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Old 01-27-2010, 10:32 PM   #1
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VA (volt amps)


I have an electric heater which I will connect to a contactor and a 24 volt 2 pole themostat. I am searching for a 120V to 24 volt plug in type transformer and I am confused. I see 24 volt AC transformers, all with VA (volt amps) specs. Some are at 20VA, some at 40VA and 50VA. How important is the VA and how should it influence my thermostat choice?
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Old 01-27-2010, 10:34 PM   #2
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In very basic terms VA is equivalent to watts.
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Old 01-27-2010, 11:06 PM   #3
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You would need to match the VA to the requirments of the contactor. 24 volt 20 VA would supply 24 volts at .8 amps, 24 volts 40 VA would supply 24 volts at 1.7 amps.
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Old 01-28-2010, 12:54 PM   #4
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VA (volt amps)


hayewe farm---in A=(VA*PF)/V, what power factor are you using or did your amps results come from a table? also what is the value of V? 24 volts?
Thanks!
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Old 01-28-2010, 06:14 PM   #5
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VA =volts times amps. 24 volts times .8 amps =19.2 VA I did round the amps it should be .83333333333333 amps.
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Old 01-28-2010, 06:47 PM   #6
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VA (volt amps)


Thanks for your help.
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Old 01-29-2010, 10:55 AM   #7
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Quote:
Originally Posted by hayewe farm View Post
VA =volts times amps. 24 volts times .8 amps =19.2 VA I did round the amps it should be .83333333333333 amps.
Are you sure that's what it is? I see UPSes and transformers rated in VA's all the time but the wattage is actually a bit lower. For example I have a 1 KVA UPS at home but it's actually 600W max. I've always wondered how VA's work as some stuff is rated in them.
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Old 01-29-2010, 01:06 PM   #8
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The industry standard for rating small UPS units (1000 VA or so, or smaller) is to give a VA rating 65% greater than the watts rating, or a watts rating 60% of the VA rating.

AC equipment containing coils or capacitors is likely to draw amperes at a given voltage so that VA is greater than the watts consumed. The ratio of watts to VA is called the power factor. Personal computers often have a capacitor power supply and a power factor in the 60% range.

For DC equipment and for AC equipment that is purely resistive (such as non-motorized heaters and incandescent lights) VA equals watts, or the power factor is 1.

You may not exceed the watts rating of the UPS or transformer.
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Old 01-29-2010, 02:15 PM   #9
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You could get one of these units, which has the transformer, and relay built-in.
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