HVAC nameplate ratings -lower voltage=lower amperage?

[switchleg45]

I, as well as many of my colleagues, would like to know: If the nameplate rating on an HVAC air handling unit w/heater kit is "Volts: 240/208, Amps 40.0/34.6"...the fundamental rule that states "as voltage is increased amperage decreases" seems to not fit. Can one of you highly knowledgeable engineers enlighten me? Can you direct me to information sources I can study to help understand this conflict?

Doing load calcs for Seisco tankless water heater install. AHJ says he will not allow the 40% demand per optional calc, only the 75% demand per the standard calc.

 

[don_resqcapt19]

For all practical purposes, electric heat has a fixed resistance. If you lower the voltage you lower the wattage and the current. You are producing 25% less heat at 208 as compared to 240.

 

[jim dungar]

...the fundamental rule that states "as voltage is increased amperage decreases" seems to not fit. Can one of you highly knowledgeable engineers enlighten me?

There is no fundamental rule that says "as voltage is increased amperage decreases", just as there is no rule that says 'current takes the path of least resistance'.
You have remembered a rule that is valid given certain conditions and then forgotten the conditions.

For a fixed output, raising the voltage decreases the amperage, however very few electrical 'outputs' are fixed.
As Don said, in the case of a heater, the thing that is fixed is the resistance, the output (load) is the heat which varies.