air handler name plates

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kbsparky

Senior Member
Location
Delmarva, USA
You don't have to find the resistance here.

If you have a resistance load of 40 Amps@240 Volts.

Simply take the voltage proportion against the rated Amps:

208/240 = .86666 Now multiply that against the rated Amps of 40 = 34.666

To find out the new output in watts using the lower supply voltage, then carry it one step further: 208x34.666 = 7210 watts.

Rule of thumb when connecting a 240 rated heater to a 208 source = 25% drop in watts output.

This was the subject of one question on my licensing exam:
Given: 2000 watt baseboard heater unit rated for 240 Volts.
Connect this same heater to a 120 Volt source.

What will be the wattage output of this heater so connected?

  • 2000
  • 1500
  • 1000
  • 500
Using the formula of Watts = VoltsxAmps we can allocate proportions thus:

Volts/2 x Amps/2 = watts/4

2000 Watts divided 4 = 500.

Now connect this same heater to a 208 Volt source. What will be the output in watts?

  • 2000
  • 1500
  • 1000
  • 500

.8666 Volts x .8666 Amps = .75 watts.

75% of 2000 = 1500 watts.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
There's a big difference between using simple Ohm's Law math for a varying voltage across a constant-impedance load, and making an equipment change, which means varying the impedance, for different design voltages.

When we say that voltage varies inversely with current for a given power level, we're talking about changing the load's characteristics to suit a change in supply voltage, because the goal is to be able to deliver a given load.

The design power output is the constant, and the load impedance is the direct variable. Then, the voltage we actually supply, and the resultant current, still follow Ohm's law. Note that most motor types don't follow this behavior.

A motor attempts to deliver the necessary torque the load demands, and will use more current as it's slowed by the load in an attempt to speed back up. That's why overloaded motors get hot; the increased current.

When we're talking about what happens when we vary the voltage across a given resistance, with no equipment change, then the load impedance is the constant; the supply voltage, and the resultant current, are variables.

Did you just swallow an engineering textbook?
 

ronaldrc

Senior Member
Location
Tennessee
You don't have to find the resistance here.

If you have a resistance load of 40 Amps@240 Volts.

Simply take the voltage proportion against the rated Amps:

208/240 = .86666 Now multiply that against the rated Amps of 40 = 34.666

To find out the new output in watts using the lower supply voltage, then carry it one step further: 208x34.666 = 7210 watts.

Rule of thumb when connecting a 240 rated heater to a 208 source = 25% drop in watts output.

This was the subject of one question on my licensing exam:

Using the formula of Watts = VoltsxAmps we can allocate proportions thus:

Volts/2 x Amps/2 = watts/4

2000 Watts divided 4 = 500.



.8666 Volts x .8666 Amps = .75 watts.

75% of 2000 = 1500 watts.



Yelp or use my calculator

Larry thats what Bob said.


Here a Calculator for converting Wattage when the voltage is higher or lower.
Bottom two calculators on this web page.

http://home.comcast.net/~ronaldrc/wsb/calculators2.htm
 
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