formula

[skittleslove]

how do i figure the watts of a water heater rated 240v 10 kw but supplied with 208v?

[ March 10, 2003, 09:25 AM: Message edited by: skittleslove ]

 

[jim dungar]

Re: formula

Basic formulas for resistance heating:
P=I*E
E=I*R

Working the math the hard way
R=E^2/P -> Rheater=Eold^2/Wold -> Rheater=240^2/10,000 ->Rheater=5.76Ohms

Pnew=Enew^2/Rheater -> Pnew=208^2/5.76 -> Pnew=7,511W -> 7.511KW

Working the math the easy way
Pnew=Pold*((Enew/Eold)^2) -> 10KW*(208/240)^2 -> Pnew=7.52KW

 

[luke warmwater]

Re: formula

Even easier(this is single phase, I take it?):
R=E(E/P)
R=240(240/10000)
R=5.76ohms
I=E/R
I=208/5.76
I=36.11amps
36.11x208=7511.11watts

Easier yet:
(208/240)squared=.7511
.7511xwatts
.7511x10000= 7511watts= 7.51Kw

 

[brad troxel]

Re: formula

Even easier yet,
Watts, is Watts, is Watts,
you will always have 10kw, your currrent will change with your voltage. You size your wire by the wattage of the equipment, your watts on the water heater won't change unless you buy a different wattage element. Look at the element at the hardware stores, anywhere from 2500w to 10kw.

 

[lady sparks lover]

Re: formula

figure out the wattage, and divide by the voltage. that should work.

 

[don_resqcapt19]

Re: formula

Brad,
Look at the math in the previous posts. Ohm's law does apply. The heating element has a fixed resistance and both the current and watts will change when you change the voltage.
Don

 

[roger]

Re: formula

Brad, as Don said look at the earlier posts.

We recently wired a 180kw 240v 3ph steam generator with 208v 3ph. This did allow us to change wire size, and at this size wire, there is a difference if bidding a job.

Roger