 kW rating on your oven indicates the maximum power it will draw under any circumstances. The breaker is underrated but it still works because you don't use all functions of your oven at full power for long enough.

Long answer:

Units:

Wattage (or power) is the energy consumption over time. One Watt (unit of power) equals to one Joule (unit of energy) per second (unit of time): 1 W = 1 J/s (Consequently, you can also call a Joule a Wattsecond.)

For household electricity, it is common to use Watthours instead of Joules or Wattseconds. Obviously, 1 Watt (unit of power) is one Watthour (alternative unit of energy) per hour (unit of time): 1 W = 1 Wh/h

Of course, you can use the usual SI prefixes, so one kilowatt is 1000 watts and one kilowatthour is 1000 watthours: 1 kW = 1000 W; 1 kWh = 1000 Wh. As there a 3,600 seconds in one hour, 1 Wh = 3,600 Ws = 3.6 kJ.

Wattage (or power) also is the voltage multiplied with the electrical current (or amperage). One Watt (unit of power) equals to one Volt (unit of voltage) times one Ampere (unit of electrical current).

Wiring:

In the UK, the normal household voltage is 240 V. (Nominally, it has recently been changed to 230 V ± 10 %. The actual voltage should still be closer to 240 V for now.) You will generally have a live wire (also known as phase, hot or active wire) at 240 V (alternating current) and a neutral wire (also known as cold wire) at 0 V, which is ultimately connected to ground (also known as earth). This is also true for high-power devices.

In the US, the normal household voltage is 120 V. That's not enough to power high-power devices such as ovens or showers. In the US, you will often have a setup where you have two live wires, one at +120 V and one at -120 V, yielding a total voltage of 240 V.

Alternating Current:

With alternating current, the voltage is not constantly at 240 V. That value is the average voltage, calculated as the root mean square (rms). The actual voltage is a sine wave with peaks at -339.4 V and +339.4 V (± 240 V * sqrt(2)).

Because it's a wave, the term amplitude comes in. The amplitude is simply the "height" of a wave. There are, of course, different ways to calculate the height/amplitude:

With 240 V power, the rms amplitude (average height) is 240 V, whereas the peak-to-peak amplitude (total height) is 339.4 V - (-339.4 V) = 678.8 V.

Your setup:

The 30 A rating on your oven breaker means that it will allow the connected appliances to continuously draw an electrical power of at *least* 30 A * 240 V = 7200 W = 7.2 kW. It will, however, also allow higher power consumption for a short time. For example, a modern breaker with 'B' characteristics is allowed to provide up to 1.45 times the rated power for up to one hour.

The 12 kW rating on your oven indicates the maximum theoretical power consumption of your oven at any time. Most of the time, it will draw much less power.

While 12 kW is more than 7.2 kW, that does not mean your breaker will constantly trip. You will normally not use full power (all cookers, all functions of the oven) for more than an hour.

Optimally, however, the rating on the oven would be lower than the rating of the breaker (and the wires). Then it would be impossible to trip the breaker in normal operation unless there's a defect.

The 45 A rating on your shower breaker indicates a maximum constant power of 45 A * 240 V = 10,800 W = 10.8 kW. That's enough for a shower rated at 8 kW.

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Point of order: it's 230V in the UK (same as the rest of Europe) and 120V in the US. See en.wikipedia.org/wiki/Mains_electricity_by_country – Niall C. Dec 15 '12 at 22:59
Thank's for the correction. I've edited my answer accordingly. IIRC, the UK has recently switched from nominal 240 V to nominal 230 V but the actual voltage is still 240 V, which is within the allowed tolerance. – cfaerber Dec 15 '12 at 23:47
Hah! Sneaky way to put one over on the Eurocrats. :) – Niall C. Dec 16 '12 at 0:19

Good question. I don't think I have a direct answer but here's some information that may lend itself to an answer.

Watts = Amps * Volts

For example, a 30 amp dedicated circuit to your oven * 120 volts (U.S. houses often have somewhere between 110 and 120 volts coming in) = 3,600 watts delivered to your oven. Hmm...

45 amps to your shower (water heater) * 120 volts = 5,400 watts to your shower.

Something isn't adding up since you said your oven is rated at 12,000 watts and shower at 8,000 watts. Even if you live outside the US and get 240 volts that would double your wattage but that's still not enough.

Either way, I suspect that what you're seeing is not that higher wattage devices need lower current (amps). I think there are two factors at play:

1. Higher wattage devices generally need higher current because it's hard to multiply a fixed voltage (110 to 240) by a low current (amps) and get a high wattage. You need high current to get high wattage.
2. Some devices, like electric ovens and water heaters, need especially high current so that they can very quickly heat up an element.

Perhaps a water heater just needs especially high current to heat a high volume of water even when compared to an oven which only has to heat a relatively low volume of air.

Disclaimer: I'm not an electrician so I may be way off.

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