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Old 04-07-2013, 04:04 AM   #1
jdlev
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Default Question about element amperage draw and 240 vs 120?

I was reading the information in the general overview of an electric brew build by coderage (very nice article by the way), and had a quick question.

I don't understand why if you have a 5500w element that is made for 240v, that if you put that on a 120v line, why the maximum wattage you can attain is 1375? He says that if you go from a 240v element and run it on a 120v line, you have to divide the usable watts by 4. Why would you do that instead of 2?

Another question I had, if we have a 2000w 120v element, and ran it on a 15amp breaker, I assume it would trip the breaker, no? Because 2000w/120v = 16.x amps? I guess what I am really asking is does the load determine how many amps are drawn (ie the element) or is it the breaker (in which case the breaker would only allow 15 amps to flow through the circuit, and only opeatre the elment at 1800w)?

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Old 04-07-2013, 04:45 AM   #2
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The reason that the power output of the element is 1/4 the power on 120V is due to the fact that the element is a resistive element. So, let's take your element for example.

P(Watts)=V(Volts)^2/R(Resistance in Ohms)

or another way of putting it

R=V^2/P

so your 5500W element will have 240^2/5500= 10.47 Ohms Resistance

When running at 240V, the current across the element is I = V/R = 240/10.47 = 22.92 A or about 23 A.

If you then reduce the voltage to 120V, the current is correspondingly halved also: I = V/R = 120/10.47 = 11.46A.

Now, we calculate the power produced:

P = VI = 120*11.46 = 1375.2W

The key to the 1/4 power is that because of the constant resistance, when you reduce the voltage by half, you also reduce the current flowing through the element also by half, hence the 1/4 power.

As for your second question, your first assumption is correct. That element will trip a breaker rated at 15 A. Breakers work by opening the circuit when the excess amperage heats up a bi-metal strip inside the breaker. Like your turn signal in your car (which operates in a similar fashion), when this bi-metal strip heats up, one side of the strip will contract faster than the other essentially bending the strip. When the strip bends enough, it operates the switch, opening the circuit.

There are also newer types of circuit breakers that use an electromagnet to open the circuit. These electromagnets will gain magnetism with increasing amperage. Once the amperage rises above a safe level (the rated amperage of the breaker) the electromagnet actuates the breaker, thereby opening the circuit.

I know that is a little longer answer than you anticipated, but at least now you have a little better understanding of how these things work!

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Old 04-08-2013, 01:46 PM   #3
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Thanks a ton ty. Very informative!

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Old 04-08-2013, 01:57 PM   #4
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so, if I had a point of use water heater that came with a 2000watt element, I would have to have a 20amp circuit/breaker/wiring. but if I replaced the element with a 1500watt, I could use a 15 amp circuit but the water would heat more slowly?

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Old 04-08-2013, 02:23 PM   #5
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Quote:
Originally Posted by jdlev View Post

Another question I had, if we have a 2000w 120v element, and ran it on a 15amp breaker, I assume it would trip the breaker, no? Because 2000w/120v = 16.x amps?
It might not, at least not right away. Breakers are designed to operate in different ways depending on the application. A typical 15 amp breaker will sustain higher currents for a period of time with the period of time depending on the amount of overage. They are often rated in terms of a specific 'I-squared-T' which is the product of the square of the current multiplied by the time for which the over current flows. Heater operated breakers work this way. For example a typical molded home panel type Square D breaker may (depending on manufacturing variations) tolerate a load 20% greater than it's rating indefinitely, a load double it's nameplate rating for up to 100 seconds, a load 6 times it's nameplate rating for 10 seconds and so on. This allows the breaker to do things like operate appliances with motors (which often have an inrush current of double the running load) without tripping and while still protecting the house wiring (I^2*t is proportional to the energy delivered to the wiring).

Modern breakers are equipped with both bi-metal heater and magnetic tripping devices. The typical Square D breakers mentioned above will trip instantly (or as instantly as an electo-mechanical device can) if the load is 8 - 20 times the labeled rating.

This does not mean you should plan to put a 16.667 amp (2000 watt 120V) load on a 15 amp circuit even though you might 'get away with it'.
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Old 04-08-2013, 04:59 PM   #6
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Quote:
Originally Posted by cageybee View Post
so, if I had a point of use water heater that came with a 2000watt element, I would have to have a 20amp circuit/breaker/wiring. but if I replaced the element with a 1500watt, I could use a 15 amp circuit but the water would heat more slowly?
A 2000 watt element draws 16.6 amps (2000/120).
A 1500 watt element draws 12.5 amps (1500/120).
If your goal is to drop below 15 amps while loosing as little heat as possible, replace the 2000 watt element with a 1650 watt element (1650/120 = 13.75 Amps).
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