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Old 06-29-2014, 07:59 PM   #1
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


Looking at a new contactor - single pole, 24v coil

VAC...........FLA....LRA......RES
240/277....30.....180......40

Can I assume the "40" is a resistive amps number and A/C fan, blower and compressor motors are not resistive loads?


Last edited by justplumducky; 06-29-2014 at 08:04 PM.
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Old 06-30-2014, 12:29 AM   #2
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


Volt-ampere (VA) is a measurement of power in a direct current ( DC ) electrical circuit. The VA specification is also used in alternating current ( AC ) circuits, but it is less precise in this application, because it represents apparent power , which often differs from true power .
In a DC circuit, 1 VA is the equivalent of one watt (1 W). The power, P (in watts) in a DC circuit is equal to the product of the voltage V (in volt s) and the current I (in ampere s):
P = VI
In an AC circuit, power and VA mean the same thing only when there is no reactance . Reactance is introduced when a circuit contains an inductor or capacitor . Because most AC circuits contain reactance, the VA figure is greater than the actual dissipated or delivered power in watts. This can cause confusion in specifications for power supplies. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 watts, unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is 1/2 to 2/3 of the VA rating.
When purchasing a power source such as an uninterruptible power supply ( UPS ) for use with electronic equipment (including computers, monitors, and other peripherals), be sure the VA specifications for the equipment are used when determining the minimum ratings for the power supply. The VA figure is nominally 1.67 times (167 percent of) the power consumption in watts. Alternatively, you can multiply the VA rating of the power supply by 0.6 (60 percent) to get a good idea of its power-delivering capability in watts.

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Old 06-30-2014, 06:03 AM   #3
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


Quote:
Originally Posted by Doc Holliday View Post
Volt-ampere (VA) is a measurement of power in a direct current ( DC ) electrical circuit. The VA specification is also used in alternating current ( AC ) circuits, but it is less precise in this application, because it represents apparent power , which often differs from true power .
In a DC circuit, 1 VA is the equivalent of one watt (1 W). The power, P (in watts) in a DC circuit is equal to the product of the voltage V (in volt s) and the current I (in ampere s):
P = VI
In an AC circuit, power and VA mean the same thing only when there is no reactance . Reactance is introduced when a circuit contains an inductor or capacitor . Because most AC circuits contain reactance, the VA figure is greater than the actual dissipated or delivered power in watts. This can cause confusion in specifications for power supplies. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 watts, unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is 1/2 to 2/3 of the VA rating.
When purchasing a power source such as an uninterruptible power supply ( UPS ) for use with electronic equipment (including computers, monitors, and other peripherals), be sure the VA specifications for the equipment are used when determining the minimum ratings for the power supply. The VA figure is nominally 1.67 times (167 percent of) the power consumption in watts. Alternatively, you can multiply the VA rating of the power supply by 0.6 (60 percent) to get a good idea of its power-delivering capability in watts.
Thx Doc. I had google and found that online also, because I was wanting to know how to calculate the current a xfmr puts out (found: VA divided by Volts). I found that same article, but don't see how it applies to my contactor question.
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Old 06-30-2014, 08:42 AM   #4
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


I believe the RES is the Resistive amps that the contacts are rated to handle the current load of. Some will give hp ratings at different voltages, and FLA as you described, But the RES is a higher value primarily because there is no inrush current to be worried about in a resistive load. At least that is my understanding. Perhaps a site like FURNAS may have more information.
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Old 06-30-2014, 09:54 AM   #5
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


It is resistive load. Contactors are used to turn on elements in some electric furnaces and that is a pure resistive load.
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Old 06-30-2014, 04:35 PM   #6
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


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It is resistive load. Contactors are used to turn on elements in some electric furnaces and that is a pure resistive load.
So the "40" number for a resistive load, which I assume means a relatively constant load (heat strips on an electric furnace), as opposed to a variable speed (variable load) PSC Motor, means 40 Amps?

Thanks HVACDave, I'll check out that site.
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Old 06-30-2014, 05:01 PM   #7
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


Most contactors can handle a higher resitive vs inductive load.
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Old 06-30-2014, 07:47 PM   #8
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Does The RES Spec On The Side of Contactor refer to Resistive Amps?


Yes, resistive is strip heaters. Motors are inductive loads.

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