Ohm's Law

Ohm's Law defines the relationships between (P) power, (E) voltage, (I)
current, and (R) resistance. One ohm is the resistance value through which
one volt will maintain a current of one ampere.


( I ) Current is what flows on a wire or conductor like water flowing down a
river. Current flows from points of high voltage to points of low voltage on
the surface of a conductor. Current is measured in (A) amperes or amps. 

( E ) Voltage is the difference in electrical potential between two points in
a circuit. It's the push or pressure behind current flow through a circuit,
and is measured in (V) volts. 

( R ) Resistance determines how much current will flow through a component.
Resistors are used to control voltage and current levels. A very high
resistance allows a small amount of current to flow. A very low resistance
allows a large amount of current to flow. Resistance is measured in  ohms. 

( P ) Power is the amount of current times the voltage level at a given point
measured in wattage or watts.




Sizing Guide - How to Calculate Heat Load for your EnclosureTo determine the
correct model for your application, it is first necessary to determine the
total heat load to which the control panel is subjected. This total heat load
is the combination of two factors  heat dissipated within the enclosure and
heat transfer from outside into the enclosure.
To Calculate Btu/hr.

First, determine the approximate watts of heat generated within the
enclosure.  Watts x 3.41 = Btu/hr.

Then, calculate outside heat transfer as follows:a. Determine the area in
square feet exposed to the air, ignoring the top of the cabinet. 
b. Determine the temperature differential between maximum surrounding
temperature and desired internal temperature. Then, using the Temperature
Conversion Table (below), determine the Btu/hr./ft.2 for that differential.
Multiplying the cabinet surface area times Btu/hr./ft.2 provides external
heat transfer in Btu/hr. 

Add internal and external heat loads for total heat load. 
 
Temperature Conversion Table 
Temperature 
Differential F Btu/hr./ft2 
5 1.5 
10 3.3 
15 5.1 
20 7.1 
25 9.1 
30 11.3 
35 13.8 
40 16.2 
 
 

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Example:  Internal heat dissipation: 471 Watts or 1606 Btu/hr.
 
 Cabinet area: 40 ft.2
 
 Maximum outside temperature: 110F
 
 Desired internal temperature: 95F
 

The conversion table (above) shows that a 15F temperature
differential inputs 5.1 Btu/hr./ft.2

40 sq. ft. x 5.1 Btu/hr./ft.2 = 204 Btu/hr. external heat load.

Therefore, 204 Btu/hr. external heat load plus 1606 Btu/hr. internal heat
load = 1810 Btu/hr. total heat load or Btu/hr. refrigeration required to
maintain desired temperature.

In this example, the correct choice is a 2000 Btu/hr. Cabinet Cooler System.
Choose a Cabinet Cooler model by determining the NEMA rating of the enclosure
(type of environment), and with or without thermostat control.