Air handler calculation

[Tav1980]

Hello!

I have a question about an air handler calculation that I am pretty sure I know the answer to, but just need a sanity check.

On the nameplate, with the heating kit that is installed, it says the MCA is 49.1/55.6. It is a dual voltage air handler that can handle either 208 V or 240 V.

Going with the principal higher voltage equals lower amps, am I right to assume that the 49.1 amps correlates to 208 V and the 55.6 amps correlates to when 240 V is used?

Thanks in advance!

 

[hbiss]

Correct.

-Hal

 

[wwhitney]

How to answer this question depends on whether the load is mostly motors or mostly resistance heaters.

For a resistance heater load, the current will be proportional to the voltage, so the lower current would be for the lower voltage. A motor load often behaves as a constant power load, in which case the lower current would be for the higher voltage.

I suspect you're in the case where most of the load is a big heater, and the motor contribution is small. In that case, the lower current corresponds to the lower voltage.

This assumes your dual current information is from a single nameplate on a single heater, as opposed to comparing two different heaters, each of which is designed and labeled for a different voltage. As in the latter case, the heater resistance would differ between the two units.

Cheers, Wayne

 

[LarryFine]

Going with the principal higher voltage equals lower amps, am I right to assume that the 49.1 amps correlates to 208 V and the 55.6 amps correlates to when 240 V is used?

No. Those numbers correlate because current varies proportionately with voltage for a resistive load.

The only times current varies inversely with voltage is when doing the math or with inductive loads.

 

[infinity]

Going with the principal higher voltage equals lower amps, am I right to assume that the 49.1 amps correlates to 208 V and the 55.6 amps correlates to when 240 V is used?

Yes the 49.1 amps correlates to the lower voltage. That is opposite of what is in bold. The reason being as mentioned the resistive load increases the current with the increase in voltage.

 

[LarryFine]

To expand a bit, when juggling numbers, it's important to know and remember which are the variables and which are the constants.

There's a big difference between varying the voltage applied to a given load ("constant" impedance) and designing a given load ("constant" power) for a different applied voltage.

And, when having theoretical discussions, we generally adopt an all-other-things-being-equal position, and keep in mind that real-world conditions do affect the results a bit.