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Thread: What's the difference between 240 volts and 220 volts?

  1. #16
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Resistive load... P=E*I but I=E/R, so P=E^2/R.

  2. #17
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Mikey View Post
    Resistive load... P=E*I but I=E/R, so P=E^2/R.
    also

    P= I squared times R

    BUT

    of course this only holds true in prue resistive circuits with DC voltages.
    For AC circuits we must also do a Power Factor calculation.

  3. #18
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Quote Originally Posted by jwelectric View Post
    also

    P= I squared times R

    BUT

    of course this only holds true in prue resistive circuits with DC voltages.
    For AC circuits we must also do a Power Factor calculation.
    Or use the RMS voltage if it's an AC circuit with a resistive load. The possibilities are endless...

  4. #19
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by hj View Post
    Devices with motors, or more specifically compressors such as freezers and drinking fountains seem to cause GFCI's to trip randomly.
    Could you give us the source for this information. In more than 29 years of experience in the field with GFCI devices I have never run into this except through conversations where no data was produced to back such information.


    Quote Originally Posted by hj View Post
    This is why a freezer ir freezer should NEVER be plugged into a GFCI. It can trip and without a temperature alarm the user will not know about it until the contents have thawed and spoiled.
    Under the 2008 code cycle this would mean that freezers can no longer be in unfinished basements, attached or detached garages or any remote (storage) buildings served electrically from another building.

    Quote Originally Posted by hj View Post
    I had one customer who had it happen, but since the outlet was dead they just ran an extension cord to a working outlet. When I was there for a different problem, the situation came up and I told them it sounded like a GFCI problem, even though that outlet was not a GFCI one. After an extensive search, I found the tripped GFCI under a bench in their bathroom. They had not even known it was there.
    Did you also check out the freezer to find the reason there was a difference between the two conductors?

    Quote Originally Posted by hj View Post
    It is also the reason that many receptacles for certain appliances are installed with a single device outlet without GFCI protection, so that an extension cord cannot be used in that outlet when the location would normally require a GFCI for the extension cord.
    Again this will no longer be allowed by the 2008 code

    Quote Originally Posted by hj View Post
    As far as 220/240 is concerned most devices will tolerate a 10% variation between the design voltage and the supplied voltage, without creating problems.
    Not meaning to sound mean or anything but this is not a true statement. The 240 GFCI devices conform to the same standard as a 120 volt device. Both trip when there is a 5 milliamp difference between the two current carrying conductors.

    Quote Originally Posted by hj View Post
    Resistive devices such as water heaters lose capacity in direct relationship to the loss of voltage.
    I wouldn’t totally agree with this statement either. The lower voltage does not mean that a water heater won’t heat water to the desired temperature it just means it will take it a few seconds longer to get the job done.

  5. #20
    Engineer Furd's Avatar
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    You're all wrong!

    In the case of the water heater let's assume a 4500 watt element operating at 240 volts. Since this is virtually a totally resistive load we can ignore impedance and power factor.

    Current draw in the above example would be 18.75 Amperes. By Ohm's law the resistance would be 12.8 Ohms. The resistance of the heating element is a fixed quantity and is based on the materials and method of manufacture.

    Now let's change the voltage to 120. The resistance is the same (12.8 Ohms) so the Amperage has now dropped to 9.38 Amperes, exactly 1/2 of the amperage at 240 volts, which certainly seems logical. HOWEVER, multiplying that 9.38 Amperes by the 120 volts to get the power gives us an answer of 1125 Watts, exactly 1/4 of the wattage at 240 volts. Therefore, the BTU output of the heating element is also 1/4 as much when run on 120 volts as compared to the design voltage of 240.

    Since there are 3414 BTUs per kilowatt it is easy to see that at 120 volts the heat output is 3840 BTUs and at 240 volts it is 15,363 BTUs or that at the lower voltage the heat output is, like the wattage, 1/4 of the design output.


    As for compressors (refrigerators) causing GFCIs to trip...my refrigerator has been connected to a GFCI circuit breaker for more than eight years and I have NEVER had the breaker trip. Yes, I have tested the GFCI function and it works as it should.

  6. #21
    Moderator & Master Plumber hj's Avatar
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    Default heaters

    Taking longer to heat is what I meant by losing capacity. A 4500 watt, 240v element will draw approximately the same amps at 208 volts, but the wattage will be less, approx. 3800 watts.

  7. #22
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Quote Originally Posted by Furd View Post
    You're all wrong!
    I beg to differ.


    Since there are 3414 BTUs per kilowatt it is easy to see that at 120 volts the heat output is 3840 BTUs and at 240 volts it is 15,363 BTUs or that at the lower voltage the heat output is, like the wattage, 1/4 of the design output.
    or, as I said earlier,

    Quote Originally Posted by Mikey
    Actually, it's a voltage-squared relationship, I think.

  8. #23

    Default I want to add a tankless

    I was reading your post and I understand what you are saying about 208VAC vs. 240VAC. The tankless water heating unit I was looking at needs 240VAC to operate but at 208 it will reduce the units ability by 25%. I am using it at the kitchen sink & dish washer. would you recomend not doing that? and will it not do what I need? I am moving my kitchen to an outside wall from the middle of the house and have to make the run outside and around the house which is a block home on a slab (1950s build home in Florida).

    Thank you for any advice you can give.

  9. #24
    Moderator & Master Plumber hj's Avatar
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    Default power

    You are misstating my answer about the 10% power differential. It has nothing to do with a GFCI, it is whether a 240 volt device will operate properly at 220 volts. It would but 208 volts would be outside its tolerances.

    You also intentionally misunderstand the statement about water heater capacity, i.e., recovery not volume. A 4500 watt/240 volt element only produces 3200 watts at 208 volts. Meaning it takes approximately 20% longer to create the same volume of hot water. There would also be a reduction if it were used with 220 volts. which is why the elements in a water heater should be ordered according to the power available for the best usage.

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