Wednesday, December 17, 2014

Ohms Law and Wattages for Electric Heaters and Elements

Flow Factor and Ohms Law

Electric Heaters and heating elements become hot due to a voltage being pushed through a wire.  The resistance in the wire measured in Ohms creates the heat.  The more resistance the hotter the wire gets. The more voltage pushed through the wire, the hotter the wire gets. Simply the smaller the wire (or the longer the wire run) the more resistance it gives to the voltage passing through.

So as voltage increases the wattage will increase four fold.  As the voltage decreases the wattage will decrease by a factor of 4 as well.  The below table shows the percentage of drop or gain with different applied voltages to a heating element.

Rated at 480 volts the expected wattage % is
575 applied  144%
480 applied  100% (Rated Watttage)
440 applied   84%
240 applied   25%
120 applied    6%

Rated at 277 volts the expected wattage % is
277 applied 100% (rated Wattage)
240 applied  75%
230 applied  69%
208 applied  56%
120 applied  19%

Rated at 240 volts the expected wattage % is
277 applied 133%
240 applied 100% (rated wattage)
230 applied  92%
208 applied  75%
120 applied  25%  (1/4 wattage)

Rated at 120 volts the expected wattage % is
240 Applied  400%  (4 times wattage - Dangerous) 
208 Applied  300%  (3 times wattage - Dangerous)
120 Applied  100%  (rated Wattage)
110 Applied   84%

This is all based on Ohm's Law or I=E/R

Simple Ohms Law equations are:

Watts = VOLTS x AMPS
Volts = AMPS x OHMS

A Conductor Wire's resistance is constant based on wire size and length.
A heater's watt rating is based on the wire used in the heating element and the voltage applied.
To get a 15 kilowatt rated heater run at 480 volts; a wire with a resistance of 15.4 Ohms is used.
(If there are three elements in the heater, then each is rated at 5 kw and the resistance of the wire would be 46.)

Knowing amps is very important for large immersion heater loads to size contractors, SCR's, Fusing and Thermostats.

Contact Flow Factor for all sizing help with heaters and accessories.